Implications of point defect accumulation on UO2 thermal conductivity and fission gas release under accelerated fuel irradiation

Evaluation of thermal properties is a crucial factor for nuclear fuel performance. During reactor operation, the accumulation of fission products and irradiation-induced lattice defects are responsible for degradation in thermal conductivity. This affects fuel temperature and fission gas release (FGR) among other multiphysics processes important for economics and safety analysis. However, the point defects (PD) induced thermal conductivity reduction are not treated mechanistically in the current fuel performance codes (FPC). In this study, we analyze the implications of PD accumulation described using a rate theory (RT) model on the lattice thermal conductivity of UO₂ by adopting Lucuta thermal conductivity correlation (LC). We demonstrate that fission rate-dependent point defect concentrations have the largest impact on in-pile thermal conductivity in the periphery of light water reactor fuel below a temperature threshold governed by the migration barrier of defects. The reduction of thermal conductivity in the low -temperature rim region acts as additional thermal resistance and leads to a temperature notably larger than suggested by LC specifically at low burnups. These effects are anticipated to have notable impacts on fuel during the accelerated conditions. The impact of incorporating a point defect-informed approach to thermal conductivity is assessed through a detailed analysis of fission gas release behavior and fuel microstructure evolution. Finally, a fission rate-dependent correction to the Lucuta correlation is proposed as part of this analysis. The modified Lucuta correlation demonstrates higher FGR compared to the original correlation, although the accelerated irradiation process leads to a reduction in overall FGR. Once additional multiphysics mechanisms tightly coupled to temperature profile are introduced it becomes harder to deconvolve the impact of point defects.