Modelling of External Cooling for In-Vessel Corium Retention in VVER-440/213 Type Nuclear Power Plants

Research and development programs of high safety significance have been going on for nuclear power plants of VVER-440/213 type to apply the in-vessel corium retention concept. The in-vessel retention (IVR) concept is based on the external reactor vessel cooling (ERVC) with the main objective to prove that the reactor pressure vessel (RPV) integrity can be preserved in accident sequences leading to core melt. One of the bases of programs was the SARNET project of European Commission, which focused on confirming the capability of the ASTEC code to simulate IVR, calculating thermal load caused by the corium. The ERVC concept is applied to the Paks nuclear power plant of VVER-440/213 type. For the experimental modelling of the ERVC, the CERES (Cooling Effectiveness on Reactor External Surface) facility was designed and constructed. The facility is a scaled down model of the cooling system intended to apply to the Paks NPP with 1:40 scaling ratio for the vessel external surface and 1:1 for the elevations giving the driving force for natural circulation. The heat load supplied to the model is provided by electric heaters. A large number of temperature, pressure, level, void and flow measurements are installed. A RELAP model of the CERES facility was developed and tested by pre-test results.