Recent investigations into the performance and economics of mixed thoria-urania (ThO2/UO2) fuel cycles in light water reactors indicate that there may be advantages to using these fuels at high burnups. The Idaho National Engineering and Environmental Laboratory (INEEL) modified FRAPCON-3, a U.S. Nuclear Regulatory Commission-sponsored software package developed by Pacific Northwest National Laboratory for use on mixed thoria-urania fuels. The modifications constituted the first stage of fuel performance evaluations supported by the Nuclear Energy Research Initiative (NERI) project titled Advanced Proliferation Resistant, Lower Cost, Uranium-Thorium Dioxide Fuels for Light Water Reactors. The goal of this NERI project is to develop mixed ThO2/UO2 fuels that can be operated to a relatively high burnup level in current and future commercial power reactors.

This paper describes in detail the INEEL's modifications to the FRAPCON-3 thermal conductivity subroutine FTHCON and the techniques used to validate the modifications. The paper presents the general fuel design criteria used to model mixed thoria-urania fuel and a steady-state analysis of a mock thoria-urania fuel using the FRAPCON-3Th code. The paper also presents the data analyses for the mock thoria-urania fuel and offers suggestions for future upgrades and improvements to the code.