Hydride Formation Process for the Powder Metallurgical Recycle of Zircaloy from Used Nuclear Fuel
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INTRODUCTION
THE United States Department of Energy is developing next generation processing methods to recycle uranium and transuranic (TRU) isotopes from spent nuclear fuel.[1] One of the most prominent methods under development is the Uranium Extraction Plus (UREX+) system of processes designed to separate uranium, TRUs, and selected fission products for recycle and specialized disposal.[2–4] As part of this effort, a 3-year demonstration project was initiated to develop a near-term storage option for TRU oxides isolated through UREX+ processing.[5,6] More specifically, a Zircaloy matrix cermet was considered as a storage form for TRUs with the understanding that the cermet also has the ability to serve as an inert matrix fuel form for TRU burning after intermediate storage. One of the goals for this project was to develop the processing methods required to transform used Zircaloy cladding and the effluent TRU nitrate solutions into a zirconium matrix cermet storage form. This involved the development of three distinct processes: (1) the recycle of used Zircaloy into a metal powder, (2) conversion of the TRU effluent into mixed oxide microspheres, and (3) hot extrusion of the combined Zircaloy and TRU powders into a Zircaloy matrix cermet. This paper describes process development results for the Zircaloy ADAM J. PARKISON, Graduate Student, and SEAN M. McDEAVITT, Assistant Professor, are with the Department of Nuclear Engineering, Look College of Engineering, Texas A&M University, College Station, TX 77843-3133. Contact e-mail: [email protected] Manuscript submitted March 22, 2010. Article published online November 4, 2010 192—VOLUME 42A, JANUARY 2011
recycle process with an emphasis on the temperaturedependent kinetics of the hydride formation reaction. The results presented here may be used to design a system to demonstrate the Zircaloy recycle process that was the contextual motivation of this research. The recycle of zirconium from used nuclear fuel cladding is desirable, in part, because the used hulls become radioactive waste after reprocessing and they represent a very large waste volume. The costs and repository requirements associated with their disposition pose a significant waste management burden to any reprocessing scenario. Zircaloy cladding becomes radioactive in service due to neutron activation and implantation of radioactive isotopes from the fission process.[7] As the fuel service continues to higher and higher burnup, the Zircaloy is further degraded and contaminated via fuel-cladding chemical interactions. As a result, it is not likely that spent Zircaloy can be easily discarded as low level waste under existing regulations, so the partial recycle of the Zircaloy ‘‘waste’’ into a TRU storage form has the potential to reduce the ultimate volume of high level waste attributable to reprocessing. Even more, the complete recycle of Zircaloy may prove to be beneficial, regardless of whether the TRU storage form is implemented. As a further point of interest, the zirconium cladding is already ‘‘nuclear gr
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