Abstract

In order to better understand the stresses to which fuel rods are subjected, we need to improve our knowledge of the fluid flow inside the core and the fuel assembly, and we are particularly interested in the first spacer grid region, as fuel rod fretting has sometimes been observed. It has been seen experimentally in previous years that rotating mixing grids on EDF’s lock-up (a fuel assembly section subjected to fluid flow, with 2 spacer grid and 2 mixing grids) could let to different vibration levels with some fuel assembly types. This seems to confirm that the influence of fluid flow is of primary importance for fuel rod vibration (and thus fretting). A series of calculations are thus run with our incompressible Navier-Stockes solver, Code_Saturne with a classical RANS turbulence model. We limit to a scale of few assemblies for practical reasons. At this scale, most of the features of the fuel rods, nozzles, and guides tubes are represented, though the geometry of the spacer grids is still much simplified, and details such as debris-trapping grids are ignored. We have analysed the axial and transverse velocities for configuration with different fuel assembly types, and calculated an approximation of efforts on individual fuel rods. Local scale results are mainly qualitative, but they already enable us to obtain a better understanding of the effect of nozzle shape or heterogeneous fuel assemblies on the fluid flow. The nozzle geometry (and fuel rod cap positions) have a major influence on the levels of transverse velocity attained. We have run a good number of sensitivity verifications and built a solid methodology, when defining these calculations. To better validate local scale calculations, EDF R&D has built a small lower fuel rod assembly mock-up “BORA 3×7” (3×3 assemblies with 7×7 rods each). This mock-up is being used to obtain the velocity data. The calculations described in this paper are not refined enough to be able to directly correlate fluid flow and fuel cladding fretting in a quantitative manner, but are a step in this direction and may already improve our understanding of the local loads and spatial load variations a fuel assembly is subjected to.

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