CONVERSION OF IRRADIATED URANIUM METAL TO UO2 FOR SAFE INTERIM STORAGE AND FINAL DISPOSAL

摘要

The operation of early test reactors in the 1950s and‘60s led to the creation of relatively small inventories inthe range of several tons of irradiated metallic uraniumfuel at a multitude of sites. Various waste managementstrategies have been adopted for these legacy fuels,ranging from reprocessing to extended interim storage.After several decades, some interim storage facilities haveproven vulnerable to the intrusion of moisture, which isitself associated with the degradation of storage safety viaoxidation and hydriding of the metallic fuel as well as theproduction of potentially explosive concentrations ofhydrogen gas. This paper presents a new route to stabilizeand facilitate disposal of metallic uranium fuels.The Studsvik Small Scale Conversion processconverts irradiated metallic U to UO_2 pellets, similar toirradiated LWR UO_2 fuel. The process thus aligns thedifficult to handle metallic uranium with existing backendpathways for UO_2-based fuels. The controlled conversionto UO_2 allows for safer interim storage, in addition tomaking the fuel more compatible with the extensiveresearch and final disposal pathways developed for UO_2-based fuels. The main steps of the process consist ofcalcination/oxidation of the U metal to U3O8/UO3,compaction/pressing to suitable geometry and reductionto UO_2. In this study, the oxidation of irradiated (~6kWd/kgU) and unirradiated metallic uranium sampleswas performed and the oxidation products prepared intopellet compacts. These compacts were then subjected todifferent thermal treatments between 600 °C and 1500 °Cunder inert and reducing atmospheres. Post treatmentcharacterization by XRD was used to confirm oxidestoichiometry. Leaching tests under oxidizing conditionsusing simulated groundwater (10:2 NaCl:NaHCO_3) werealso performed in order to assess the compatibility of theconverted compacts with LWR fuel direct-disposaldesigns. Green pellets and higher oxides demonstratedpoor leaching resistance, while incrementalimprovements were observed with pellets of increasingdensity. The results indicate that an oxidation conversiontreatment can be successfully employed to stabilize legacyirradiated metallic uranium for continued interim storageor final disposal.