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--> 3 Waste-Package Performance Criteria The second charge of the statement of task involves the examination of the waste-package performance criteria developed by the Department of Energy (DOE) to meet anticipated waste acceptance criteria for disposal of aluminum spent fuel and to identify other factors that DOE might consider. In the context of this charge, the term waste package refers to the "road-ready" package discussed in Chapter 1. It consists of a metal disposable canister1 that contains several aluminum spent fuel elements or the treated equivalents (e.g., metal ingots produced by melt and dilute treatment). As explained in more detail later in this chapter, the waste-package performance criteria comprise the physical, chemical, and thermal characteristics that this waste package must meet to be acceptable for shipment to and emplacement in a repository container. Waste package performance criteria are being developed by DOE-Savannah River staff, and they are based on waste acceptance criteria (WAC) that are being created by another part of DOE as part of the repository development program. This program is discussed in more detail in the following section. This chapter provides a short review of the waste-package performance criteria that are under development by DOE-Savannah River to meet anticipated repository WAC. The chapter also provides comments on some of the technical work being carried out by or under the direction of DOE-Savannah River to demonstrate conformance with these WAC. 1 A stainless steel canister whose primary purpose is to protect the spent fuel or the treated equivalent during interim storage, shipping, and handling operations (see Figure 2.3).
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--> Several sources of information were used to develop this chapter. The main sources of information are the presentations made at the two information-gathering meetings (Chapter 1 and Appendix B) and the following documents: Chapter 10, Part 60 of the Code of Federal Regulations (10 CFR 60), Disposal of High-Level Radioactive Wastes in Geologic Repositories (USNRC, 1997); Mined Geological Disposal System Waste Acceptance Criteria, published by the Management and Operating Contractor for the DOE-Office of Civilian Radioactive Waste Management (TRW, 1997a); Alternative Aluminum Spent Nuclear Fuel Treatment Technology Development Status Report (WSRC, 1997a); Total System Performance Assessment Sensitivity Analysis of U.S. Department of Energy Spent Nuclear Fuel (CRWMS, 1997a); Acceptance Criteria for Interim Dry Storage of Aluminum-Alloy Clad Spent Nuclear Fuels (Sindelar and others, 1996); OCRWM2 Data Needs for DOE Spent Nuclear Fuel (TRW, 1997b). Background The Nuclear Waste Policy Act as amended3 designates Yucca Mountain, Nevada,4 as the candidate site for a spent fuel and high-level waste repository. Several federal agencies have responsibilities under this 2 Office of Civilian Radioactive Waste Management. 3 Public Law 97-425 (1982) as amended by P.L. 100-203 (1987) and P.L. 102486 (the Energy Policy Act of 1992). 4 Yucca Mountain is located in southern Nevada adjacent to the Nevada Test Site. The candidate repository would be constructed in a bedded tuff several hundred feet above the ground water table. The repository is being designed to contain 70,000 MTHM of spent fuel and vitrified high-level waste from reprocessing.
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--> act for ensuring the safe disposal of spent fuel and high-level waste. The administrator of the U.S. Environmental Protection Agency (EPA) is responsible for promulgating standards for protection of the general environment and the public from releases of radioactive materials from the repository. The U.S. Nuclear Regulatory Commission (USNRC) has the responsibility for establishing the technical requirements that it will use in evaluating applications for authorization to construct a repository, receive and emplace spent fuel or high-level waste, and close and decommission the facility once waste emplacement is completed. The act specifies that these technical requirements are "not to be inconsistent" with EPA standards.5 The responsibility for developing and operating a repository at Yucca Mountain lies with DOE.6 To meet its obligations under the act, DOE and its management and operating contractor7 are carrying out a detailed characterization program at Yucca Mountain to determine whether the site is suitable for a repository. The program includes geological, geochemical, and engineering studies of the site, including performance assessment (PA) studies to obtain bounding estimates of the long-term behavior of radioactive waste emplaced in the repository. DOE- 5 Standards and regulations were developed for Yucca Mountain during the 1980s by the EPA (in 40 CFR 191) and the USNRC (in 10 CFR 60). But certain provisions of the EPA standards were remanded by judicial action, and the EPA was subsequently directed by the Congress (in the Energy Policy Act of 1992) to develop a separate set of standards for Yucca Mountain. These standards have not yet been issued. Thus, at present, DOE is faced with the difficulty of developing repository and waste package designs to conform to EPA standards that do not yet exist. 6 Specifically, the act designates the director of DOE's Office of Civilian Radioactive Waste Management as the party responsible for carrying out these tasks, acting under the general supervision of the Secretary of Energy. 7 In subsequent discussions, the DOE Office of Civilian Radioactive Waste Management and its management and operating contractor will be collectively referred to as DOE-Yucca Mountain.
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--> Yucca Mountain also is developing repository and waste-package designs8 and detailed WAC for radioactive wastes to be emplaced in the repository. The WAC comprise the physical, chemical, and thermal characteristics that spent fuel, high-level waste, and associated disposable canisters must conform to for disposal in the repository. These criteria cover the characteristics of the waste and associated disposable canisters that affect the safety of the "back end" of the disposal process—that is, receipt of the waste at Yucca Mountain, transfer of the waste into disposal containers, transport of the waste into the repository, emplacement of waste in the repository drifts, and repository performance. The criteria are based on USNRC regulations (e.g., 10 CFR 60) as well as requirements imposed by DOE-Yucca Mountain to protect worker and public health during repository operations and to meet long-term repository performance requirements. Before DOE-Savannah River can ship its inventory of aluminum spent fuel to Yucca Mountain, it must obtain a certification from DOE-Yucca Mountain that its wastes conform with the WAC. To this end, DOE-Savannah River is working with DOE-Yucca Mountain to determine which WAC are likely to apply to aluminum spent fuel and what data are likely to be needed to demonstrate conformance. The certification process will involve the submission of an application that contains these required data by DOE-Savannah River to DOE-Yucca Mountain for review and approval. The format and prescribed information to be included in the application have yet to be determined. 8 Specifically, DOE-Yucca Mountain is designing disposal containers, which may consist of two or more corrosion-resistant metallic layers and are expected to maintain their integrity for thousands of years. All spent fuel and high-level waste will be placed in one of these containers before being emplaced in the repository, and each container is designed to hold a number of spent fuel assemblies or high-level waste glass logs (see Figure 2.3).
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--> Waste Acceptance Criteria The waste-acceptance criteria developed by DOE-Yucca Mountain are provided in the document Mined Geological Disposal System Waste Acceptance Criteria (TRW, 1997a)—that was made available for this review. This document provides the WAC for the following types of waste: Intact Spent Fuel. Spent nuclear fuel in which cladding integrity is maintained and none of the structural components have been compromised. Spent Fuel in Disposable Canisters. Spent nuclear fuel that lacks structural integrity or has fuel cladding degradation that could adversely affect repository performance and therefore must be delivered to the repository in a disposable canister. High-Level Waste. Waste generated from reprocessing activities that has been vitrified in borosilicate glass logs. Other Radioactive Waste. Waste that is not spent nuclear fuel or high-level waste. The aluminum spent fuel must conform to the criteria in the second category, because DOE-Savannah River has decided to place the waste in disposable canisters that do not need to be reopened before emplacement in the repository outer container that is being designed by DOE-Yucca Mountain (e.g., WSRC, 1997a, p. 3.3). A list of the applicable WAC for spent fuel in disposable canisters is given in Table 3.1. These are grouped into the following categories: general or descriptive criteria; physical or dimensional criteria for canistered waste; chemical compatibility for canistered waste; and thermal, radiation, and pressurization criteria.
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--> TABLE 3.1 Waste Acceptance Criteria for SNF in Disposable Canisters Criterion No. Title General and Descriptive Criteria for Non-Intact SNF 2.1.1 Compliance with Nuclear Waste Policy Act Definition of SNF 2.1.2 Minimum Cooling Time Since Reactor Discharge 2.1.3 Provision that SNF be a Solid 2.1.4 Provision that Wastes Other than Intact SNF be Canistered 188.8.131.52 Canistering of Degraded or Damaged SNF 184.108.40.206 Canistering of SNF Debris and Corrosion Products 220.127.116.11 Canistering of Non-Fuel Components 2.1.5-2.1.19 Placeholder for Future Text General and Descriptive Criteria for Canistered Waste 2.1.20 Provisions for Disposable Canister Materials 2.1.21 Requirement that Canisters be Sealed 2.1.22 Limits on Free Liquids in Canistered SNF 2.1.23 Maximum Allowable Quantity of Particulates 2.1.24 Limits on Pyrophoric Materials 2.1.25 Limits on Combustible, Explosive, or Chemically Reactive Waste Forms 2.1.26 Provision for Unique, Permanent Canister Labeling 2.1.27 Provision for Tamper-Indicating Devices (TID) on Canisters Not Seal-Welded 2.1.28 Physical Condition Of Disposable Canisters Physical and Dimensional Criteria for Canistered Waste 18.104.22.168 Dimensional Envelope for Disposable Single-Element Canisters 22.214.171.124 Dimensional Envelope for Disposable Multi-Element Canisters 126.96.36.199 Dimensional Envelope for Disposable DOE-Owned SNF 188.8.131.52 Weight of Disposable Single-Element Canisters 184.108.40.206 Weight of Disposable Multi-Element Disposable SNF Canisters 220.127.116.11 Weight of Disposable DOE-Owned SNF Canisters 18.104.22.168 Capability to Lift Commercial SNF Canisters 22.214.171.124 Capability to Lift DOE-Owned SNF Canisters Chemical Compatibility Criteria for Canistered Waste 126.96.36.199 Limits on Radionuclide Inventories in Single-Element Canisters 188.8.131.52 Limits on Radionuclide Inventories in Multi-Element Canisters 2.3.21 Limits on Total Fissile Material in a Disposable Canister 2.3.22 Limits on Disposable Canister Criticality Potential 2.3.23 Limits on Organic Materials in Canistered SNF
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--> Criterion No. Title Thermal, Radiation and Pressurization Criteria for Canistered Waste 2.4.20 Limits on Total Thermal Output for Disposable Canisters 2.4.21 Limits on Disposable Multi-Element Canister Thermal Design 2.4.22 Limits on Disposable Canister Surface Contamination 2.4.23 Provisions for Canister Internal Pressure 2.4.24 Limits on Disposable Canister Leak Rates SOURCE: TRW (1997a). The first two groups of criteria address basic definitions (e.g., what spent fuel is), requirements (e.g., spent fuel cooling time after discharge from a reactor), and disposable canister characteristics (e.g., physical dimensions, weight, construction). These criteria are fairly straightforward and should be easy to conform to through careful documentation and design. The criteria in the last two groups are potentially problematical, because they will require data collection and analysis to demonstrate conformance and because the criteria are uncertain at present and subject to change in the future. Several of these criteria are discussed in more detail in a later section. Other Requirements Although the focus of this chapter is on waste acceptance criteria, it is important to recognize that at least three other sets of requirements must be satisfied before the aluminum spent fuel or its processed equivalent can be shipped to Yucca Mountain for disposal. First, after treatment, the aluminum spent fuel will be placed in interim dry storage at Savannah River until it can be shipped to a repository, and it must conform to a set of interim storage criteria being developed by DOE-Savannah River. For planning purposes, DOE-Savannah River is assuming that interim dry storage will last for up to 50 years, and it is in the process of establishing criteria (Sindelar and others; 1996, WSRC,
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--> 1997a) for interim storage that will set limits on fuel corrosion, deformation, cladding integrity, and fission-product release. A summary of these criteria is given in Table 3.2. These criteria were established to minimize fuel corrosion and to maintain criticality control and fuel handleability during the interim storage period. TABLE 3.2 Criteria for Interim Storage of Aluminum Spent Fuel at Savannah River Criterion No. Description 1 Free water remaining within the sealed storage canister after drying is limited to maintain the hydrogen content less than 4% by volume. 2 The lag storage, treatment, and canister storage environments shall limit general corrosion or pitting corrosion to less than 0.0076 cm (0.003 in.) in depth in SNF cladding or in exposed fuel material. 3 The canister storage environment shall preclude the plastic deformation of SNF elements to less than 2.54 cm (1.0 in.) over a fuel assembly length of 91.44 cm (3.0 ft.) and deformation not to exceed 75% of the clearance space between the fuel assembly and storage grid throughout the period of storage. 4 The interim storage environments shall prevent rupture of the SNF cladding due to creep or due to severe embrittlement. 5 Canisters shall be backfilled with helium to 1.5 times atmospheric pressure at room temperature. 6 The storage facility shall be capable of handling canisters from 10 to 15 ft. in length. 7 The interim storage environment shall prevent the SNF cladding temperatures from exceeding 200 °C. SOURCE: WSRC (1997a).
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--> Second, the road-ready packages must meet U.S. Department of Transportation and USNRC regulations that govern the shipment of nuclear materials. These regulations, which are provided in 10 CFR 71, include requirements for transport configurations, criticality evaluations, accident testing, and quality assurance plans. A detailed consideration of these requirements is outside the current statement of task. Third, the repository itself must meet certain USNRC radionuclide release limits given in 10 CFR 609 and yet-to-be-established EPA dose and possibly ground water standards.10 To ensure that the repository will comply with these standards, DOE-Yucca Mountain is modeling the long-term performance of the waste forms to be emplaced in the repository, including the aluminum spent fuel waste forms,11 through its PA program.12 9 10 CFR 60 requires in part that the wastes be emplaced in packages that provide substantially complete containment for 300 to 1,000 years, and that releases from the repository be less than 1 part in 100,000 parts after 1,000 years. 10 As noted in footnote 5 in this chapter, the EPA will be releasing safety standards for Yucca Mountain in the future. At the present time it is very uncertain what those standards will contain. 11 That is, the physical and chemical form of the disposal product. For direct co-disposal treatment, the waste form consists of aluminum spent fuel. For melt and dilute treatment, the waste form consists of uranium-aluminum alloy ingots. For conventional reprocessing treatment, the waste form consists of vitrified glass logs. 12 Currently, the PAs for Yucca Mountain are primarily parametric analyses. It is not clear to the P.I. whether the PA will have a controlling role in repository licensing or, like probabilistic risk assessment in reactor licensing, it will have a minor role, especially given that the data base for PA is so much smaller than the data base for reactor analysis. Irrespective of its use in repository licensing, the PA will play an important role in establishing WAC for the repository, and one would hope for a strong and transparent relationship between the WAC, transportation requirements, and performance of the repository.
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--> Response to Second Charge in Statement of Task As noted at the beginning of this chapter, the second charge in the statement of task involves an examination of the waste-package performance criteria being developed by DOE-Savannah River for aluminum spent fuel and the identification of other criteria that should be considered. Most of the comments in this section will be addressed to the performance criteria that have been developed by DOE-Savannah River in response to the WAC published by DOE-Yucca Mountain (TRW, 1997a), but additional comments will be made about other criteria that could have a significant impact on the selection of a treatment option. To address the second charge, answers are provided to the following three questions: Have all of the important waste-package performance criteria been identified by DOE-Savannah River? Are there other performance criteria that should be considered? Is the work under way by DOE-Savannah River appropriate to demonstrate conformance with the various criteria and requirements? Consultants Francis Alcorn, Robert Bernero, and Valerie Putman provided comments that were helpful in responding to these questions. Their reports are provided in Appendix D. The answer to the first question concerning the identification of important waste-package criteria (Table 3.1) is a qualified ''yes." DOE-Savannah River staff appear to be working closely with their counterparts at DOE-Yucca Mountain to ensure that the important WAC have been identified and that the right kind of work is being done to demonstrate conformance. Perhaps the best evidence of this close working relationship is the analysis being done on nuclear criticality by DOE-Yucca Mountain under a contract from DOE-Savannah River. DOE-Savannah River also
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--> appears to have access to draft documents being prepared by DOE-Yucca Mountain that could affect the acceptability of aluminum spent fuel for disposal at the repository. The answer is qualified because, as noted earlier in this chapter, many of the WAC are preliminary and could change significantly as DOE-Yucca Mountain refines the waste package and repository designs. Thus, a continuance of the ongoing dialogue between DOE-Savannah River and DOE-Yucca Mountain will be essential to track and respond effectively to any future changes. The answer to the second question—should other criteria be considered?—has three parts: (1) for the WAC, (2) for the interim storage criteria, and (3) for the transportation criteria. The answer to the first part of the second question is "no." The current WAC for the candidate repository at Yucca Mountain are very clearly laid out (TRW, 1997a), and the information received during this study from DOE-Savannah River (e.g., WSRC, 1997a) indicates that all of the potentially applicable WAC have been identified and are being addressed through ongoing work. As noted above, however, many of the WAC are preliminary and could change significantly as waste package and repository designs are refined by DOE-Yucca Mountain. Again, a continuation of the dialogue between DOE-Yucca Mountain and DOE-Savannah River will be essential to track and respond effectively to any future changes. The answer to the second part of the second question—are there other criteria that should be considered for interim storage (Table 3.2) in addition to those that are required for repository acceptance?—is "no." Most of the criteria seem reasonable given the current plans that DOE-Savannah River has to store, retrieve, and process (as necessary) the fuel to put it into road-ready form. One of the criteria, however, appears to be unnecessary. Specifically, criterion 3 in Table 3.2, which sets limits for plastic deformation of the aluminum spent fuel in the disposable canister, seems overly restrictive and potentially costly. The justification given for this requirement is that it will "provide for ready removal of the fuel from a canister and handleability of the fuel" (WSRC, 1997a, p. 3.4). It is not
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--> clear why DOE-Savannah River would ever want to remove spent fuel from a disposable canister under normal operating conditions. If the fuel was treated properly before placement in the canister there should be no need to retrieve it prior to shipment to the repository. Even under "abnormal" conditions such as a tipover accident the canister could be sectioned to remove the spent fuel. DOE-Savannah River is encouraged to reexamine the cost and potential benefit of this criterion in view of the unlikely need for future fuel removal. The third part of the second question—are there other transportation criteria that should be considered?—cannot be answered at this time. Based on discussions with DOE-Savannah River staff, relatively little work has been done to date on establishing criteria to meet transportation requirements. The transportation requirements given in 10 CFR 71 could affect the design of the disposable canister into which the spent fuel or its processed equivalent will be placed for shipment to and emplacement in the repository. DOE-Savannah River should not encounter any significant problems meeting the requirements in 10 CFR 71—highly enriched uranium (HEU) spent fuel is shipped across the country and around the world routinely, and the aluminum research reactor spent fuel now stored at Savannah River was shipped from offsite at some time in the past. DOE-Savannah River must review the shipping requirements before it finalizes the design of its disposable canisters. The answer to the third question—is the work under way by DOE-Savannah River appropriate to demonstrate conformance?—is a very qualified "yes." Based on the presentations received at the two information-gathering meetings, the documents reviewed, the tour of the Savannah River Technology Center where much of this work is being done, and discussions with the personnel there, the development program under way to demonstrate conformance with the WAC appears to be properly focused and appropriate to the task. This answer is qualified, however, because the short schedule for this project did not allow an in-depth review of all of the ongoing work in the aluminum spent fuel
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--> program, and because the WAC on which current activities at Savannah River are based may change once EPA standards and USNRC regulations are issued. The remainder of this section provides a few comments on WAC developed by DOE-Yucca Mountain that are potentially limiting for treatment option selection either because data collection or analysis will be required to demonstrate conformance or because the criteria are uncertain and subject to change in the future. The following sections provide brief discussion of three such criteria and a discussion of the efforts under way at DOE-Savannah River to document conformance. The criteria are 184.108.40.206 (limits on radionuclide inventories in multi-element canisters), 2.3.21 (limits on total fissile material in a disposable canister), and 2.3.22 (limits on disposable canister criticality potential). A complete list of criteria is provided in Table 3.1. 220.127.116.11: Limits on Radionuclide Inventories in Multi-Element Canisters Calculated radionuclide inventories in disposable multi-element canisters for a given group of waste shall not average more than the levels listed in [Table 3.3] to be accepted into the MGDS [Mined Geological Disposal System] (all numbers TBV [to be verified]. There are no restrictions on radionuclides excluded from this list. Note: It must be recognized that acceptance limits for radionuclides must be considered preliminary until additional Performance Assessment [PA] analyses are performed, and the repository PA is accepted by the [US]NRC as part of granting a repository operating license. Currently, there is no environmental release standard to which acceptance limits can be set (40 CFR 191 standards are used, even though 40 CFR 191 has
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--> been remanded), there are a wide range of assumptions used in the Performance Assessment models that can significantly alter acceptance ranges (these assumptions have varying degrees of concurrence from the [US]NRC), and a number of important mechanisms that will influence acceptance ranges are not yet adequately modeled by the current Performance Assessment codes. Radionuclide inventories footnoted with an asterisk [in Table 3.3] represent those that are the largest contributors to total repository release rates and are, along with their parent radioisotopes, the least likely to have higher acceptance limits in future versions of this document. (TRW, 1997a, p. 4-17-4-18.) This criterion is of particular concern for direct co-disposal treatment for two reasons.13 First, DOE-Savannah River will have to characterize the aluminum spent fuel to demonstrate that it meets the limits in the table. Second, the limits shown in the table are subject to change as the PA models are verified or new EPA standards or USNRC regulations are promulgated. A future move to risk-based regulation, for example, could introduce a new set of radionuclide inventory criteria. The simplest way to demonstrate that spent fuel meets the limits in Table 3.3 is through direct calculation using the fuel property data, which are determined during fuel fabrication, and the reactor operation history to calculate fuel burnup and isotope production. This approach is preferred because it is sufficiently accurate, relatively inexpensive, and technically straightforward. However, the direct calculation approach may be problematical for some of the foreign research reactor fuel and the 13 Waste form characterization is not a problem for melt and dilute treatment or conventional reprocessing, because the physical and chemical properties of the waste form can be controlled during processing.
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--> TABLE 3.3 Limits on Radionuclide Inventories in Multi-Element Canisters Isotopea Concentration Average (Ci/waste package)b 227Ac 1.79 x 10-4 241Am 3.73 x 104 242MAm 2.16 x 102 243Am 2.48 x 102 14C* 1.38 x 101 36Cl 1.11 x 10-1 244Cm 1.16 x 104 245Cm 3.36 246Cm 6.95 x 10-1 135Cs* 5.13 129I 3.43 x 10-1 93MNb 1.82 x 101 94Nb 8.24 59Ni* 2.36 x 101 63Ni 3.10 x 101 237Np* 4.35 231Pa 3.30 x 104 210Pb 6.75 x 10-6 107Pd* 1.26 238Pu 3.05 x 104 239Pu 3.56 x 104 240Pu 5.26 x 103 241Pu 3.39 x 105 242Pu 2.01 x 101 226Ra* 2.50 x 10-5 228Ra 3.10 x 10-9 79Se* 4.41 151Sm 3.53 x 103 126Sn 8.50 99Tc 1.42 x 102 229Th 3.54 x 106 230Th 3.59 x 103 232Th 4.35 x 10-9 233U 7.01 x 10-1 234U 1.34 x 101 235U 1.68 x 10-1 236U 2.72 238U 3.07 93Zr 2.38 x 101 a Isotopes with an asterisk represent the largest contributors to total repository release rates in current versions of the PA. b Curies per waste package. SOURCE: TRW (1997a).
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--> older domestic research reactor fuel, because the data to perform such calculations may be unavailable, incomplete, or inaccurate. In these cases it may be necessary to use direct measurement techniques such as gamma spectrometry or radiochemical techniques, which can add expense and time to the characterization process. It may also be possible to use bounding analyses based on average values as is done for power reactor fuel. DOE-Savannah River has initiated a program to evaluate the availability and quality of existing fuel property and reactor operation history data for representative aluminum spent fuel types to determine if such data are adequate to meet existing acceptance requirements (WSRC, 1997a). This activity is necessary, cost effective, and timely. Fuel receipt and storage schedules and costs could be affected significantly by the level and amount of characterization required. The relatively small investment of time required to determine the availability of such data should pay off handsomely in the future in terms of increased throughput and reduced handling as the fuel is received at Savannah River for treatment and interim storage. This activity should continue to receive a high priority. 2.3.21: Limits on Total Fissile Material in a Disposable Canister There are no limits on total fissile material in disposal canisters based on current waste-package and performance assessment analyses. Note: Analyses to establish limits on total fissile material are ongoing but were not available in time for inclusion in this version of the MGDS WAC. Until such limits are available, this text represents a placeholder for a future quantified criteria. (TRW, 1997a, p. 4-20.)
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--> DOE-Yucca Mountain has placed no limits on the amount of fissile material14 in waste packages, but the guidance language provided above suggests that it intends to establish such limits in the future. Limits on fissile material could be imposed either as a matter of policy, to reduce the potential for criticality, or to limit the release of radionuclides from the repository to meet regulatory or safeguard standards. Enriched aluminum spent fuel contains less fissile material than typical commercial spent fuel,15 so fissile material limits are not likely to be a problem if DOE-Yucca Mountain sets a single limit for all spent fuel to be disposed in the repository. If DOE-Yucca Mountain determines that lower fissile material limits are required for aluminum spent fuel, or if lower limits are required for all HEU fuel, then disposal of such fuel using direct co-disposal treatment may not be possible. 2.3.22: Limits on Disposable Canister Criticality Potential Canistered SNF [spent nuclear fuel] entering the MGDS shall be shown to have a calculated keff of 0.95 or less, after allowance for bias in calculation methods and uncertainty in the empirical data used to validate the method of calculation assuming the following conditions: All canister basket structure (other than components made from titanium, zircalloy, or other 14 The important fissile isotope in enriched aluminum spent fuel is uranium-235 (235U). The important fissile isotopes in commercial spent fuel are 235U and plutonium-239 (239Pu). 15 A canister of commercial spent fuel will contain about 80 kg of 235U and 60 to 70 kg of 239Pu + 241Pu (Oak Ridge Light Water Reactor Radiological Database). A disposal container of HEU aluminum spent fuel will contain a maximum of about 30 kg of 235U and only small amounts of 239Pu relative to commercial spent fuel (based on data from CRWMS, 1997b).
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--> extremely corrosion-resistant materials) have collapsed and degraded into component corrosion products (e.g., FeO2 from carbon steel basket materials). All supplemental neutron absorber materials (e.g., boron), except hafnium, have degraded and are no longer part of the waste package. Assembly hardware has degraded and all fuel assemblies are touching in a optimum reactivity condition (assuming a corrosion resistant zircalloy clad fuel).16 SNF reactivity has increased to the peak levels in the early years after reactor discharge. (TRW, 1997a, p. 4-20.) One of the most important objectives in designing the repository and waste canister is to prevent the possibility of criticality events17 from occurring during or after the emplacement of spent fuel.18 Current USNRC regulations in 10 CFR 60 require DOE-Yucca Mountain to demonstrate with a 5 percent margin of safety19 that the probability of a criticality event in the repository is less than one in a million (1 x 10-6) for 10,000 years following disposal. The USNRC is revising this rule and may impose different or additional requirement s, for example, that the dose consequences of a criticality event also be determined. 16 This criterion is not relevant to aluminum spent fuel. 17 A criticality event is a self-sustaining nuclear reaction, much like that in a nuclear reactor. In a repository, such an event can occur when the fissile materials from the spent fuel (e.g., 235U) are brought into certain geometric configurations in the presence of water. A criticality event in the repository would result in the release of energy and neutrons but might not be detectable at the surface. 18 See the comments of consultants Francis Alcorn and Valerie Putman in Appendix D for more details. 19 Conventionally expressed as keff ≤ 0.95.
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--> DOE-Yucca Mountain intends to demonstrate compliance with this regulation through careful attention to repository design, the placement of disposal canisters in the repository, the internal configuration of spent fuel in the waste packages, and possibly limits on fissile materials in the waste packages (criterion 2.3.21). The upper-limit probability of a criticality event for a given set of design criteria can be determined through calculation using worst-case assumptions for waste package degradation, geochemical reactions, and ground water flow. Examples of the types of worst-case assumptions used in the calculations are shown in the bulleted paragraphs in the WAC (see the italicized text above). The criticality potential criterion will require the most attention for direct co-disposal of aluminum spent fuel because of its 235U enrichment relative to spent commercial fuel. DOE-Savannah River appears to recognize the importance of this criterion and has developed a cooperative program with DOE-Yucca Mountain to establish the technical viability of the direct co-disposal option with respect to criticality. This work is described in WSRC (1997a). Work is currently under way at DOE-Yucca Mountain to assess the criticality potential of aluminum spent fuel for the direct co-disposal option and is occurring in three phases: Phase 1, which examines the criticality potential of degraded fuel in an intact disposal canister (i.e., the inner canister in Figure 2.3); Phase 2, which examines the criticality potential when the degraded fuel is released from the disposable canister into the co-disposal waste package (the outer container in Figure 2.3); and Phase 3, which examines the criticality potential when the degraded fuel is released from the co-disposal waste package into the repository. Briefings were received from DOE on phases 1 and 2 of this work during the two information-gathering sessions, and the written report of the phase 1 work, which was completed in mid-1997 (CRWMS, 1997b), was reviewed by the P.I. and consultants. The work to date appears to be technically sound, and neither the P.I. nor the two criticality consultants
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--> invited to the second information-gathering meeting (Appendix D) identified any significant major flaws in the design or execution of these analyses.20 The phase 1 work suggests that aluminum spent fuel will conform to criterion 2.3.22 if neutron absorbers (e.g., borated stainless steel) are added to the waste packages, and the phase 2 work described in the information-gathering sessions seems to indicate that neutron absorbers also can be used to meet this criterion for the degraded container scenario. Additional work will be required to confirm this result only if direct co-disposal treatment is selected, because the melt and dilute treatment and conventional reprocessing can be designed to produce waste forms that do not contain HEU. Not enough work has been done on the criticality potential external to the container (i.e., the phase 3 study mentioned previously) to determine whether aluminum spent fuel—especially HEU fuel—will conform to criterion 2.3.22 for direct co-disposal. If it cannot, the direct co-disposal treatment option may be eliminated for all or certain types of aluminum spent fuel.21 Conclusions DOE-Savannah River appears to have identified all of the significant waste acceptance criteria for aluminum spent fuel and is engaged in the proper process (through close consultation with DOE-Yucca Mountain) to demonstrate conformance. As noted in the foregoing discussion, however, several of the WAC are poorly defined at present or 20 The criticality consultants (Francis Alcorn and Valerie Putman) did note several minor concerns about this work in their reports (Appendix D), but the P.I. assumes that these will be addressed by DOE before the criticality work is completed. 21 Of course, changes to the design of the repository, including engineered barriers, also could influence the acceptability of HEU spent fuel for disposal.
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--> may be subject to significant future change. In fact, for several reasons including lack of an EPA standard and corresponding USNRC regulations, it may be quite some time before DOE-Savannah River knows with certainty whether direct co-disposal is viable for all fuel types. The current state of uncertainty has significant implications for the ''path forward" for selecting spent fuel treatment options. Three initial conclusions based on these facts are offered below: A single treatment option may not be suitable for all types of aluminum spent fuel. The aluminum spent fuel program will need to maintain flexibility in selecting treatment options until there is more complete information on the WAC and other requirements. A path forward that involves phased decision-making in the selection and implementation of alternative treatment options is indicated. These conclusions are developed in greater detail in the last chapter of this report.
Representative terms from entire chapter: