Thorium-based mixed oxide fuel in a pressurized water reactor: A burnup analysis with MCNP

Abstract

A three-dimensional MCNP model of a Westinghouse-type 17×17 PWR was used to assess the feasibility of enhancing the current fuel supply with a thorium-plutonium mixed oxide fuel (ThMOX). A conventional UO2-fueled configuration was compared to a configuration which incorporated ThMOX fuel. The safety of the ThMOX configuration was compared to that of the UO2 configuration at several time steps of interest within all three cycles (beginning of cycle, peak excess reactivity of cycle, and end of cycle) using the following metrics: axial and radial nuclear hot channel factors FZNandFRN, moderator and fuel temperature coefficients (MTC and FTC), delayed neutron fraction (βeff), and shutdown margin (SDM). Additionally, the performance of the ThMOX configuration was assessed by tracking cycle lengths, the amount of plutonium destroyed, and fission product poison concentration.The ThMOX configuration’s FZN (1.455±0.222) was slightly less favorable than that of the UO2 configuration (1.515±0.246), though its FRN was statistically identical (3.623±0.179 compared to 3.430±0.066 for UO2). The delayed neutron fraction was another area of concern for the ThMOX configuration. It was lower than that of the UO2 configuration, but the lowest βeff occurred at the beginning of Cycle 3 (3.324E−03±8.919E−07). The βeff of the UO2 configuration was 46% higher (6.177E−03±1.510E−06) at this time step. Similarly, the ThMOX configuration’s SDMs were less favorable than those of the UO2 configuration, though even the lowest (3971±43pcm) remained well within the NRC prescribed limit of 1300pcm. However, the MTC of the ThMOX configuration was more favorable for all time steps except the beginning of Cycle 3, and even then it was negative as required by the NRC (−2.325±0.00098pcm/°F). The FTC results were also more favorable for the ThMOX configuration. Its highest FTC value (−6.718±0.001pcm/°F at the peak of Cycle 1) was more negative than all but the lowest FTC of the UO2 configuration (−7.335±0.002pcm/°F at the end of Cycle 3).Overall, incorporating ThMOX fuel into the fuel supply for a conventional PWR seems feasible from a neutronics standpoint. However, this analysis would benefit from a more detailed burnup simulation and optimization of the ThMOX fuel composition. Also, a full thermal hydraulic analysis in conjunction with a neutronics analysis is required.