Recent investigations into the performance and economics of mixed thoria-urania fuel cycles demonstrate potential advantages at high burnup. Initial neutronic and fuel behavior calculations for several ThO2-UO2 mixtures being considered for use in commercial nuclear power plants are described. The Monte Carlo N-Particle-Origen2 Coupled Utility Program (MOCUP) was used to analyze the reactivity characteristics and isotopic concentrations Of unit fuel pins/cells and lattice/assembly models as a function of burnup and reactivity. Neutronic results for a three-batch 6-yr cycle for each of three proposed ThO2-UO2 mixtures with the UO2 enriched to 19.5% U-235 are presented. Neutronic results show that fuels fabricated from ThO2-UO2 MiXtUres can reach all average discharge burnup of lip to 70 MWd/kgHM, which will increase the time between refueling and decrease the production of weapons-grade plutonium by a factor of 3 as compared to all-urania fuel. A version of FRAPCON-3, modified to handle pure thoria and ThO2-UO2 mixtures, was used for the fuel performance and behavior calculations. The new version called FRAPCON-3Th includes the updated material property models for thermal conductivity, specific heat capacity, emissivity, thermal expansion, modulus of elasticity, and melting temperature to predict fuel behavior for pure ThO2 or ThO2/UO2 mixed fuel. For a concentration of 75% ThO2/25% UO2, initial fuel performance parameters (peak centerline temperature, gap conductance, thermal expansion, etc.) predicted operating conditions are better than those of current UO2 fuel. A ThO2-ThO2/UO2 thermal conductivity model is still in the development stage. For all fuel calculations, an interim model that interpolates between the Belle and Berman predicted thermal conductivity using a correction factor for radiant heat transport and the MATPRO-predicted thermal conductivity for UO2 was applied.
