Development and preliminary verification of a two-step Monte Carlo-diffusion depletion scheme for sodium-cooled fast reactors

Sodium-cooled fast reactors (SFRs) have a hard neutron spectrum and complex resonance interference phenomena, making traditional computational methods for pressurized water reactors (PWRs) unsuitable. In this work, a Monte Carlo (MC)-Diffusion depletion scheme specifically developed for SFRs is proposed. This scheme follows a two-step calculation process: generating homogenized macroscopic cross sections (XSs) using the MC method, followed by three-dimensional core calculations with a nodal diffusion solver. To enhance the accuracy of control rod worth calculations, Super-Homogenization (SPH) corrections are utilized. Additionally, an independent depletion solver module is integrated to achieve core burnup calculations. This scheme is validated against the OECD/NEA benchmark for a 1000 MWth metal-fueled sodium-cooled reactor. Validation results show reasonable agreement for key parameters such as core k_eff, Doppler constant, sodium void worth, and control rod worth. However, to fully assess the feasibility and general applicability of this method, further analysis involving a broader set of benchmarks is necessary. The two-step MC-diffusion depletion scheme significantly improves computational efficiency compared to direct MC simulations, reducing CPU hours from 7464.4 to 781.24 and demonstrating its potential for fast reactor core design and fuel management. This work also investigates the error cancellation phenomenon in the two-step MC-diffusion scheme.