A unified thermal conductivity model of LWR MOX fuel considering its microstructural characteristics

LWR MOX fuel, fabricated either by direct mechanical blending of UO₂ and PuO₂ powders or by two stage mixing, inevitably has Pu-rich particles dispersed in the matrix of fuel pellet, whose Pu concentrations are higher than pellet average one and whose size distribution depends on specific fabrication method. This paper describes a mechanistic thermal conductivity model of MOX fuel by considering this inhomogeneous microstructure and explains the wide scattering of measured MOX's thermal conductivity. The developed model has been incorporated into a KAERI's fuel performance code, COSMOS, and then evaluated using measured data for irradiated MOX fuel. The measured temperatures were obtained from both homogeneous MOX at beginning of life and inhomogeneous MOX at high burnup. The COSMOS predicts the thermal behavior of MOX fuel well except for irradiation test accompanying substantial fission gas release. This over-prediction of fuel temperature, that is, the under-prediction of thermal conductivity, in the MOX fuel experiencing substantial fission gas release suggests that the introduction of a recovery factor would be required in the term that considers the effect of burnup on thermal conductivity. (author)