Embrittlement and DBTT of High-Burnup PWR Fuel Cladding Alloys

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Structural analyses of high-burnup (HBU) fuel require cladding mechanical properties and failure limits to assess fuel behavior during long-term dry-cask storage and transportation. Pre-storage drying-transfer operations and early stage storage subject cladding to higher temperatures and pressure-induced tensile hoop stresses relative to in-reactor operation and pool storage. Under these conditions, radial hydrides may precipitate during slow cooling and provide an additional embrittlement mechanism as the cladding temperature decreases below the ductile-to-brittle transition temperature (DBTT). Graphic and photographic details of the testing are presented and analysis provided. (Additionally, results indicate that although the drying process for casks containing HBU fuel assemblies may require multiple drying cycles to minimize cask water content, no effects of multiple drying cycles were observed on radialhydride lengths or DBTT with HBU ZIRLO™ subjected to 90-MPa peak hoop stress at 400°C.) The work permits recognition of areas of importance for prioritization of future work efforts necessary to describe likely performance of HBU fuel cladding; the data obtained on HBU PWR cladding alloys will be shared with NRC and industry and contribute directly to help establish the technical basis for extended storage and transportation of HBU fuel.