MOX fuel effects on the isotope inventory in LWRs

Although today in most of the nuclear power reactors UO2 fuel is used, currently about 40 reactors in Europe are licensed to use MOX, and over 30 are making use of this right. While the typical isotopic compositions (mostly including only the minor actinides and the major fission products) of MOX fuel is well known and described in the literature, it is difficult to find detailed data how the concentrations change for the isotopes which are important in the source term (noble gases, iodine, cesium) with respect to Pu composition and Pu content. This paper presents the results of systematic calculations to determine how the isotopic composition of MOX fuel fed into PWRs and BWRs is changing during irradiation depending on the initial plutonium composition and content and the level of burnup. For most of the fission products the differences are in the range of a few percent. This is due to the fact that the results are for a given burnup and furthermore, the differences in the cumulative fission yields are only a few 10%. Differences larger than 10% at 50 MW day/kg HM can be only found for a few isotopes, e.g. 135Xe, 149Sm, 151Sm, 155Gd, 157Gd. For comparison purposes, the isotope concentration ratios between MOX fuel and conventional UO2 fuel are also provided.