Monte Carlo-Based Simulation of Reactivity and Transmutation in the CEFR Sodium-Cooled Fast Reactor

As a representative Generation IV sodium-cooled fast reactor (Gen-IV SFR), neutron physics characteristics studies of the China Experimental Fast Reactor (CEFR) core are crucial for its safety case. In this study, a three-dimensional core model of the CEFR was developed using the Monte Carlo-based MCNP5 code, with its reliability validated through five neutronics benchmark experiments. Based on this model, the fundamental neutronics characteristics of minor actinide (MA) transmutation in the sodium-cooled fast reactor were investigated. The results demonstrate that as the minor actinide (MA) loading fraction in the core increases from 0% to 8%, the effective multiplication factor (Keff) exhibits a significantly nonlinear decrease, accompanied by a corresponding reduction in neutron flux, necessitating increased fuel enrichment to maintain core criticality. Opposite impacts on reactivity are observed for different MA nuclides: 237Np, 241Am, 243Am and mixed MA reduce Keff, whereas 244Cm and particularly 245Cm significantly enhance Keff. The reactivity change rate sharply decreased from −1242.5 to −312.7 pcm/wt%, clearly demonstrating saturation effects in MA neutron absorption. Crucially, reactivity remained deeply negative across all operational scenarios, with safety requirements being satisfied even at maximum MA loading levels, confirming the inherent safety of the proposed approach.