Non-isotropic thermal behavior of an MHTGR fuel block: Impact upon reactivity feedback

This report discusses lumped parameter and detailed multi-dimensional thermal analyses of a New Production Modular High Temperature Gas Cooled Reactor (NP-MHTGR) fuel block were conducted that indicated that during a power transient, the target temperature would rise significantly later than the fuel temperature. This behavior, which is due to radiative, convective and conductive heat transport phenomena within the fuel block coupled with the significantly different thermal physical properties of the fuel block materials, leads to the potential for a delayed positive contribution to the temperature coefficient of reactivity of the NP-MHTGR core during TOP events. These results have indicated the need for additional experimental and analytical studies in order to more fully assess the design, operational and safety implications of this phenomenon. In addition, experiments in the TREAT facility are planned to provide additional data to assist in the capabilities to predict the reactivity feedback characteristics of the NP-MHTGR core. These studies will be the subject of a future paper