Electrometallurgical Techniques for DOE Spent Fuel Treatment: Final Report (2000)

The Committee on Electrometallurgical Techniques for DOE Spent Fuel Treatment was formed in September 1994 in response to a request made to the National Research Council (NRC)¹ by the U.S. Department of Energy (DOE). DOE requested an evaluation of electrometallurgical processing technology proposed by Argonne National Laboratory (ANL) for the treatment of DOE spent nuclear fuel (SNF).² Electrometallurgical treatment of spent reactor fuel involves a set of operations designed to remove the remaining uranium metal and to incorporate the radioactive nuclides into well defined and reproducible waste streams.

Over the course of the committee’s operating life, this charge has remained constant. Within the framework of this overall charge, the scope of the committee’s work—as defined by its statement of task—has evolved in response to further requests from DOE, as well as technical accomplishments and regulatory and legal considerations. As part of its task, the committee has provided periodic assessments of ANL’s R&D program on the electrometallurgical technology.

Electrometallurgical technology (EMT) consists of electrorefining the reactor fuel in an electrochemical cell. The fuel, in metallic form, is selectively dissolved at the anode while nearly pure uranium metal is deposited at the cathode, leaving fission products, fuel cladding material, plutonium and other transuranic elements partially at the anode and partially in the molten salt electrolyte. Thus the fuel is separated into three components; metallic uranium, a metallic waste form from the anode, and a highly radioactive salt mixture that subsequently can be converted to a ceramic waste form. In 1995, ANL proposed the use of electrometallurgical technology for treatment of all spent nuclear fuel in the DOE inventory.³ Treatment would convert the fuel to components suitable for waste disposal as well as separate out any material that might be of use in future DOE operations. Electrometallurgical technology was suggested to offer the potential that it could produce the same waste forms for all of spent fuels in the DOE inventory, thus providing substantial cost savings for qualification of these wastes for disposal in a geologic repository. The ANL proposal was further based on the presumption that some form of treatment would be needed for part of the SNF inventory because the significant reactivity of certain spent fuels

would preclude direct disposal in a geologic repository. Of particular concern was sodium-bonded fuel such as that from the EBR-II reactor at ANL’s site in Idaho.

A key step in the ANL’s proposal was treatment of all the EBR-II spent fuel as a demonstration of the technology. However, in response to a lawsuit, DOE scaled down the size of the demonstration so that it could be carried out under the scope of the existing Environmental Impact Statement (EIS).⁴ As a result, the demonstration project was refocused to treat only part of the EBR-II spent fuel. As work progressed on the EBR-II spent fuel, the committee’s technical evaluation of electrometallurgical technology became increasingly focused on the demonstration project—which provided the primary source of data on which the committee could make its assessments.

The committee, throughout its three phases of operation, has consistently recommended that ANL adopt a set of criteria by which the success of the use of electrometallurgical technology could be judged.^5,6 For the demonstration project, ANL adopted a set of four success criteria, together with specific technical goals to meet each of the criteria.⁷ These success criteria are discussed in greater detail in Chapter 6.

During its tenure, the committee has operated under three different statements of task. Although in each phase it addressed a different aspect of the use of EMT for the treatment of SNF, the committee in all three phases operated under the general charge of evaluating the technical viability of electrometallurgical technology for treatment of DOE spent nuclear fuel.⁸ This technical evaluation was performed based on the R&D pursued by ANL in its EBR-II electrometallurgical demonstration project.

In phase one, the committee evaluated ANL’s research related to EMT that had been performed up to that point (i.e., the Integral Fast Reactor Program, see Chapter 2). In addition, the committee received briefings from experts in EMT, which provided a basis for the committee’s assessment of ANL’s R&D plan. Committee members also were briefed on ANL’s decision to utilize EBR-II spent nuclear fuel for the EMT demonstration program. In phase one of its work, the committee did not address whether EMT should be adopted as a component of the national strategy for handling nuclear materials. Although it evaluated EMT in light of other technical options, the committee refrained at this stage from suggesting which option should be pursued.

The committee in phase one produced two reports on ANL’s program plan for electrometallurgical technology:

• A Preliminary Assessment of the Promise of Continued R&D into an Electrometallurgical Approach for Treating DOE Spent Fuel (1995, Report 1),⁹ and

• An Assessment of Continued R&D into an Electrometallurgical Approach for Treating DOE Spent Nuclear Fuel (1992, Report 2).

The committee recommended that ANL proceed with its development plan for an EMT demonstration.

In July 1995, following a request by DOE, the NRC extended the tenure of its Committee on Electrometallurgical Techniques for DOE Spent Fuel Treatment. During this time, the EBR-II fuel treatment demonstration project had been delayed to allow for the completion of an environmental assessment (EA).¹⁰ As a result of the EA, the amount of fuel in the demonstration was reduced and separation of plutonium during the processing was eliminated but the program was allowed to continue; any increase in scope would require that a full environmental impact statement be prepared.

This change in scope as a result of the EA had an impact on the committee’s statement of task as it entered phase two (see Appendix A).

An objective of the committee was to learn how the modified scope, as defined by the EA, affected the adequacy of the demonstration and the potential application of the electrometallurgical technology to process DOE spent fuel.

In the second phase of its work the committee produced four reports: three status reports, and a fourth report on the issue of electrometallurgical treatment of excess weapons plutonium. This topic is discussed in detail in Chapter 2.

• An Evaluation of the Electrometallurgical Approach for Treatment of Excess Weapons Plutonium (1996, Report 3),

• Electrometallurgical Techniques for DOE Spent Fuel Treatment: A Status Report on Argonne National Laboratory’s R&D Activity (1996, Report 4),

• Electrometallurgical Techniques for DOE Spent Fuel Treatment: Fall 1996 Status Report on Argonne National Laboratory’s R&D Activity (1997, Report 5), and

• Electrometallurgical Techniques for DOE Spent Fuel Treatment: Status Report on Argonne National Laboratory’s R&D Activity Through Spring 1997 (1997, Report 6).

The committee entered its third phase of activity in 1998, focusing on specific aspects of the original charge (see Appendix A).¹¹

The three reports issued in phase three all dealt with the scientific progress made in ANL’s electrometallurgical demonstration project.

• Electrometallurgical Techniques for DOE Spent Fuel Treatment: Spring 1998 Status Report on Argonne National Laboratory’s R&D Activity (1998, Report 7),

• Electrometallurgical Techniques for DOE Spent Fuel Treatment: Status Report on Argonne National Laboratory’s R&D Activity as of Fall 1998 (1999, Report 8), and

• Electrometallurgical Techniques for DOE Spent Fuel Treatment: An Assessment of Waste Form Development and Characterization (1999, Report 9).

In addition, in phase three the committee analyzed a number of other issues related to the demonstration project. In its Report 7 the committee evaluated a set of success criteria developed by ANL in response to a

previous recommendation made in Report 5 of the committee and judged the criteria as adequate for determining the success of the EBR-II Spent Nuclear Fuel Demonstration Project. These success criteria and the committee’s evaluation of progress in meeting them are presented in Chapter 6.

In the same report the committee also evaluated electrometallurgical treatment of SNF in light of other possible treatment technologies. The committee’s evaluation of these alternatives is reproduced in depth in Chapter 3.

In its ninth report, the committee examined the issue of waste forms produced by the electrometallurgical process. This topic was not explicitly part of the committee’s statement of tasks for phase three but was addressed as a result of discussions between representatives from DOE and NRC. The ramifications of these waste forms are discussed in detail in Chapter 4, where the electrometallurgical waste streams are described.

This, the committee’s tenth and final report, fulfills its tasks to assess the viability of electrometallurgical technology for treating DOE spent nuclear fuel and to monitor the scientific and technical progress of the ANL program on electrometallurgical technology, specifically within the context of ANL’s demonstration project on electrometallurgical treatment of EBR-II SNF. Significant results are summarized and all recommendations of the first nine reports are reported in Appendix D. The electrometallurgical process, the equipment used, and the waste forms that result from this treatment are discussed and evaluated in detail. The committee also presents its evaluation of ANL’s performance relative to the success criteria for the demonstration project, which have served as the basis for judging the efficacy of using electrometallurgical technology for the treatment of EBR-II spent nuclear fuel. Finally, this report addresses post-demonstration activities related to ANL’s electrometallurgical demonstration project, and makes related recommendations in this area.

A total of approximately 2,000 metric tons of spent nuclear fuel has accumulated throughout the DOE complex.¹² The DOE has more than 150 different types of SNF stored at more than 200,000 units at DOE, non-DOE, and university facilities across the United States.¹³ The fuels located at commercial nuclear reactors are currently believed to be suitable for direct disposal in a geologic repository.¹⁴

The SNF that is, or is scheduled to become, part of the DOE inventory is stored at nine DOE sites, eight miscellaneous facilities performing reactor and fuel development or testing and isotope generation, three “special case” commercial facilities, 33 U.S. universities, and 49 foreign research reactors.

SNFs are broadly classified as production fuels, special fuels, or naval fuels. Production fuels are located mainly at the DOE’s Hanford Reservation and include the N-reactor fuels, which account for about 80% of the total DOE SNF inventory. Special fuels include both low- and high-enrichment fuels from a variety of reactors used in a wide range of research, development, and testing activities. Naval fuels are those developed and used for naval propulsion and for related R&D activities.

Electrometallurgical technology was originally proposed by ANL as a process with the potential to successfully treat all DOE spent fuels. Among the earlier incentives to proceed with R&D on EMT was its potential for handling a variety of different spent fuels, such as N-reactor fuel from Hanford, Molten Salt Reactor Experiment (MSRE) residues, and Savannah River Site fuels. The EBR-II termination program employed EMT for the treatment of EBR-II driver fuel and blanket assemblies.¹⁵ This was the basis for ANL’s electrometallurgical demonstration project.¹⁶ In its fourth report, the committee recommended that “upon satisfactory completion of the demonstration with EBR-II fuel, the electrometallurgical technique should be evaluated in the broader context of alternative technologies for processing spent nuclear fuel.”¹⁷

The electrometallurgical technology developed by ANL is potentially applicable to a fairly wide variety of spent fuel types besides the EBR-II fuel used by Argonne in its development and demonstration of the technology, for example Fermi-1 blanket fuel or Fast Flux Test Facility sodium-bonded fuel. In principle, electrometallurgical separations in alkali metal chloride media could be used for most of the DOE SNF, if used in conjunction with appropriate head-end processes. For most fuels, such as oxides, the fuel would first have to be converted to a suitable metallic form before the electrorefining could be applied. The purpose of specially tailored head-end processes would be to convert the fuels to metal and free them from elements such as aluminum and carbon that, above certain threshold-level concentrations, are incompatible with the electrometallurgical process as currently operated.

DOE initially proposed that the EBR-II driver fuel and at least half of the blanket fuel be treated via the electrometallurgical process.¹⁸ It is expected that, in addition to processing EBR-II fuel, the electrometallurgical technology could process undamaged metallic fuels from the Hanford N-reactor without any head-end treatment. N-reactor fuels constitute about 80% by mass of DOE’s SNF inventory, and EBR-II fuels about 1%.

However, a significant fraction of the N-reactor fuel elements have a breach in their cladding, and these failures have resulted in oxidation of the uranium metal as well as the zirconium cladding. These failed fuel elements (and the accompanying sludge that has formed in Hanford’s K-basin East) would have to be treated by head-end processing such as chemical purification and oxide reduction prior to electrorefining.

In addition to N-reactor fuel, Hanford has a wide variety of other SNFs stored on site. These fuels amount to only about 1.5% of the mass of N-reactor fuel, but the amount is not negligible (33 MT). About half of this amount by mass is the Shippingport pressurized water reactor Core II, treatment of which by the electrometallurgical process would require a special head-end step to remove a layer of graphite from the fuel wafers that contain the fissile material. Another (small) portion of the Hanford SNF contains aluminum that is metallurgically bonded to the uranium. It is unlikely that this fuel could be processed directly by the electrometallurgical process without additional head-end treatment to remove the aluminum. Aluminum’s tendency to form low-melting eutectic mixtures and volatile species would present a significant challenge for containment and effluent treatment in the electrorefining process.

The electrometallurgical technology under development at ANL is derived from many years of research and development on molten salt systems for the production of materials for nuclear reactors and weapons as well as from activities in battery development. This technology for treating DOE’s SNF consists of several unit operations:

• Head-end treatment, including fuel disassembly and steps such as oxide reduction, if required;

• Electrorefining; and

• Treatment of effluent electrorefining streams, including the ceramic and metal waste forms produced by this electrorefining, and processing of the uranium deposited at the steel cathode.

It is the technological aspects of these operations that the Committee on Electrometallurgical Techniques for DOE Spent Fuel Treatment has evaluated in the course of its work from 1995 to the present.