Evaluation of fluid flow in the lower core of a PWR with Code_Saturne

TitleEvaluation of fluid flow in the lower core of a PWR with Code_Saturne
Publication TypeJournal Article
Year of Publication2007
AuthorsFournier, Y, Vurpillot, C, Béchaud, C
JournalNuclear Engineering and Design
AbstractIn 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. The first spacer grid region is of particular interest, as fuel rod fretting has sometimes been observed at that level. Entry conditions depend on the geometry of the lower core plate and of the assembly nozzles. Distribution of flow in the downcomer and lower plenum is also a factor. A series of calculations are thus run with the incompressible Navier–Stokes solver, Code_Saturne, using a classical RANS turbulence model. The first calculations involve a global geometry, including part of the cold legs, downcomer, lower plenum, and lower core of a pressurized water reactor. The level of detail includes most obstacles below the core. The lower core plate and the fuel rod assemblies above it cannot be well represented within a practical mesh size, so that a head loss model is used. Different types of assemblies can be represented through different head loss coefficients. We make full use of Code_Saturne ’s non-conforming mesh possibilities to represent a complex geometry, being careful to retain a good mesh quality. Steady-state or near steady-state results are obtained, which may be used as realistic entry conditions for full core calculations at assembly width resolution, and beyond those, mechanical strain calculations. We are especially interested in more detailed flow conditions and in the lower core area, so as in the future to quantify vibrational input. This requires a much higher resolution, which is limited to a scale of a few assemblies for practical reasons. At this scale, most of the features of the fuel rods, nozzles, and guide tubes are represented, though the geometry of the spacer grids is still much simplified, and details such as debris-trapping grids are ignored. Different meshes are used for different fuel types. For the moment, a constant velocity upstream of the lower core plate is used as an inlet condition. We have also built a small lower fuel rod assembly mock-up (1/5 scale 7 ×× 7 tube, 3 ×× 3 assemblies) with which we plan to obtain detailed flow information, and better qualify the use of our CFD codes with regards to this type of application.