MODELING AND PARAMETRIC STUDIES OF THE EFFECT OF PU-MIXING HETEROGENEITY ON FISSION-GAS RELEASE FROM MIXED-OXIDE FUELS OF LWRS AND FBRS

A simple, practical method has been proposed for evaluation of the effect of heterogeneity in Pu-mixing on fission gas release (FGR) behavior in mixed oxide (MOX) fuels. In contrast to the usual PuO2 spherical particle model, the proposed model uses the measurable (by electron probe microanalysis, etc.) statistical frequency distribution of local Pu-content in as-fabricated MOX fuels in combination with the usual fission gas release model based on the diffusion theory for homogeneous UO2 fuel. From parametric studies using a binomial distribution model for the expression of the statistical frequency distribution of local Pu-content, it was confirmed that the enhancement of FGR in MOX fuels is caused by the simultaneous effects of (a) the fission-rate-enhanced diffusion of fission gas atoms and (b) the local (on the scale of fission fragment range) fission rate peaking in the fuel matrix, resulting from the Pu-mixing heterogeneity. The degree of FGR enhancement is greater in the MOX fuels of lower average Pu fraction under a given average power density, and is more apparent at lower operating temperatures as long as the temperature is high enough to cause the FGR. Since FBR-MOX fuels are of high average Pu fraction (approximately 20 wt%) and are used at relatively high operating temperatures compared with LWR-MOX fuels, the FGR enhancement due to their heterogeneity is insignificant.