Integrated multiscale experiment and model analysis of radially resolved microstructure and thermal conductivity in mixed oxide fuel

The thermal conductivity of mixed oxide (MOX) fuel depends on complex microstructural, chemical, and thermomechanical processes. Due to large thermal variations across the annular fuel pellet of sodium fast reactors, many significant microstructural alterations occur across short distances, which greatly impact local thermal conductivity. Using novel experimental methods that provide high spatial resolution enables capturing these localized microstructural trends affecting the thermo-physical properties of nuclear fuel. In this study, radial measurements of porosity, elemental composition, and thermal conductivity of mixed oxide nuclear fuel pellets at various burnups (6 - 19 % FIMA) have been acquired and are analyzed with a multiphysics fuel performance model. The model includes equations capturing heat generation and diffusion, porosity evolution, grain growth, fission gas behavior, and microstructure dependent thermal conductivity. This coupled experimental and modeling effort provides insight into how burnup and irradiation temperature lead to intricate microstructure evolution impacting the properties of the nuclear fuel. We quantify and discuss the accuracy of the implemented models. The porosity and dissolved fission product profiles resulting from burnup were identified as having the most significant impact on thermophysical properties. Validity of the overall multiphysics model was assessed using radially resolved experimental data revealing central void evolution, porosity, grain growth, dissolved fission gas, and thermal conductivity. This work provides a pathway for improving localized thermal conductivity models and the predictive capabilities of fuel performance codes.