Post-irradiation examinations of UO₂ composites as part of the Accident Tolerant Fuels Campaign

Enhanced thermal conductivity uranium dioxide composites containing silicon carbide (UO₂−SiC) and diamond (UO₂-diamond) have been irradiated to low burnup. The conditions of this irradiation test and subsequent postirradiation examinations are discussed. These irradiations evaluate fuel microstructure and potential fuel cladding interaction of UO₂ composites, which have been proposed as accident tolerant fuel candidates. Both non-destructive and destructive techniques have been used to evaluate fuel integrity, fission gas release, fission product distribution, burnup, fuel swelling and cladding strain. Examination of the UO₂-SiC pellets revealed enhanced cracking when compared to UO₂ pellets irradiated under similar conditions. Instability of the SiC whiskers in the uranium dioxide matrix was observed in the pellet central region, where the local temperatures exceeded 1300°C. The microstructure of the UO₂-diamond was severely disrupted during irradiation, resulting in local migration of cesium along the fuel stack and increased fission gas release when compared with the expected release from the Vitanza curve at corresponding values of burnup and irradiation temperature. The postirradiation examination results cast doubt on the suitability of these additives to improve UO₂ fuel performance in a way that would lead to enhanced accident tolerance.