Mechanistic grain growth model for fresh and irradiated UO₂ nuclear fuel

The purpose of this work is to develop a mechanistic model of grain growth in UO₂ fuel during reactor operation. The model development builds on published experimental data on UO₂ grain growth, as well as atomistic and mesoscale simulation results. We begin by developing new fits with temperature T (in K) for the average grain boundary (GB) energy and mobility in UO₂ using literature data, where the GB energy γ¯=(1.56−5.87×10⁻⁴T)±0.3J/m² and the GB mobility M¯=(2.14±0.15×10⁻⁷)exp(−(290±22kJ/mol)/RT)m⁴/(Js). We then discuss the pinning of grain boundaries by porosity, including porosity left over after sintering and fission gas bubbles that form during operation. We present our mechanistic model and validate it using existing grain growth data. Finally, we implement the model in the BISON fuel performance code and quantify its impact for constant and transient power cases. Our model produces similar results to an empirical model for the constant power case, but it predicts more grain growth than the empirical model for the transient power case. We attribute this discrepancy to the new mechanistic model's ability to account for the impact of the temperature and power history.