Integrated sub-element mesh-based R2S shutdown dose rate calculations in OpenMC

Shutdown dose rate (SDR) analysis is essential for the design and maintenance of fusion energy systems, yet existing workflows for the Rigorous 2-Step (R2S) method often require code modifications, external coupling tools, and approximations in space and energy. We present a mesh-based R2S capability implemented directly in the open-source Monte Carlo code OpenMC that enables high-fidelity SDR calculations on both computational solid geometry (CSG) and computer-aided design (CAD) models. The workflow uses a novel mesh–material tally filter and direct microscopic transmutation reaction-rate tallies to define activation regions at the level of distinct materials within each mesh element, so that each material present in an element is activated separately and assigned its own decay photon source rather than being homogenized with the others. This sub-element treatment is supported on structured and unstructured meshes. Activation is performed with OpenMC’s depletion module and produces decay photon sources whose energy distributions are defined directly by the contributing nuclides and their atom densities rather than by multigroup binned spectra, enabling a novel capability to break down the shutdown dose rate by contributing radionuclide. All steps of the workflow, including neutron transport, activation, decay photon source construction, photon transport, and activation-region volume calculation, are automated through a single Python driver class and parallelized using a combination of distributed and shared-memory techniques. The new capability is validated against the Frascati Neutron Generator (FNG) shutdown dose rate benchmark, where mesh-based R2S results are shown to agree with experimental measurements and previously validated cell-based R2S and Direct 1-Step (D1S) calculations, provided sufficient spatial resolution is used. Application to a CAD-based model derived from the Eos neutron source stellarator demonstrates applicability to complex geometries with large numbers of activation regions and reveals dose-rate levels that directly inform shielding and material-selection decisions for future fusion facilities.