Implementation and application of PyNE sub-voxel R2S for shutdown dose rate analysis

Abstract

PyNE R2S is one of the mesh-based R2S implementations with the capability of performing shutdown dose rate (SDR) analysis directly on CAD geometry with Cartesian or tetrahedral meshes and supports advanced variance reduction for fusion energy systems. However, the assumption of homogenized materials of PyNE R2S with Cartesian mesh throughout a mesh voxel introduces an approximation in the case where a voxel covers multiple non-void cells. This work implements a sub-voxel method to add fidelity to PyNE R2S with Cartesian mesh during the process of activation and photon source sampling by performing independent inventory calculations for each cell within a mesh voxel, and using the results of those independent calculations to sample the photon source more precisely. The PyNE sub-voxel R2S has been verified with the Frascati Neutron Generator (FNG)-ITER and ITER computational shutdown dose rate benchmark problems. The results of the sub-voxel R2S show a satisfactory agreement with the experimental values or reference results. The PyNE sub-voxel R2S has been applied to the shutdown dose rate calculation of the Chinese Fusion Engineering Testing Reactor (CFETR). In conclusion, the sub-voxel R2S is a reliable tool for shutdown dose rate calculation and obtains more accurate results with the same voxel size than voxel R2S.