• No evidence to support that the large void space will form surrounding the second phase stringers though the size of stringers (usually > 100 µm). The result indicates that the extra risk of forming extra-large void space (> 50 µm) around the second phase stringers is low even at the highest burn-up. • Second phase stringers have plate-type morphology that aligns with the rolling direction of fuel plate in fabrication. • Irradiation induced porosities surrounding uranium carbides could interconnect to form large void space (> 5 µm) on UC/UMo interface as burn-up level increasing. • The size of pores inside of uranium carbides is smaller than UMo matrix due to the low diffusivity of vacancies in UC and the grain refinement at higher fission density. Post-irradiation microstructure characterization plays an important role in qualifying the low-enriched uranium (LEU) monolithic U-10 wt%Mo plate-type fuel for United States high-performance research reactors (USHPRRs) program. Inhomogeneous features resulting from manufacturing and irradiation processes, including carbides, second phase stringers, and extensive void spaces caused by the combining of small porosities, may increase the risk of heat concentration in local regions of the fuel plate over the operating conditions. In this study, characteristics of carbides, stringers, and porosity after multiple levels of irradiation at varying fission densities were studied by electron microscopes to decipher the morphology of pores and the porosity evolution in U-10 wt%Mo. For carbides, the result shows that porosities start forming on UMo grain boundaries, then on UMo/carbides interfaces as the burn-up going higher. However, the porosities surrounding carbides grow larger than the ones on UMo grain boundaries. The porosities around the uranium carbides could interconnect to form larger void space. The study revealed that the void spaces larger than 5 µm were found around uranium carbides after high burnup, while no evidence was observed to support the similar voids formed near second phase stringers even though the size of the stringers (> 50 µm) was much larger than uranium carbides (< 20 µm). The evolution of porosities suggests that the formation of second phase stringers may not create more significant porosities compared to regular uranium carbides regions during fuel operating conditions.
Teng et al. (Sun,) studied this question.