Preliminary validation and application of shutdown dose rate calculation method based on cell-in-mesh

Shutdown dose rate (SDR) analysis plays a critical role in ensuring radiation safety during reactor maintenance, transportation, and decommissioning. Traditional methods such as the direct one-step (D1S) method and the rigorous two-step (R2S) method face limitations in accuracy and implementation, especially for compact and complex geometries like vehicle-mounted micro-nuclear power systems.

This study aims to develop and validate a cell-in-mesh-based R2S method for SDR calculations, with enhanced sampling efficiency and spatial resolution. The goal is to enable accurate prediction of post-shutdown radiation fields for both benchmarking and practical reactor applications.

An improved R2S methodology was implemented by integrating nested cell-in-mesh geometry with a Monte Carlo (MC) transport framework. Photon source sampling was optimized using bounding box division and local mesh-based distribution sampling. The method was validated using the ITER shutdown dose rate benchmark and applied to the Megapower microreactor model, which employs HALEU fuel, heat pipe cooling, and composite shielding.

The developed method produced SDR distributions with statistical deviations below 2% and matched international benchmark results within 4% deviation. In the Megapower case, the highest dose rate (16.3 mSv/h) at a radial location 30 cm occurred near the heat pipe outlet, primarily due to activated structural materials and neutron streaming along the heat pipe path.

The cell-in-mesh-based R2S method improves the accuracy and resolution of SDR calculations without significantly increasing computational costs. It is suitable for advanced shielding analysis of compact nuclear systems and provides a reliable tool for guiding safety design, maintenance planning, and decommissioning strategies.