Full 3D equivalence between the NEM code and the Serpent 2 Monte Carlo code

This study addresses the challenge of accurately simulating reactor core behavior in the presence of strong heterogeneities using nodal methods. These methods, com- monly employed in reactor simulations due to their computational efficiency, rely on the diffusion approximation, which simplifies neutron transport but may introduce errors in heterogeneous configurations. To mitigate these errors, Generalized Equiva- lence Theory (GET) introduces Assembly Discontinuity Factors (ADFs) that correct for discrepancies between homogeneous and heterogeneous fluxes at node interfaces. Traditional methods of computing ADFs often rely on infinite lattice assumptions, which can be inaccurate near boundaries or in highly heterogeneous cores. To en- hance the accuracy of nodal diffusion calculations, this study introduces a full 3D equivalence approach, utilizing the Serpent 2 Monte Carlo code to generate ho- mogenized nuclear data compatible with an NEM-based diffusion solver. A custom subroutine was developed to compute discontinuity factors in all spatial directions, ensuring consistency with the NEM framework. Validation through several bench- mark models demonstrated that the incorporation of ADFs significantly improves the estimation of axial power distributions, particularly near boundaries where flux discontinuities are most pronounced. Furthermore, the multiplication factor showed strong agreement with reference Serpent calculations, with notable improvements when DFs were applied. These results highlight the value of the proposed approach in enhancing the fidelity of nodal simulations, especially for complex, highly hetero- geneous reactor systems.