Verification and validation of the R2Smesh approach for the calculation of high resolution shutdown dose rate distributions

The well validated Rigorous-2-Step (R2S) approach for the calculation of decay gamma shutdown dose rates has been extended utilizing mesh tally capabilities of the MCNP5 code. Contrary to other similar approaches, the new R2Smesh system pays special attention to achieving highly accurate and reliable results for neutron flux intensities and spectral shape in the initial MCNP5 neutron transport step. To this end, non-uniform fine and coarse meshes are adopted. Also several enhancements to the performance of activation calculations by FISPACT2007 and to sampling from repeated structures geometry have been implemented. The R2Smesh system has been verified by application to an ITER (International Thermonuclear Experimental Reactor) computational benchmark. An extensive validation exercise has been conducted on the FNG (Frascati Neutron Generator) shutdown dose rate benchmark. In this case several variants of the fine/coarse mesh spacing have been tested showing evidence for convergence toward R2S values obtained with the original cell-based approach. It thus has been shown that R2Smesh is validated for ITER applications and offers additional capabilities for high resolution shutdown dose rate calculations.