Characterization of Irradiated Metal Waste from the Pyrometallurgical Treatment of Used EBR-II Fuel
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BEGINNING with the development of melt refining as a reprocessing technology for metallic nuclear fuel, process engineers have strived to simplify the reprocessing flow sheet while still achieving sufficient recoveries with minimal waste accumulation.[1] Melt refining marked the transition from a laboratory-based recovery method to an engineering-scale process while utilizing non-aqueous techniques. With simplicity as the cornerstone, pyrometallurgical reprocessing techniques like melt refining have attracted considerable interest in the nuclear community for the last 50 years. Combining the benefits of pyrometallurgical techniques with electrochemistry, the integral fast reactor (IFR) concept enhanced the reprocessing of used nuclear fuel considerably.[2,3] Keeping it simple, the IFR concept employs only two waste streams; a ceramic-based and a metalbased one depending on the reactivity of the separated fuel constituents.[4] Those separated constituents that are not reactive to the electrochemical salts are considered noble and are primarily retained by the fuel cladding following electrochemical operations.[5,6] The metal waste stream therefore consists of noble metal fission products such as zirconium (Zr), molybdenum (Mo), ruthenium (Ru), rhodium (Rh), technetium (Tc), BRIAN R. WESTPHAL, W.M. MCCARTIN, T.P. O’HOLLERAN, T.S. YOO, K.C. MARSDEN, K.J. BATEMAN, and M.N. PATTERSON, Engineers, S.M. FRANK, D.G. CUMMINGS, J.J. GIGLIO, and P.A. HAHN, Chemists, are with the Idaho National Laboratory, P.O. Box 1625, Idaho Falls, ID 83415. Contact e-mail: [email protected] Manuscript submitted May 9, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
palladium (Pd), and tellurium (Te) as well as the iron (Fe)-based cladding. The cladding hulls also contain adhering salt and a minor amount of unreacted residual fuel, primarily Zr if the fuel is a uranium (U)-10Zr (wt pct) binary alloy due to the relative thermochemical stabilities of Zr and U in the electrolyte.[7] It is for this reason that an Fe-Zr alloy was defined for the metal waste stream.[8] A composition of 15 wt pct Zr is targeted for ease of processing and may require trim Zr as an additive if sufficient Zr is not available as fuel residual. Development of the metal waste form has progressed from the initial surrogate test program[8,9] to production-scale irradiated operations and includes compositional and microstructural evaluations for phase stability,[10–25] corrosion testing,[26–33] mechanical as well as thermophysical property testing, and process qualification.[34,35] As a result of the extensive developmental test program, the following conclusions can be stated: (1) the intermetallic ZrFe2 phase incorporates the noble metals and actinides exclusively, with the exception of technetium which may also be present in the iron solid solution, (2) the metal waste alloy is as corrosion resistant as borosilicate glass based on a variety of durability tests including immersion, electrochemical, galvanic, hydration, and toxicity, and (3) the alloy is a viable high-level
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