Modelling of the Surroundings of MOX Fuel in PWRs

Abstract

A strong perturbation of the neutron flux results from the MOX fuel insertion in UO₂ PWR cores. It is always anticipated that modelling of such cores is more difficult than of those fueled by either UO₂ or even only MOX fuel assemblies. The effect of various models of the surroundings on a PWR MOX assembly is studied in this work. Three different MOX assembly surroundings are considered here, in the first it is assumed that the MOX assembly is surrounded by similar eight MOX assemblies (reflective boundary model). The considered MOX assembly is surrounded by tight UO₂ fuel assemblies, tach has the UO₂ core reload average enrichment (macro-cell model) in the second case. In these two models it is assumed that the MOX assembly neighbours have the same burnup status as the considered MOX assembly. The third case is bad an that the considered MOX assembly was followed by its actual assembly neighbors in each irradiation time (actual surroundings model). The MOX assembly reactivity, local power peaking, bran worth, moderator temperature and Doppler coefficients at different burnup status are compared for the three different cases. These parameters are evaluated at HFP and withdrawn control rods. Similar comparison was performed at HZP for inserted and withdrawn control rods. In general, it is shown that the results obtained using the macro-cell model are much closer to the more accurate results obtained using the actual surroundings model than those obtained using the reflective boundary model. However, that evaluated assembly power peakings using reflective boundary model are somewhat closer to those of the actual surrounding model than those evaluated using he macro-cell model. Nevertheless, neither macro-cell nor reflective boundary model is capable for predicting the assembly pin power distribution.