Confinement Dependency on Added Stiffness Effect on a Fuel Assembly Under Axial Flow

Safety measures are required to insure the drop of control rods and that the core is cooled when the fuel assemblies of a Pressurized Water Reactor (PWR) are subjected to a seismic excitation. A way to insure these two criteria is to prevent the spacer grids from buckling. The reactor core made of fuel assemblies is subjected to an axial water flow to cool the reactor. The flow modifies the dynamical behaviour of the fuel assemblies. Tests made on a real fuel assembly highlighted an added stiffness effect under axial flow. In previous studies simulations were compared to experiment involving by-passes significantly larger than the distance between two fuel assemblies in a PWR core. Thus, one could wonder if the observations made on a fuel assembly with large by-passes are representative of core geometry. Simulations using a fluid-structure model of the core to a seismic excitation have been proposed. A parametric study has been conducted to observe the effect of confinement on the added stiffness effect for several confinements and bulk velocities. Simulations showed that the added stiffness reaches a maximum for a confinement around 20 mm, and that the added stiffness should be negligible in a real core configuration. (authors)