Dynamical Nonlinear Modelling of a Pressurised Water Reactor Fuel Assembly Subjected to an Axial Flow

Safety measures are required to insure the drop of control rods and that the core is cooled when the fuel assembly spacer grids strike each other during seismic excitation of a Pressurized Water Reactors. A way to insure these two criteria is to prevent the spacer grids from buckling. Engineers need special tools for designing and maintaining reactor cores. The reactor core made of fuel assemblies is subjected to an axial water flow to cool the reactor. The flow strongly modifies the dynamical behaviour of the fuel assemblies; therefore the identification of the fluid forces is important to provide a relevant modelling of the fuel assemblies' behaviour. The modelling proposed in this paper is based on a porous medium approach. This approach gives access to an equivalent fluid model and an equivalent structure model both defined on the whole domain. Motion equations for the equivalent fluid and the equivalent structure are first established separately. For the fluid part, global fluid flow equations through the rod bundle are obtained by local spatially averaging the Navier Stokes equations written with an Arbitrary Lagrangian Eulerian approach. The resulting equivalent fluid is characterized by an equivalent velocity and an equivalent pressure both defined in the whole domain. The structure equations are also space averaged, the fuel assembly is modelled as an equivalent structure satisfying a Timoshenko beam model with a nonlinear behaviour. The flow in the by-passes surrounding the fuel assemblies are accounted for by a leakage flow modelling. The increase of stiffness with the flow rate is reproduced by the simulations. Simulations accounting for one and three fuel assemblies are performed for various values of by-passes to observe the influence of by-pass and configurations on this increase of stiffness. (authors)