Sub-channel analysis with F-COBRA-TF – Code validation and approaches to CHF prediction

Different simulation scales are applied in AREVA NP's thermal hydraulics department to predict the flow conditions in nuclear reactor cores: there are codes which regard one (or more) fuel assemblies as one lumped sub-channel. Sub-channel codes resolve each individual channel by one radial node. CFD codes use a much finer discretization in order to resolve the local flow conditions within a sub-channel, e.g. behind a spacer grid.The main focus of the present paper lies on the advanced sub-channel code F-COBRA-TF which has been established as a new part in the company's range of thermal hydraulic codes and customized to the industrial needs recently. Furthermore, an impression of the application of multiphase CFD within the course of fuel assembly optimization will be given.