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AN ASSESSMENT OF CONTINUED R&D INTO AN ELECTROMETALLURGICAL APPROACH FOR TREATING DOE SPENT NUCLEAR FUEL 3 THE DOE SPENT FUEL INVENTORY The DOE has more than 150 different types of SNF stored as more than 200,000 units in storage at DOE, non-DOE, and university facilities across the United States.1 The SNF that is, or is scheduled to become, part of the DOE inventory is stored at the following sites: Nine Doe sites: Idaho National Engineering Laboratory (INEL), including ANL-West and the Naval Research Facility Brookhaven National Laboratory Savannah River Site (SRS) Los Alamos National Laboratory Oak Ridge National Laboratory Sandia National Laboratories Hanford Reservation ANL-East Eight miscellaneous facilities performing reactor and fuel development or testing and isotope generation; Three “special case” commercial facilities; Thirty-three U.S. universities; and Forty-nine foreign research reactors. A majority of DOE SNFs were originally stored in cooling ponds; it was expected that they would be processed within 2 to 7 years after discharge from the reactor. To date, some DOE SNF has been in storage for more than 20 years. The DOE SNF inventory is estimated to correspond to 2,618 metric tons of heavy metal (MTHM). If one considers the entire SNF inventory in the United States, including fuels located at commercial powers reactors, DOE SNF accounts for only about 3 percent of the total by MTHM. The fuels (predominantly uranium oxide) located at commercial nuclear reactors are currently believed to be suitable for direct disposal in a geologic repository.2 In 1992, the DOE began efforts to address the disposition of SNF and to develop an integrated, long-term SNF management program. The DOE Spent Nuclear Fuel Program has produced a comprehensive database containing a detailed inventory of DOE-owned SNF, as well as a plan of action to resolve SNF vulnerabilities that have been identified. Currently in preparation are several environmental impact 1 Fillmore, Denzel L., and Kenneth D. Bulmahn, Characteristics of Department of Energy Spent Nuclear Fuel, in Proceedings of the Topical Meeting on DOE Spent Nuclear Fuel, Salt Lake City, Dec. 13-16, 1994, American Nuclear Society, LaGrange Park, Ill., pp. 313ff. Unless otherwise noted, all data in this section are from this source. 2 International Fuel Cycle Evaluation—Summary Volume, page 227, published by the International Atomic Energy Agency, Vienna, Austria, 1980.
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AN ASSESSMENT OF CONTINUED R&D INTO AN ELECTROMETALLURGICAL APPROACH FOR TREATING DOE SPENT NUCLEAR FUEL statements (EISs), including site-specific ones, that will provide the framework within which the DOE must operate to safely, reliably, and efficiently manage and prepare for disposal of its inventory of SNF. Issues being addressed include transportation, characterization, stabilization, interim storage, and technology development for ultimate storage. CATEGORIZATIONS OF SPENT NUCLEAR FUEL Historical Grouping Historically, for purposes of reprocessing and recovery, DOE SNFs have been grouped by enrichment level and by cladding and construction. Enrichment levels vary for the different fuel forms (construction), nominally classified as oxide, aluminum alloy, metal or metal non-aluminum alloy, carbide, hydride, naval, or other. Cladding can be described in terms of four main subgroups: zirconium or Zircaloy, stainless steel, aluminum, and graphite. INEL Grouping In the Spent Fuel Background Report3 from INEL, SNF is broadly classified into three categories: (1) production fuels, (2) special fuels, and (3) naval fuels. Production Fuels Most of the production fuels are located at DOE's Hanford Reservation and include the N-reactor and single pass reactor (SPR) fuels, with the N-reactor fuels alone accounting for about 80% of the total DOE fuel inventory. N-reactor fuel elements consist of two concentric tubes made of uranium metal coextruded into Zircaloy cladding. Two basic types of N-reactor fuel are differentiated by their uranium enrichment. In one type (Mark IV), the pre-irradiation enrichment levels in both tubes is about 0.95% U-235; in the other type (Mark IA), pre-irradiation enrichment levels are about 0.95% and 1.25% U-235 in the inner and outer tubes, respectively. Approximately 70% (by MTHM) of the fuel currently in storage is Mark IV, the remainder being Mark IA. Special Fuels The category of special fuels includes both low- and high-enrichment fuels from a variety of reactors used in a wide range of research, development, and testing activities. Fuel materials include uranium oxides, metal alloys, mixed oxides, metals, carbides, and others. Cladding materials include zirconium, aluminum, and stainless steel. Fuel forms vary among bundled or individual rods, pieces of rods, plates, carbide pellets in graphite blocks, solidified salts, and even core debris from the Three Mile Island (TMI) reactor. The largest contributors to the DOE inventory of special fuels include TMI core debris (83 MTHM), pressurized water reactor (PWR) fuel assemblies from Virginia Power and Electric and Nevada Power Corporation's Engine Maintenance Assembly and Disassembly commercial plants (38 MTHM), Fermi-1 blanket fuels (34 MTHM), and graphite fuel (24 MTHM) from the Fort St. Vrain reactor. 3 Spent Fuel Background Report, Idaho National Engineering Laboratory report SNF-5800-450-004, March,
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AN ASSESSMENT OF CONTINUED R&D INTO AN ELECTROMETALLURGICAL APPROACH FOR TREATING DOE SPENT NUCLEAR FUEL Of the different fuel types within the special fuels category, much of it can be described as light water reactor (LWR)-type fuel used for testing and demonstration of PWR or boiling water reactor concepts. TMI core debris consists of severely damaged PWR fuel assemblies and other core material placed in stainless steel canisters. In general, LWR fuel assemblies contain uranium dioxide (UO2) pellets in sealed metal (Zircaloy, stainless steel, or Inconel). The pre-irradiation enrichment for LWR fuels is nominally about 3 to 3.5% U-235. Several reactor designs also included assemblies that contain criticality-control materials (e.g., poison rods, moderators, or control rods). Another example of special fuels are those for the Advanced Test Reactor (ATR) program. ATR was an experimental test reactor designed to produce a more heterogeneous neutron flux in the core than could be obtained in a normal PWR. The ATR fuel is contained in plates made of an aluminum-uranium matrix clad with aluminum, and is highly enriched (93% U-235). The plates contain none of the control components found in many PWR fuel assemblies, although some contain boron carbide (B4C) mixed with the fuel as a burnable poison to minimize radial power peaking and to extend the life cycle of the fuel element. EBR-II fuel is also included in this category. Naval Fuels Naval fuels are those developed and used for naval propulsion and for related R&D activities. Although its design is classified, naval fuel can be categorized as being composed of highly enriched uranium. About 10 MTHM of high-burnup naval fuels are estimated to be in the inventory. EIS Grouping For the purpose of environmental impact statements, SNF inventories are divided into eight categories: Naval fuel; Aluminum-clad fuel (except for fuel from the General Atomics Corporation 's Training, Research, and Isotope production reactor); Hanford production fuel; Graphite fuel; Commercial fuel located at DOE facilities; Stainless-steel-clad research reactor fuel; Zirconium (Zircaloy)-clad research reactor fuel; and Other. This categorization is convenient because the fuel elements in each category are stored at just one or a few DOE sites. But in some cases these categories group together fuel types with radically different characteristics.
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AN ASSESSMENT OF CONTINUED R&D INTO AN ELECTROMETALLURGICAL APPROACH FOR TREATING DOE SPENT NUCLEAR FUEL Treatment-Oriented Grouping An alternative way of categorizing DOE's SNF is found in the DOE Spent Nuclear Fuel Technology Integration Plan.4 In this approach, DOE SNF is assigned to 53 fuel categories in an effort to reflect more than just physical differences. It was felt that these categories would better correspond with the technology needed to reduce existing vulnerabilities and stabilize and prepare SNF for interim storage and possibly for long-term disposal. Figure 4 summarizes DOE SNF throughout all DOE sites on the basis of total mass, volume, number of storage units, uranium mass, mass of fissile materials, and mass of heavy metal. With respect to the total inventory, the majority of the fuel is located at three major sites: INEL, SRS, and Hanford. N-reactor fuel is the largest contributor according to almost every measure, but it is not the only DOE SNF that needs attention. Of the approximately 150 SNF types in the DOE inventory, 25 types constitute 98% of the total mass. Nevertheless, solving the disposal problems for these fuels will still leave 125 fuel types that require a path to final disposition. Over the next 40 years, the DOE SNF inventory is anticipated to increase by 42% in total mass but by only 3.6% in MTHM. The biggest contributors to the increase are naval SNF, followed by foreign research reactor SNF. By the year 2035, naval SNF will account for almost 50% of the total mass within the inventory. TREATMENT, STORAGE, AND DISPOSITION OF SPENT NUCLEAR FUEL Spent nuclear fuels that will probably require some sort of treatment prior to disposition for interim or long-term storage are those for which potential chemical reactions in the environment of a repository are of concern. Generally, these are metal, metal non-aluminum alloys, and carbide fuels. The most pressing concerns identified in 1993 by a special working group in the DOE include:5 Approximately 2100 MTHM of metallic N-reactor fuel located in the K-basins at Hanford; Approximately 165 MTHM of spent fuel and target assemblies in the disassembly and canyon basins at SRS; and Approximately 2.7 MTHM of metal-clad (Zr, Al, or stainless steel) SNF at the Fuel Storage Facility at INEL, as well as the EBR-II fuels, which are uranium metal fuels using Na metal for bonding to the Zr cladding. 4 DOE Spent Nuclear Fuel Technology Integration Plan, Department of Energy report SNF-PP-FS-002, December, 1994. 5 Spent Fuel Working Group Report on Inventory and Storage of the Department 's Spent Nuclear Fuel and Other Reactor Irradiated Nuclear Materials and Their Environmental, Safety and Health Vulnerabilities (3 volumes), U.S. Department of Energy, December, 1993.
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