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ELECTROMETALLURGICAL TECHNIQUES FOR DOE SPENT FUEL TREATMENT: An Assessment of Waste Form Development and Characterization Executive Summary INTRODUCTION The Committee on Electrometallurgical Techniques for DOE Spent Fuel Treatment has functioned since January 1995 to evaluate Argonne National Laboratory's (ANL's) spent fuel demonstration project for the Department of Energy.1 Over this period, the committee has operated in three phases and has released eight reports detailing various aspects of the demonstration project as it has proceeded. The present report is the ninth in this series and will be followed by a tenth, and final, report to be released after the completion of ANL's demonstration project. The Committee on Electrometallurgical Techniques for DOE Spent Fuel Treatment has three parts to its charge for phase 3: Continue its ongoing evaluation of Argonne National Laboratory's (ANL's) demonstration project, and issue a final report at the end of the demonstration; Review the viability of electrometallurgical technology in light of technical progress in other possible treatment technologies; and Evaluate the criteria developed by ANL and the U.S. Department of Energy (DOE) to determine the success of the demonstration project. The committee in its seventh report2 addressed the second and third aspects of its charge. The committee 's eighth report,3 the present (ninth) report, and the committee's final report all have addressed and will address the first aspect of the committee's charge for phase 3. The committee's previous report4 focused on the chemical and electrometallurgical process steps of electrometallurgical technology (EMT) and on ANL's progress in the Experimental Breeder Reactor-II (EBR-II) demonstration project. The committee stated in that report that it would defer the part of its charge involving assessment of the waste forms being developed by ANL for disposition of the products of the EMT treatment. The present report discusses these waste-form issues as they relate to the completion of the demonstration project, as well as postdemonstration testing that will be necessary for placement of these waste forms in a geologic repository. 1 The committee's charge for phases 1 and 2 is included in Appendix A. 2 National Research Council, Electrometallurgical Techniques for DOE Spent Fuel Treatment: Spring 1998 Status Report on Argonne National Laboratory's R&D Activity, National Academy Press, Washington, D.C., 1998. 3 National Research Council, Electrometallurgical Techniques for DOE Spent Fuel Treatment: Status Report on Argonne National Laboratory's R&D Activity as of Fall 1998, National Academy Press, Washington, D.C., 1999. 4 National Research Council, Electrometallurgical Techniques for DOE Spent Fuel Treatment: Status Report on Argonne National Laboratory's R&D Activity as of Fall 1998, National Academy Press, Washington, D.C., 1999.
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ELECTROMETALLURGICAL TECHNIQUES FOR DOE SPENT FUEL TREATMENT: An Assessment of Waste Form Development and Characterization BACKGROUND Electrometallurgical processing technology produces two waste streams: 5 the metallic waste form (MWF) and the ceramic waste form (CWF). The novel structure and composition of the MWF require a verification approach that can demonstrate its suitability as a final waste form for geologic disposal. R&D and evaluation led to the selection of glass-bonded sodalite as the reference CWF. The committee in this report assesses waste qualification activities related to the MWF and CWF produced by this process and provides an evaluation of the technical consistency and comprehensiveness of the EMT activities that support ultimate acceptance of EMT waste forms by DOE's Office of Civilian Radioactive Waste Management (RW). WASTE FORM QUALIFICATION AND ACCEPTANCE DOE, through its Office of Civilian Radioactive Waste Management (DOE-RW), is assessing the viability of permanent disposal of spent nuclear fuel (SNF) and high-level waste (HLW) in a deep geologic repository at Yucca Mountain, Nevada.6 The performance and compatibility of the ANL waste forms must be assessed within a system context of overall repository safety. DOE asked the committee to evaluate ANL's progress in taking appropriate steps for obtaining the necessary regulatory approvals in the future. The EMT Program has developed a waste qualification program that is patterned after the protocols used for the waste qualification of defense HLW (DHLW) borosilicate glass.7 To date, both commercial spent fuel and vitrified defense HLW have been subjected to detailed characterizations conducted with respect to their performance in a geologic repository.8 For the EMT waste forms, preliminary evaluation is being performed by the DOE-RW, and no final decision on the EMT waste form has been made by RW. Issues remain that affect the ability of the EMT Program to fully document its plans and schedule for achieving a future waste-acceptance decision. First, the EMT Program is concluding a directed demonstration phase that supports issuance of an Environmental Impact Statement (EIS) regarding continued application of the EMT to process the remaining inventory of EBR-II spent fuel. A final consideration is that the initial draft of RW's Acceptance Criteria document should be issued for review in 1999. This new document may modify the actual waste-acceptance strategies and waste-acceptance criteria that the EMT Program is currently following. 5 This process also produces a uranium stream. ANL does not consider uranium a waste product. 6 Viability Assessment of a Repository at Yucca Mountain, DOE/RW-0508, U.S. Department of Energy Office of Civilian Radioactive Waste Management, Washington, D.C., 1998. 7 DWPF Waste Acceptance Reference Manual (U), WSRC-IM-93-45, Westinghouse Savannah River Company, Savannah River Site, Aiken, SC, 1993. 8 Mined Geologic Disposal System Waste Acceptance Criteria Document, B00000000-01717-4600-00095 REV 00, TRW Environmental Safety Systems, Inc., Las Vegas, NV, 1997, pp. 5-1 – 5-8.
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ELECTROMETALLURGICAL TECHNIQUES FOR DOE SPENT FUEL TREATMENT: An Assessment of Waste Form Development and Characterization Finding: From interactions with RW, ANL has developed a strategy appropriately based on RW's waste acceptance criteria for the characterization of its MWF and CWF for eventual acceptance by RW. This strategy encompasses its characterization protocols, including short-term test procedures, for its ceramic and metal waste forms. Conclusion: Continued interaction between ANL and RW will become even more important in the postdemonstration phase. Conclusion: There remains uncertainty regarding which DOE organization will be charged with the ultimate responsibility for performance-confirmation testing of waste forms suitable to support a repository licensing decision. As this uncertainty in responsibility could lead to costly duplication of effort and lack of consensus among DOE organizations regarding data supporting future decisions, DOE should take the lead in achieving a documented resolution to this issue. Metal Waste Forms The electrometallurgical treatment of spent EBR-II reactor fuel involves a set of operations designed to disassemble driver and blanket fuel pins, to refine and recover the uranium metal contained therein, and to segregate the radioactive waste components. For the entire EBR-II spent fuel inventory, the base metal waste stream composition is stainless steel containing approximately 15 weight % zirconium, labeled SS-15Zr by ANL personnel.9 Thus, MWF testing at Argonne National Laboratory-West (ANL-W) has focused primarily on this and similar alloys. The corrosion resistance of SS-15Zr alloys has been determined using immersion tests, electrochemical tests, and accelerated corrosion tests (vapor hydration, high-temperature immersion, and product consistency tests). Good progress seems to have been achieved in the identification of the various microstructures of SS-15Zr-type materials. Noble metal-rich precipitates have not been observed. Most of the corrosion tests have shown either no corrosion or only slight tarnish. However, it was found that corrosion rates were greatly accelerated by exposure to steam. Surface corrosion was observed in the pulsed-flow immersion test of the SS-15Zr MWF. No data have been presented to the committee thus far for “standard” SS-15Zr samples. Corrosion appears to be retarded by the formation of a passivating oxide layer that may trap the fission products and actinides, limiting their release. Finding: Some of the corrosion products, which may sequester radionuclides, might remain on the sample surface and might not be detected by solution analysis. Conclusion: Corrosion behavior data for the SS-15Zr MWF standard need to be obtained. 9 D. P. Abraham, Metal Waste Form Handbook, NT Technical Memorandum No. 88, Argonne National Laboratory, Argonne, IL, 1998, p. 3.
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ELECTROMETALLURGICAL TECHNIQUES FOR DOE SPENT FUEL TREATMENT: An Assessment of Waste Form Development and Characterization Recommendation: Surface analysis by x-ray photoelectron spectroscopy (XPS) or Auger electron spectroscopy (AES) should be performed for selected samples to determine the chemical composition of passivation filings and/or corrosion products. Because a large number of samples to be tested differ only slightly in minor alloying elements, it is recommended that only a few of these samples be subjected to full characterization. These samples should be selected using a statistical analysis approach. The corrosion resistance of the MWF appears to be dominated by the passivation behavior of the alloy, and dissolution is not considered to be a dominant release mechanism of the radionuclides. Finding: Results from corrosion testing of the MWF in rather benign environments suggest that the corrosion behavior of the MWF is similar to that of stainless steel. Finding: At the present time, ANL has not indicated how it plans to conduct crevice corrosion studies. Finding: ANL has carried out a number of corrosion tests using mild solutions. Under these conditions, significant corrosion damage to the MWF is not expected. Recommendations: Instead of continuing to conduct a large number of corrosion tests using mild conditions, it would be better to subject a few carefully selected samples to additional evaluation by surface analysis to determine the chemical composition of the corrosion products. It may be better to concentrate on a few key samples, expose them at higher temperatures, and then obtain electrochemical and surface analysis data. Ceramic Waste Form Testing and Plans for Qualification ANL reported10 on the detailed testing to support CWF qualification using “scoping” tests.11 These tests are carried out to provide data for assessment of the CWF's dissolution and to provide data for the development of a model to predict waste form stability in the repository. Finding: ANL's tests of several months' duration indicate that the CWF dissolves at a rate equal to or less than that of the reference DHLW borosilicate glass. Conclusion: If dissolution remains the dominant release mechanism under actual 10 Presentation to the committee by William Ebert, National Academies Beckman Center, Irvine, CA, January 28, 1999. 11 For background on the development of these testing methods as they apply to the ceramic waste form produced by ANL's demonstration project, see L. J. Simpson, D. J. Wronkiewicz, and J. A. Fortner, Development of Test Acceptance Standards for Qualification of the Glass-Bonded Zeolite Waste Form Interim Annual Report: October 1995 – September, 1996, ANL Technical Memorandum No. 51, Argonne National Laboratory, Argonne, IL, 1997.
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ELECTROMETALLURGICAL TECHNIQUES FOR DOE SPENT FUEL TREATMENT: An Assessment of Waste Form Development and Characterization repository conditions, then the release performance of the CWF will be at least comparable to that of DHLW borosilicate glass. ANL reported12 on modeling long-term behavior based on short-term data. The model is based on a transition state theory approach for the rate of silicate mineral dissolution.13,14 ANL15 discussed accelerated alpha damage testing on simulated CWF doped with 0.2 to 2.5 weight % 238Pu or 239Pu. Finding: During the conduct of the alpha-decay tests, plutonium oxide was observed as nanocrystals in the grain boundaries. Conclusion: Plutonium may not be in the sodalite phase.16 Its presence in potentially colloid-sized products may have implications for the long-term release behavior of plutonium and any other radionuclides that also segregate into such colloid-sized phases. The EMT Program's waste-form qualification program is based on adaptation of models and test protocols developed for DHLW borosilicate glass. Recommendation: The EMT Program should continue to evaluate and demonstrate that test protocols and conceptual models developed for monolithic single-phase borosilicate glass can adequately represent the behavior of the nonhomogeneous multiphase EMT CWF. There is to date little or no direct evidence quantifying the distribution of radionuclides among the phases. Finding: The Material Characterization Center test (MMC-1) and product consistency test (PCT) designed to model the release behavior of inert, major components of the CWF may be irrelevant with respect to evaluating the release of plutonium and other actinides partitioned into separate oxide phases. Conclusion: The committee believes that ANL is taking appropriate steps to coordinate its waste-qualification program with the DOE-RW repository program. It remains undemonstrated, however, that direct adaptation of test procedures and models developed for measuring the rate of general corrosion of the matrix of homogeneous, vitrified HLW forms are sufficient for evaluating the performance of the heterogeneous, crystalline CWF under long-term repository conditions. 12 Presentation to the committee by Thomas Fanning, National Academies Beckman Center, Irvine, CA, January 28, 1999. 13 S. Glasstone, K. Laidler, and H. Eyring, The Theory of Rate Processes, McGraw-Hill, New York, 1935. 14 A. C. Lasaga, and R. J. Kirkpatrick, eds., “Transition State Theory” in Kinetics of Geochemical Processes, Reviews in Mineralogy, Mineralogical Society of America, Washington, D.C., 1981. 15 Presentation to the committee by Stephen Johnson, January 28, 1999. 16 Sodalite is the name of a group of alumino-silicate framework materials formed by linkage of SiO4 and AlO4 tetrahedra that form internal cavities that can be occupied by chloride or other ions.
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ELECTROMETALLURGICAL TECHNIQUES FOR DOE SPENT FUEL TREATMENT: An Assessment of Waste Form Development and Characterization Conclusion: These continuing concerns are not expected to jeopardize the timely completion of the EBR-II demonstration project in 1999, but attention should be devoted to their resolution prior to extending the EMT process past the demonstration. When waste form acceptance criteria for geologic repository placement are adopted by RW, test procedures for the waste forms produced by the electrometallurgical process may require modification. The committee believes, however, that the test procedures used for the MWF and CWF are appropriate for the completion of ANL's demonstration project. ADDITIONAL CONSIDERATIONS RELATED TO THE DEMONSTRATION PROJECT Disposition of the Uranium Product Stream Produced by the EMT Process The EMT Program staff has discussed applicable product purity levels with the staff of the Oak Ridge National Laboratory's Y-12 Plant for Y-12 acceptance of the uranium metal. If DOE decides that a commercial disposition option is desirable, some additional purification must be sought. To date, these options have not been studied. The Quantity of EMT Waste Forms Produced from EBR-II Spent Nuclear Fuel The committee notes that both the quantity and radionuclide inventory of EMT waste forms are extremely small relative to those of commercial SNF and DHLW. DOE should evaluate whether small quantities, both in terms of volume and radionuclide inventory, of novel waste forms should be characterized and qualified to the same level of detail as major waste forms. However, because final qualification criteria have not been set, it is difficult to assess whether the testing is excessive or not at this stage. Conclusion: Alternative, conservatively bounding strategies for assuring safe disposal of such relatively small quantities of novel HLW may result in significant cost avoidance while still protecting public safety. However, if EMT is to be used for other DOE SNF, a full qualification of the waste forms would be required.
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