Thermophysical properties of thorium mononitride from 298 to 1700 K

Nitride fuel forms offer higher actinide density, melting point, and thermal conductivity compared to traditional oxide fuels, and such properties afford a reduction in core size for comparable reactor designs. Preliminary findings show that additions of thorium mononitride (ThN) into uranium mononitride (UN) greatly improves the thermomechanical and neutronic performance of the fuel. This study is part of a series of experiments designed to precisely characterize the properties of thorium mononitride, for which little data is available in the published literature. The purpose of this work was to create high purity, dense (>92%) monoliths of ThN for use in thermophysical property measurements. This was achieved by traditional cold pressing and high temperature sintering under inert atmospheres. Sintered test specimens were used for the measurement of the coefficient of thermal expansion, heat capacity, thermal diffusivity, and thermal conductivity of ThN as a function of temperature from 298 to 1700 K. The thermal conductivity of ThN was found to exceed that of UN by up to a factor of 5 in temperatures relevant to the operation of nuclear reactors. The results of this work will facilitate more detailed thermodynamic, mechanical, and neutronic modeling of mixed UN and ThN fuel forms to aid in the design of thorium nitride reactors and fuel cycles.