Verification of the RMC-SaraGR Nuclear Design Code System Based on the HTTR Benchmark

Abstract In order to perform a detailed nuclear analysis for the small modular prismatic HTGRs, the RMC-SaraGR nuclear design code system has been developed. The verification of the code system using the HTTR benchmark is reported in this paper. The detailed HTTR models have been established with the continuous-energy Monte Carlo code RMC to provide the reference solutions and the 25-group cross sections, which are further corrected by the super homogenization (SPH) factors before being used for the homogeneous core transport calculations by SaraGR or the multi-group RMC (RMC-MG). Various core configurations including fully-loaded core with/without control rods (CRs) insertion and annular cores with different number of fuel columns loaded have been modeled. Compared with the benchmark data, the reference solutions provided by RMC show a 100∼ 500 pcm overestimation of keff for the fully-loaded cores, which may be attributed to the nuclear data uncertainty and the model bias. When the CR block is homogenized as a whole region, the calculation results from RMC-MG and SaraGR agree well but both show a significant keff overestimation of more than 1000 pcm compared with the RMC reference solution for the fully-loaded core with all CRs withdrawn, which is likely to be related to the strong neutron streaming effect through the void holes of the CR blocks. Therefore, a local mesh refinement is imposed on the CR blocks for an explicit modeling of the CR holes, which reduces the keff discrepancy to less than 500 pcm. Most results agree well with the reference solutions and are within an uncertainty of the benchmark values, indicating that the RMC-SaraGR code system is a feasible tool for the nuclear analysis of the prismatic HTGRs.