3d core thermalhydraulic phenomena in pwr sblocas and iblocas

System thermalhydraulic codes have 3D models in porous medium approach which were initially devoted to the prediction of very large scale 3D effects during LBLOCAs. Such 3D modules initially used a very coarse nodalization including only a few hundreds of meshes in the whole pressure vessel. Today the computer power allows 3D simulations with a much finer nodalization in many transients. A core modelling with one mesh per assembly may become a standard practice in near future. This allows to look at much finer multi-dimensional physical processes. As part of a general methodology a detailed PIRT is made to identify the dominant phenomena occurring in a PWR core in small break and intermediate break LOCAS with particular attention to uncovered core situations and to the peak clad temperature (PCT). Processes which have a significant impact on the PCT include the interfacial friction, the radial mixing of phases below the swell level, the crossflows in the single-phase vapou uncovered zone which are produced by radial differences in gravity and friction pressure losses. Turbulent diffusion of momentum and heat with the impact of spacer grids and dispersion effects of momentum and heat in the porous medium with the impact of the mixing vanes are also considered. Then processes are quantitatively evaluated using existing data, using scaling analysis, using some simulations and sensitivity tests. At last a validation matrix is defined to cover all dominant processes. The role of finer scale simulations in the whole process is also presented. As a final objective, a reduction of code prediction uncertainty is expected.