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4 Spent DOE Nuclear Fuel The Department of Energy (DOE) manages an assortment of spent nuclear fuel (SNF) types that vary in both materials of construction and in quantity. DOE spent fuel was generated in military and civilian reactor development, research, and fuel testing programs. The inventory also includes irradiated fuel and target1 assemblies that were placed in storage when DOE stopped reprocessing nuclear fuel for production purposes in 1992. Altogether there are over 250 different fuel types that have different enrichments, fissile materials, cladding, and geometries (DOE, 2000c). The fuels range from the Hanford N-Reactor SNF (about 85 percent of the total, see Figure 4.1), to the “cats and dogs” stored at the Idaho National Engineering and Environmental Laboratory (INEEL), which approach 150 different types. Currently a major effort is underway to retrieve N-Reactor SNF—some of which is damaged or deteriorating—from storage in pools at Hanford’s K-Basin, dry the fuel, put it in multi-canister overpacks, and place it in a newly constructed dry storage facility. Similar efforts to ensure safe interim2 storage are underway at other sites (NRC, 1998). DOE has recognized that small quantity “orphan” SNF, which is not included in these programs, may become an obstacle to site closure (Chambers and Kiess, 2002). The various DOE SNFs have been characterized broadly into categories based on fuel content, composition, and cladding material type, 1 Most of DOE’s nuclear materials were created in nuclear reactors through the capture of neutrons by various target isotopes, e.g., U-238 (see Appendix A). Using separate fuel (driver) and target assemblies increased production efficiency. DOE manages most irradiated targets as SNF. The committee does not distinguish between fuels and targets when referring to SNF. 2 Interim storage is temporary storage that is begun before its eventual duration is known.
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Figure 4.1 Fuel from the Hanford N-Reactor comprises 85 percent of DOE’s spent nuclear fuel inventory. The fuel consists of metallic uranium assemblies (two concentric cylinders with zirconium-based cladding) that are 66 cm long and 5 cm in diameter. In fresh fuel, as shown here, the U-235 enrichment is 1.25 percent. Source:DOE Richland. as shown in Table 4.1. While the heterogeneity of DOE spent fuel will make its long-term management expensive and complex, the total inventory of DOE spent fuel—approximately 2,500 metric tons of heavy metal (MTHM)3—amounts to only about 5 percent of the current inventory of commercial spent fuel from power reactors. Furthermore, the current inventory of about 44,000 MTHM of commercial SNF will approximately double to about 84,000 MTHM by 2020.4 The quantities of DOE spent fuel are also small compared to the 340,000 cubic meters of high-level waste that resulted from reprocessing spent fuel and target materials, mainly at the Savannah River Site (SRS), South Carolina; the Hanford Site, Washington; and INEEL (DOE, 2001a). Disposition Options and Challenges DOE plans to dispose of its SNF along with commercial SNF and vitrified high-level waste in a repository at Yucca Mountain in Nevada. Because DOE has only recently begun to prepare a license application for Yucca Mountain, significant uncertainty exists in what the waste acceptance criteria will be for many of the categories of DOE spent fuel. Commitments for waste form characteristics made during the 3 Quantities of nuclear fuel are reported in terms of the mass of heavy metals, principally uranium but in some cases including plutonium, used in their fabrication. 4 See http://www.eia.doe.gov/cneaf/nuclear/pageforecast/projection.html.
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Table 4.1 U.S. Department of Energy Spent Nuclear Fuels Group Fuel Type Typical Fuel Total (MTHM) Percent of total 1 Zircaloy clad naval Surface ship/submarine assemblies 65 2.6 2 Plutonium/uranium alloy Enrico Fermi Reactor (Fermi) Core 1 & 2 9 0.4 3 Plutonium/uranium carbide Fast Flux Test Facility (FFTF) test fuel assembly 0.1 0 4 Plutonium/uranium oxide and plutonium oxide Fast Flux Test Facility (FFTF) driver fuel assembly 12 0.5 5 Thorium/uraniurn carbide Fort St. Vrain 26 1.1 6 Thorium/uraniurn oxide Shippingport Light Water Breeder Reactor 50 2.0 7 Uranium metal N-Reactor 2,100 85.0 8 Uranium oxide Three Mile Island core debris 180 7.1 9 Aluminum-based fuel Foreign research reactor pin cluster 21 0.8 10 Unknown Miscellaneous 5 0.2 11 Uranium-zirconium hydride Training, research, and isotope General Atomic (TRIGA) 2 0.1 TOTAL 2,470 99.8 Source: Duguid et al., 2002; BSC, 2001. Yucca Mountain licensing phase will affect the accuracy required for characterizing the spent fuel isotopic and chemical composition prior to waste acceptance and the nature of any spent fuel conditioning that will be required. For security, it is expected that DOE SNF will have to meet the “spent fuel standard” proposed by the National Academy of Sciences, namely that through a combination of size, weight, intensity of radiation, and fissile material content the material would be no more attractive for theft than commercial SNF (NAS, 1995). The management of DOE spent fuel will first involve its continued interim storage for another 10 years or more and transportation to facilitate the consolidation of the materials in a smaller number of DOE sites. Some residues from this spent fuel, such as corrosion products found in the K-Reactor Basin at the Hanford site, will be treated as transuranic waste and sent to the Waste Isolation Pilot Plant facility in New Mexico for permanent disposal (McKenney and Walton, 2001).
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The handling and conditioning of spent fuel for disposal will also generate low-level radioactive waste that DOE must disposition. While reprocessing DOE spent fuel could potentially create a source of fissile and target material for reuse in future nuclear energy production, the heterogeneity and complexity of most of the inventory would make it a significantly less attractive source material for commercial fuel fabrication than the much larger, homogeneous quantities of commercial spent fuel that will be in storage, should the commercial demand for reprocessing emerge. The highly enriched uranium (HEU) spent fuels in the DOE inventory do retain a significant investment in enrichment that could be recovered by reprocessing and down-blending to create low-enrichment power reactor fuel. However, this HEU has substantial concentrations of U-236, a neutron-absorbing isotope, and U-232, which has a gamma-ray-emitting daughter, thallium-208. The presence of these isotopes substantially complicates using DOE HEU in low-enriched uranium power reactor fuel. Most types of DOE spent fuel have important characteristics that are different from commercial spent fuel, which comprises most of the waste intended to be disposed in Yucca Mountain. These are primarily differences in the chemical forms of the fuel, the cladding materials that encase it, and the isotopic composition of the fuel. The different characteristics affect the spent fuel’s chemical stability and potential for gas generation, the decay heat generation and the potential for thermal damage under different storage and accident conditions, the potential for inadvertent nuclear criticality, the potential doses for workers, and the attractiveness of the material for theft. Assessments are further hampered when current instruments and records are inadequate to characterize the chemical and isotopic composition of the fuel sufficiently. The areas where the unique characteristics of the DOE spent fuels influence safety and security, and thus create opportunities for research, can be subdivided into three categories: (1) chemical and thermal stability, (2) nuclear criticality, and (3) material protection, control, and accounting. Chemical and Thermal Stability Commercial spent fuel consists of ceramic pellets contained in zirconium-alloy tubes, called fuel pins or rods, which are highly corrosion resistant due to the requirement for protracted operation in high-temperature water inside the intense radiation environment of a reactor core. Several classes of DOE spent fuel have lower chemical stability than commercial spent fuel. These include metallic fuels and fuels with aluminum cladding. The cladding of some of these spent fuel materials is already corroded, some severely (see Figure 4.2). Also included in this category is the damaged fuel that was recovered from the Three
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Figure 4.2Most of DOE’s spent nuclear fuel is stored underwater to provide cooling and radiation shielding. A variety of corrosion processes, such as the pitting shown here, are degrading many of the fuels. Source:DOE National Spent Fuel Program. Mile Island Reactor. These materials have the potential to corrode more readily in both storage and repository environments and to generate radiolytic hydrogen during storage if not properly conditioned to remove water. The fissioning or “burnup” of DOE SNF is lower but much more variable than that of commercial SNF, for which maximizing burnup for energy production is important economically. While the technical challenges of removing decay heat are thus lower, thermal safety analysis requires accurate characterization of the isotopic composition of the spent fuel. Conditioning to improve the fuel’s stability also requires the capability to accurately characterize the chemical state of the fuel. Nuclear Criticality Several of the DOE spent fuel categories have significantly different criticality potentials during storage, handling, and disposal than commercial spent fuel, primarily due to differences in the isotopic composition of the fuel. Evaluations focus on worker safety because criticality can create dangerous radiation fields. Disposal regulations (10 CFR 63) require that criticality in a repository be considered and analyzed. For repository disposition, HEU spent fuels are the most important spent-fuel category that differs substantially from commercial spent fuel in criticality potential. The DOE HEU spent fuels include research-reactor spent fuel, naval spent fuel, and Fort St. Vrain gas-cooled reactor spent fuel.
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Material Protection, Control, and Accounting Several of the DOE spent-fuel categories contain weapons-usable materials in forms that are much more readily recovered than the plutonium and U-235 in commercial spent fuel. Commercial spent fuel has low uranium enrichment, relatively small plutonium concentrations (around 1 to 2 percent), and high radiation levels. These characteristics make commercial spent fuel an unattractive source of uranium for enrichment, and the recovery of plutonium is difficult and expensive. HEU spent fuel, and unirradiated or low-irradiation mixed oxide fuels containing plutonium, require more rigorous material protection, control, and accounting (MPC&A) to compensate for their lower intrinsic resistance to theft. Also, during any type of chemical stabilization or processing of spent fuels, accurate characterization of the isotopic and chemical composition of the fuel is required to permit accurate accounting for inputs and outputs from the process. These MPC&A issues have substantial similarities to the issues for the plutonium scraps and residues also considered in this report (see Chapter 3). A previous committee provided research recommendations to the EMSP for improving sensor technology and remote monitoring techniques (NRC, 2002). Research Needs and Opportunities The Environmental Management Science Program should support research to help ensure safe and secure storage and disposal of DOE SNF. Research should emphasize materials characterization and stabi lization, including developing a better understanding of corrosion, radiolytic effects, and accumulated stresses. This research should be directed toward determining a limited number of basic parameters that can be used to evaluate the long-term stability of each of the types of DOE SNF in realistic storage or repository environments. Material Characterization The primary research challenge and opportunity in the area of characterization is nondestructive assay of plutonium and other radioactive isotopes in the high-radiation environment typical of most spent fuels. There are three areas in which characterization is necessary to ensure the suitability and stability of DOE SNF for long-term storage or disposal. The first is to characterize the chemical and materials properties of the spent fuel to allow its stability for interim storage and disposition to be assessed. Important mechanisms that can degrade spent-fuel stability
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range from radiolytic gas generation to biocorrosion to matrix dissolution. Stresses can accumulate from the fuel’s thermal history and from other effects such as swelling due to oxidation or radiolytic displacements and transmutations. Research is needed to identify parameters and methods of measuring them that better characterize the potential for these forms of degradation to occur. The second is to characterize the isotopic composition of the fuel sufficiently to predict decay heat generation rates during storage and disposal, to assess the potential for criticality, and to provide an adequate description of the spent fuel to assess its ability to meet repository waste acceptance criteria. The isotopic composition is critical to the postclosure performance assessment of the repository, which is an essential part of the licensing case. For both commercial and defense spent fuel, the estimates of inventory depend on the fuel’s in-reactor history, which may be poorly known. Consequently, conservative assumptions regarding burn-up and other factors may be required, but at the expense of an overly conservative design. If means were available to assay nondestructively the isotopic content of spent fuel, a potentially significant uncertainty could be reduced. The specific repository acceptance requirements for spent-fuel isotopic characterization may change as Yucca Mountain licensing progresses, and changes should be noted to ensure that appropriate characterization research is performed. The third is to characterize the inventories of weapons-usable isotopes in the spent fuel to provide input for the materials accounting. Research is needed to develop improved methods to assay weapons-usable isotopes in spent fuel and could greatly facilitate materials accounting for fuel-conditioning methods that involve bulk processing of the fuel materials. Such methods could be tailored to the specific bulk-processing methodology. For example, the dry reprocessing methods being developed in South Korea to convert spent pressurized water reactor fuel into new fuel for heavy-water reactors provide a novel accounting method for plutonium. Neutrons from curium isotopes are measured at all processing stages. Because such dry reprocessing methods are incapable of separating plutonium and curium, the method allows accurate tracking of the plutonium inventory (Greenspan et al., 1998). Stability in Storage For spent fuels of relatively low chemical stability, such as DOE aluminum-clad fuels, some conditioning is likely to be needed to ensure that they remain stable enough to meet safe storage requirements until they can be emplaced in a disposal facility (NRC, 1998). A wide variety of potential degradation mechanisms exist: radiolytic gas generation,
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biocorrosion, pitting corrosion, interactions with other materials in storage containers, oxidation, matrix dissolution, and hydriding. There are opportunities for research to better understand these degradation methods and to identify inexpensive approaches to arrest them without resorting to more complex conditioning methods that would alter the entire fuel. Stability for Disposal Except for some corrosion products and ancillary low-level waste, the materials in DOE spent fuel are expected, ultimately, to be disposed in a deep geologic repository. What remains uncertain for many of the DOE waste forms is the degree of conditioning that will be required before the spent fuel can meet repository acceptance criteria. In this context, if the waste acceptance criteria that ultimately emerge are quite broad, then the economically optimal disposition approach will be to dispose of DOE spent fuel directly to a repository, with minimum conditioning. Conversely, highly restrictive waste acceptance criteria might lead to reprocessing all of the DOE spent fuel to generate streams of high-level waste, plutonium, and uranium that are essentially identical to much larger quantities of commercial material the United States must manage in any case. The likely outcome is somewhere in between—some DOE spent fuels will require substantial conditioning, and others very little. From the perspective of repository acceptance, minimal conditioning may prove to be problematic for HEU fuels, due to criticality issues, and for aluminum-clad fuels, due to chemical stability issues. Research is needed to identify and further develop conditioning methods that could facilitate repository acceptance. Further advances to the “melt and dilute” bulk-processing method, which reduces the criticality potential for HEU, would be an example. In this process, HEU spent fuel is melted down under carefully controlled conditions to drive off the most volatile and mobile fission products. The melt is then diluted with depleted uranium to prevent possible criticality and render the SNF unattractive as weapons material. Research to further develop lower-temperature reprocessing options, where the spent fuel is dissolved in a molten salt or an aqueous solution, and separate streams of well-characterized materials are created, may help to address the specific issues of high enrichment and low cladding chemical stability that distinguish many DOE spent fuels from commercial spent fuel. There are opportunities for collaboration with the new DOE Advanced Fuel Cycle Initiative (AFCI) to identify research that would make the reprocessing approach viable for some DOE spent
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fuels that would otherwise have difficulty meeting repository waste acceptance criteria (DOE, 2003). Developing a scientific basis for assessing the performance of DOE SNF in a repository environment and qualifying various DOE spent fuels for repository acceptance may be less expensive than reprocessing these spent fuels to create well characterized and understood waste streams. Such research will need to be integrated with ongoing repository licensing processes, to develop approaches to meet the waste acceptance criteria as these emerge, and to provide feedback to defining the waste acceptance criteria so these criteria do not inadvertently exclude materials for reasons that are not justified from the perspective of safety. Examples of appropriate research in this area could include the development of less soluble neutron poisons, and modeling of the interactions between spent nuclear fuel and waste glass if they are to be co-disposed in a single waste container.
Representative terms from entire chapter: