Neutronic calculation of deformed cores: Development of a time-dependent diffusion solver in CAST3M, a mechanics dedicated finite element code

Neutron transport is driven by the geometrical arrangement of materials in nuclear reactor cores and is therefore impacted by any modification of their geometry. Nevertheless, non-trivial distortions able to noticeably modify the neutron behavior in LWRs (light water reactors) are unlikely to happen and difficult to compute. These effects are therefore rarely considered in LWRs. They are however more likely to affect fast reactors. As a consequence, in the frame of the development of the fourth generation, the interest for new methodologies able to compute the impact of geometric distortions of fast reactor cores is growing. In this paper, we present a straightforward approach based on mesh deformation to tackle this problem. This work was done with CAST3M, a finite element code dedicated to structural mechanics. A diffusion equation solver, for both critical and time-dependent calculations, was implemented in CAST3M. The use of this code makes mesh handling and data transfer very easy. In the following, we first shortly describe this solver; then we test it by comparing it with APOLLO3® on cases without mesh deformation. Finally, radial distortions (flowerings) are considered: the geometrical representation of deformed cores is discussed, and results are compared to TRIPOLI-4 ones and to those of a totally different method, subject of a previous paper, based on pixels and implemented in APOLLO3®. A good consistency between these different approaches is found.