Effective Thermal Conductivity of Tri-Isotropic (TRISO) Fuel Compacts

The Next Generation Nuclear Plant (NGNP) program objective is to develop a new type of nuclear reactor that produces process heat instead of electricity. The process heat can be used in the production of hydrogen and many other industrial processes. As part of the NGNP program a new type of nuclear fuel is being developed. The fuel is a composite construction of specially coated fuel particles and graphite pressed together in a cylindrical compact. Thermal conductivity is an important thermophysical property of the fuel that needs to be measured. Knowledge of the thermal conductivity of the fuel will provide accurate prediction of fuel performance and safety assessment of the nuclear reactor. The composite nature of the fuel compact requires the thermal conductivity measurement be performed over the entire length of the compact in a non-destructive manner. No existing measurement system is capable of performing such a measurement. The objective of this study was to characterize the thermal conductivity of the fuel compact. This was done both experimentally and numerically. Previously a measurement system was built to measure the thermal conductivity of the fuel compact. That measurement system had to be validated to ensure the accuracy of the measurements. Once the system was validated it was used to measure multiple surrogate fuel compacts. The numerical work was performed using a finite element model developed in COMSOL Multiphysics. The purpose of the numerical model was to compare the thermal conductivity of the fuel compacts to previously established models that predict the thermal conductivity of two phase composites. The results from the experimental study provide an estimate of the expected thermal conductivity of the fuel compacts. The numerical results provided a possible model that can be used to predict the thermal conductivity of the nuclear fuel compacts