Study of neutronic phenomenology of heterogeneous moderator voiding in a partially MOX-loaded PWR

Assessing neutronic effects of a moderator loss in a Pressurized Water Reactor (PWR) is a physical and computational challenge. It is however a crucial safety issue to be able to predict the reactivity variation during such an event, especially when Mixed Oxides Fuel (MOX) is used. In the past, a lot of work have been dedicated to understand the phenomenology of homogeneous moderator voiding. In this paper, we investigate the neutronic phenomenology of the less familiar case where the moderator loss happens only in a region of the core. In this case, we talk about heterogeneous voiding. A two-dimensional 3 x 3 cluster, composed of eight Uranium Oxides (UOX) assemblies surrounding a MOX assembly was used. We compared the void effects in the central MOX assembly in simplified homogeneous and heterogeneous voiding configurations. In the homogeneous configuration, UOX assemblies are voided as well, while in the heterogeneous case UOX assemblies are still flooded. The physical realism of those configurations have not been assessed in this paper. Neutronic parameters are computed using TRIPOLI-4 Monte-Carlo code and JEFF-3.1.1 library. We show that the reactivity of the voided MOX assembly is higher when surrounding UOX assemblies are flooded: the difference can reach value as high as 9,500 pcm. We highlight discrepancies of spatial and spectral effects in the MOX assembly between both voided configurations. The influence of each isotope in the MOX fuel on both voiding conditions is finally discussed. (authors)