A three-dimensional two-energy group heterogeneous intrusive Reduced Order Model of xenon oscillations in PWRs

Paper i proceeding, 2023

An intrusive reduced order model based on a modal expansion method is presented to study xenon oscillations in pressurized water reactors. The model is based on the neutron diffusion equation and uses a two-group, three-dimensional heterogeneous formalism with nodal macroscopic nuclear cross sections while featuring a simple arbitrary feedback term to model the response to changes in the macroscopic cross sections. The comparison between the model and an equivalent one-group homogeneous model shows that the computed time-dependent evolution of the eigenmodes of the system differs significantly. The underlying equations of the two models, which describe the deviations of neutron flux, iodine concentration, and xenon concentration from the equilibrium condition, are investigated to identify the reasons behind the discrepancies. The terms of the equations containing products between the spatial eigenmodes and the stationary neutron flux or xenon concentration are most affected by the increase in energy and spatial resolution. The use of one or two energy groups causes the most considerable discrepancies. Nevertheless, spatial resolution is important in calculating the coupling of the eigenmodes when spatial offsets characterize the equilibrium neutron flux distribution. As a result, the -group heterogeneous model predicts a more unstable system with respect to xenon oscillations.