Assessment of the CATHARE 3D capabilities in predicting the temperature mixing under asymmetric buoyant driven flow conditions

Coolant mixing in the reactor vessel of a pressurized water reactor constitutes a key parameter for predicting the core behaviour under abnormal asymmetric cooling conditions. This issue was recently investigated within the experimental framework of the OECD/PKL-2 project. The aim was twofold: to assess the coolant mixing in the vessel downcomer and the core lower plenum under buoyant asymmetric cooling loops, as well as to evaluate the capabilities of computer codes in simulating such phenomenon. It is commonly known that CFD codes are applied for this objective using millions of nodes and large computer resources. However, thermal-hydraulic system codes having the 3D vessel models could also be considered for such purpose. Indeed, these codes benefit from large validation background, require less computational resources and include 3D capabilities that are up to now not fully exploited. In this framework the CATHARE 3D models are assessed against two asymmetrical buoyant mixing experimental tests performed at the ROCOM facility. The outcomes of the current study show good prediction potentialities of the 3D thermal-hydraulic system codes. However their applicability to NPP scale has to be further investigated experimentally and analytically as well.