Enhancing the effective temperature model for typical UO2 fuel in criticality calculations

In current neutron transport equation calculations, a common strategy to enhance computational efficiency is approximating the fuel temperature distribution as uniform. This approach defines a uniform temperature, known as the effective temperature ($T_{\text{eff}}$), which preserves the reactivity of the model corresponding to the actual temperature gradient. Based on the actual temperature distributions within the UO₂ pellet of the AFA-3G fuel, the present study introduces a weighting coefficient for the volume-averaged temperature to extend the Chabert-Santamarina model. This extended model, as a generalized formulation of both the Rowlands and Chabert-Santamarina models, demonstrates superior performance to the current five effective temperature models in terms of deviation fluctuation and central values. This conclusion is verified through independent simulations, with OpenMC employing the Windowed Multi-Pole (WMP) database and RMC utilizing point-wise ACE library or data derived from Gaussian-Hermite quadrature for online Doppler broadening. Therefore, the present refined effective model enhances the accuracy of effective temperature and the resulting reactivity. Furthermore, the deviation of using a uniform effective temperature across the assembly remains within the acceptable uncertainty range (3 $\sigma$).