Investigation of the flow redistribution upstream of grid-like obstacles separated by a variable gap

International audienceIn nuclear Pressurized Water Reactors, fuel assemblies are slender structures composed of many rods held together by regularly spaced grids. Intense in-core conditions may sometimes result in permanent deformations impairing the reactor normal operation or even challenging its safety. Fluid-structure interactions are the central plot of this multiphysical storyline, and the water gaps between adjacent fuel assemblies act as the main character. More specifically, the flow redistribution upstream of grids, from the water gap to the rod bundle or the other way around, affects the force which is exerted by the coolant on the grids. Analytical models accounting for this effect can be found in the literature. The present article describes a simple experiment dedicated to providing validation data to such models. Mock-up grids were 3D printed and disposed in a hydraulic loop with a variable distance between them. For each configuration, the mean axial velocity was probed at hundreds of locations inside the water gap using an LDA acquisition system. The flow rate was then computed with several methods compensating the lack of velocity measures in unreachable areas. This experimental campaign helped to gain insight into the flow behavior across the grids of a fuel assembly. Notably, near proportionality was demonstrated between the water gap width and the flow rate through it. It was also observed that the redistribution cross-flows take place in a very limited region upstream of the grids