Executive Summary

The Committee on Electrometallurgical Techniques for DOE Spent Fuel Treatment is in the third phase of its continuing evaluation of the Argonne National Laboratory (ANL) research and development program for electrometallurgical technology (EMT).1 This report focuses on the processing technology and corresponding processing equipment utilized in ANL's demonstration project.2

SPENT FUEL OPERATIONS

Summary of Progress Through October 1998
High-Throughput Electrorefiners

Initial testing of the 25-inch (inner diameter) high-throughput electrorefiner (HTER) at ANL-E revealed several problems that required modification. A very large deposit of uranium metal adhered to the cathodes in the HTER and made removal by the ceramic “scrapers” impossible. These scrapers also caused uranium metal hold-up in the space between the anode baskets. Solutions attempted for these problems included the addition of more scrapers, and current reversal, or stripping, carried out periodically to remove the high-density uranium metal from the anode and deposit it in the anode basket.

Additional modifications of the 25-inch HTER included the aforementioned increase in the number of ceramic scrapers, and the placement of the scrapers on the leading edge of the anode basket, rather than the trailing edge. The placement of the fuel in the anode basket was also modified.

FINDING

The current approach to development of the Mark-V anode-cathode module (ACM) appears to be highly empirical, and the committee perceives that there is a lack of understanding about some of the fundamental electrochemical and mechanical factors affecting the ACM.

RECOMMENDATIONS
  1. ANL should broaden its perspective regarding the Mark-V ACM by seeking information about the following:

    1. Physical, morphological, and mechanical characteristics (e.g., plasticity) of the uranium/salt mixture produced during HTER operating conditions;

    2. Electrochemical behavior of uranium in molten LiCl-KCl under HTER operating conditions; and

    3. Useful strategies from the metal electrowinning industry that can be applied to the electrometallurgical process as applied to uranium.

1  

Abbreviations and acronyms are defined in Appendix G.

2  

The committee's charge is given in Appendix A.



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ELECTROMETALLURGICAL TECHNIQUES FOR DOE SPENT FUEL TREATMENT: STATUS REPORT ON ARGONNE NATIONAL LABORATORY'S R&D ACTIVITY AS OF FALL 1998 Executive Summary The Committee on Electrometallurgical Techniques for DOE Spent Fuel Treatment is in the third phase of its continuing evaluation of the Argonne National Laboratory (ANL) research and development program for electrometallurgical technology (EMT).1 This report focuses on the processing technology and corresponding processing equipment utilized in ANL's demonstration project.2 SPENT FUEL OPERATIONS Summary of Progress Through October 1998 High-Throughput Electrorefiners Initial testing of the 25-inch (inner diameter) high-throughput electrorefiner (HTER) at ANL-E revealed several problems that required modification. A very large deposit of uranium metal adhered to the cathodes in the HTER and made removal by the ceramic “scrapers” impossible. These scrapers also caused uranium metal hold-up in the space between the anode baskets. Solutions attempted for these problems included the addition of more scrapers, and current reversal, or stripping, carried out periodically to remove the high-density uranium metal from the anode and deposit it in the anode basket. Additional modifications of the 25-inch HTER included the aforementioned increase in the number of ceramic scrapers, and the placement of the scrapers on the leading edge of the anode basket, rather than the trailing edge. The placement of the fuel in the anode basket was also modified. FINDING The current approach to development of the Mark-V anode-cathode module (ACM) appears to be highly empirical, and the committee perceives that there is a lack of understanding about some of the fundamental electrochemical and mechanical factors affecting the ACM. RECOMMENDATIONS ANL should broaden its perspective regarding the Mark-V ACM by seeking information about the following: Physical, morphological, and mechanical characteristics (e.g., plasticity) of the uranium/salt mixture produced during HTER operating conditions; Electrochemical behavior of uranium in molten LiCl-KCl under HTER operating conditions; and Useful strategies from the metal electrowinning industry that can be applied to the electrometallurgical process as applied to uranium. 1   Abbreviations and acronyms are defined in Appendix G. 2   The committee's charge is given in Appendix A.

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ELECTROMETALLURGICAL TECHNIQUES FOR DOE SPENT FUEL TREATMENT: STATUS REPORT ON ARGONNE NATIONAL LABORATORY'S R&D ACTIVITY AS OF FALL 1998 ANL should evaluate the potential impact of the higher salt content of the Mark-V HTER product on the performance of the cathode processor. ANL should evaluate the effects of cathode surface roughness on the adhesion of the uranium deposit; other materials or metallic coatings that might reduce adhesion of uranium on the ACM cathode should be considered. Cathode Processor and Casting Furnace Unit Efforts continued at ANL-W to evaluate the performance of both the cathode processor and the casting furnace unit processes. Reduction of the pressure in the cathode processor to about 1 torr reduced the operating temperature from 1225 °C to about 800 °C. This lower pressure also resulted in more efficient distillation of fission products from the uranium. ANL-W is working with an outside vendor to produce larger beryllia crucibles, which are required because of the marginal performance of the baseline material in the crucible when uranium, process salt, and cadmium are present simultaneously in the cathode processor. As of October 1998, 30 batches of uranium and 7 batches of cladding hulls had been processed through the cathode processor from irradiated EBR-II fuel. RESEARCH AND DEVELOPMENT IN SUPPORT OF ARGONNE NATIONAL LABORATORY 'S DEMONSTRATION PROJECT Metal Waste Form Testing and Plans ANL's metal waste form (MWF) test plan calls for a total of 856 tests. The MWF test plan consists of attribute tests, characterization tests, accelerated tests, and service condition tests. A large number of the samples that will be tested differ only very slightly in minor alloying elements. Metal Waste Form Release Rate The object of this work is to develop a model of the radioisotope release rate for the stainless steel MWF. A number of potential degradation mechanisms were discussed in the committee's meetings at ANL-W and ANL-E. FINDINGS Since no corrosive attack on the MWF has been observed in the immersion tests conducted thus far, it is unlikely that tests under mild conditions will yield any additional information. A general observation has been the lack of information from these tests under mild conditions. As a result, it is questionable whether the large number of MWF tests (856) is necessary. The proposed study of crevice corrosion requires careful design of an artificial crevice with consideration of the proposed application of the alloys used in the MWF.

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ELECTROMETALLURGICAL TECHNIQUES FOR DOE SPENT FUEL TREATMENT: STATUS REPORT ON ARGONNE NATIONAL LABORATORY'S R&D ACTIVITY AS OF FALL 1998 ANL's observation that the MWF behaves like stainless steel is critical in that the overall MWF test program is modeled after tests used for stainless steel. MWF tests to date have been conducted under mild conditions, and little attack has been observed. RECOMMENDATIONS Surface analysis by X-ray photoelectron spectroscopy (XPS) or Auger electron spectroscopy (AES) should be performed for selected samples drawn from the characterization tests. The committee notes that ANL-E has one of the leading experts in this area. It is recommended that only a few of these samples be fully characterized. ANL needs to refocus near-term testing by reducing product consistency testing under relatively mild conditions and instead emphasizing product performance testing under more stringent conditions that may reveal significant corrosion effects and address success criterion 2, goal 2 (Appendix B). ANL should evaluate how the MWF performance model will be used and whether ongoing MWF testing will provide information needed for developing the performance model. Ceramic Waste Form Testing and Development Considerable progress has been made in the demonstration-scale ceramic waste form (CWF) operations. Key accomplishments include production of 61 demonstration-scale hot isostatic pressing (HIP) cans, production of salt-loaded zeolite within process specifications using the V-mixer, completion of a mill/classifier process test matrix, use of a crusher for pre-mill salt processing, and routine production of dried zeolite at demonstration scale. A cooperative effort between ANL-W and the Australian Nuclear Science and Technology Organisation will provide the first demonstration of radioactive waste processing with HIP in a remotely controlled hot cell. Hot Isostatic Pressing The HIP treatment is the last step in a procedure for the production of ceramic waste forms. ANL-W representatives stated that despite the increased amount of disposal solid waste form produced by hot uniaxial pressing (HUP) and pressureless sintering, both provide attractive alternatives to HIP. Subsequent to the committee's meeting at ANL-E, representatives from ANL confirmed that HUP is not being considered as an alternative for HIP, but that ANL intends to evaluate pressureless sintering for post-demonstration use in early 1999.

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ELECTROMETALLURGICAL TECHNIQUES FOR DOE SPENT FUEL TREATMENT: STATUS REPORT ON ARGONNE NATIONAL LABORATORY'S R&D ACTIVITY AS OF FALL 1998 RECOMMENDATION Assuming that an increased amount of glass in the waste form is acceptable, then in addition to HUP, conventional cold pressing and sintering should also be considered as a viable processing option. Additional Ceramic Waste Form Development Issues The issue of reaction between excess UCl3and the initial zeolite 4A host remains unresolved. A related concern is the potential for reaction of PuCl3with the zeolite. Tests to date, however, show no indication of such a reaction. The mechanism of transformation of salt-loaded zeolite 4A to sodalite may affect whether radionuclides larger than 2.2 Å will be retained in the sodalite. RECOMMENDATION The committee believes that characterization of the ceramic waste form should be accelerated in order to determine the mechanism of transformation of salt-loaded zeolite 4A to sodalite. Demonstration-Scale CWF Processes An empirical process model based on important processing variables is being developed by ANL's program on EMT. Local charge balance may be an important aspect of the long-term structural stability of zeolite and sodalite host matrices for radionuclides. Electrometallurgical Treatment of Oxide and Aluminum Alloy Fuels Oxide Fuels Laboratory-scale work has been conducted to investigate the limits for reduction of PuO2, elucidate the kinetics of UO2 reduction, develop an electrowinning cathode for handling metallic lithium, and evaluate possible anode materials. Although several anode materials have been tested, the committee believes that further progress will be enhanced through analysis of published material in this area. Work is in progress to study oxide fuel reduction kinetics, optimum fuel basket design for oxide fuel reduction and electrorefining, and development of methods for handling metallic lithium. Aluminum Alloy Spent Fuels The feasibility of electrometallurgical treatment of aluminum alloy spent fuels has been demonstrated in laboratory-scale experiments. An engineering-scale aluminum electrorefiner has been installed for further testing.

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ELECTROMETALLURGICAL TECHNIQUES FOR DOE SPENT FUEL TREATMENT: STATUS REPORT ON ARGONNE NATIONAL LABORATORY'S R&D ACTIVITY AS OF FALL 1998 OVERALL PROGRESS IN ANL'S DEMONSTRATION PROJECT The committee concludes that the accomplishments reported by ANL concerning its EMT demonstration project are an accurate measure of progress toward the goal of meeting the success criteria. The committee believes that the conditions utilized to meet success criterion 2, goal 2 are too mild to yield sufficient information about the metal waste form under more stringent conditions. Problems with the scrapers of the ACM for the Mark-V HTER must be resolved if this equipment is to be used successfully.

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