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 eff ect of heterogeneity in Pu-mixing on fission gas release (FGR) behavio r in mixed oxide (MOX) fuels. In contrast to the usual PuO2 spherical particle model, the proposed model uses the measurable (by electron pr obe microanalysis, etc.) statistical frequency distribution of local P u-content in as-fabricated MOX fuels in combination with the usual fis sion gas release model based on the diffusion theory for homogeneous U O2 fuel. From parametric studies using a binomial distribution model f or the expression of the statistical frequency distribution of local P u-content, it was confirmed that the enhancement of FGR in MOX fuels i s caused by the simultaneous effects of (a) the fission-rate-enhanced diffusion of fission gas atoms and (b) the local (on the scale of fiss ion fragment range) fission rate peaking in the fuel matrix, resulting from the Pu-mixing heterogeneity. The degree of FGR enhancement is gr eater in the MOX fuels of lower average Pu fraction under a given aver age power density, and is more apparent at lower operating temperature s as long as the temperature is high enough to cause the FGR. Since FB R-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 insignifi cant.