U3Si2-SiC fuel performance analysis in BISON during normal operation

Abstract

U3Si2-SiC is an accident-tolerant fuel concept currently under development by Westinghouse that has potential to improve economic performance and safety of light water reactors. Since U3Si2 fuel suffers from poorer hydrothermal corrosion performance relative to UO2, it is paramount in an U3Si2-SiC fuel rod to lower the stress in the cladding to prevent any inward leakage of coolant. This motivates a detailed fuel-clad analysis of U3Si2-SiC during normal operation. The SiC cladding is modeled with a layer of SiCf/SiC composite followed by a layer of monolithic SiC. The dependence of thermomechanical properties for each SiC layer on temperature and neutron fluence is considered in the BISON fuel performance code. Particularly, pseudo-ductile deformation and anisotropic swelling models of SiCf/SiC composite are updated. While SiC thermal conductivity is commonly known to decrease as a consequence of irradiation damage, its additional degradation by mechanical damage is also modeled. Owing to limited availability of U3Si2 irradiation behavior models and experimental data under power reactor conditions, several assumptions are made to perform the initial detailed assessment of the fuel system. The results show that the fuel rod designs with larger pellet radius, thinner SiC cladding and larger initial gap width (thus larger cladding inner radius) can be beneficial to lower cladding stress during power operation and reactor shutdown.