The role of UC inclusions in the development of fission gas bubble superlattice neutron-irradiated monolithic U-10Mo fuels

Uranium Carbide (UC) inclusions are the most prevalent impurities in Uranium-Molybdenum (U-Mo) fuel and are considered undesirable because they could potentially affect fuel performance. This work revealed that, like grain boundaries, UC inclusions could help facilitate the formation of the fission gas bubble superlattice (GBS). The GBS is a highly organized complex defect structure that can effectively store fission gases, thereby inhibiting fuel swelling. Transmission electron microscopy (TEM) showed that GBS self-organization can initiate at the UC/ U-Mo interfaces in U-10Mo fuel irradiated to low fission density. This study also revealed that the UC/U-Mo phase boundary in U-10Mo irradiated to low fission density is wavy and periodic in morphology and that GBS formation is semi-coherent with the UC boundary. Because the initiation of GBS preferentially occurs at grain boundaries and UC/U-Mo phase boundaries, it is expected that following GBS collapse at higher fission densities, high burnup structure development may initiate first at grain boundaries and UC/U-Mo interfaces.