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Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×

2
Transportation Package Safety

Package performance—the ability of a transportation package1 to maintain a high level of containment effectiveness in long-term normal use and under extreme mechanical forces and thermal loading conditions (i.e., thermomechanical conditions) generated during severe accidents—is a crucial issue for transportation safety and key to understanding and quantifying transportation risks. The regulatory requirements for the design, fabrication, certification, and maintenance of these packages are substantially more rigorous than those for transporting most other types of hazardous materials (e.g., hazardous chemicals). Accordingly, the packages used to transport spent fuel and high-level waste are designed to withstand extreme thermomechanical conditions without a significant loss of containment. The slogan “safety is built into the package” is commonly used by package manufacturers and vendors to describe the ability of these packages to maintain their containment effectiveness under most conceivable accident conditions.

This chapter provides a summary of investigations carried out in the United States and several other countries to examine the performance of transportation packages. The chapter provides a brief overview of the re-

1  

Transportation containers loaded with spent fuel or high-level waste are referred to as packages in international standards, whereas the containers themselves without their contents are referred to as packagings. For simplicity, the term packages is used throughout this report. The terms casks and flasks (the latter term is commonly used in the United Kingdom) are sometimes used synonymously to refer to such packages.

Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×

quirements for spent fuel and high-level waste transportation packages; describes some key investigations of package loading conditions that have furthered technical understanding of package performance; and offers findings and recommendations about current standards and regulations and about improvements in package performance.

There is extensive literature on package performance in response to extreme thermomechanical conditions. Some of this work appears in the “gray” literature,2 which may or may not be peer reviewed. There is also extensive classified literature on package performance in response to potential terrorist acts.3 As discussed in Chapter 1, the committee did not review this classified literature or perform an in-depth examination of package performance in response to terrorist acts. The committee does, however, comment on this issue in Chapter 5.

2.1 TRANSPORTATION PACKAGE DESIGNS AND REGULATIONS

Packages for the transport of spent fuel and high-level waste are designed to meet three basic requirements both during normal conditions of transport4 and during a range of hypothetical accident conditions established in 10 CFR Part 71:5

  1. Prevent an unsafe configuration (i.e., accidental criticality6) of spent fuel.

  2. Prevent or limit the release of radioactive contents.

  3. Limit dose rates on external package surfaces to acceptable levels.

A wide range of package designs have been developed to meet these general performance requirements.

2  

The U.S. Interagency Gray Literature Working Group defines gray literature as “foreign or domestic open source material that usually is available through specialized channels and may not enter normal channels or systems of publication, distribution, bibliographic control, or acquisition by booksellers or subscription agents.” (Gray Information Functional Plan, January 18, 1995, accessed at http://www.osti.gov/graylit/whatsnew.html). This literature includes technical reports, conference proceedings, and business documents.

3  

In addition, some unclassified literature on this topic was removed from public circulation after the September 11, 2001, terrorist attacks.

4  

Normal conditions of transport subject packages to minor mishaps due to rough handling or exposure to weather. Such conditions would not be expected to compromise the vital containment functions of the package.

5  

Title 10, Part 71 of the Code of Federal Regulations, Packaging and Transport of Radioactive Material. The hypothetical accident conditions are described in subpart 73 (i.e., 10 CFR 71.73).

6  

That is, a configuration that allows the establishment of a self-sustaining nuclear chain reaction as occurs in an operating nuclear reactor.

Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×

Most packages consist of a hollow cylindrical body that is open at one end (Figure 2.1). The body is typically constructed of multiple layers of the following materials: Steel is used to provide structural strength and durability; steel, lead, depleted uranium, or concrete is used to provide shielding against gamma radiation; and water, borated polymers, or concrete is used to provide shielding against neutrons.

The package closure system consists of one or two steel lids that are attached to the open end of the package body with steel bolts. Elastomer or

FIGURE 2.1 Generic truck and rail spent fuel packages. In these particular designs, lead is used as the shielding material. Other materials such as steel and concrete are used in other package designs. SOURCE: Sprung et al. (2000).

Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×

metal seals are used between the body and lids to obtain airtight seals. The package body and lids may contain sealable openings, such as pipes and tubes, to allow for the removal of water, the addition of inert gases, leak testing of lid seals, and monitoring of internal pressures after the package is closed. The lid, lid bolts, pipes, and valves are usually recessed to protect them from damage.

The package is also designed with impact limiters to absorb mechanical forces generated in the event of transport accidents and to provide thermal protection for the lid seals in case of fires. Impact limiters, which are usually attached to the ends of packages, are typically constructed of wood, rigid foam, or honeycombed metal. Metal fins may also be machined into (or welded onto) the exterior surfaces of the package body and closures for additional impact protection and heat dissipation.

The interior of the package contains a basket to hold the spent fuel assemblies in a fixed configuration to ensure criticality control and minimize damage to the fuel during transport. In packages designed to hold multiple spent fuel assemblies, the baskets are typically constructed of materials containing neutron absorbers (e.g., borated metals) to provide a further margin of criticality safety. In some package designs, these baskets are placed into a stainless steel canister with a welded lid, which in turn is placed into a transportation overpack. Packages containing such canisters are sometimes referred to as canister-based packages. Packages without such canisters are sometimes referred to as bare-fuel packages because the fuel basket is placed directly into the package body.

Packages designed for transport by legal-weight trucks7 typically carry between about 0.5 and 2 metric tons (0.55 and 2.2 short tons) of spent fuel. These packages are about 3 feet (0.9 meter) in diameter and weigh up to about 25 metric tons (28 short tons) when loaded. Packages designed to be transported by train can hold about 10 to 18 metric tons (11 to 20 short tons) of spent fuel. The packages typically have diameters of about 8 feet (2.4 meters) and can weigh 150 metric tons (165 short tons) or more when loaded.

The number and types of spent fuel assemblies that can be carried in a package are, of course, determined by its size. Package size, in turn, depends on transportation mode (i.e., rail versus truck). Legal-weight truck package size is limited primarily by highway weight regulations, whereas rail package size is usually not weight limited but instead limited by railcar clearance requirements. Regulatory limits on radiation levels on the exterior surfaces of packages (10 CFR 71.47) also restrict the age and burn-up of spent fuel that can be carried in the package.

7  

A truck having a total gross weight (i.e., including cargo) of 80,000 pounds (36,300 kilograms) or less.

Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×

The International Atomic Energy Agency (IAEA) has established standards for the safe transportation of radioactive materials. These standards were first issued in 19618 and have undergone several revisions and amendments; the latest edition was issued in 2005 (IAEA, 2005a). The IAEA standards establish recommended requirements for a number of different package types, each designed to transport specific quantities and types of radioactive materials:

  • Excepted packages are designed for transport of very small quantities of radioactive material such as radiopharmaceuticals.

  • Industrial packages are designed for transport of low-specific-activity materials such as uranium ore and low-level radioactive wastes.

  • Type A packages are designed for transport of materials of limited radioactivity—for example, uranium hexafluoride and fresh nuclear fuel.

  • Type B packages are designed for transport of larger quantities of radioactive material including spent fuel, high-level waste, and mixed oxide fuel.9

  • Type C packages are designed for air transport of quantities of radioactive material exceeding a defined (large) threshold including, for example, plutonium and mixed oxide fuel.

This report is concerned with Type B packages.

IAEA safety standards are recommendations and are not legally binding on member states. However, the United States and many other member states adopt these standards, either in whole or in part, in their own national regulations. U.S. regulations for the packaging and transport of radioactive materials are provided in 10 CFR Part 71 (see footnote 5). The Nuclear Regulatory Commission (USNRC) and Department of Transportation (DOT) recently modified these regulations (USNRC, 2004a) to reflect the 1996 amended version of the IAEA standards (IAEA, 2000).10,11

8  

The first comprehensive set of regulations for transporting radioactive materials by common carrier were established by the Interstate Commerce Commission in 1947–1948. These regulations were drafted by the National Research Council’s Subcommittee on Shipment of Radioactive Substances (see NRC, 1951). These and subsequent U.S. regulations served as an important basis for the establishment of IAEA standards (see Pope, 2004).

9  

Mixed oxide fuel, or MOX, contains uranium and plutonium.

10  

The updated regulations did not incorporate the IAEA standards for Type C packages, because U.S. federal law mandates more stringent requirements for air shipments of plutonium.

11  

The IAEA issued updated safety standards in 2003 and 2005 (see http://www-ns.iaea.org/standards/documentpages/transport-of-radioactive-material.htm). USNRC and DOT staff were discussing whether to update U.S. regulations to bring them into conformance with these updated standards when this report was being finalized for publication in December 2005.

Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×

Type B packages that are to be used for transporting spent fuel and high-level waste in the United States are required to be certified by the USNRC. To receive a certification, the applicant (i.e., the package manufacturer or vendor) must demonstrate to the Commission’s satisfaction that the package design will meet the testing requirements in 10 CFR Part 71, the key elements of which are described in Sidebar 2.1. Under these regulations, packages are required to survive a free-drop test onto an essentially unyielding surface (Sidebar 2.2) as well as a puncture test, an immersion test,12 and a thermal test with less than the specified loss of containment effectiveness (see Sidebar 2.1).

The USNRC permits quantitative analysis (e.g., computer simulations using finite element models), scale-model (typically one-quarter or one-half scale; see Sidebar 2.3), and full-scale testing of packages or package components, and comparisons with existing approved package designs to be used to demonstrate compliance with the regulations. Testing of full-scale packages13 is not a requirement of the regulations.

Testing of full-scale spent fuel packages is not carried out routinely because of the cost. A full-scale package can cost more than a million dollars, and it is generally not reusable after undergoing full-scale testing in accordance with USNRC regulations. The paucity and costs of suitable testing facilities are also impediments. There are no package testing facilities in the United States capable of handling large truck or rail packages. A new facility was recently opened in the Horstwalde region in Germany, which is located near Berlin.14 Two full-scale 9-meter (30-foot) free-drop tests on rail packages were carried out at this facility in late September 2004 in conjunction with the 14th International Symposium on Packaging and Transportation of Radioactive Materials (PATRAM 2004). A subgroup of the committee’s members visited this facility and witnessed a full-scale test of a 180 metric ton (198 short ton) rail package (Figure 2.2).

USNRC regulations also require that transportation packages be de-

12  

Package immersion is not discussed at much length in this chapter because the committee judges it to be of a lower concern than the thermomechanical conditions generated during truck and train accidents.

13  

There is sometimes confusion about what is meant by “full scale” in regard to package testing. The regulatory tests described in Sidebar 2.1 are full-scale tests because they are carried out using actual Type B packages. These tests are referred to as certification tests. The term full-scale can also apply to tests made on actual Type B packages under simulated accident conditions such as those described in Section 2.3. These tests are referred to as demonstration tests.

14  

This facility is operated by the German Federal Institute for Materials Research and Testing (BAM).

Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×

signed and manufactured using an approved quality assurance program.15 Packages must be designed to standards that include conservative assumptions and design margins for material properties such as yield stress and ductility. This requirement provides for a built-in “safety margin” (see Sidebar 2.4) and offers increased confidence that packages will survive thermomechanical conditions somewhat more severe than regulatory requirements.

The USNRC has certified several spent fuel storage and transportation package designs for use in the United States. These include storage-only packages as well as packages that are designed for both transportation and storage (see JAI, 2005).

IAEA standards and USNRC regulations for Type B packages (see Sidebar 2.1) were not derived from a comprehensive bounding analysis of all possible extreme thermomechanical conditions resulting from package mishandling and accidents. Rather, their development, which dates from the early 1960s as noted previously, was based on then-available data on typical impacts (e.g., drops from cranes, other mechanical accidents, vehicular collisions) and thermal environments (e.g., fires from spilled fuel) to which a package might be exposed in the course of transport (see Pope, 2004). As such, they do not necessarily reflect the most extreme conditions that might be encountered during spent fuel or high-level waste shipments.

Nevertheless, during the committee’s information-gathering meetings, several industry presenters asserted that it is very unlikely that a certified Type B package for spent fuel or high-level waste would fail under any credible loading conditions that might be encountered during transport. This assertion is based on the confidence that these presenters place in the combination of rigorous regulatory requirements for package certification, the built-in margins of safety in current package designs (see Sidebar 2.4), and the worldwide decades-long record of spent fuel transport without a significant package failure (see Chapter 3). However, other presenters pointed out that the spent fuel transport experience in the United States is limited, and that the planned large future shipping campaigns to a federal repository could expose transportation packages to a wider range of loading conditions and longer-term use than have been experienced to date.

15  

10 CFR 71.101 defines the term quality assurance as those “planned and systematic actions necessary to provide adequate confidence that a system or component will perform satisfactorily in service. Quality assurance includes quality control, which comprises those quality assurance actions related to control of the physical characteristics and quality of the material or component to predetermined requirements.” Quality assurance requirements apply to the design, fabrication, assembly, testing, maintenance, repair, modification, and use of the proposed package.

Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×

SIDEBAR 2.1
U.S. Regulations for Type B Transport Packages

Type B packages for transporting spent fuel and high-level waste are designed to withstand severe accident conditions without a loss of containment or an increase in external radiation to levels that would endanger emergency responders or the general public. USNRC regulations for Type B packages contain requirements for both “normal conditions of transport” and “hypothetical accident conditions.” Under normal transport conditions, the regulations require that Type B packages maintain their containment effectiveness as given by the following three conditions:

  1. No loss or dispersal of radioactive contents to a sensitivity of 10−6A2 per hour.a

  2. No substantial reduction in packaging effectiveness.

  3. No substantial increase in external surface radiation levels.

For hypothetical accident conditions, the regulations allow for some degradation of the packages’ containment effectiveness as given by the following four conditions:

  1. No escape of krypton-85 exceeding 10 A2 in 1 week.

  2. No escape of other radioactive material exceeding a total amount A2 in 1 week.

  3. No external radiation dose rate exceeding 10 millisieverts per hour (1 rem per hour) at 1 meter (about 40 inches) from the external surface of the package.b

  4. Compliance with these requirements may not depend on filters or mechanical cooling systems.

These release and radiation limits are designed primarily to protect emergency responders and members of the public in case of an accident. The values for A2 are tabulated in 10 CFR Part 71 and are radionuclide specific. For krypton-85, the A2 value from Table A-1 in 10 CFR Part 71 is 10 terabecquerels (TBq; approximately 270 curies).c The regulations do not place any limits on the physical form of the radioactive materials (e.g., particulate size) that could be released in a hypothetical accident.

The USNRC is responsible for certifying the design of Type B packages. The requirements for certification (10 CFR 71.73) are derived directly from IAEA standards:

Evaluation for hypothetical accident conditions is to be based on the sequential application of the tests specified in this section, in the order indicated, to determine their cumulative effect on a package or array of packages…. [E]xcept for the water immersion tests, the ambient air temperatures before and after the tests must remain constant at that value between −29°C and +38°C which is the most unfavorable for the feature under consideration. The initial internal pressure within the containment system must be the maximum normal operating pressure, unless a lower internal pressure, consistent with the ambient temperature presumed to precede and follow the tests, is more unfavorable.

a  

The regulations also provide guidance on determining A2 activity values for mixtures of radionuclides. See the glossary (Appendix D).

b  

These increased doses would result from structural damage to the package shielding.

c  

The fission product krypton-85 was selected for use in this regulation because it is the only noble gas that exists in significant quantities in irradiated fuel that has been cooled for several years. While it is not a significant contributor to dose, it is likely to be among the most mobile of radionuclides in a spent fuel package.

Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×

The following tests are specified in the regulations (10 CFR 71.73; see figure below):

  • A free-drop test in which the specimen is dropped through a distance of 9 meters (about 30 feet) onto a flat, essentially unyielding horizontal surface (see Sidebar 2.2), with the package striking the surface in the position expected to produce maximum damage. A package dropped from this height strikes the ground at a speed of about 13 meters per second (48 kilometers per hour, or about 30 miles per hour).

  • A puncture test in which the specimen used in the free-drop test is dropped through a distance of 1 meter (about 40 inches) onto the upper end of a 6-inch (15.2-centimeter) diameter solid, vertical, cylindrical mild steel bar mounted on an essentially unyielding horizontal surface. The package is dropped onto the bar in a position that is expected to produce maximum damage.

  • A thermal test in which the same specimen is fully engulfed in a hydrocarbon-fuel fire with an average flame temperature of at least 800°C (about 1475°F) for a period of 30 minutes.

  • An immersion test in which a separate, undamaged specimen is subjected to a pressure head equivalent to immersion in 15 meters (about 50 feet) of water. Additionally, 10 CFR 71.61 also specifies that for packages designed for transport of spent fuel with activity exceeding 37 petabecquerels (PBq; 106 curies), the undamaged containment system must be able to withstand an external water pressure of 2 megapascals (290 pounds per square inch) for one hour without collapse, buckling, or in-leakage of water. This pressure corresponds to a water depth of about 200 meters (650 feet), typical of maximum water depths on the U.S. continental shelf.

The velocity of package impact in the free-drop test described above is lower than many real world crashes (e.g., the Central Electricity Generating Board’s [CEGB’s] full-scale crash described in Section 2.3.3). Nevertheless, the impact forces on the package are much higher in the free-drop test because the impact surface is “essentially unyielding.”

Illustration of the hypothetical accident conditions in 10 CFR Part 71. SOURCE: Modified from a USNRC circular.

Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×

SIDEBAR 2.2
What Is an “Essentially Unyielding” Surface?

An essentially unyielding surface is used for impact tests on Type B packages under both International Atomic Energy Agency standards (IAEA, 2000) and U.S. regulations (10 CFR Part 71). The standards and regulations require that the impacting surface be essentially unyielding, that is, sufficiently massive and stiff to produce maximum damage to the specimen being tested. Such surfaces are usually constructed of a thick reinforced concrete slab with a steel plate floated onto its surface (i.e., slid onto the concrete surface while still wet). The concrete provides a large reaction mass and the steel plate provides stiffness. For tests of Type B packages, the slab and plate should have a combined mass at least 10 times that of the specimen being tested (IAEA, 2002).

An article such as a spent fuel transportation package that is dropped onto such an unyielding surface will be subjected to higher impact forces and will consequently experience greater deformation than if the same article is dropped onto a surface that is itself deformed by the impact (i.e., a yielding surface). An article dropped onto an essentially unyielding surface in the 9-meter (30-foot) regulatory drop test (Sidebar 2.1), for example, will be traveling at about 13 meters per second (30 miles per hour, or 48 kilometers per hour) when it impacts the surface. The deformation of the article caused by this impact will be essentially identical to the deformation resulting from the head-on collision between two identical copies of that article if each is traveling at 13 meters per second. This is comparable to the difference between a collision of a moving vehicle and a parked car versus a head-on collision between two moving vehicles. The head-on collision between moving vehicles results in much greater damage.

Gonzales et al. (1986) compared the hardness of an essentially unyielding target to several other target types in a series of impact tests. These tests involved impacts of a 2500 kilogram (5500 pound) cylindrical steel test unit resembling a

They also questioned the adequacy of regulations that do not require full-scale testing to demonstrate package performance.

There is, in fact, a good deal of quantitative information available on the performance of transportation packages under extreme loading conditions. This information comes from modeling and full-scale testing studies carried out in the United States and Europe over the past three decades. A summary of selected studies is provided in the following sections.

2.2 PACKAGE PERFORMANCE MODELING STUDIES

Computer simulation models are routinely used to estimate the thermomechanical behaviors of truck and rail packages under a range of extreme loading conditions that would not be practicable to obtain from actual

Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×

spent fuel transportation package onto four targets: desert soil, a concrete runway, a concrete highway, and an unyielding target. The unyielding target was constructed of a 56 metric ton (62 short ton), 3.6-meter (11.8-feet) thick reinforced concrete slab with a steel face plate. The table below shows the results of these tests for an impact speed of 13 meters per second (30 miles per hour), which corresponds to the impact speed for the 9-meter (30-foot) regulatory free-drop test.

In the table, the second column shows how far the test unit penetrated into each of the targets, and the third column shows the maximum strain in the test unit. Penetrations ranged from 48 centimeters (19 inches) for the soil target to 0 centimeters for the unyielding surface. The maximum strains experienced during the impact of the test unit on the unyielding surface were several times greater than for the other target types. The reason for this is simple: The other targets absorbed some of the impact forces, whereas the test unit absorbed essentially all of the impact forces when impacted against an unyielding surface. In fact, in only one case—impact onto the unyielding target—was the strain great enough to produce permanent deformation of the test unit.

Experimental Results Obtained by Gonzalez et al. (1986)

Target

Test Unit Penetration, centimeters (inches)

Maximum Strain, microstrainsa

Soil

48 (19)

90

Concrete highway

10 (4)

400

Concrete runway

0.6 (0.25)

500

Unyielding target

0

3500

aMicrostrains are measured in parts per million, for example, centimeters per 106 centimeters.

testing of full-scale articles. “Generic” truck and rail packages, which incorporate the salient design features of certified packages, are typically investigated with such models. The loading conditions used in these models are usually derived from historical accident reconstructions, but hypothetical accident scenarios may also be used to investigate conditions that exceed those of any known historical accident. The committee selected a number of modeling studies for discussion in the following subsections. These are summarized in Table 2.1.

2.2.1 Modal Study

The modal study (Fischer et al., 1987) examined the expected responses of spent fuel transportation packages to thermomechanical conditions de-

Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×

SIDEBAR 2.3
Scale Modeling

The term scale modeling is conventionally taken to mean the testing of either an article that is a miniaturized version of a large structure (see figure), or a large component of a complex structure. The major benefit of scale modeling is that it reduces material, fabrication, and testing costs, thus allowing more variables to be explored, and a multiplicity of data to be obtained, for the same cost as a single test on the full-scale structure. This is of particular value when the structure is very expensive, the article needs to be tested to a point at which it has no further utility, and/or the loads that must be applied exceed the capacities of readily available testing apparatuses. Typically models of at least one-quarter scale are used in engineering tests of spent fuel transport packages, although models as small as one-eighth scale have been used for initial scoping tests.

The failure condition for the full-scale structure generally can be estimated from the results of scale-model testing through the application of well-known scaling laws. However, there are a number of pitfalls in this procedure. The most important is that the weak points in a complex structure are generally associated with joining processes—bolts, welds, and adhesives—that may not scale precisely. For example, in a weldment, the heat-affected zone sizes and mechanical properties, and the residual stresses that are induced, will depend on the number of weld passes that are made and the heat input rate that is used, and these parameters depend on the thicknesses of the materials that are joined. Similarly, the strength of an adhesive joint will depend on the sizes and surface quality of the materials involved, as well as the thickness of the adhesive layer.

Structures that utilize many component materials that are off-the-shelf items (e.g., bolts, seals, gaskets) may be hard to scale because these items may not be available in the exact size or quality that is needed. Also, bolts and other fasteners that may be required for the actual structure are not always available in the same metallurgical form in scaled-down sizes. Additionally, there are scaling issues in monolithic and composite structures. Metallic parts may have different thermomechanical properties because of differences in heat treatments, which are affected by article sizes. Scaling of composite laminates can be done in a practical manner only by reducing the number of plies, essentially making it into a different material.

Finally, cracks and other potential initiators of brittle failure also do not scale because it is their absolute size that is of first-order importance, with their relative sizes generally being of second-order importance. In its simplest form, crack instability and growth is generally governed by a parameter of the form K = σ[πc]1/2 β(c/h), where K is the stress intensity factor, c denotes the crack size, σ is the applied stress, and β is a function of the ratio of the crack size to one of the component dimensions (h). For small- to moderate-sized cracks, β is on the order of unity. Although the mathematical relationships for corrosion pits, dents, and gouges are not as well established as in fracture mechanics, their qualitative behavior will be similar to cracks. This can make it very difficult to duplicate the failure mechanism in a small-scale test.

Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×

This does not mean that scale modeling is without value. Rather, it means only that direct application of the results of small-scale testing to the full-scale article should be done with great caution. In cases where some features of the test article cannot be modeled accurately at reduced scale (e.g., the valve assemblies used on spent fuel transport packages), it may be possible to combine a simplified reduced-scale model to determine decelerations and then separately test the full-scale component when subjected to the appropriately scaled decelerations. Another approach is to use scale modeling as a test bed for the calibration and validation of a computational analysis simulation of the structure. Then, with the further assurance gained from viable predictions of the results of a small but representative set of independent “proof-of-concept” tests made on a full-scale structure, the computer simulation can be used with confidence, and in a highly cost-effective manner, for further evaluations of the performance of the structure in a broad range of anticipated and accident service conditions.

Scale modeling is routinely used in spent fuel transport package testing and certification. A good technical discussion of scaling laws and properties for materials in pristine condition is provided in Donelan and Dowling (1985). Recent work on scaling laws for materials with flaws is provided by Bazant (2004).

Full-scale (background) and 1/2-, 1/4-, and 1/8-scale models (foreground) of the Magnox flask. SOURCE: Magnox Electric Ltd.

Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×

FIGURE 2.2 The German Federal Institute for Materials Research and Testing (BAM) carried out two 9-meter regulatory drop tests in September 2004 at its recently completed testing facility near Berlin. One of the tests was conducted on the180 metric ton (198 short ton) Mitsubishi rail package shown in these photos. The top photo shows the package orientation for this test. The package was hoisted and dropped onto the steel plate embedded in the floor. The bottom photo shows the package after the test. SOURCE: Photo by K.D. Crowley.

Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×

SIDEBAR 2.4
Margin of Safety

To reduce the possibility of catastrophic failure, engineering structures in which the weight of the structure is not a critical concern (e.g., buildings, dams, bridges, power plants, storage tanks) are generally designed with large “margins of safety.” These safety margins are generally achieved in two ways. First, conservative values of the mechanical properties of the materials used in the structure (i.e., values that underestimate the potential strength of the materials) are selected for use in the design. Second, “worst-case” assumptions are made on the applied loads that the structure must resist, and/or an arbitrary “factor of safety” is introduced to artificially inflate the expected loads.

Factors of safety range from as low as 1.4 for natural gas transmission pipelines located in unpopulated areas to 10 or more in especially sensitive applications, with a value of 3 being a typical choice for noncritical structures. The cumulative effect of conservative design choices and the imposition of a factor of safety on each structural component means that the actual margin of safety, while not specifically known, could be well above the safety margin for each individual component. In a typical design, loads sufficient to cause structural failure could be more than four to five times greater than the load anticipated in actual, normal service.

Large safety margins are necessary because of the multitude of uncertainties that could affect the integrity and durability of a structural design. These uncertainties include the potential for fabrication defects from inadequate workmanship and less than satisfactory material properties and joining techniques; in-service mechanical damage, fatigue and environmental degradation (e.g., corrosion); and unexpectedly severe operating conditions due to accidents or sabotage. Structures that are designed with large safety margins are very likely to be able to resist catastrophic failure over their intended service lifetime, even when the expected operating conditions are substantially exceeded once or many times during service.

In contrast, in engineering structures for which the weight of the structure is a critical concern (e.g., aircraft), the safety margins are generally much lower. Airplanes are typically designed and tested to achieve a margin of safety of only about 50 percent over the maximum anticipated operating loads. In these applications, considerable effort is placed on analytical projections of times to failure based on anticipated defects or other forms of damage, and rigorously scheduled inspections to determine if or when damage has come to exist at critical locations within the structure. This approach has come to be known as damage tolerance methodology.

There are built-in margins of safety for the design of spent fuel transportation packages. USNRC guidance for the design of spent fuel transportation packages (USNRC, 1978) adopts portions of the American Society for Mechanical Engineers (ASME) code for boilers and pressure vessels. The ASME code specifies the use of highly ductile materials that accommodate unusually high stresses through deformation rather than fracture. The code also specifies maximum stress limits for these materials that are well below their yield strengths. Vessels that are properly designed, manufactured, and maintained to these codes should perform as intended even under conditions that exceed design specifications because of this built-in margin of safety.

Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×

TABLE 2.1 Summary of Selected Studies on Package Performance Modeling

Test Description

Results in Brief

Reference

Modeling the performance of a generic truck and rail package under mechanical and thermal loading conditions derived from historical accident records

A very small number of rail accident scenarios could result in package releases in excess of regulatory limits

Modal study: Fischer et al. (1987)

Modeling the performance of two generic truck packages and two generic rail packages for impacts at various orientations against unyielding and yielding surfaces at 30, 60, 90, and 120 miles per hour (48, 96, 144, and 192 kilometers per hour)

No package penetration at any orientation or impact speed against yielding or unyielding surfaces; package seals may leak during high-speed impacts against unyielding surfaces in excess of regulatory test limits

Spent fuel shipment risk reexamination study: Sprung et al. (2000)

Comparison of the thermomechanical conditions from severe historical accidents to regulatory test limits and other modeling studies

No accident produces mechanical loads in excess of the 9-meter regulatory free-drop test. Two accidents could have produced thermal loads in excess of the IAEA 30-minute fully engulfing fire test

Fischer et al. (1987); Ammerman et al. (2002, 2003); Ammerman and Ginn (2004)

Modeling of thermal performance of a rail package in the July 2001 Howard Street tunnel fire near Baltimore, Maryland

No package releases would have occurred

USNRC (2003a)

Analysis of the thermal conditions in the December 1984 Summit Tunnel fire near Manchester, England, and the likely effect on a fuel package

Thermal conditions would have exceeded those of the regulatory thermal test such that package seals might have failed should a package have been exposed in this fire

UK Department of Transport (1986)

rived from the historical record of truck and train accidents in the United States. These data were compiled from government and private databases. The data included accident speeds and impact angles, the hardness of impacted objects (i.e., other vehicles, wayside terrain), and the frequency and duration of accident-associated fires. The data were used to develop a suite of historical accident scenarios and associated accident probabilities that

Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×

FIGURE 2.3 The event tree for train accidents from the 1987 modal study (Fischer et al., 1987). The numbers shown at each branch are probabilities for the accident branch based on an analysis of historical data. The accident scenarios marked with an asterisk were determined to produce consequences that would approach or exceed regulatory limits. SOURCE: Fischer et al. (1987, Figure 2-5).

Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×

SIDEBAR 2.5
Radioactive Material Releases in Severe Transportation Accidents

There are two barriers to the release of radioactive materials from spent fuel packages into the environment during a severe accident. The first is the package itself. As described in this chapter, Type B packages used to transport spent fuel are designed to withstand severe accidents without a significant loss of containment or an increase in external radiation to levels that would endanger emergency responders or the general public.

The second barrier is the fuel rod cladding. The cladding for most commercial nuclear fuel is made from a zirconium metal alloy, referred to as zircaloy, which is fabricated into long (3.5 to 4.5 meter [11.5 to 14.75 feet]) tubes (Sidebar 1.3). These tubes, which contain the uranium dioxide fuel pellets, are pressurized and sealed to resist collapse or leaks when placed into the high-pressure operating environment of the reactor core. Fuel rods are bundled together into fuel assemblies using metal structural supports. These supports also help to prevent the fuel rods from collapsing and buckling in a severe transportation accident.

The release of significant quantities of radioactive materials from a loaded spent fuel transport package into the environment during a severe accident would occur only if the package and one or more fuel rods were breached (small amounts of radioactive contamination from the external surfaces of the fuel rods [crud] could be released from the package if the package seals were compromised, even if the fuel rods maintained their integrity). Fuel rod breaching could potentially occur by two processes: mechanical rupture or thermal creep. The former could occur if impact forces exceed the mechanical strength of the cladding, causing it to buckle. The latter could occur at elevated temperatures due to time-dependent elongation of the cladding along fracture planes. High burn-up fuel rods may be more susceptible to breaching because of cladding embrittlement resulting from their longer residence in the reactor.

If breached, the fuel rods would depressurize, and radioactive material could

could be displayed as “event trees” (Figure 2.3). Quantitative analyses were carried out to assess the effects of the loading conditions represented by these event trees on generic train and truck transportation packages that were similar in design to the packages in service in the mid-1980s. A more complete discussion of this study is provided in Chapter 3.

The analysis involved a two-stage screening process. Phase 1 screening used dynamic linear stress analysis and standard transient heat-transfer models to screen out those accident scenarios in which the thermomechanical conditions did not exceed the regulatory testing requirements in 10 CFR Part 71 (Sidebar 2.1). For these scenarios, any radioactive material releases from the packages were assumed to fall below regulatory limits. Approximately 99.4 percent of all truck accidents and 99.7 percent of the rail accident scenarios analyzed fell into this category (Fischer et al., 1987,

Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×

be released into the interior of the transportation package by depressurization flow. Two types of radioactive materials could be released: (1) gaseous materials (e.g., radioactive noble gases such as krypton-85 and volatile materials such as cesium-137) produced by fission reactions while the fuel is in the nuclear reactor; and (2) fine particles of the fuel itself, referred to as fuel fines, which are created by mechanical fracturing. The particle-size distributions of the fuel fines will depend on the burn-up of the fuel and the magnitude of the mechanical forces on the fuel pellets during the accident.

Only fuel fines smaller than the size of the cladding breach can be released from the fuel rod into the package; larger particles can also clog the cladding breach and reduce the quantity of fine-particle releases into the package. Most of the released fines would be deposited onto the interior surfaces of the package. Some of the remaining airborne fines (which typically comprise only a few percent of the fines released into the package; see Sprung et al., 2000, p. 7-30) and radioactive gases could be released into the environment, but only if all of the package barriers (i.e., the package lids and, if present, the inner canister) were breached. Volatile components such as cesium-137, if present, would condense upon cooling. The quantity of materials released from the package would depend on the size of the breaches and the presence of a driving force (e.g., depressurization) to propel material out of the package. Once air pressure between the package interior and outside environment was equalized, further material releases would occur by much slower diffusion processes.

The process for the release of radioactive materials from transportation packages containing high-level waste is similar to that for spent fuel with three notable exceptions. First, high-level waste does not contain fission-produced noble gases; those gases were removed from the waste during processing. Second, high-level waste to be transported in a vitrified (glass) form is contained in stainless steel canisters (see Sidebar 1.3), rather than zircaloy cladding. Third, the canisters are not pressurized, so there are no large depressurization forces to drive radioactive material releases from the canister into the package or the environment.

p. 9-2). Phase 2 screening involved more sophisticated analyses of package responses and radiological releases for the relatively small number of accident scenarios that exceeded the 10 CFR Part 71 testing limits. This screening employed nonlinear dynamic stress analysis models to estimate package deformation and transient thermal models that took into account the phase change accompanying the melting of lead shielding in the package at high temperatures. These analyses assumed that the packages contained five-year-cooled pressurized water reactor fuel having a burn-up of 33,000 megawatt-days per metric ton, which was typical of spent fuel at that time.16 The analyses also considered breaches of the spent fuel cladding due

16  

Present-day fuel burn-ups are typically between 50,000 and 60,000 megawatt-days per metric ton.

Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×

to both impact and thermally induced creep (see Sidebar 2.5). The radiological effects considered included releases of radioactive materials from the package as well as increased radiation doses resulting from damage to the package shielding.

Based on the phase 2 screening, Fischer et al. (1987) concluded that roughly 0.3 to 0.6 percent of extreme accidents would result in radioactive releases that approach or slightly exceed the regulatory limits in 10 CFR Part 71 (Sidebar 2.1), with less than 0.001 percent of the truck and 0.012 percent of the rail accident scenarios actually having releases that would exceed regulatory limits. Because these conditions pushed the capabilities of the computer codes, there was no attempt made to model the releases for the most extreme accidents. Instead, estimates of the releases for these very extreme accidents were extrapolated from the release behavior during less extreme accidents.

2.2.2 Reexamination Study

In a “reexamination study,” Sprung et al. (2000) updated the modal study analyses using different package designs and modeling approaches. This study examined the performance of four generic transportation packages: steel-lead and steel-depleted uranium truck packages and steel-lead and monolithic steel train packages. These packages were similar in design to the USNRC-certified packages that were in use in the late 1990s. A detailed description of this study is provided in Chapter 3.

Mechanical performance was estimated using a three-dimensional finite element code (PRONTO 3D17), which was developed by Sandia National Laboratories for modeling large deformations in nonlinear mechanical behavior for materials subjected to very high strain rates. The code was used to estimate the mechanical response of the four packages to end, center-of-gravity over corner, and side impacts onto unyielding and yielding surfaces at speeds of 30 (the impact speed for the regulatory free-drop test; see Sidebar 2.1), 60, 90, and 120 miles per hour (about 48, 96, 144, and 192 kilometers per hour). The package impact limiters were assumed to be in place but fully crushed before impact occurred. This is a very conservative assumption; in an actual accident the impact limiters would be expected to absorb most of the impact forces by crushing, as they are designed to do.

Impacts onto unyielding surfaces provide the most rigorous test of package performance. Based on their modeling analysis, Sprung et al. (2000)

17  

PRONTO 3D is a Lagrangian finite element code developed by Sandia National Laboratories that is roughly comparable to LS DYNA 3D, a code that is used worldwide for transport package design and verification analyses.

Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×

concluded that package impacts onto an unyielding surface would have produced strains lower than those required for package penetration at all of the modeled impact speeds and orientations. The models indicate that the seals on the truck packages maintained their integrity in all but possibly the 120 mile per hour impact; however, for the latter impact, the seal leak areas would have been small. The models also suggest that for rail packages, some seal leakage could occur for some impact orientations at impact speeds onto unyielding surfaces as low as 60 miles per hour and possibly at all orientations at speeds of 120 miles per hour.

A one-dimensional heat transport code was used to estimate the time required to cause the failure of the elastomeric package seals and rupture fuel rods when the package was subjected to a fully engulfing optically dense fire at 800°C (1472°F) (the regulatory thermal test; see Sidebar 2.1) and 1000°C (1832°F). Failure temperatures for elastomeric seals were estimated from data available in the literature. These data suggested that the seals would experience rapid degradation at temperatures exceeding 350°C (662°F). For the 800°C fire (1472°F), it was found that the minimum time to the 350°C (662°F) seal degradation temperature was just over one hour for one of the truck packages, with the maximum time being almost 2.5 hours for a rail package. For a 1000°C (1832°F) fire, the minimum and maximum times to degradation were about 0.6 and 1.4 hours, respectively, for a truck package and a rail package. As noted previously, packages are required by USNRC regulations to withstand a 30-minute, fully engulfing fire (see Sidebar 2.1).

2.2.3 Historical Accident Reconstructions

Additional investigations have been undertaken to reconstruct the thermomechanical conditions from a number of historical accidents that, had they involved spent fuel or high-level waste transportation packages, could have provided a severe test of package performance. It should be emphasized that none of these accidents actually involved shipments of spent fuel or high-level waste. The modal study (Fischer et al., 1987) developed estimates of the thermomechanical conditions for four severe accidents selected from a database of 400 train and truck accidents in the United States that were known to have produced extreme loading conditions. These are summarized in Table 2.2. The authors concluded that only one of the four accidents—the September 1982 Livingstone, Louisiana, train derailment and fire—could have resulted in any releases of radioactive materials. Whether releases would have occurred depended on where the package was placed on the train relative to the location of the fire, which was allowed to burn for several days.

Ammerman and colleagues (Ammerman et al., 2002, 2003; Ammerman

Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×

TABLE 2.2 Severe Accident Scenarios Examined in the 1987 Modal Study

Date

Location

Description

Conclusions About Accident Severity by Fischer et al. (1987)

January 1979

Hunter, Alabama

Five railcars plunged off a rail bridge into the muddy bottom of a river about 23 meters (75 feet) below

No radioactive material releases or increases in external radiation expected

March 1981

San Francisco, California

A tractor trailer traveled through a bridge railing and fell onto a soil surface about 19.5 meters (64 feet) below the bridge

No radioactive material releases or increases in external radiation expected

April 1982

Oakland, California

A truck fire in a highway tunnel involving about 33,300 liters (8800 gallons) of gasoline

No radioactive material releases or increases in external radiation expected

September 1982

Livingston, Louisiana

A train derailment and fire fed by plastics and petroleum products; fires burned for several days

Package releases could have exceeded regulatory limits depending on where the package was located in the fire

 

SOURCE: Fischer et al. (1987).

and Ginn, 2004) from the Sandia National Laboratories examined the thermomechanical conditions for 12 historical accidents, some of which had been identified by the State of Nevada as potentially being severe enough to compromise the containment effectiveness of spent fuel transportation packages (Table 2.3). Accident loading conditions were reconstructed from National Transportation Safety Board reports and newspaper accounts. These conditions were compared to the loads experienced by transportation packages during regulatory testing (e.g., the 9-meter drop test; 30-minute thermal test); to the accident scenarios estimated in the modal study (Fischer et al., 1987); and to the 2000 reexamination study estimates (Sprung et al., 2000).

The authors concluded that the thermomechanical conditions described in Table 2.3 were encompassed by the event trees used in the Sprung et al. (2000) study (see Chapter 3). They also concluded that none of these acci-

Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×

TABLE 2.3 Severe Accident Scenarios Examined by Sandia National Laboratories

Date

Location

Description

Conclusions About Accident Severity by Ammerman and Colleagues

June 1983

Greenwich, Connecticut

Two trucks and two cars plunged off an interstate highway bridge into a river about 21 meters (70 feet) below

Impact would have been less severe than the 9-meter drop test onto an unyielding surface

August 1985

Checotah, Oklahoma

Transported military ordnance (2000-pound Mk-84 bombs) exploded after a truck accident

Detonation of military ordnance would not have caused package failure

July 1986

Miamisburg, Ohio

A train carrying yellow phosphorus and molten sulfur derailed and caught fire

Fire would not have exceeded the 30-minute regulatory thermal test

April 1987

Amsterdam, New York

Several cars and a truck plunged off an interstate highway bridge, falling about 24 meters (80 feet) into a rain-swollen creek

Impact would have been less severe than the 9-meter drop test onto an unyielding surface

December 1988

Memphis, Tennessee

A tanker truck carrying about 9500 gallons of propane caught fire and exploded

Package would have experienced only superficial damage

February 1989

Helena, Montana

A runaway train collided with a locomotive at 15 to 25 miles per hour, causing two large explosions from hazardous cargo

Fire would not have exceeded the 30-minute regulatory thermal test

February 1989

Akron, Ohio

One railcar carrying butane ruptured, releasing its contents in the form of a fireball

Fire would not have exceeded the 30-minute regulatory thermal test

May 1989

San Bernardino, California

A train derailed at high speed (100 miles per hour); a gas pipeline failed catastrophically following cleanup of the derailment

Impact would not have caused package breach or release of contents

Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×

Date

Location

Description

Conclusions About Accident Severity by Ammerman and Colleagues

July 1989

Freeland, Michigan

A derailed freight train carrying flammable materials burned for several days

Fire could have exceeded the 30-minute regulatory thermal test, but conditions would not have exceeded those shown by Sprung et al. (2000) to be necessary to cause package seal failure

October 1989

Oakland, California

The upper level of a viaduct collapsed onto the lower deck

Collapse of viaduct onto a truck package would not have been severe enough to cause seal failure, but package shield could be somewhat compromised

December 1994

Cajon, California

A runaway freight train struck the rear of another train at a speed of about 45 miles per hour

No significant damage to package would have occurred

February 1996

Cajon Junction, California

The derailment of a freight train caused a fire that burned for several days

Fire conditions would not have exceeded those shown by Sprung et al. (2000) to be necessary to cause package seal failure

 

SOURCE: Ammerman et al. (2002, 2003); Ammerman and Ginn (2004).

dents would have produced thermomechanical conditions that exceeded the regulatory test conditions in 10 CFR Part 71.

The USNRC undertook a detailed thermal analysis of the July 2001 fire in the Howard Street tunnel in Baltimore, Maryland, that resulted from the derailment of a train carrying hazardous materials. The fire was fed by a tanker railcar carrying about 28,600 gallons (106,300 liters) of liquid tripropylene. A National Institute for Standards and Technology (NIST) study of the fire (McGrattan and Hamins, 2003) used detailed numerical simulations to develop estimates of temperatures in the tunnel for the most severe portion of the tripropylene fire, which occurred between the time of its ignition and the rupture of a water main within the tunnel about three

Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×

hours later.18 The study estimated that peak temperatures in the narrow flaming region in the tunnel reached about 1000°C (1832°F) and that the tunnel walls reached peak temperatures of about 800°C (1472°F). The hot gas layer near the fire had average temperatures of about 500°C (932°F).

Tunnel temperatures were also estimated by analyzing oxide layer thickness, metal loss, and metal melting on railcar components recovered after the fire (Garabedian et al., 2002). This analysis suggested that gas temperatures in excess of 800°C (1472°F) existed for more than 30 minutes near the fire source. At 20 meters (66 feet) from the fire source, the analysis suggested that maximum surface temperatures of 600°C (1112°F) could have been reached for much less than 30 minutes.

USNRC staff modeled the thermal behavior of a specific USNRC-approved spent fuel package19 subjected to these estimated “extraregulatory” thermal conditions. The package and its cradle were modeled using a two-dimensional finite element code for two scenarios: first assuming a one-railcar (20-meter, or 66-foot) separation between the package and fire source, as would be required by DOT regulations had spent fuel and hazardous materials been transported together; and second assuming a 5-meter (16.4-foot) separation. Both scenarios were analyzed for 150 hours of fire exposure at the maximum temperature conditions estimated by the NIST model.

The committee received a briefing from USNRC staff on the results of this analysis, which can be summarized as follows (see also Bajwa, 2002; USNRC, 2003a): For the first scenario, the temperature of the fuel element cladding exceeded regulatory limits of 570°C (1058°F)20 after about 166 hours of fire exposure. For the second scenario, the fuel cladding would have reached 570°C (1058°F) after 37 hours of exposure. Calculations were also carried out to estimate the stresses on the welded canister resulting from fire exposure. Those calculations indicated that the welded canis-

18  

The NIST study noted that the distribution of tripropylene fuel within the tunnel, and thus the duration of the tripropylene fire, are difficult to estimate. The study suggests that the tripropylene fire was extinguished sometime between 3 and 12 hours after ignition either from a lack of fuel or from water suppression. Smoldering of combustible materials contained in closed boxcars on the train continued for several days after the tripropylene fire was extinguished.

19  

The Holtec Hi-Star MPC package was modeled. This rail package is designed to hold five-year-old pressurized water reactor spent fuel assemblies with maximum burn-ups of 45,000 megawatt-days per metric ton. It has a bolted external closure and an internal welded canister. For the purposes of the USNRC analysis, the spent fuel assemblies were assumed to generate the maximum internal heat (20 kilowatts) allowed by the package design.

20  

The 570°C (1058°F) regulatory limit was established to prevent fuel cladding failure from thermal creep during storage. The actual burst temperature for zircaloy fuel rods is about is about 750°C (1382°F) (see USNRC, 2003b).

Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×

ter would have maintained its integrity. Consequently, USNRC staff concluded that no radioactive material would have been released from this package in this fire.

It is noteworthy that this analysis contains several significant “conservatisms” (i.e., assumptions that resulted in more dire predictions of package performance than might have occurred in an actual fire): The maximum fire temperatures in the tunnel were assumed to have been maintained for 150 hours—more than 6 days. The actual duration of the tripropylene fire in the Howard Street tunnel was estimated by NIST (McGrattan and Hammins, 2003) to last from 3 to 12 hours. A two-dimensional thermal model was used in the analysis. This model ignored axial direction heat transfer, which could have reduced the peak temperatures.21 Also, the package was assumed to have the maximum allowed internal heat load from spent fuel decay heat.

The USNRC is extending its Howard Street tunnel fire analyses to examine the performance of two additional spent fuel packages: a TN68 rail package mounted on a railcar and NAC-LWT truck package in an ISO (International Organization for Standardization) container mounted on a railcar. Both are bare-fuel packages (see Section 2.1). These packages are currently certified by the USNRC for use in the United States, and the transport of truck packages by rail, which is one of the scenarios being examined, is allowed under current regulations.

A draft report containing the Howard Street tunnel fire analyses (Adkins et al., 2005) was made available to the committee in early September 2005, after the committee held its last meeting for this study.22 Just prior to the committee’s final meeting in July 2005, the State of Nevada also provided a preprint of a paper describing a thermal analysis of a generic steel-lead-steel

21  

Marvin Resnikoff of Waste Management Associates criticized the USNRC’s analysis on the basis that it did not use a three-dimensional thermal model and did not explicitly model the bolts and seals on the external closure. While the committee agrees that additional details in the models would have been informative, it also judges them unlikely to have changed the results, given that the modeling predicted that there were no failures of the internal welded canister of the package.

22  

This analysis assumed that the packages were located 20 meters (66 feet) from the fire source and that the fire burned for seven hours, a shorter time than the original analysis. According to the draft paper, the analysis shows that the maximum temperatures on the seals of the TN68 and NAC-LWT packages would have exceeded their rated service temperatures, making it possible for the release of radioactive materials to occur. An analysis was also carried out to estimate the radioactive releases from these packages. They were characterized in the paper as “very small—less than an A2 quantity” (see Sidebar 2.1) and consisting of non-fixed radioactive material (crud) from the external surfaces of the fuel rods. The draft paper indicates that the fuel cladding would have maintained its integrity.

Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×

truck package exposed to a fully engulfing hydrocarbon fire (Greiner et al., 2005).23 Because these papers were provided so late in the study, the committee was unable to analyze, discuss, and integrate them into this report.

The United Kingdom Department for Transport (1996) analyzed the thermal loading conditions in the Summit rail tunnel fire near Manchester, England, on December 20, 1984. The fire resulted from the derailment of 10 tank cars carrying gasoline. The fire burned for about four days (Figure 2.4) and completely destroyed several tanker cars. The analysis showed that fire conditions in the tunnel exceeded those required in the regulatory thermal test, which suggested that there could have been releases of radioactive materials had a spent fuel transportation package been involved in the derailment and fire. As a result of this analysis, an operational rule was established that prohibited English trains carrying spent fuel packages and trains hauling flammable materials from crossing in rail tunnels.

2.3 FULL-SCALE PACKAGE TESTING UNDER EXTREME CONDITIONS

Full-scale testing on transportation packages under severe extraregulatory conditions has been carried out in both the United States and the United Kingdom. In the United States, these tests have been carried out under the sponsorship of the Atomic Energy Commission and its successor agencies, the Energy Research and Development Administration (ERDA) and the USNRC. In the United Kingdom, one test has been carried out by the British Central Electricity Generating Board (CEGB). In addition, the USNRC plans to carry out an additional test on a rail package when funds are made available by Congress. These studies are described in the following sections, and the results are summarized in Table 2.4.

2.3.1 Sandia National Laboratories Air-drop Tests

Two air-drop tests were conducted by Sandia National Laboratories in 1975 to provide a demonstration of the ruggedness and survivability of shielded containers in a manner that was thought to be better appreciated by the general public than a regulatory test (Waddoups, 1975). The test

23  

According to this analysis, the elastomeric seal for this generic package would reach its melting temperature (referred to as the “temperature of concern” in the paper) in about two hours if the impact limiter is attached to the lid end of the package and about 0.7 hour without the impact limiter. The paper did not provide an analysis of the consequences of exceeding the seal melting temperature in terms of possible releases of radioactive materials from the package.

Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×

FIGURE 2.4 Photos from the December 1984 Summit Tunnel fire near Manchester, England. The top photo shows two fire plumes emerging from tunnel ventilation shafts. The bottom photo is an interior view of the tunnel. Part of the tunnel ceiling has collapsed onto one of the tank cars. SOURCE: Photos taken by a member of the West Yorkshire Fire Brigade or Manchester Fire Brigade (used with permission of www.todchat.com).

Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×

TABLE 2.4 Full-scale Package Tests Described in this Chapter

Test Description

Results in Brief

Reference

600-meter (2000-foot) air drop of two small packages onto hard soil

Less severe damage observed than for a 9-meter free-drop test onto an unyielding surface

Waddoups (1975)

Crash of a truck carrying a 20 metric ton (22 short ton) package mounted on a trailer into a massive reinforced concrete barrier at 98 kilometers per hour (61 miles per hour) and 135 kilometers per hour (84 miles per hour)

Superficial package damage for 98 kilometers per hour (61 miles per hour) test; deformation of package with small amount of water leakage observed for 135 kilometers per hour (84 miles per hour) test

Huerta (1977)

Crash of locomotive into 25 metric ton (28 short ton) package mounted on a trailer at 130 kilometers per hour (81 miles per hour)

Package was deformed, and a small leak was detected when the package was pressurized

Huerta and Yoshimura (1983)

Crash of a 68 metric ton (75 short ton) package mounted on a railcar into a massive reinforced concrete barrier at 131 kilometers per hour (82 miles per hour)

Superficial package damage

Huerta (1981)

9 meter free-drop tests of a package onto its side and corner

Water spray from lid-body joint at impact releasing up to a few liters of water

IME (1985)

Crash of a locomotive into a package mounted on a railcar at 160 kilometers per hour (100 miles per hour)

Superficial damage with an internal pressure drop corresponding to the loss of about 0.5 liter (0.1 gallon) of water through the package seal

IME (1985)

Full-scale testing of a rail package mounted on a rail carrier car placed at 90 degrees to a simulated rail crossing, subjected to a collision with a locomotive and several freight cars traveling at 60 miles per hour, followed by a fully engulfing, optically dense, hydrocarbon fire for a duration of one-half hour post-collision

Test has not yet been carried out

USNRC (2003c, 2004b,c,d, 2005a,b)

Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×

involved 600-meter (almost 2000-foot) air drops of two “obsolete”24 packages: a Pratt and Whitney 1 package25 and an OD-1 Oak Ridge Research Reactor Spent Fuel Carrier.26 These drops were made onto a hard prairie (a hard, dry, sandy silt soil) at the Sandia Edgewood Test Range in New Mexico.

The packages impacted the prairie at speeds exceeding 100 meters (350 feet) per second and created deep impact craters (in one case exceeding 2 meters [7 feet] in depth). One of the packages experienced superficial damage, while the other experienced some bulging and shifting of its internal lead shielding. Waddoups (1975) noted that the impact velocities for these drop tests reached speeds of about 230 and 246 miles per hour (103 and 110 meters per second), many times greater than the 30 mile per hour (about 13 meter per second) speeds in the 9-meter regulatory drop test. However, the hard prairie surface at the test site was not “essentially unyielding,” as evidenced by the deep craters created by the impacts. Based on a comparison of damage to one of the packages, Waddoups (1975, p. 15) concluded that “the 30-foot drop test onto an unyielding surface is a more severe environment than the 2000-foot drop onto hard soil.”

2.3.2 Sandia National Laboratories Crash Testing

ERDA (predecessor agency to the Department of Energy) sponsored a full-scale testing program at Sandia National Laboratories to obtain a better understanding of the behavior of transport packages in severe accident environments (Jefferson and Yoshimura, 1977). This program had two primary objectives: (1) assess and demonstrate the validity of analytical modeling and scale modeling for predicting the damage to transport packages in accidents; and (2) develop quantitative information on the conditions in extreme accident environments.

This full-scale test program was carried out in three separate phases: (1) use of computational methods to predict the conditions in accident environments and the potential damage to shipping containers in such

24  

Both packages were considered obsolete because they were not designed to meet fire standards with an acceptable loss of shielding. They also were not as rugged as then-licensed packages.

25  

This package had a 0.622-meter (25-inch) outside diameter, 0.9065-meter (36-inch) outside height, and a weight of 3054 kilograms (3.4 short tons). The package is smaller than many of the packages currently in use to transport commercial spent fuel.

26  

This package had a 0.8-meter (32-inch) outside diameter, 1.2-meter (48-inch) outside height, and a weight of 7410 kilograms (8.2 short tons). The package also is smaller than many of the packages currently in use to transport commercial spent fuel.

Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×

environments; (2) determination of physical damage mechanisms through scale-model testing; and (3) full-scale testing of representative hardware to validate the computational analysis methodology.

Several criteria were considered in selecting the test scenarios. These included the desire to expose transport packages to realistic and severe accident environments, tractability of the scenarios to mathematical analysis and scale-model testing, cost-effectiveness, and the likelihood of successful execution. The last criterion eliminated scenarios that were difficult to replicate such as skids into barriers. The cost criterion prompted the use of out-of-service transport packages, used tractors, and a military surplus locomotive in the tests. Three full-scale test scenarios were eventually selected:

  1. Impacts of tractor-trailer rigs carrying spent fuel transport packages into a concrete barrier at nominal speeds27 of 100 kilometers (62 miles) per hour and 130 kilometers (about 80 miles) per hour.

  2. Impact of a locomotive into a spent fuel transport package mounted on a truck trailer at a simulated grade crossing at a nominal speed of 130 kilometers (about 80 miles) per hour.

  3. Impact of a spent fuel transport package mounted on a railcar into a concrete barrier at a nominal speed of 130 kilometers (about 80 miles) per hour, followed by exposure to a fire.

Prior to performing these tests, Sandia carried out both analytical and one-eighth scale-model tests to predict the response of the vehicles and transport packages under each of these impact conditions. Analyses were conducted using “lumped parameter models” of the transport systems in which the vehicle system and package are represented as a series of loads and couplings. A limited amount of finite element modeling was also carried out to elucidate the details of package deformation.

Scale-model testing was carried out in two phases. First, scale-model packages were impacted directly against rigid barriers to identify and quantify potential damage mechanisms. Then, scale models of the entire transport system were tested to understand total system response. The latter tests helped researchers determine the appropriate vehicle-package configurations for the full-scale testing described in the following sections.28

27  

Actual test speeds varied slightly from these nominal speeds in some cases.

28  

Videos of these tests are available on the Department of Energy’s web site at http://www.ocrwm.doe.gov/newsroom/videos.shtml.

Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×
Tractor-trailer Impact Tests

The tractor-trailer impact tests (Huerta, 1977) were carried out using an obsolete spent fuel transport package weighing 20,500 kilograms (45,000 pounds) that was mounted on a trailer in a head-on position. The trailer was attached to a standard tandem-axle tractor. The package contained an unirradiated fuel assembly and was filled with water. Conventional balsawood impact limiters were mounted on each end of the package. The tractor-trailer was crashed into a massive (626 metric tons [690 short tons]) reinforced concrete barrier backed by more than 1500 metric tons (1650 short tons) of soil. The target was described by Jefferson and Yoshimura (1977, p. 13) as “essentially unyielding” and of a weight greatly exceeding what would be encountered along normal truck routes.

The tests were carried out at Sandia’s sled test-track facility on January 18 and March 16, 1977 (Figure 2.5). For each of the two tests, a tractor-

FIGURE 2.5 High-speed (135 kilometers per hour [84 miles per hour]) crash of a spent fuel package mounted on a truck trailer into a massive barrier carried out at Sandia National Laboratories in 1977. The truck cab was destroyed in the crash, but the package remained attached to the trailer. SOURCE: Sandia National Laboratories.

Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×

trailer was accelerated to the target by a rocket sled mounted on guide rails behind the trailer. The sled was disengaged from the trailer prior to impact to allow the vehicle to coast into the barrier at the predetermined speed. The same transport package was used in both full-scale tests. It was instrumented with accelerometers, triaxial strain gauges near the front (impact) end, and passive water pressure sensors inside the package to measure peak pressures. The tests were recorded by high-speed photography.

On the first test, the trailer impacted the concrete barrier at a speed of 98 kilometers per hour (61 miles per hour). The tractor and the front end of the trailer were completely destroyed. The package remained attached to the trailer throughout the test, although the front package tie-down failed and the rear tie-down was damaged. The package suffered only superficial damage. There was no water leakage from the package, and the fuel assembly was intact and undamaged. The package experienced a peak deceleration of about 18 times the acceleration of gravity (i.e., 18 g’s) based on a velocity-time analysis of the crash photos (see Sidebar 2.6).

On the second test, the vehicle hit the concrete barrier at 135 kilometers per hour (84 miles per hour). At this speed the vehicle had approximately double the kinetic energy29 of the first test. Nonetheless, the response of the tractor-trailer was similar to the first test, and the package remained attached to the trailer. The front impact limiter was partially crushed and displaced, allowing the package to impact the rigid barrier. The front end of the package was slightly deformed and the package length was reduced by about 6 centimeters (2.4 inches). The impact created a 0.95-centimeter (0.4-inch) gap between the lead shielding and the outer shell at the back of the package. A small amount of water seepage (two drops per minute) was observed at the package head (Jefferson and Yoshimura, 1977, p. 29). Mechanical means had to be employed to remove the package head and a large force applied to remove the fuel assembly because the package had deformed. Some of the fuel rods were buckled by the impact.

Rail Grade-crossing Impact Test

The grade-crossing test (Huerta and Yoshimura, 1983) involved a crash of a locomotive traveling at 130 kilometers per hour (81 miles per hour) into a tractor-trailer holding a spent fuel transport package at a simulated grade crossing (Figure 2.6). The test was carried out on April 24, 1977,

29  

The kinetic energy of a body in motion is equal to 1/2 mv2, where m is the body mass and v is the body velocity. Because the package masses are the same in both tests, the ratio of the kinetic energies is equal to the ratios of the test velocities squared: that is, (135/98)2 ≈ 1.90.

Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×

SIDEBAR 2.6
Impact Severity

The accelerations and strains that are routinely measured during package impact tests provide quantitative information for the verification of computation simulation analyses. Analyses of this kind are particularly important for comparing the relative severity of full-scale crash tests and regulatory free-drop tests.

The tests described in this chapter were designed to generate large forces on transport packages by accelerating them to a known speed and then impacting them against rigid barriers. The forces generated by the impact can be calculated using Newton’s equation:

During an impact, the package undergoes a rapid and negative change in A (i.e., it decelerates) as its velocity goes to zero. It is possible to determine a nominal (average) value for A directly by measuring the change in position of the package as a function of time. This measurement is typically made using photographs taken during the crash by high-speed cameras, which record at up to 3000 frames per second. Instruments, called accelerometers, also can be mounted on the package to provide this information. Unfortunately, the highest decelerations and forces occur locally at the point of impact where measurements are very difficult.

Estimates of A as a function of time are usually given in the form of a smooth curve (see figure), where acceleration is expressed relative to the acceleration imparted by Earth’s gravity field. The units of measurement are expressed in g’s, (1 g = 9.8 meters per second squared, or 32 feet per second squared). Because the mass of the package is known and does not change during the test, the average forces (F) acting on the package can be determined directly using the above equation. The peak of the curve provides a good estimate of the peak force of the test.

The forces imparted during the test will cause both the package and the barrier to deform. If the deformations are below the elastic limit they will be temporary, and the deformed materials will return to their original shapes after the forces are removed. If the deformation is above that limit, the deformations will be permanent. This elastic limit is material specific but tends to be less than 1 percent strain for steel objects.

using a 2545-kilogram (56,000-pound) stainless steel and lead package30 containing an unirradiated fuel assembly. The package was mounted to the trailer with heavy steel bands. The trailer in turn was attached to a used gasoline tractor. A military surplus locomotive weighing 109,000 kilo-

30  

This package was constructed of a 2.54-centimeter (1-inch) thick outer stainless steel shell and a 1.9-centimeter (0.75-inch) thick inner stainless steel shell with a 21.3-centimeter (8.37-inch) thick sandwich of lead shielding. The package head was attached by eight 2.54-centimeter (1-inch) diameter stainless steel bolts.

Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×

The amount of deformation can be measured directly by installing strain gauges on the package. These devices provide an estimate of the maximum local (i.e., where the strain meter is installed) changes in dimension (strain) of the object. Strain is usually expressed in units such as microstrains (see table in Sidebar 2.2).

Curve showing the variation in package acceleration as a function of time after impact in the Sandia National Laboratories grade-crossing test. SOURCE: Huerta and Yoshimura (1983, Figure 24, p. 25).

grams (240,000 pounds) was used in the tests. The package was instrumented with strain gauges and accelerometers, and the crash was recorded by high-speed photography.

The test geometry was such that the heavy locomotive frame, which was constructed of I-beams and welded steel plates, impacted the package below its centerline. Upon impact, the package plowed through about 3 meters (10 feet) of the lighter locomotive superstructure above the heavy frame and then became detached from the trailer, which became wrapped around the front of the locomotive. The impact forces launched the pack-

Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×

FIGURE 2.6 High-speed (130 kilometers per hour [81 miles per hour]) crash of a locomotive into a package mounted on a truck trailer carried out at Sandia National Laboratories in 1977. The top photo shows the impact of the locomotive and the trailer. The end of the package can be seen near the center of the photo. The bottom photo shows the package after the test. SOURCE: Sandia National Laboratories.

Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×

age into the air. It hit the ground about 46 meters (150 feet) from the point of impact and tumbled for another 15 meters (49 feet). The package attained a maximum horizontal velocity in flight of about 60 meters per second (about 50 miles per hour).

The impact produced two indentations in the package where it was struck by the frame of the locomotive. The cooling fins on the package were crushed and the outer package shell was bulged inward by about 2.5 centimeters (1 inch). A small leak in the package head was detected when the package was pressurized following the test (Jefferson and Yoshimura, 1977, p. 37). The inside cavity of the package was undeformed, however, and although some of the fuel rods had bowed slightly, the assembly was otherwise undamaged. The maximum deceleration on the package was about 33 g’s based on a velocity-time analysis of the high-speed photos. Two accelerometers mounted on the package gave peak readings of about 90 g’s and 200 g’s, but these readings may have been affected by package rotation, which reached a peak of about 1500 rotations per minute. The peak strain readings were below the yield strain for the package material.

Railcar Impact Tests

The third full-scale Sandia test involved the high-speed crash of a railcar-mounted spent nuclear fuel package into the same concrete barrier used in the truck crash tests (Huerta, 1981). The railcar system used in this test was constructed around 1960 but was no longer in use at the time of the tests. The package weighed about 68,000 kilograms (150,000 pounds). It was mounted in a steel-frame railcar with a package encasement system of about the same weight.

The transport package was larger than that used in the rail-crossing test, but it had a similar construction.31 It was designed to carry 10 spent fuel assemblies in water. For the purposes of this test, the package was loaded with nine mock assemblies and one unirradiated assembly, and it was filled with water. The package was mounted in the railcar with its closure end facing forward (i.e., toward the impact end of the railcar). The railcar and package were extensively instrumented with strain gauges and accelerometers. The crash was recorded by high-speed photography using both stationary and railcar-mounted cameras.

The test was conducted on September 27, 1977 (Figure 2.7). The railcar impacted the barrier at a speed of 131 kilometers per hour (82 miles per

31  

It had two stainless steel shells, 3.5 centimeters (1.375 inch) thick on the outside and 0.95 centimeter (0.375 inch) thick on the inside. The lead shielding was sandwiched between these shells. The package head was attached to the body with 24 high-strength bolts.

Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×

FIGURE 2.7 High-speed (131 kilometers per hour [82 miles per hour]) crash of a spent fuel package mounted in a railcar into a massive barrier carried out at Sandia National Laboratories in 1977. The railcar was destroyed in the crash, but the package sustained only superficial damage. SOURCE: Sandia National Laboratories.

hour). The impact crushed both the front end of the railcar and a spacer device that was placed at the forward end of the package. The package suffered some external damage to its cooling fins, but was otherwise undeformed. The package remained leaktight after the test. The fuel rods themselves were undamaged, although one of the support brackets was slightly distorted.

High-speed photography was used to estimate the deceleration-time curves for the package. Maximum decelerations were calculated to be 32 g’s. The strain gauge data from the package and fuel showed that maximum strains were below the elastic limits for these materials, which is consistent with the observation of no permanent deformations of those objects.

Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×

2.3.3 British Central Electricity Generating Board Tests

In the early 1980s, the British CEGB undertook a testing program aimed at improving understanding of the performance of spent fuel transport packages. The program had three objectives:

  1. Understand how to assess package impact resistance and demonstrate compliance with regulatory requirements.

  2. Estimate the probabilities of transport accident scenarios.

  3. Demonstrate to the public that the Generating Board’s packages that meet regulatory requirements will withstand severe accident conditions.

The impact performance of transportation packages was investigated over a period of four years through a carefully planned progression of analytical studies, scale-model testing, drop testing, and a full-scale crash test. A discussion of these tests is provided in Blythe et al. (1984) and an Institution of Mechanical Engineers (IME, 1985) report.

CEGB selected the Magnox package for this testing program. This package has been used since the 1950s for transporting Magnox fuel32 to the Sellafield site for reprocessing. By the early 1980s this package was being used for most of CEGB’s fuel movements to Sellafield as well as shipments to Sellafield by the Scotland Electricity Board and British Nuclear Fuels Limited (BNFL). The package has undergone several design improvements since being introduced into service. It has a monolithic cuboid body with a bolted lid with welded steel fins for cooling. A photograph of this package is shown in Sidebar 2.3. The package can hold up to 400 Magnox fuel elements and 1 metric ton (1.1 short tons) of water for heat transfer and radiation shielding. The loaded weight of the package is about 48 metric tons (53 short tons). The packages are transported mostly by train on specially designed railcars called flatrols.

Analytical studies carried out under the CEGB project indicated that because of their massive construction, the package body and package lid would be unlikely to sustain major damage in an accident. Any damage would likely be minor (e.g., bent cooling fins). These studies also suggested that any package releases would most likely be caused through impacts that result in bolt extension and decompression of the elastomer seals at the lid closure (Dallard, 1985, p. 49).

Analytical studies and scale-model testing undertaken by CEGB had shown that the maximum deformation would be sustained in a drop test in

32  

Magnox fuel contains uranium metal encased in a magnesium alloy “can.” The United Kingdom now operates 6 Magnox reactors, down from a peak of 26. The first reactor (Calder Hall) began operating in 1956 and is now shut down.

Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×

which there is no package rotation at impact. To achieve this condition, the drop tests were designed so that the center of gravity of the package was located directly above the impact point. The drop tests were designed to produce maximum deformation of the lid-body joint, because this would have a direct effect on containment integrity. Tests were conducted using one-quarter- and one-half-scale models and a full-scale package (shown in Sidebar 2.3). The packages were instrumented by transducers to measure forces on the bodies, lid displacements, and internal water pressures. Strain gauges were fitted to the closure bolts to measure elongations. High-speed photography was used to capture a detailed visual record of each test.

Two full-scale tests were carried out: In the first, the package was dropped onto the corner of the lid, and the second public demonstration test, the package was dropped onto the lid edge (Figure 2.8). The same package was used for both tests. The “public demonstration” drop test was conducted in March 1984. The package was filled with steel bars to simulate the Magnox fuel, and it also contained the other internal components and water in a manner that would be typical in an actual package shipment. It was pressurized to 100 pounds per square inch (6.9 bars) for leak testing.

A detailed discussion of the results of these tests is given in IME (1985). There was generally good agreement between the measured accelerations and displacements in the scale-model and in the full-scale drop tests. It was concluded that package behavior can be characterized accurately in scale-model tests “provided that all the important features are accurately represented in the models” (Barnfield and Donelan, 1985, p. 82; see Sidebar 2.3).

A small decrease in internal package pressures (ranging from about 1.5 to 6 pounds per square inch [0.1 to 0.4 bar]) was measured in both the scale-model and the full-scale drop tests. High-speed photography of the full-scale and half-scale tests showed a water spray from the lid-body joint lasting about 20 milliseconds. The water loss from one of the full-scale tests was estimated to be on the order of a few liters. Calculations based on this observation suggested that the associated radiological releases would have been less than about 5 percent of the amount permissible under IAEA regulations for accident conditions.33

The next phase of the CEGB study involved the identification of package transport impact hazards along transport routes. The objective of this hazard analysis was to estimate the probability of occurrence of various accident scenarios for use in designing the full-scale impact tests. Most spent fuel transport in the United Kingdom is carried out using rail, so the analysis focused on the identification of potential hazards along current and future rail routes used to transport packages to Sellafield. CEGB recog-

33  

These are given by the A2 values described in Sidebar 2.1.

Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×

FIGURE 2.8 Preparation of a Magnox spent fuel package (behind the workers on the scaffold) for the March 1984 CEGB free-drop test. The package was subjected to a 9-meter (30-foot) drop onto an unyielding surface. SOURCE: Magnox Electric Ltd.

nized that most people would have difficulty comparing the severity of a 9-meter drop test required by the regulations to a rail accident that might occur when a package was being transported at high speeds on the British rail system. This test was designed to provide a graphic demonstration of such a rail accident.

Information was collected on topography, geology, tunnel and bridge abutments, mobile hazards such as road vehicles and aircraft, and previous railway collisions. This information was used to construct event trees (e.g., see Figure 2.3) that could be used to assess the probability of future acci-

Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×

dent occurrences. The hazard analysis identified a range of potential events and estimated their probabilities of occurrence. The most likely accident scenarios range from derailments (5 chances in a hundred per year [5 × 10−2 per year]) to the impact of a package onto rock from a height exceeding 20 meters (70 feet) (1 chance in 100 million per year [1 × 10−8 per year]).

Two scenarios were considered initially for the full-scale crash test (Hart et al., 1985a, p. 116) based on the hazard analysis: (1) package-flatrol impact following a fall of 20 meters (65 feet); and (2) a package-flatrol striking a bridge or tunnel abutment at a speed exceeding 20 meters per second (about 45 miles per hour), which was the speed limit for trains carrying spent fuel packages at the time these tests were carried out.34 A third scenario was added because it was frequently mentioned as a cause of public concern: a derailed package-flatrol struck by a train traveling at a closing speed of greater than 20 meters per second (45 miles per hour). Other possible scenarios were eliminated from consideration because they were thought to have a very low probability of occurrence. These included aircraft impacts, explosions, and blasts.

The third scenario (train crash into a derailed package-flatrol) was finally selected for demonstration because model tests showed that of the three scenarios, this one would inflict the most damage on the package closure. The testing was limited to one full-scale crash because of cost and logistical considerations.

The crash test was carried out using the heaviest locomotive in service, which had an operating weight of about 140,000 kilograms (310,000 pounds) and a maximum operating speed approaching 45 meters per second (100 miles per hour). Three passenger coaches were hooked to the locomotive to “add realism to the test” (Collins et al., 1985, p. 206), even though calculations suggested that they would not increase its severity.

The test configuration chosen was similar to that used for the drop test: an impact on the package-closure joint with the center of mass of the locomotive aligned with the center of mass of the package to minimize rotational forces. The same package body used in the drop tests was used for this crash test, but it was fitted with a new lid. The package was mounted on a flatrol railcar, which was turned on its side and laid diagonally across the track so that the front coupler on the locomotive was aligned with the package-closure joint (Figure 2.9). The locomotive and package were instrumented with accelerometers and strain gauges, and the test was recorded using high-speed photography.

The test was carried out at a test track at Old Dalby in Leicester,

34  

This speed limit has since been raised to 65 miles per hour (104 kilometers per hour).

Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×

FIGURE 2.9 Configuration for the July 1984 full-scale crash test of a Magnox spent fuel package carried out in Leicester, England by the CEGB. The Magnox package was mounted on a flatrol railcar, which was turned on its side and laid diagonally across the track as shown in this photograph. The wires leading away from the package are connected to instruments for monitoring conditions during the early phases of the crash. SOURCE: Magnox Electric Ltd.

England on July 17, 1984, and was shown live on national television.35 In the crash, which was described as “first and foremost a visual spectacle” (Hart et al., 1985b, p. 234; Figure 2.10), the package suffered only superficial damage. The peak recorded acceleration, about 49 g’s, occurred about 10 milliseconds after impact. The pressure within the package was checked after the crash and was found to have decreased by 0.012 megapascal, corresponding to a fluid loss of about 0.5 liter (Hart et al., 1985b, p. 234).

A significant observation from this test is that the peak force on the package during the test (29 meganewtons [MN]) was considerably less than the peak force in the 9-meter drop test (75 MN) on the same package. In other words, this visually spectacular crash was actually a much less severe mechanical test of package containment than the 9-meter free-drop test used in the IAEA standards and USNRC regulations (see Sidebar 2.1).

35  

A video of this test is available on the Department of Energy’s Web site at http://www.ocrwm.doe.gov/newsroom/videos.shtml.

Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×

FIGURE 2.10 Full-scale crash test of a Magnox spent fuel package carried out in Leicester, England by the CEGB in July 1984. This photograph was taken just after the locomotive, which was traveling at 100 miles per hour (45 meters per second), collided with the package and flatrol. The collision has lifted the package and flatrol off the ground. SOURCE: Magnox Electric Ltd.

2.3.4 Package Performance Study

In 1999, the USNRC initiated a five-year project, referred to as the Package Performance Study, which had the following three objectives (USNRC, 2003c, p.4):

  1. Assess whether finite element analysis is a valuable tool for characterizing package and fuel response in extreme thermomechanical environments.

  2. Demonstrate the inherent safety of spent fuel package design using public outreach as a significant element.

  3. Refine dose and risk estimates to the public and workers through the collection of additional empirical data and improved transportation statistics.

USNRC staff embarked on an effort to obtain public input to inform this study. Initially, the staff held public meetings and solicited comments through its Web site to identify concerns that could be addressed in this

Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×

study. An analysis of these comments by Sandia National Laboratories (Sprung et al., 2001) identified several common concerns: Some commenters asserted that the regulatory testing requirements in 10 CFR Part 71 were unconvincing as a demonstration of a transport package’s performance under severe accident conditions. They wanted more realistic full-scale testing to demonstrate package performance. Some members of the public also thought that the accident statistics used in the analytical studies of package performance in severe accidents (e.g., the 1987 modal study) needed to be reanalyzed in light of increased truck traffic and vehicular speed limits on interstate highways.

USNRC staff then held additional public meetings to help them assess the accuracy of the Sprung et al. (2001) analyses of public comments and to obtain additional comments on a set of proposed package tests. These comments were used to develop a set of draft protocols for extraregulatory tests of rail and truck packages (USNRC, 2003c). Two draft protocols were proposed:

  1. High-speed (75 miles per hour [120 kilometers per hour]) impact tests involving drops of rail and truck packages from a tall tower onto an essentially unyielding surface. The rail package drop would be a package-corner-over-center-of-gravity test with the impact limiter in place. The truck package drop would be onto the package body in a so-called back-breaker orientation that bypassed the impact limiters.

  2. A fire test for both the rail and the truck packages that would involve a fully engulfing, optically dense hydrocarbon fire of a greater-than-30-minute duration.

Both of these tests are extraregulatory in the sense that they would exceed the test requirements in 10 CFR Part 71 (Sidebar 2.1). The 9-meter free-drop test, for example, produces, neglecting windage, a terminal speed of 13.3 meters per second, which is approximately 30 miles per hour (or approximately 48 kilometers per hour). This impact speed is much lower than the 75 mile per hour (120 kilometers per hour) impact speed envisioned in the staff’s draft test protocol.

USNRC staff proposed to perform full-scale testing on two packages that are currently certified in the United States. These packages are similar in construction to the generic packages that were used in Sandia’s spent fuel shipping risk reexamination (Sprung et al., 2000).

These draft testing protocols were put out for additional public comment. USNRC staff identified several groups of suggestions from the public comments on the draft testing protocols (USNRC, 2004c): full-scale testing should be conducted to regulatory limits; tests should be based on realistic accident scenarios; they should be designed to test packages to failure; and

Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×

they should address terrorist acts. USNRC staff commented in its recommendations to the Commissioners (USNRC, 2004c, p. 3) that having realistic accident scenarios and testing to failure are incompatible goals. Staff also noted that terrorism was being addressed in other studies currently under way within the USNRC that “are not suitable for the public participatory approach.”

USNRC staff rejected the suggestion to test packages to failure as unrealistic and unworkable for the following reasons: There is no readily agreed-upon definition of package failure among various stakeholders; package failure is a design-specific issue that would have little generic application to risk insights; and there are no realistic accident scenarios that are sufficiently severe to lead to package failure.

USNRC staff also decided to eliminate extraregulatory testing. In its place, staff recommended to the Commissioners that one or more of the following tests be carried out on full-scale truck and train packages:

  • A test of a rail package to the regulatory limit. This would subject a rail package with impact limiters to the regulatory tests in 10 CFR Part 71.

  • A test of a truck package with its impact limiters to the same regulatory limits, except that the 2-megapascal pressure test might be eliminated if the truck and rail packages are tested in combination.

  • A full-scale crash demonstration of a rail package with impact limiters and its railcar with a simulated bridge abutment at about 75 miles per hour (120 kilometers per hour), followed by a fire from a ruptured tank car.

  • A full-scale crash test of a truck package with impact limiters and its trailer by a locomotive traveling at about 75 miles per hour (120 kilometers per hour) on a grade crossing followed by a fire.

The staff proposed to the Commissioners a testing protocol involving various combinations of these tests, as well as an option to perform the extraregulatory tests originally outlined in USNRC (2003c). In December 2004, the Commissioners approved a modified full-scale test that will be carried out when funds are made available by Congress (USNRC, 2004d). USNRC staff was directed to plan for a demonstration test involving a single rail spent fuel package involved in a “viable” transportation accident. The Commissioners directed that

the test should consist of a simulated rail crossing with a train traveling at an appropriate speed colliding at a ninety degree angle with a transportation cask on its rail carrier car in a normal transportation configuration…. The test will consist only of the collision and the natural results of that collision. No separate fire testing or immersion testing will be conducted on the cask.

Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
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The Commissioners directed USNRC staff to prepare an information paper that provided details and estimated costs of the test.

The requested information paper was submitted to Commissioners in March 2005 (USNRC, 2005a). The staff proposed a test involving a full-scale package of the kind that is likely to be used by the Department of Energy (DOE) in its Yucca Mountain transportation program. The package would contain surrogate fuel elements and would be mounted on a rail carrier car placed at 90 degrees to a simulated rail crossing. The rail package would be subjected to a collision with a locomotive and several freight cars traveling at 60 miles per hour (96 kilometers per hour). In accordance with the Commissioners’ December 2004 directive, no fire or immersion testing was included in the proposed testing plan.

The Commissioners approved this proposed test in June 2005 (USNRC, 2005b). In addition, the Commissioners reversed their earlier instructions concerning the thermal test, directing the staff to “add a fire test scenario … involving [a] fully engulfing, optically dense, hydrocarbon fire for a duration of one-half hour post-collision.” The Commissioners directed staff to inform it of the details and estimated costs for the proposed fire test. They also noted that the proposed test plan provided by staff (USNRC, 2005a) “is not the final word on this issue, as the project is subject to additional modifications and Commission direction once additional information becomes available.” The Commissioners directed staff to engage with higher-level management in DOE to request financial support for the demonstration test and to request increased funding from Congress if DOE is unable to provide support.

2.4 CRITICAL ASSESSMENT OF PACKAGE PERFORMANCE

It is clear to the committee that package performance under severe accident conditions is a major concern for transportation safety among many members of the public, especially those who live and work along shipping routes. Finding a way to resolve this issue continues to be a challenge to regulators in the United States and may eventually become a challenge for DOE and the private sector in their commercial spent fuel transportation programs.

The packages used to transport spent fuel and high-level waste are designed to contain their radioactive contents under normal transport conditions and to withstand accident conditions without an increase in external radiation to levels that would endanger emergency responders or the general public. The package performance standards developed by the IAEA and embodied in USNRC regulations (Sidebar 2.1) were developed to help ensure that properly manufactured, certified packages provide such containment effectiveness. The 9-meter regulatory free-drop test onto an essen-

Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×

tially unyielding surface, for example, is a more severe mechanical test of package performance than has been produced in any of the severe full-scale crash tests described in this chapter (see Section 2.3).

The safety of transporting spent fuel and high-level waste depends to a great extent on the “inherent” safety of transportation packages to contain their contents even under severe accident conditions. The committee observes that while the full-scale crash tests described in this chapter have imposed mechanical conditions on packages that are less severe than the regulatory free-drop test, some of these tests have resulted in small releases from package containments.36 However, these releases would not have exceeded regulatory limits, which have as their goal the protection of emergency responders and the general public. The committee judges that the regulatory free-drop test imposes mechanical forces that are severe enough to bound conditions likely to be encountered in foreseeable real-life accidents. At the same time, the committee understands that any releases of radioactivity could have substantial social risk implications (see Chapter 3).

The accident reconstruction and modeling studies described in Section 2.2.3 suggest that there may be a very small number of credible accident conditions involving very long duration, fully engulfing fires37 that are potentially capable of damaging the seals on transportation packages if such fires are allowed to burn in an uncontrolled manner for long periods of time. Such damage could potentially lead to the release of radioactive material from the package through the processes described in Sidebar 2.5.

The potential for radioactive material releases from packages involved in such fires and the consequences of such releases are incompletely understood at present. USNRC staff has completed a solid technical analysis of the response of one package design to a realistically extreme thermal event resulting from the Howard Street tunnel fire. Additionally, staff are completing analyses of the performance of two additional package types for this fire exposure (see footnote 22 in this chapter). The committee judges that additional analyses of this type are needed to better understand package performance for realistic accident conditions involving very long duration fires.

36  

The releases described in this chapter consisted of water that leaked from the package interiors. This water may have been slightly contaminated with radioactive material from the fuel rod cladding (crud), but the fuel itself remained intact. It is important to note that in the United States, packages now in use to transport commercial spent fuel are filled with inert gas, not water. Inert gas is a much less efficient medium than water for transporting radioactive contamination out of leaking packages.

37  

The committee uses this term to describe fires that burn for periods of hours (or longer). Such fires could produce thermal loading conditions that exceed those for the regulatory thermal test specified in 10 CFR 71.73. Some of the real-world accidents described in this chapter involved fires that burned for hours to days.

Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×

The number of additional analyses might be small in number, especially if they focused on fire scenarios that would essentially bound expected real-world accident conditions,38 and if they examined the performance of a representative set of package designs that are likely to be used in future large-quantity shipping programs. The analyses have to address both the containment effectiveness of packages in response to very long duration fires and the consequences of any radioactive releases that result from package seal failures.

The purpose of these analyses would be to inform changes, if needed, to regulatory and operational practices to reduce the likelihood of occurrence and potential consequences of accidents involving very long duration fires. At least two options are available to regulators and implementers if such changes are needed:

  1. Operational steps to reduce the likelihood of occurrence of long-duration fires during spent fuel transport, and/or

  2. Design or testing requirements to improve the thermal resistance of transportation packages.

The first option would clearly be preferable from a cost and implementation standpoint. The objective of taking such operational steps would be to reduce the potential for accidents between trucks or trains carrying spent fuel and other vehicles carrying large quantities of flammable materials, especially flammable liquids, in places where it might be difficult to mount an effective firefighting response.39 Operational controls are used routinely by railroads to control transportation hazards. Such controls would probably be more easily implemented and enforced when shipments of spent fuel and high-level waste are made by dedicated train.

The second option would be more difficult and expensive to implement, because it very likely would require expensive changes to regulatory testing requirements and some existing package designs. However, there would be no reason to change the testing requirements or package designs if effective operational controls could be implemented.

The committee received several comments at its meetings on the testing of transportation packages under extreme loading conditions, especially

38  

The historical accident record provides perhaps the best available information to identify bounding accident scenarios. For example, the Livingston, Louisiana, fire (see Table 2.2) and Summit Tunnel fire (see Section 2.2.3 and Figure 2.4) are possible examples of bounding scenarios.

39  

Tunnel fires are of special concern because of access restrictions that can delay or prevent an effective firefighting response. Such delays could allow the fires to burn in an uncontrolled manner for long periods of time.

Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
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with respect to the USNRC’s Package Performance Study. Many commenters asserted that the regulatory testing requirements in 10 CFR Part 71 are unconvincing as a demonstration of a transport package’s performance under severe accident conditions. They wanted realistic full-scale testing to be conducted to demonstrate package performance. The State of Nevada suggested that comprehensive full-scale testing would improve the overall safety of the package and vehicle system and enhance confidence in risk analysis techniques. Full-scale testing was proposed as a way to potentially increase the acceptance of spent fuel shipments by state and local officials and members of the general public and to “potentially reduce adverse social and economic impacts caused by public perception of transportation risks” (Hall, 2003, p. 3).

Full-scale testing can be used to determine how packages will perform under both regulatory and credible extraregulatory conditions. To be of value, however, the committee believes that extraregulatory tests must be designed to closely mimic conditions that would reasonably be expected to be encountered in actual service. The test program undertaken by the CEGB in the 1980s (Section 2.3.3) provides an excellent example of the appropriate use of full-scale testing. In that program, the full-scale test was the end point of a much larger deliberative investigation of the conditions that could be encountered during transport of spent fuel. The full-scale test was used both to validate the analytical and scale-modeling work that had been carried out beforehand, and at the same time to provide a demonstration of the containment effectiveness of the packages in a way that could be appreciated by the general public.

The USNRC’s Package Performance Study has elements of an appropriate full-scale testing program. The design for a full-scale test was developed through a deliberative process that involved technical analysis and provided many opportunities for public comment. However, none of the tests that emerged from this process were selected by the USNRC for execution. In fact, the scenario that was selected (a collision between a locomotive and a package mounted on a railcar placed at 90 degrees to the rail crossing in an upright position; see Section 2.3.4) duplicates many features of the rail-crossing impact test carried out by Sandia National Laboratories more than 25 years ago. One very important difference between the Sandia test and the currently proposed test is the packages selected for testing. The Sandia test used an obsolete truck transport package. The proposed test would use a rail package that is currently certified and of the type to be used by DOE for transport of spent fuel to the federal repository. If executed properly, this full-scale test will be valuable for validating scale-model and computer simulations of package performance and thereby increasing confidence in current regulatory testing approaches. The test is also likely to be

Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×

visually powerful and could provide the public with useful information about the performance of transportation packages.

The committee also endorses package testing studies that integrate full-scale testing, scale-model testing, and finite element and associated structural analysis methods to demonstrate compliance with regulatory standards. These analysis methods allow a much wider variety of failure scenarios to be examined than is possible with full-scale testing alone. The analysis codes (e.g., LS DYNA, PRONTO) have been well benchmarked for these applications. Full-scale testing has two primary uses in this integrated test regime. First, it can be used to validate the computer codes for each package design, thus providing additional confidence in package performance under real-world conditions. Second, it provides a direct demonstration of a package’s ability to meet specific regulatory test conditions.

Several participants at the committee’s information-gathering meetings strongly urged that testing to failure should be carried out on all package designs that will be used to transport spent fuel and high-level waste to Yucca Mountain. However, they were not clear on what would constitute a failure. When pressed for specifics, the committee understood that what was generally meant was that full-scale testing should be carried out to destruction, presumably to establish the ultimate strength of transportation packages. The State of Nevada specifically recommended that the USNRC undertake an evaluation of the costs and benefits of destructive testing of a randomly selected production model cask (Hall, 2003, p. 3).

The principal argument in favor of destructive testing is that it can provide information on the magnitudes of thermomechanical loads required to eliminate containment effectiveness, thus establishing a “safety factor” for a given package design. There are significant drawbacks to this approach, however. Most importantly, each of the tests would involve only one thermal or mechanical loading condition out of the set of many possible conditions, and the selected loading condition would likely far exceed what could reasonably be expected to occur even in the most extreme real-world accidents. Moreover, a separate package might have to be acquired for each test, which would greatly increase the costs of a testing program for even a single package design.

It is important to recognize that any transportation package could be destroyed if no limits are placed on the loads that act upon it. Moreover, the failure of a package, in the sense that it can no longer perform its intended containment function, will generally occur under conditions that are much less severe than needed for destruction. Consequently, even if costs were not prohibitive, testing to destruction would provide little or no insight into the conditions that would cause a loss of package containment under real service conditions.

Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×

Even full-scale testing to failure could be problematical. Because examining the multitude of possible accident conditions and failure scenarios through full-scale testing is clearly impractical, the committee judges that the bounding approach that the IAEA has established is entirely appropriate. The approach is one that reflects four plausible accident-like conditions: free drop, puncture, thermal exposure, and water immersion (see Sidebar 2.1).

The important question to be answered by testing is not whether a package could be made to fail; as noted previously, it would certainly be possible to design tests that would accomplish this goal. Rather, the question that needs to be answered is whether there are credible accident conditions that would result in releases of radioactivity to the environment that would endanger emergency responders or the general public. It is clear from the modeling and full-scale tests described in this chapter that transportation packages are extremely rugged. The committee judges that packages designed, fabricated, used, and maintained under current regulatory standards are very unlikely to encounter loading conditions under real-world conditions, with the possible exception of very long duration fires, that would lead to releases in excess of regulatory limits. The committee recognizes, however, that even minor releases from package containment might have important social implications. These are discussed in Chapter 3.

2.5 PACKAGE PERFORMANCE FINDINGS AND RECOMMENDATIONS

The committee offers the following findings and recommendations based on the analysis of package performance provided in this chapter.


FINDING: Transportation packages play a crucial role in the safety of spent fuel and high-level waste shipments by providing a robust barrier to the release of radiation and radioactive material under both normal transport and accident conditions. International Atomic Energy Agency package performance standards and associated Nuclear Regulatory Commission regulations are adequate to ensure package containment effectiveness over a wide range of transport conditions, including most credible accident conditions. However, recently published work suggests that extreme accident scenarios involving very long duration, fully engulfing fires might produce thermal loading conditions sufficient to compromise containment effectiveness. The consequences of such thermal loading conditions for containment effectiveness are the subject of ongoing investigations by the Nuclear Regulatory Commission and other parties, and this work is improving the understanding of package performance. Nonetheless, additional analyses and experimentation are needed to demonstrate a bounding-level understand-

Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×

ing of package performance in response to very long duration, fully engulfing fires for a representative set of package designs.


RECOMMENDATION: The Nuclear Regulatory Commission should build on recent progress in understanding package performance in very long duration fires. To this end, the agency should undertake additional analyses of very long duration fire scenarios that bound expected real-world accident conditions for a representative set of package designs that are likely to be used in future large-quantity shipping programs. The objectives of these analyses should be to

  • Understand the performance of package barriers (spent fuel cladding and package seals);

  • Estimate the potential quantities and consequences of any releases of radioactive material; and

  • Examine the need for regulatory changes (e.g., package testing requirements) or operational changes (e.g., restrictions on trains carrying spent fuel) either to help prevent accidents that could lead to such fire conditions or to mitigate their consequences.

Strong consideration should also be given to performing well-instrumented tests for improving and validating the computer models used for carrying out these analyses, perhaps as part of the full-scale test planned by the Nuclear Regulatory Commission for its package performance study.

Based on the results of these investigations, the Commission should implement operational controls and restrictions on spent fuel and high-level waste shipments as necessary to reduce the chances that such fire conditions might be encountered in service. Such effective steps might include, for example, additional operational restrictions on trains carrying spent fuel and high-level waste to prevent co-location with trains carrying flammable materials in tunnels, in rail yards, and on sidings.


FINDING: The committee strongly endorses the use of full-scale testing to determine how packages will perform under both regulatory and credible extraregulatory conditions. Package testing in the United States and many other countries is carried out using good engineering practices that combine state-of-the-art structural analyses and physical tests to demonstrate containment effectiveness. Full-scale testing is a very effective tool both for guiding and validating analytical engineering models of package performance and for demonstrating the compliance of package designs with performance requirements. However, deliberate full-scale testing of packages to destruction through the application of forces that substantially exceed credible accident conditions would be marginally informative and is not

Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×

justified given the considerable costs for package acquisitions that such testing would require.


RECOMMENDATION: Full-scale package testing should continue to be used as part of integrated analytical, computer simulation, scale-model, and testing programs to validate package performance. Deliberate full-scale testing of packages to destruction should not be required as part of this integrated analysis or for compliance demonstrations.

Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
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Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
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Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
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Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
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Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
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Page 96
Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
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Page 97
Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×
Page 98
Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×
Page 99
Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×
Page 100
Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×
Page 101
Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×
Page 102
Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×
Page 103
Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×
Page 104
Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×
Page 105
Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×
Page 106
Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×
Page 107
Suggested Citation:"2 Transportation Package Safety." Transportation Research Board and National Research Council. 2006. Going the Distance?: The Safe Transport of Spent Nuclear Fuel and High-Level Radioactive Waste in the United States. Washington, DC: The National Academies Press. doi: 10.17226/11538.
×
Page 108
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This book looks to provide an independent, objective, and authoritative analysis of the transportation of spent nuclear fuel and radioactive waste in the United States, while simultaneously examining risks and identifying current and future technical and societal concerns for such specialized transportation. Going the Distance? also gives comparisons between health and safety risks for transporting spent fuel and radioactive waste and other risks that confront members of society. Comparisons are provided for routine radiological transport, which has the potential to produce chronic radiation exposures and latent cancer, and severe accident risks, which have the potential to produce acute radiation sickness and death, as well as latent cancer. This book will inform readers about the risks of spent fuel and high-level waste transportation.

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