*4.2.2 Heat conduction and conjugate heat transfer*

The temperature distribution in NTPs is of great importance. First, it is the most important driver for neutronics feedback in thermal LEU-fueled reactors, and second, temperature values and differences (cold to hot) are large and margins to failure are typically small. During normal operation, most heat is transferred to the hydrogen via conjugate heat transfer. However, some of the heat is transferred from the fuel through the insulator and multiple gas gaps to the moderator and even to the reflector. Heat transfer through the gas is mostly facilitated by radiation. In addition to heat transfer from the fuel, some of the fission heat is deposited nonlocally in the moderator and reflector; it is therefore required to model a significant portion of the core to obtain an accurate understanding of the temperature distribution in the moderator and reflector.

Heat conduction in an NTP needs to consider the change in thermal properties with temperatures. The temperatures over the time of a startup transient and at different locations within the reactor vary significantly. The material properties relevant for thermal analysis of the problem (e.g., thermal conductivity, specific heat, density) vary as a function of the temperature, thereby requiring an accurate model that can account for the temperature dependence of these properties. NTPs use a significant number of special purpose materials (e.g., porous ZrC insulator, refractory metal matrix with uranium inclusions) and the thermal properties of these materials need to be available.

Heat transfer in open spaces of the reactor (e.g., plena and exhaust nozzle) must also model thermal radiation in complex geometries. MOOSE provides heat conduction, gap heat transfer, and net radiation transfer capabilities within its heat conduction module. The material system in MOOSE has the ability to use general temperature-dependent material properties supplied as polynomial fits, lookup
