Effect of Simplifications of Core Analysis Model on Neutronics Characteristics of HTTR

This study evaluates the effects of simplifying the detailed core structures in the high temperature engineering test reactor (HTTR) analysis model on reactivity and fuel-rod-wise fission reaction rate. Using the continuous-energy Monte Carlo code MVP, we compared the neutronics characteristics of a detailed core analysis model and a simplified model. The simplified model is commonly employed in deterministic methods. In this study, the modeling focused on the side shielding block, permanent reflector, top shielding block, handling hole, and the control rod. For simplification, the side shielding block and the reactor pressure vessel (RPV) were excluded, and a vacuum boundary condition was applied to the outer surface of the permanent reflector. The permanent reflector was approximated by arranging the hexagonal graphite blocks. The handling holes were removed, and the graphite density was uniformly reduced. The top shielding blocks were excluded, and a vacuum boundary condition was applied to the outer surface of the top replaceable reflectors. The control rods were modeled with axially uniform sleeves and neutron absorbers, while excluding the shock absorber on the bottom. For the keff, we found that the effect of each simplification was approximately 0.15%Δk/k or less. For the fuel-rod-wise fission reaction rates, root mean square (RMS) errors were comparable to the statistical errors. Furthermore, the maximum and minimum relative errors remained within ±2.2% in the high-power regions.