EXECUTIVE SUMMARY

INTRODUCTION

In September 1994, in response to a request from the Department of Energy (DOE), the National Research Council (NRC) established the Committee on Electrometallurgical Techniques for DOE Spent Fuel Treatment. The committee was charged with evaluating the technical advantages and disadvantages of a proposed Argonne National Laboratory (ANL) R&D program for the use of electrometallurgical techniques to treat DOE spent nuclear fuel. The committee's preliminary report was issued in February 1995,1 and a more extensive report was published in July 1995.2 In July 1995, the DOE requested that the Committee on Electrometallurgical Techniques for DOE Spent Fuel Treatment continue its activity by carrying out two tasks: first, to monitor the scientific and technical progress of the ANL's program on electrometallurgical techniques for the treatment of DOE spent nuclear fuel, including both the redirected research program at ANL-East and the fuel treatment program at ANL-West associated with the ongoing shutdown of the Experimental Breeder Reactor II (EBR-II), and second, to evaluate the scientific and technological issues associated with extending this research and development program to handle plutonium, should the DOE decide that an electrometallurgical treatment option for the disposition of excess weapons plutonium (WPu) is worth pursuing. This report has been prepared in response to the second task.

BACKGROUND

The electrometallurgical processing technique, formerly called pyroprocessing, was investigated by ANL in conjunction with its Integral Fast Reactor (IFR) program and was designed to recycle IFR and spent oxide fuels from light water reactors (LWRs) into new IFR fuel. The new fuel would contain substantial quantities of uranium, plutonium, other actinides, and long-lived fission products that then could be “burned ” in the fast reactor. With the termination of the IFR program, the electrometallurgical process at ANL was redirected and modified with the goal of being able to treat spent nuclear fuels within the DOE inventory. Initially, the electrometallurgical process under development at ANL (see Figure 1 in Chapter 2) was designed to separate actinide elements from fission products present in spent fuels and to place the waste products in a form suitable for disposal.

The key element of the process is the electrorefining step, in which the metal to be processed is oxidized at the anode and deposited at a cathode in a condition of greater purity by electrotransport through a suitable molten salt electrolyte. It is in this step that the actinide-fission product separation occurs as a consequence of the different oxidation-reduction properties of two different cathodes. Relatively pure uranium is deposited at a steel cathode, and the transuranic fraction is collected at a molten cadmium cathode.

ANL has recently modified its flow sheet by eliminating the cadmium cathode, which would leave the transuranic elements in the molten salt. This modification would eliminate one of the product streams and the corresponding need for its interim storage. In the new scheme, the transuranic elements would remain with the fission products and be disposed of in a glass-bonded zeolite (GBZ) waste form.

1  

A Preliminary Assessment of the Promise of Continued R&D into an Electrometallurgical Approach to Treating DOE Spent Fuel, National Research Council, Washington, D.C., February 1995.

2  

An Assessment of Continued R&D into an Electrometallurgical Approach for Treating DOE Spent Nuclear Fuel, National Research Council, National Academy Press, Washington, D.C., July 1995.



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AN EVALUATION OF THE ELECTROMETALLURGICAL APPROACH FOR TREATMENT OF EXCESS WEAPONS PLUTONIUM EXECUTIVE SUMMARY INTRODUCTION In September 1994, in response to a request from the Department of Energy (DOE), the National Research Council (NRC) established the Committee on Electrometallurgical Techniques for DOE Spent Fuel Treatment. The committee was charged with evaluating the technical advantages and disadvantages of a proposed Argonne National Laboratory (ANL) R&D program for the use of electrometallurgical techniques to treat DOE spent nuclear fuel. The committee's preliminary report was issued in February 1995,1 and a more extensive report was published in July 1995.2 In July 1995, the DOE requested that the Committee on Electrometallurgical Techniques for DOE Spent Fuel Treatment continue its activity by carrying out two tasks: first, to monitor the scientific and technical progress of the ANL's program on electrometallurgical techniques for the treatment of DOE spent nuclear fuel, including both the redirected research program at ANL-East and the fuel treatment program at ANL-West associated with the ongoing shutdown of the Experimental Breeder Reactor II (EBR-II), and second, to evaluate the scientific and technological issues associated with extending this research and development program to handle plutonium, should the DOE decide that an electrometallurgical treatment option for the disposition of excess weapons plutonium (WPu) is worth pursuing. This report has been prepared in response to the second task. BACKGROUND The electrometallurgical processing technique, formerly called pyroprocessing, was investigated by ANL in conjunction with its Integral Fast Reactor (IFR) program and was designed to recycle IFR and spent oxide fuels from light water reactors (LWRs) into new IFR fuel. The new fuel would contain substantial quantities of uranium, plutonium, other actinides, and long-lived fission products that then could be “burned ” in the fast reactor. With the termination of the IFR program, the electrometallurgical process at ANL was redirected and modified with the goal of being able to treat spent nuclear fuels within the DOE inventory. Initially, the electrometallurgical process under development at ANL (see Figure 1 in Chapter 2) was designed to separate actinide elements from fission products present in spent fuels and to place the waste products in a form suitable for disposal. The key element of the process is the electrorefining step, in which the metal to be processed is oxidized at the anode and deposited at a cathode in a condition of greater purity by electrotransport through a suitable molten salt electrolyte. It is in this step that the actinide-fission product separation occurs as a consequence of the different oxidation-reduction properties of two different cathodes. Relatively pure uranium is deposited at a steel cathode, and the transuranic fraction is collected at a molten cadmium cathode. ANL has recently modified its flow sheet by eliminating the cadmium cathode, which would leave the transuranic elements in the molten salt. This modification would eliminate one of the product streams and the corresponding need for its interim storage. In the new scheme, the transuranic elements would remain with the fission products and be disposed of in a glass-bonded zeolite (GBZ) waste form. 1   A Preliminary Assessment of the Promise of Continued R&D into an Electrometallurgical Approach to Treating DOE Spent Fuel, National Research Council, Washington, D.C., February 1995. 2   An Assessment of Continued R&D into an Electrometallurgical Approach for Treating DOE Spent Nuclear Fuel, National Research Council, National Academy Press, Washington, D.C., July 1995.

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AN EVALUATION OF THE ELECTROMETALLURGICAL APPROACH FOR TREATMENT OF EXCESS WEAPONS PLUTONIUM CISAC Findings The general issue of the disposition of excess weapons plutonium has been thoroughly addressed by the National Academy of Sciences (NAS) Committee on International Security and Arms Control (CISAC) 3 and by its associated Panel on Reactor-Related Options for the Disposition of Excess Weapons Plutonium (the “Reactor Panel”).4 The disposition options explored by CISAC were “minimized accessibility” (i.e., the creation of physical, chemical, radiological, and isotopic barriers to reduce the material's accessibility for use in a weapon) and “elimination” (i.e., the removal of the material completely from human access, for example, by allowing the material to fission in a reactor until less than a critical mass remained). To compare options, CISAC introduced the concept of the “spent fuel standard.” This concept was not intended “to imply a specific combination of radiation barrier, isotopic mixture, and degree of dilution of plutonium” but rather to denote a “condition in which the WPu has become roughly as difficult to acquire, process, and use in nuclear weapons as it would to be to use plutonium in commercial spent fuel for this purpose. ”5 Since the ratio held in civilian relative to military stockpiles is believed to be about 3:1, the Reactor Panel concluded that “there would be very little security gain from special efforts to completely eliminate the WPu, or render it much less accessible even than the plutonium in spent fuel, unless society were prepared to take the same approach with the global stock of civilian plutonium.” The Panel on Reactor-Related Options examined pyroprocessing as an alternative approach for plutonium disposition. At the time the evaluation was made, the panel cited several disadvantages that it felt effectively excluded the electrometallurgical technique as a viable option in the near term. The panel concluded that the “pyroprocessing approach is not competitive with either vitrification in borosilicate glass or the use of mixed uranium-plutonium oxide (MOX) in existing reactors, both of which would be likely to involve lower costs, lower technical uncertainties, and shorter delay.”6 In the period since CISAC and its Reactor Panel evaluated the pyroprocessing approach, ANL has modified its process (see Chapter 2, Figure 2) to capture the plutonium and other transuranic elements in a zeolite matrix along with most of the fission products. This approach contrasts with the originally proposed scheme that was considered by CISAC and its Reactor Panel, in which the plutonium, other transuranics, rare-earth fission products, and some uranium were to be reduced to metals at a molten cadmium cathode and finally cast as metal ingots. 7 In each of these schemes, the waste form would include both the plutonium and radioactive fission products, thereby providing the rationale for plutonium disposition in accord with the “spent fuel standard.” ANL has proposed using the electrometallurgical technique for disposition not only of weapons “pits” (the plutonium components of nuclear weapons, named by analogy with the pit of a fruit such as a peach), but also for the non-pit materials that include plutonium in any other shape or chemical form. 3   Management and Disposition of Excess Weapons Plutonium, National Academy of Sciences Committee on International Security and Arms Control (CISAC), National Academy Press, Washington, D.C., 1994. 4   Management and Disposition of Excess Weapons Plutonium: Reactor-Related Options, Panel on Reactor-Related Options for the Disposition of Excess Weapons Plutonium, Committee on International Security and Arms Control (CISAC), National Academy Press, Washington, D.C., 1995. 5   See the report cited in footnote 4, p. 73. 6   See the report cited in footnote 4, p. 221. 7   As noted in the report cited in footnote 4, p. 220 (footnote 3), the CISAC Reactor Panel analysis utilized the same IFR flow sheets that were considered in another NRC study (Nuclear Wastes: Technologies for Separation and Transmutations, National Research Council, National Academy Press, Washington, D.C., 1995).

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AN EVALUATION OF THE ELECTROMETALLURGICAL APPROACH FOR TREATMENT OF EXCESS WEAPONS PLUTONIUM Efforts of the Fissile Materials Disposition Program The DOE, through its Fissile Materials Disposition Program (FMDP), is making an extensive and ongoing effort to examine options for the disposition of excess weapons-usable nuclear materials (principally plutonium and highly enriched uranium). The options currently being considered by the FMDP fall into one of the following three groupings: Plutonium burning in once-through reactors as MOX fuel followed by disposal of the spent fuel in a repository, Immobilization or fixation in an acceptable matrix to create an environmentally benign waste form for direct disposal in a repository, and Disposal in deep boreholes (with or without prior fixation). During the last year, the FMDP Immobilization Task Team has evaluated and eliminated during the first phase of its study a number of disposition options associated with the second grouping.8The electrometallurgical approach continues to be under consideration by the FMDP Immobilization Task Team.9, 10 The FMDP is preparing its Programmatic Environmental Impact Statement (PEIS) for Fissile Materials Disposition. The PEIS is the primary document that describes in detail the potential environmental impacts of technologies being considered for achieving the objectives of the FMDP. ANL has assumed that any WPu disposition operations would be integrated with the treatment of DOE spent fuels and that the same “hot” cells and some of the same equipment would be used for both. MODIFICATIONS OF THE SPENT FUEL PROCESSING FLOW SHEET The essential steps of the flow sheet for electrometallurgical processing of spent fuel are spent fuel element chopping; lithium reduction (for fuel other than metallic)/lithium regeneration; electrorefining; disposition of uranium metal product; and disposition of chloride waste streams. The lithium reduction step has been incorporated to allow application of the technology to a broader range of fuels. The head-end fuel pin chopping operations have been demonstrated for a variety of fuels. The lithium reduction of oxides and the associated regeneration of the by-product lithium oxide have been less well demonstrated. This step in the flow sheet will need to be fully demonstrated with actual spent fuel prior to attempting the reduction of nonmetallic plutonium. Considerable work has been done on electrorefining, which has been demonstrated at the laboratory scale. Scale-up to production-size equipment and throughputs has yet to be fully demonstrated. Plutonium Disposition Flow Sheet Option ANL has indicated that “the plutonium immobilization process is visualized as being conducted in the same facilities, in the same electrorefiner, and at the same time as the spent fuel treatment” (Appendix B, 8   DOE Fissile Material Disposition Program, Screening of Alternate Immobilization Candidates for Disposition of Surplus Fissile Materials, Lawrence Livermore National Laboratory, L-20790-1, 1996. 9   DOE Fissile Material Disposition Program, Alternative Team Technical Data Document: Electrometallurgical Treatment Alternative, Lawrence Livermore National Laboratory, L-20220-1, Predecisional Draft, 1995. 10   Also see comments of William Magwood as summarized in Appendix A, pp. 34-35, of this report.

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AN EVALUATION OF THE ELECTROMETALLURGICAL APPROACH FOR TREATMENT OF EXCESS WEAPONS PLUTONIUM p. 39). For the proposed treatment of plutonium, the preferred flow sheet for the electrometallurgical process (seeFigure 2, Chapter 2) contains only three effluent streams: (1) the relatively pure uranium metal fraction;11 (2) miscellaneous metal wastes containing the noble metal fission products; and (3) zeolites containing actinides (including plutonium) and rare-earth and other fission products. The zeolites would be converted to a glass-bonded zeolite waste form, and the miscellaneous metals would be formed into ingots of a corrosion-resistant metal waste form. ANL believes that both waste forms would be suitable for disposal in a geologic repository and is also investigating an alternative flow sheet (see Figure 3 in Chapter 3) that would retain the cadmium cathode and direct the actinides into the metallic waste form. For the application of the electrometallurgical treatment technology to surplus fissile material disposition, ANL proposes the addition of CsCl from capsules at DOE 's Hanford Reservation to create the radiation barrier to meet the “spent fuel standard.” The pretreatment for removal of plutonium from weapons components would consist of hydriding followed by either dehydriding or chlorination. The hydriding step would eliminate security issues associated with weapons pits, including their classified shape. If the hydride were converted to metal, the resulting metallic plutonium would be fed directly into the electrorefiner. If it were chlorinated, the plutonium chloride could be fed directly into a salt blend tank prior to sorption on zeolite. Another source of plutonium metal to be fed into the electrorefiner would be the lithium reduction of plutonium compounds. Poorly characterized plutonium compounds would require significant pretreatment to make them suitable for lithium reduction. Large amounts of impurities such as silica, magnesia, and many other materials are present in some of the DOE material, and the effects of these impurities must be evaluated if plutonium oxides and oxide-like materials are to be considered as potential feed materials. An option has also been presented by ANL for adding plutonium trichloride (from plutonium processing salts) into the molten salt as it is fed into the zeolite ion exchange step of the process. Most of the pyrochemical salts in the DOE holdings are composed of either a NaCl/KCl eutectic matrix with a relatively high assay of actinides, or CaCl2 with relatively small amounts of dispersed metallic plutonium and some plutonium oxide. Introduction of this additional salt feed stream via a salt blending tank would require some pretreatment type of chlorination operations to assure that all of the actinides were present as chlorides in the salt. Modified Flow Sheet: Conclusions Metallic plutonium from weapons components is compatible with the several flow sheets for electrometallurgical processing. Significant pretreatment would be required for all forms of plutonium other than metal from weapons components before the material would be suitable for further electrometallurgical processing. Interactions between the electrometallurgical process salt and non-metallic plutonium feed streams, specifically pyrochemical salts, might adversely affect electrometallurgical processing and/or waste form performance. The lithium reduction/regeneration portion of the proposed process has not been adequately demonstrated on the different fuels to allow adoption as a pretreatment step for plutonium disposition. 11   The uranium metal produced in the electrorefining step has up to the present time been designated as a product to be stored, but not as a waste. Its final disposition remains to be decided.

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AN EVALUATION OF THE ELECTROMETALLURGICAL APPROACH FOR TREATMENT OF EXCESS WEAPONS PLUTONIUM Modified Flow Sheet: Recommendations Pretreatment requirements for the nonmetal plutonium feed streams should be determined and, if possible, R&D should be started to validate the treatment and subsequent compatibility with the electrometallurgical process. The effects of major impurities such as additional salts (NaCl/KCl and CaCl2) and other impurities such as Si, Mg, and C on the performance of the electrometallurgical treatment operations should be evaluated. WASTE FORMS AND CHARACTERISTICS Expected Waste Forms Two general “waste” forms are under investigation in conjunction with electrometallurgical processing of spent fuel and the technology's adaptation to disposal of excess weapons plutonium. The first and preferred waste form is a glass-bonded synthetic zeolite that incorporates radionuclides, including the fission products and possibly transuranic element (TRU) components via ion exchange with molten salt from the electrorefining process. The second general waste form is a metallic waste composed predominantly of the cladding material into which the noble metal fission products from the spent fuel are incorporated via solid solution. Currently, both waste forms are being evaluated as to their suitability for incorporation of excess weapons plutonium, including their ability to comply with the “ spent fuel standard ” for the final disposition of plutonium. Zeolite Waste Form Physical Characteristics A molten chloride stream containing chlorides of plutonium, residual uranium, fission products other than noble metals, and transuranic elements would be produced by the simultaneous electrometallurgical treatment of excess weapons plutonium, DOE spent nuclear fuel, and in some cases additional 137Cs now in storage at Hanford. The resulting molten chloride stream would be passed though successive columns of salt-loaded synthetic Linde Type A (LTA) zeolite for essentially complete removal of plutonium, uranium, fission products, transuranic elements, and added 137Cs. Further treatment of the resulting loaded zeolite, including the addition of a borosilicate glass binder, would produce a glass-bonded zeolite waste form. Zeolite Column Loading There is considerable uncertainty about concentrations of fission products, transuranic elements, residual uranium, and the amount of 137Cs to be added to the molten salt feed to the zeolite columns. The chemistry of ion exchange and salt occlusion in the zeolite has been studied in the absence of large quantities of radioactive materials, but the technology has not been demonstrated on a large scale.

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AN EVALUATION OF THE ELECTROMETALLURGICAL APPROACH FOR TREATMENT OF EXCESS WEAPONS PLUTONIUM Glass-Bonded Zeolite Production Fabrication of the GBZ waste is being studied at the 10- to 100-g batch scale, and screening tests are under way to select the best glass frit to produce a waste form with optimum leach resistance and mechanical properties. A long development period with iterative testing and optimization of the GBZ fabrication process may be necessary. Waste Form Testing and Acceptability The GBZ characteristics will determine the actual mechanism (e.g., ion exchange, dissolution) for release of radioelements from the zeolite waste form over the long time periods relevant to geologic disposal. Work to date on radiation damage has been done largely if not exclusively via gamma irradiation of laboratory specimens. The committee has additional concerns about unexamined radioactive decay effects, including charge-balance discrepancies arising from radioactive decay of incorporated radionuclides. Questions also remain regarding chemical stability (including the effects of impurities on the stability of the zeolite and borosilicate glass phase), thermal stability, long-term waste form performance, and criticality concerns. The committee concludes that for disposition of excess WPu, issues related to waste form acceptability are of the highest level of importance relative to all other aspects of development of the electrometallurgical technique. However, the committee is not convinced that a sufficiently aggressive program is being pursued to demonstrate waste form performance in a timely manner. Cladding Metal Waste Form Cladding metals (zircaloy and stainless steels) are obtained from the electrorefining process as a separate waste stream that would also include noble metal fission product elements. Questions remain about the production and properties of metal matrices, including the number, degree of compositional uniformity, and spatial scale of separated phases, which depend on processing conditions, bulk composition, and possibly on the presence of minor alloying components. Waste Forms: Conclusions and Recommendations In the area of radioelement loading of zeolite columns, particular emphasis should be given to: establishing the range of parameters, with respect to zeolite type, configuration, and operating conditions, that give satisfactory column performance, and determining the thermal, chemical, mechanical, and radiological stability of zeolite under expected column loading conditions. For testing and evaluation of waste forms, the committee recommends the following: A schedule should be developed and implemented for demonstrating waste form performance over a time period commensurate with DOE' s plans for treatment of spent nuclear fuel (SNF) and conversion of WPu to a form suitable for ultimate disposal. Evaluation of waste form performance is of equal concern for application of the electrometallurgical technique to treatment of DOE SNF, although the latter application is governed by a different schedule. Waste-form testing should be conducted on the “as-produced” zeolite host phase for radionuclides, as well as on the GBZ waste form.

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AN EVALUATION OF THE ELECTROMETALLURGICAL APPROACH FOR TREATMENT OF EXCESS WEAPONS PLUTONIUM OTHER ISSUES AND CONSIDERATIONS: PROLIFERATION, TIME LINE, COST, AND POLICY What should be done with excess weapons plutonium, including how it can best be extracted, captured, and disposed of, is an issue of great importance, given that “[v]irtually any combination of plutonium isotopes … can be used to make a nuclear weapon.”12 Given the great concern about the weapons plutonium, the CISAC reports 13 recommended against approaches that would require significantly more time to develop, or would entail significantly greater uncertainty, than alternatives that could be available in a shorter time and with less uncertainty. This was the primary factor in the conclusion of CISAC and its Reactor Panel that the electrometallurgical approach is not competitive with the vitrification or reactor-burning options. The conclusions of CISAC and its Reactor Panel were based on an earlier version of the electrometallurgical approach.14 Although the present committee has not examined costs, it believes that the uncertainty and timeliness for the present proposed electrometallurgical technique would not alter the conclusion of the earlier reports. If the electrometallurgical technique is to be considered for disposal of excess plutonium, the feasibility of this technique must be validated. The current ANL program for treatment of EBR-II fuel appears to be the most cost-effective and timely way to make that demonstration for several, but not all, elements of the electrometallurgical technique. Were the electrometallurgical technique to be demonstrated successfully for treatment of DOE spent fuel and the issues relating to the waste forms resolved, the electrometallurgical technique could provide a potential method for handling excess plutonium at some later time. However, the committee recently has been made aware that some of the major milestones identified in its 1995 report have not been met: “[S]pent fuel treatment or processing activities using irradiated spent fuel are not authorized at this time pending completion of a further National Environmental Policy Act (NEPA) process.”15 In the absence of this necessary demonstration, the basis for evaluating the electrometallurgical approach as an option for plutonium disposition is unlikely to be available. CONCLUSIONS AND RECOMMENDATIONS The disposition of WPu involves, in part, alternative feeds compared to those used in SNF processing. These alternative feeds raise several concerns with respect to electrometallurgical processing, zeolite loading, and waste form performance. Although ANL has demonstrated an initial program in evaluating zeolite loading, considerable work remains to demonstrate this step in a large-scale, continuous operation with fully radioactive loadings on zeolite columns. Introduction of WPu in the electrometallurgical process significantly increases the demands on the technology to meet the performance requirement for waste forms relative to the use of the waste forms for ultimate disposal of fission products from SNF processing. 12   See the report cited in footnote 4, p. 32. 13   See the reports cited in footnotes 3 and 4 above. 14   See footnote 7. 15   Letter of 29 Nov. 1995 from Y.T. Chang, ANL, to R. Neuhold, DOE.

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AN EVALUATION OF THE ELECTROMETALLURGICAL APPROACH FOR TREATMENT OF EXCESS WEAPONS PLUTONIUM Consequently, greater priority should be given to the development of a strategy and a relevant test protocol to demonstrate acceptability of waste forms. This activity is of the highest importance relative to all other aspects in the development of the electrometallurgical technique for WPu disposition. The committee concurs with the earlier statements of CISAC and its Reactor Panel on excess weapons plutonium: “The existence of this surplus material constitutes a clear and present danger.”16 “The timing of disposition options is crucial to minimizing risks. ”17 The urgency of moving ahead with disposing of weapons plutonium makes scheduling considerations an important factor in deciding whether or not the electrometallurgical technique is a practicable and timely solution. The potential advantage of the electrometallurgical technique for disposition of excess plutonium depends on the availability of operational electrometallurgical process equipment. In the absence of operational process equipment, the electrometallurgical approach is not a viable candidate for plutonium disposition. Furthermore, until EBR-II fuel treatment is demonstrated and treatment of additional spent fuel undertaken, it would be imprudent to plan for use of the electrometallurgical technique for disposition of weapons plutonium. If the demonstration called for in the committee's 1995 report were unsuccessful, no further consideration of the electrometallurgical technique for plutonium disposition would be expected. It is conceivable that the possible failure of the electrometallurgical technique to process DOE spent fuel successfully could result from a feature that would not adversely affect treating WPu. However, such a determination could not be made prior to obtaining the results of the demonstration program. A decision on the use of the electrometallurgical technique for weapons plutonium disposition cannot be made until the demonstration of this technology shows whether or not this process is viable for treating DOE spent fuels. If a weapons plutonium disposition technology is to be selected for use with weapons pits before the electrometallurgical technology demonstration program is concluded, this committee recommends that the electrometallurgical technique not be included as a candidate technology. The potential of the electrometallurgical technique as an adjunct for long-term disposition of non-pit excess plutonium remains a possibility, but the technology is still at too early a stage of development to be evaluated relative to disposition alternatives such as glass or MOX. 16   See the report cited in footnote 3, p. 1. 17   See the report cited in footnote 4, p. 2.