A Proposed Code-to-Code Benchmark for the Depletion of HTGR Fuel

Several ongoing High Temperature Gas-cooled Reactor (HTGR) development projects are currently under way throughout the world, with the U.S. Department of Energy’s Next Generation Nuclear Plant representing a significant and growing activity in the United States. HTGR designs utilize graphite-moderated fuel forms and helium gas as a coolant. HTGR fuel elements are typically composed of tristructural-isotropic (TRISO) fuel particles. The TRISO particles are either dispersed in the graphite matrix of a fuel pebble for the pebble-bed design or molded into compacts/rods that are then inserted into hexagonal graphite blocks for prismatic core designs. The use of localized fuel grains within a larger fuel element results in two levels of heterogeneity that will challenge many existing lattice physics methods. Hence, advanced methods for treatment of both levels of heterogeneity effects are needed. In doubly heterogeneous (DH) systems, heterogeneous fuel particles in a moderator matrix form the fuel region of the fuel element and thus constitute the first level of heterogeneity. Fuel elements themselves are also heterogeneous with fuel and moderator or reflector regions, forming the second level of heterogeneity. The current lack of well-qualified experimental measurements for spent HGTR fuel elements limits the validation of advanced DH depletion methods. Because of this shortage of data, this benchmark has been developed as the first, simplest phase in a planned series of increasingly complex set of code-to-code benchmarks.