Thorium-based mixed oxide fuel in a pressurized water reactor: A beginning of life feasibility analysis with MCNP

Abstract

Thorium is an asset the nuclear industry does not use, and plutonium is a liability that much of the world would like to be rid of. By incorporating a thorium–plutonium mixed oxide fuel (Th-MOX) into the fuel cycle, pressurized water reactors could provide a means for the United States to address both of these issues – but only if key reactor safety parameters are not affected.The feasibility of utilizing Th-MOX fuel in a pressurized water reactor is examined under steady-state, beginning of life conditions. With a three-dimensional MCNP model of a Westinghouse-type 17×17 PWR, many possibilities for replacing one-third of the UO2 assemblies with Th-MOX assemblies were considered. The excess reactivity, critical boron concentration, and centerline axial and radial flux profiles for several configurations and compositions of a one-third Th-MOX core were compared to a 100% UO2 core. A blanket-type arrangement of 5.5wt% PuO2 was determined to be the best candidate for further analysis. Therefore, this configuration was compared to a 100% UO2 core using the following parameters: delayed neutron fraction (βeff), temperature coefficient, shutdown margin (SDM), and axial and radial nuclear hot channel factors (FZN and FRN).The one-third Th-MOX configuration showed an undesirable reduction in βeff from 0.00716±4.60E−07 for the 100% UO2 configuration to 0.00607±4.30E−07. The reduction in βeff would perhaps be ameliorated by the one-third Th-MOX configuration’s temperature coefficient of reactivity, which at −2.05±0.02pcm°F−1 is more favorable than the corresponding value of −1.42±0.02pcm°F−1 for the 100% UO2 configuration. The SDM of the one-third Th-MOX configuration is estimated to be 4079±7pcm, which is 28% lower than value of the 100% UO2 configuration. The FZN for the two cores were virtually identical. However, FRN for the one-third Th-MOX configuration (1.67±0.28) was 20% higher than the corresponding value for the 100% UO2 configuration (1.39±0.23).These preliminary results are encouraging. However, additional investigations are required to study the impact of thermal fluid feedback and the effect of burnup and poison buildup.