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1

Introduction

The Waste Isolation Pilot Plant (WIPP) is the world's first deep underground operational geological repository for the disposal of radioactive waste. The WIPP consists of an underground mined facility located in a 250 million-year-old bedded salt formation (the Salado Formation), which lies 660 meters below the surface in a semiarid desert near the community of Carlsbad, New Mexico. The WIPP repository has been established for the disposal of transuranic (TRU) waste resulting from the nation's defense program. The advantages of the WIPP as a transuranic waste disposal are listed in Sidebar 1.1. Figure 1.1, Figure 1.2 and Figure 1.3 show the location, layout, and geologic stratigraphy of the WIPP.

Transuranic waste contains alpha-emitting radionuclides that have atomic numbers greater than 92, the atomic number of uranium, the heaviest natural element. The WIPP Land Withdrawal Act (LWA) (U.S. Congress, 1992) defined TRU waste as waste contaminated with transuranic radionuclides with half-life1 greater than 20 years and activity greater than 100 nanocuries per gram. It mainly consists of contaminated protective clothing, rags, old tools and equipment, pieces of dismantled buildings, chemical residues, and scrap material. Table 1.1 and Table 1.2 provide, respectively, the inventory of major radionuclides in the WIPP and the repository inventory by waste category. More details on transuranic waste are given in Sidebar 1.2. Figure 1.4 shows pictures of typical TRU waste. Even though the backfill magnesium oxide (MgO) appears in the repository inventory, it is not considered to be waste. Water is also not part of the waste inventory. There is only a negligible amount of water in the waste, mostly water vapor and less than 1 volume percent of free liquids as allowed by the Waste Acceptance Criteria (DOE, 1999).

Packed in 55-gallon steel drums and wooden boxes, TRU waste is currently being stored at various sites across the nation. The source of the waste is the manufacture of nuclear warheads and the cleanup of the nuclear weapons sites. The risks associated with transuranic waste are related primarily to plutonium. Plutonium's long half-life (24,000 years for plutonium-239)2 and toxicity must be considered in assess-


1The half-life is the time required for half of the atoms of a radioactive substance to disintegrate.
2Plutonium-239 indicates the isotope of mass number 239 of the element plutonium. The same notation is used for other radionuclides throughout this report.


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Page 7 1 Introduction The Waste Isolation Pilot Plant (WIPP) is the world's first deep underground operational geological repository for the disposal of radioactive waste. The WIPP consists of an underground mined facility located in a 250 million-year-old bedded salt formation (the Salado Formation), which lies 660 meters below the surface in a semiarid desert near the community of Carlsbad, New Mexico. The WIPP repository has been established for the disposal of transuranic (TRU) waste resulting from the nation's defense program. The advantages of the WIPP as a transuranic waste disposal are listed in Sidebar 1.1. Figure 1.1, Figure 1.2 and Figure 1.3 show the location, layout, and geologic stratigraphy of the WIPP. Transuranic waste contains alpha-emitting radionuclides that have atomic numbers greater than 92, the atomic number of uranium, the heaviest natural element. The WIPP Land Withdrawal Act (LWA) (U.S. Congress, 1992) defined TRU waste as waste contaminated with transuranic radionuclides with half-life1 greater than 20 years and activity greater than 100 nanocuries per gram. It mainly consists of contaminated protective clothing, rags, old tools and equipment, pieces of dismantled buildings, chemical residues, and scrap material. Table 1.1 and Table 1.2 provide, respectively, the inventory of major radionuclides in the WIPP and the repository inventory by waste category. More details on transuranic waste are given in Sidebar 1.2. Figure 1.4 shows pictures of typical TRU waste. Even though the backfill magnesium oxide (MgO) appears in the repository inventory, it is not considered to be waste. Water is also not part of the waste inventory. There is only a negligible amount of water in the waste, mostly water vapor and less than 1 volume percent of free liquids as allowed by the Waste Acceptance Criteria (DOE, 1999). Packed in 55-gallon steel drums and wooden boxes, TRU waste is currently being stored at various sites across the nation. The source of the waste is the manufacture of nuclear warheads and the cleanup of the nuclear weapons sites. The risks associated with transuranic waste are related primarily to plutonium. Plutonium's long half-life (24,000 years for plutonium-239)2 and toxicity must be considered in assess- 1The half-life is the time required for half of the atoms of a radioactive substance to disintegrate. 2Plutonium-239 indicates the isotope of mass number 239 of the element plutonium. The same notation is used for other radionuclides throughout this report.

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Page 8 Sidebar 1.1 Why the WIPP? The rationale for isolating nuclear wastes through deep geologic disposal is based on a large body of U.S. and international research. The National Academy of Sciences observed in 1957 (NRC, 1957): “The best means of long-term disposal . . . is deep geological emplacement. . . .” The Academy reaffirmed and expanded on this view in NRC (1984) and in NRC (1996a). The WIPP repository is carved out of a bedded salt formation, with the following features that make it ideal for transuranic waste disposal: Dry environment. Large salt beds are found only in geologic regions that lack significant flows of groundwater. This deep, relatively dry underground environment greatly reduces the possibility that wastes could be carried out of a repository by natural processes. The saltbed at the WIPP site has been stable for 225 million years. It can be expected, with high confidence, to remain that way for many thousands of years into the future. Waste immobilization. Salt tends to “heal” itself after being mined because it gradually creeps under the pressure from overlying earth and fills any openings. After several hundred years, the salt at the WIPP is expected to close in on the waste and lock it deep below the surface. Since the mid-1970s, the Department of Energy (DOE) and its scientific adviser, Sandia National Laboratories, have studied the WIPP site to make sure it is a safe place to isolate transuranic waste. The WIPP addresses the following two key national needs: Reducing risk. As long as transuranic waste remains at storage sites, there will be some level of risk to populations near these sites. Also, workers who must maintain current sites and monitor wastes are frequently exposed to low levels of radiation. Providing disposal. The WIPP is a first-of-its-kind deep geologic disposal facility and will provide a model for radioactive waste disposal. In addition to the existing inventory of stored transuranic waste, estimated at about 2.32 million cubic feet, the WIPP will be the disposal site for more than 3.7 million cubic feet of transuranic waste expected to be generated during the next 35 years as DOE sites are closed. Under current law, the DOE is allowed to store 6.2 million cubic feet of transuranic waste at the WIPP. SOURCE: Citizens' Guide to the Compliance Certification Application (DOE, 1996b). SOURCE: Citizens' Guide to the Compliance Certification Application (DOE, 1996b).

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Page 9 ~ enlarge ~ Figure 1.1 Location of the WIPP site. SOURCE: DOE, 2000g.

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Page 10 ~ enlarge ~ Figure 1.2 The WIPP facility and stratigraphic sequence. Panel 1 is currently in use. The mining of Panel 2 was completed on October 13, 2000. SOURCE: DOE, 2000h.

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Page 11 ~ enlarge ~ Figure 1.3 WIPP stratigraphy and depths of four key formations (Castile Formation, Salado Formation, Rustler Formation, and Dewey Lake Red Beds) including the position of the WIPP repository within the Salado. The Culebra Dolomite is one of the members of the Rustler Formation. It is approximately 7-8 meters thick at the WIPP site. Because it is a relatively transmissive unit, the Culebra is important to the groundwater flow model for the WIPP site. Inset shows finer-scale stratigraphy around the repository horizon, with marker beds and other thin beds. Adapted from Jensen et al. (1993).

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Page 12 Table 1.1 Inventory of the Most Abundant Radionuclides Expected in the Repository.a Radionuclide Contact Handled (CH)-Transuranic (TRU) Waste (Ci/m3) Remote Handled (RH)-Transuranic (TRU) Waste (Ci/m3) Am-241 2.62 0.842 Ba-137m 4.53 × 10−2 28.9 Cm-244 0.187 4.45 × 10−2 Co-60 3.83 × 10−4 1.47 Cs-137 4.78 × 10−2 30.5 Pu-238 15.5 0.205 Pu-239 4.66 1.45 Pu-240 1.25 0.715 Pu-241 13.7 20.0 Sr-90 4.07 × 10−2 29.5 Y-90 4.07 × 10−2 29.5 aThe expected volumes of CH waste and RH waste are, respectively, 160,000 and 7,079 cubic meters. SOURCE: DOE, 1996. Table 1.2 Repository Inventory by Waste Category Waste Category Inventory (wt%) Iron-based metal, alloys 14 Steel container material 12 Aluminum-based metal, alloys 1 Other metal, alloys 6 Other inorganic materials 3 Vitrified 5 Cellulosics 4 Rubber 1 Plastics 3 Plastic container or liner material 2 Solidified inorganic material (including cement) 4 Solidified organic material (not including cement) 0 Solidification cement 4 Soils 4 MgO backfill 37 SOURCE: Knowles et al., 2000. ing not only the long-term risk of the WIPP, but also the potential radiation exposure of workers who handle, repackage, and transport the waste. The WIPP has been under study since the mid-1970s, began construction in January 1981, was certified by the U.S. Environmental Protection Agency (EPA) in May 1998, and received its first transuranic waste shipment from the Los Alamos National Laboratory in March 1999. The first out-of-state shipment was received in June 1999 from the Rocky Flats Environmental Technology Site, and in September 2000, the first mixed-waste shipment was received from the Idaho National Engineering and Environmental Laboratory (INEEL). Figure 1.5 shows the main waste generators and the transportation routes to the WIPP.

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Page 13 Sidebar 1.2 What Is TRU Waste And How Is It Classified? Transuranic waste is waste that contains alpha particle-emitting radionuclides with atomic numbers greater than that of uranium (92), half-lives greater than 20 years, and concentrations greater than 100 nanocuries per gram of waste. TRU waste is classified according to the radiation dose rate at package surface. As defined in the LWA, contact-handled (CH) TRU waste has a radiation dose rate at package surface not greater than 200 millirem per hour; this waste can safely be handled directly by personnel. Remote-handled (RH) TRU waste has a radiation dose rate at package surface of 200 millirem per hour or greater, but not more than 1,000 rem per hour (U.S. Congress, 1992); this waste must be handled remotely (i.e., with machinery designed to shield the handler from radiation). Alpha radiation is the primary factor in the radiation health hazard associated with TRU waste. Alpha radiation is not energetic enough to penetrate human skin but poses a health hazard if it is taken into the body (e.g., inhaled or ingested). In addition to alpha radiation, TRU waste also emits gamma and/or beta radiation, which can penetrate the human body and requires shielding during transport and handling. RH TRU waste has gamma and/or beta radiation-emitting radionuclides in greater quantities than exist in CH TRU waste (DOE, 2000a). TRU waste is further classified as TRU waste or mixed TRU waste. Mixed TRU waste contains both radioactive materials regulated under the Atomic Energy Act and hazardous chemical compounds regulated under the Resource Conservation and Recovery Act. The total activity of the waste expected to be disposed at the WIPP is estimated to be approximately 7 million curies (of which 6 million is from CH waste), including 12,900 kilograms of plutonium distributed throughout the waste in very dilute form. According to the Compliance Certification Application (CCA), the volume of CH waste expected in WIPP is 160,000 cubic meters and that of RH waste is 7,079 cubic meters (DOE, 1996). The WIPP is designed to dispose of approximately 175,000 cubic meters of transuranic waste. Total activity of the waste is estimated to be approximately 7 million curies. The largest fraction of this activity comes from approximately 12,900 kilograms of plutonium distributed throughout the waste in very dilute form. TRU waste is classified as contact-handled (CH) and remote-handled (RH) waste, according to the radioactivity at the container surface 3 (see Sidebar 1.2). According to the National TRU Waste Management Plan, the disposal of RH waste will not begin before early 2002 (DOE, 2000a). Since most of the radioactivity is coming from the plutonium in CH waste (approximately 85 percent of the total curies inventory, see Table 1.1), the disposal of RH waste should not represent a significant added risk to the repository. A further issue concerning RH waste will be discussed in relation with the emplacement of backfill in Chapter 2. This report presents the results of a National Research Council (NRC) study of operational, technical, and programmatic issues associated with the long-term performance of the WIPP. Previous studies 3This type of classification is intended for the protection of workers handling radioactive waste. Public health protection standards have also been taken into account in the design and operation of the WIPP.

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Page 14 ~ enlarge ~ Figure 1.4 Radiography of a transuranic waste drum. SOURCE: DOE, 2000i. by the NRC's committee on the WIPP covered ongoing activities in preparation for the opening of the facility. This study is the first to address the WIPP as an operational repository. The seeds for this report were planted during the preparation of the 1996 report by the previous WIPP committee (NRC, 1996a). That committee observed that the long operating period of the WIPP (at least 35 years and possibly much longer) provides an opportunity to conduct studies and investigations that would decrease some of the uncertainties about the long-term safety performance of the repository.

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Page 15 ~ enlarge ~ Figure 1.5 Defense transuranic waste generating and storage sites and primary transportation routes. SOURCE: DOE, 2000j. Thus, this committee has focused on identifying studies and investigations “that would enhance the assessment of long-term repository performance,” as noted in the statement of task in the Preface to this report. The second part of this committee's statement of task addresses potential improvements to the National Transuranic Waste Management Plan, also known as the National TRU Program. This program coordinates the management and disposal activities of TRU waste between the WIPP and the 23 generator sites. As written in the statement of task, the committee must “identify areas for improvement in the TRU waste management system that may increase system throughput, efficiency, cost-effectiveness, or safety to workers and the public.” The result is the consideration of issues having to do with waste characterization, packaging requirements, waste transportation and handling, and emergency preparedness. The two-part statement of task required very different skills and approaches: the first part is related to site performance, while the second is programmatic. The committee has chosen to structure this report into two primary sections that can be mapped directly to the two principal requirements of the statement of task. The part of the statement of task relevant to the long-term performance of the repository is addressed in the context of the repository performance confirmation program, in reference to enhancing confidence in the performance assessment models. The task relating to programmatic issues is addressed in the context of the National TRU Program. SITE PERFORMANCE AND CHARACTERIZATION To evaluate the long-term performance of the disposal system, the DOE uses a technique developed especially for predicting the behavior of geologic repositories over the thousands of years required for waste isolation. This technique is called “performance assessment.” Performance assessment (PA) is a multidisciplinary, iterative, analytical process that begins by using available information that characterizes the waste and the disposal system (the design of the repository, the repository seals, and the natural barriers provided by the host rock and the surrounding formations). To obtain certification for the WIPP,

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Page 16the DOE used the PA tool to estimate the releases of radionuclides within the first 10,000 years, based on the probabilities of relevant features, events, and processes occurring. The performance of the repository has been assessed for two main scenarios: the undisturbed repository scenario and the human intrusion scenario. If the repository is left undisturbed, the only release pathway for radionuclide release into the environment is through leakage of brines containing radioactive materials into the environment. Scenarios for the disturbed case involve releases resulting from boreholes drilled inadvertently into the waste. According to the Land Withdrawal Act (U.S. Congress, 1992), the DOE must exercise active institutional controls4 on a perimeter of land extending up to 5 kilometers from the boundaries of the WIPP site for 100 years after the closure of the repository. During this period, there will be no natural resource extraction activities in the site. Between 100 and 700 years after the closure of the repository, the site will be under passive institutional controls.5 During this period, drilling activity is expected to resume and to reach its maximum after 700 years, when the land will be released to public use and the WIPP site will be no longer controlled. Uncontrolled extraction activities would increase the probability of drilling directly into the repository. Sensitivity analyses are used by the DOE to determine which parameters of the disposal system exert the greatest effect on performance (DOE, 1996). Performance assessment calculations show that in the absence of human intrusion, brine inflow and gas generation are the most important parameters affecting the performance of the WIPP (Helton, 2000d). In the case of the disturbed scenario, the most important parameter is the borehole permeability (Helton, 2000e). Sidebar 1.3 describes the main results of the performance assessment and their implication for the long-term performance of the WIPP. For a complete review of the PA for the WIPP see Apostolakis et al. (2000). The containment requirements are set by the regulatory agency, the U.S. Environmental Protection Agency, and are listed in Sidebar 1.4. More information on regulatory compliance can be found in the previous NRC report on the WIPP (NRC, 1996a). The EPA certified the WIPP on the basis of the performance assessment included in the Compliance Certification Application (CCA). While various mechanisms and scenarios, including their uncertainties, were considered in the performance assessment, the question now is how to enhance the degree of confidence expressed by the performance assessment results. The conceptual structure and the development of scenarios for the WIPP's PA are described respectively in reference Helton et al. (2000a) and Galson et al. (2000). The uncertainties in the PA for the WIPP are analyzed in Helton et al. (2000b,c). The current committee on the WIPP believes that better knowledge of site performance and better site characterization are important in decreasing the uncertainties, and therefore possibly enhancing the confidence, in the performance assessment of the repository. The committee's approach to examining the PA was to focus on underlying assumptions and results of the performance assessment. Of particular interest to the committee was how the results could be impacted by uncertainties and relied upon EPA's certification for proof of the ability of the computer program to represent the model adequately. The issues and their uncertainties are discussed in Chapter 2 as site performance and site characterization issues. THE NATIONAL TRANSURANIC PROGRAM The National Transuranic Waste Management Plan, also known as the National TRU Program, is a plan that organizes the activities concerning storage, characterization, packaging, handling, transporta- 4 Active institutional controls imply restrictions on land access or use. 5 Passive institutional controls imply the identification of the controlled area through signs or markers; also, records are kept on the repository and land use.

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Page 17 Sidebar 1.3 Performance Assessment and Regulatory Acceptance The Environmental Protection Agency's certification of the WIPP facility was based on the performance assessment submitted as a part of the U.S. Department of Energy's Compliance Certification Application. The regulatory basis for the PA for the WIPP is described in Howard et al. (2000). The PA is a computerized, mathematical model that evaluates the performance of the WIPP repository over its lifetime. The main results of this model are shown in Figure A below, and are compared there with the acceptance criterion established by the EPA shown as the line in the upper right corner. These requirements are reported in Sidebar 1.4. The horizontal axis is a measure C, of the total radioactivity released from the repository to the biosphere during its nominal 10,000-year lifetime. The vertical axis shows the “probability of release,” that is, at any value of C, the probability that the actual release from the repository will exceed that value. Such a curve is called a “complementary cumulative distribution function (CCDF).” It expresses quantitatively the state of knowledge of the analysis team about how much radioactivity will be released from the repository over its lifetime. It is important to observe that the curve is well to the left of the regulatory acceptance boundary set by the EPA, meaning that the repository is in compliance with the regulation. A variation on this form of presentation is shown in Figure B. In this figure, a family of CCDFs is traced to show the different effects of uncertainties arising from possible human intrusions into the repository (mainly by drilling into it) and the geotechnical uncertainties (e.g., physical and chemical properties of the salt). Again, the important result is that the whole family of curves is well to the left of the EPA acceptance boundary. In addition, the curves bunched close together indicate a reasonable bound on the uncertainties and add confidence that a substantial margin of safety exists. ~ enlarge ~ Figures A (left) and B (right) Complementary cumulative distribution functions resulting from the performance assessment. In A, the probability of radionuclide release from the repository is compared to the acceptance criteria. In B, a family of CCDF curves is traced to show the effect of different uncertainties. The “summed normalized release” of radionuclides C, is related to WIPP containment requirements in Sidebar 1.4. The term “normalized” release means that the release Cj is divided by the release limit Lj. The use of the term “summed” indicates the sum of all Cj/Lj over all the radionuclides with half-life greater than 20 years. The summed normalized release represents the total radioactivity released from the repository to the biosphere during its nominal 10,000-year lifetime. More details on CCDFs can be found in NRC (1996a). SOURCE: DOE, 2000k.

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Page 18 Sidebar 1.3 Continued The committee recognizes that computing the performance of an underground repository over many millennia into the future cannot be done today with the accuracy with which, for example, the performance of an airplane wing can be simulated. Nevertheless, the results of this performance assessment are considered adequate by experts and regulators to support the decision to move waste from its surface storage to the WIPP (EPA, 1998). Sidebar 1.4 Containment Requirements Title 40 CFR 191.13 requires that “disposal systems for . . . transuranic radioactive wastes shall be designed to provide a reasonable expectation, based on performance assessments, that the cumulative releases of radionuclides to the accessible environment for 10,000 years after disposal from all significant processes and events that may affect the disposal system shall: 1. Have a likelihood of less than one chance in 10 of exceeding the quantities calculated according to Table 1 . . .; and 2. Have a likelihood of less than one chance in 1,000 of exceeding ten times the quantities calculated according to Table 1. . .”. To explain how these requirements are applied to the WIPP, let Lj be the limit shown in the above table for radionuclide j. Suppose for the moment that WIPP had only one radionuclide, j, and let Cj be the total release of that radionuclide to the environment, measured in curies per 1000 metric tons of heavy metal (MTHM), during its 10,000 year lifetime. Then the first requirement of 40 CFR 191.13 means that the probability of Cj being greater than Lj should be less than 0.1. That is: p(Cj /Lj >1) should be < 0.1 The second requirement then indicates that p(Cj /Lj >10) should be < 0.001. The actual inventory of radionuclides C, is defined as: ~ enlarge ~ with Nj being the total number of radionuclides with a half-life greater than 20 years. The requirements then become: p(C>1) should be < 0.1 p(C>10) should be < 0.001

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Page 19 Table 1. Release Limits per 1,000,000 Curies of TRU Waste per 10,000 Yearsa Radionuclide Release Limit (curies per 1000 MTHM) Americium-241 or 243 100 Carbon-14 100 Cesium-137 or 137 1,000 Iodine-129 100 Neptunium-237 100 Plutonium-238, 239, 240, or 242 100 Radium-226 100 Strontium-90 1,000 Technetium-99 10,000 Thorium-230 or 232 10 Tin-126 1,000 Uranium-233, 234, 235, 236, or 238 100 Any other alpha-emitting radionuclide with a half-life greater than 20 years 100 Any other radionuclide with a half-life grater than 20 years that does not emit alpha particles 1,000 a Containment requirements for selected isotopes as declared in Title 40 CFR 191, Appendix A (EPA, 1995). The release limits specified here scale with the quantity of waste in a repository; for this reason, they are specified in terms of curies that may be released per 10,000 years per 1,000 metric tons of heavy metal (MTHM). For a repository such as WIPP, which is intended to contain transuranic wastes, EPA has established in 40 CFR 191 that 1,000 MTHM is equivalent to 1,000,000 curies of TRU wastes with greater than 20-year half-lives. Therefore, the limits specified are applicable per million curies of TRU waste. tion, and disposal of defense-related transuranic waste to the WIPP from the 23 generator sites. The National TRU Program is administered by the DOE's Carlsbad Field Office. The goals of the National TRU Program are the following: achieving regulatory compliance among all the sites, reducing risk while maximizing rate of TRU waste disposal, reducing mortgage costs by closing the generators' sites as soon as possible, and using the WIPP effectively by coordinating the shipments with the repository's waste-handling and disposal capabilities. The issues considered in this report relate primarily to waste characterization and packaging and waste transportation. Because of their importance in the near term for achieving the beginning of operation at the WIPP, the committee focused on these issues in its interim report, reported in Appendix A1. In Chapter 3 of this final report, the committee re-visits the issues related to characterization, packaging, and transportation of the wastes, including communication systems and emergency preparedness. The issue of hydrogen gas generation, as it applies to both waste characterization and transportation, is also discussed in Chapter 3.