Reactor Physics Analysis of the VHTGR Core Wei Ji, Jeremy L. Conlin, William R. Martin, and John C. Lee Department of Nuclear Engineering and Radiological Sciences University of Michigan, Ann Arbor, MI 48109-2104, wjiz@umich.edu INTRODUCTION As part of an I-NERI project to develop safety analysis codes and experimental validation for a Very- High Temperature Gas-Cooled Reactor (VHTGR), we have developed MCNP5 [1] models to represent material heterogeneities inherent in the microsphere fuel particles and fuel compacts for a GT-MHR design [2]. We have also performed preliminary coupled nuclear-thermal- hydraulic (NTH) analysis to obtain self-consistent global power and temperature distributions using a homogenized global model for MCNP5 and three-ring core model for the RELAP5-3D/ATHENA code [3]. We present a comparison of heterogeneous and homogeneous representations of the microsphere and fuel compact cells, establishing the impact of heterogeneities on the overall VHTGR neutronics analysis. We also discuss the importance of fuel temperature feedback on the global power distribution. MCNP5 MODELING OF MICROSPHERES As part of the effort to address the double heterogeneity inherent in the VHTGR fuel design, we have assumed the same microsphere geometry and densities as used in the NGNP Point Design [2], consisting of a 10.36% enriched uranium oxycarbide fuel kernel with layers of carbon buffer, pyrolytic carbon, and silicon carbide and a packing fraction of 0.289. Homogeneous and heterogeneous microspheres were simulated, both as standalone microsphere cells as well as a fuel compact cell consisting of microsphere cells contained within a hexagonal graphite block. We have performed MCNP5 simulations of: Microsphere centered within a graphite cube with reflecting boundary conditions on the cube surface, represented both as a homogeneous and heterogeneous cell. Hexagonal graphite block with a fuel compact consisting of either homogeneous or homogeneous microsphere cells. There are reflecting boundary conditions on the outer hexagon surfaces and the top and bottom surfaces of the fuel compact. The heterogeneous microsphere cell is illustrated together with the homogeneous and heterogeneous compact cells in Figure 1. Heterogeneous Fuel Compact Cell Fuel Compact Cell Microsphere Cell (homogeneous fuel) (heterogeneous fuel) Figure 1. Heterogeneous microsphere and homogeneous and heterogeneous fuel compact cells for MCNP5 simulations Reactor Physics Advances for High-Temperature, Gas-Cooled Systems 556