Off-centering effects on MOX fuel behavior

Standard fuel performance models often assume concentric alignment of fuel pellets within the cladding, overlooking the potential effects of pellet eccentricity on reactor performance and safety. This paper investigates the thermal and porosity dynamics of hollow MOX fuel with off-centered pellets under irradiation, using a Finite Element Method (FEM) model implemented via the MFEM library. Simulations replicate experimental conditions from the PuMMA Horizon Europe project, considering the CAPRIX and TRABANT-2/2 experimental pins with high plutonium content MOX fuel. Results reveal that pellet eccentricity induces asymmetrical temperature peaks, significantly reducing the margin to melt compared to the results of axisymmetric modeling, and drives anisotropic pore migration, phenomena with critical implications for the safety and design of Gen-IV reactor fuels. Comparisons against post-irradiation examinations (PIE) confirm the model capability to semi-quantitatively replicate the observed behavior. These findings highlight the need to integrate pellet eccentricity into safety assessments and advanced fuel design strategies, especially for hollow fuels for Gen-IV reactors.