Neutronics modeling method of core deformation in a small fast reactor using the three-dimensional method of characteristics

A neutronics modeling framework for core deformation in sodium-cooled fast reactors (SFR) is developed by coupling Monte Carlo–generated multigroup cross-sections (MGXS) with a three-dimensional Method of Characteristics (3D MOC) solver. The method explicitly represents deformed geometries and is verified against continuous-energy Monte Carlo (CEMC) for a small SFR under uniform expansion, linear flowering, and outward bowing. Leveraging the strong geometric modeling capability of the MOC method, four core models with different levels of spatial heterogeneity are constructed. The transport-corrected flux-moment homogenization technique (TC-MHT) significantly improves accuracy, reducing reactivity bias from ∼500 pcm to within ∼100 pcm. Additional axial refinement in MGXS generation further lowers errors to within ±67 pcm. Reactivity feedback is predicted within ±7 pcm across models. Power distribution accuracy is also improved, with remaining discrepancies mainly attributed to homogenization near strong axial heterogeneities. Among the tested models, the duct-heterogeneous approach provides the best balance between accuracy and efficiency. The proposed method offers an effective tool for deformation-induced neutronics analysis in fast reactors.