Influence of the CATHARE2 Three-Dimensional Meshing of a 900MWe PWR Core in LOCA Conditions

The CATHARE2 code is a “Best-Estimate” system code, developed by the CEA, EDF, AREVA-ANP and IRSN, mainly used in France in the frame of realistic methodology to evaluate safety margins for Pressurized Water Reactor (PWR). Since a three-dimensional thermal hydraulic module, based on a two-fluid 6-equations model, is now available in the most recent versions of the code, a new challenge consists in analyzing whether the 3D modeling of the vessel allows to describe more accurately complex 3D phenomena occurring during Loss of Coolant Accident (LOCA) transients. This document specially studies the sensitivity of the 3D meshing of a 900MWe PWR vessel in Large and Intermediate Breaks LOCA conditions. The development of such a meshing, suitable for the CATHARE2 code, is a long and meticulous task that implies important knowledge of the PWR vessel features and the evaluation of numerous CATHARE2 parameters such as repartition of the guide tubes, volumetric and surfacic porosities or hydraulic diameters. This study also requires a good understanding of the main specific phenomena occurring during the various types of LOCA transients in order to check if the chosen CATHARE2 meshing is well adapted. These studies are successively focused on the consideration of homogenous parameters in the vessel lower head, on axial meshing refinement in the lower head as well as in the active part of the core and finally on radial and azimuthal meshing refinement in the vessel. In order to assess its results and since any experimental evolutions of hydraulics core parameters are available for such multi-dimensional transients, IRSN has considered the physical experimental results of separate effect tests, characteristic of LOCA transients and available in the literature. It has also taken into account the results of previous reference calculations performed with the CATHARE2 code. Considering different three-dimensional meshing of the vessel, the evolution of the hydraulics is observed in the whole reactor and more accurately in the core, which is submitted to a non-uniform radial power profile. In case of Large Break LOCA transient analysis, more attention is paid to the simulation of cross-flows between hot and cold channels during the reflooding phase. This study allows us to verify the strong impact of the axial refinement of the core meshing on the simulation results, leading to greater cross-flows under the quench front. The study of the Intermediate Break LOCA transient, with a delayed pumps stop, has also shown a strong impact of the vessel schematization on the hydraulic evolution. Contrary to the reference case water falling back from hot legs to the upper plenum is observed at the centre of the core and flow is less disturbed in the lower plenum when the CATHARE2 parameters are homogenized. Moreover these sensitivity tests to the 3D meshing show that cladding temperatures are dependent on axial meshing refinement.