On the modelling of fuel assembly vibrations using coarse mesh approaches

Paper in proceeding, 2022

In this paper, the stationary fluctuations of the neutron flux as compared to its static component, also called the neutron noise, induced by the vibrations of a single fuel assembly in a commercial Pressurized Water Reactor, are considered. The purpose of the work is to investigate, using two-group diffusion theory and for one-dimensional systems, whether coarse mesh approaches are able to reproduce the global behavior of the neutron noise. It is demonstrated that the neutron noise can be faithfully reconstructed after assembly homogenization by coarse mesh simulators by introducing four noise sources only. Those noise sources are positioned at the interface between the moving fuel assembly and its non-moving neighbors. The noise sources are defined in each of the two energy groups. The strengths of the noise sources nevertheless need to be adjusted in order to properly reproduce the reference solution. Because of the four introduced noise sources, four normalization conditions are necessary. Several conditions are investigated in this work. The most practical alternative is to normalize the neutron noise sources in the homogenized configuration to the same reactivity effect as the one of the neutron noise source in the non-homogenized configuration. The two additional normalization conditions are thereafter chosen by forcing the homogenized solution to be scaled to the heterogeneous solution for each of the two energy groups. Choosing two different spatial points in the direct vicinity of the vibrating fuel assembly was demonstrated to provide a more robust reconstructed neutron noise.