Quantifying the effects of depletion parameters on the PWR spent fuel reactivity based on nuclide sensitivity coefficients

Abstract

This study employs nuclide sensitivity coefficients to investigate the spent fuel reactivity of pressurized water reactors (PWRs) in terms of differences in predicted inventories under various depletion conditions. Considering a representative PWR spent fuel, the study began by selecting the top 20 nuclides of large macroscopic neutron absorption cross sections, which are expected to have a substantial effect on reactivity. The sensitivity coefficients (Δk∞/ΔNi) of each nuclide were first evaluated at various burnups and cooling times using MCNPX. These coefficients were used to facilitate a detailed analysis of the spent fuel reactivity and to identify which nuclides cause the effect in a quantitative manner. The approach was demonstrated by analyzing the effects on spent fuel reactivity of several depletion parameters, including the initial enrichment, specific power, operating history, boron concentration, fuel temperature, moderator temperature, control rod effect, and axial burnup profile. The results were generally in good agreement with those obtained from direct MCNPX criticality calculations. Two major advantages of this approach are: (1) the changes in spent fuel reactivity with various depletion parameters can be closely related to composition changes; and (2) it provides a quantitative estimate of the contribution of each nuclide to the reactivity effect based on the predicted inventory differences.