Gaseous swelling of U₃Si₂ during steady-state LWR operation: A rate theory investigation

Rate theory simulations of fission gas behavior in U₃Si₂ are reported for light water reactor (LWR) steady-state operation scenarios. A model of U₃Si₂ was developed and implemented into the GRASS-SST code based on available research reactor post-irradiation examination (PIE) data, and density functional theory (DFT) calculations of key material properties. The reliability of the model was examined by performing sensitivity analysis of the key parameters. Simplified peripheral models were also introduced to capture the fuel-cladding interaction. The simulations identified three regimes of U₃Si₂ swelling behavior between 390 K and 1190 K. Under typical steady-state LWR operating conditions where U₃Si₂ temperature is expected to be below 1000 K, intragranular bubbles are dominant and fission gas is retained in those bubbles. The consequent gaseous swelling is low and associated degradation in the fuel thermal conductivity is also limited. Those predictions of U₃Si₂ performance during steady-state operations in LWRs suggest that this fuel material is an appropriate LWR candidate fuel material. Fission gas behavior models established based on this work are being coupled to the thermo-mechanical simulation of the fuel behavior using the BISON fuel performance multi-dimensional finite element code.