Accurate resonance absorption calculation for fuel pins with non-uniform intra-pellet temperature profile based on ultra-fine-group slowing-down calculations

A new method based on the ultra-fine-group slowing down calculation and the temperature-perturbation-technique is proposed to obtain accurate resonance absorption reaction rate of the fuel pin with non-uniform temperature profiles. It is proved theoretically and numerically that the conventional model using uniform collision probabilities between sub-rings of the fuel to treat non-uniform temperature will underestimate the resonance absorption reaction rate of 238U for the inner region of the fuel. Using the heterogeneous ultra-fine-slowing down solver which is capable of treating two independent resonant absorbers, a new calculation scheme is developed based on the assumption that the influence on the spectrum along the radius from the non-uniform temperature distribution can be decomposed into the influence from each independent sub-ring. Numerical results show that the new proposed method is capable of predicting the resonant absorption reaction rate with better accuracy without the on-line calculation of collision probabilities.