RELAP5 simulations of the PASI experiment on Containment Wall Condensation

During a Design Basis Accident (DBA) or Beyond Design Basis Accident (BDBA) scenario, concerning a Loss Of Coolant Accident (LOCA), the heat extraction from the containment is related to the steam condensation on the cold surfaces, thermosiphon cooling loop and through the connection to the water tank. For these reasons the presence of a Containment Wall Condenser (CWC) as passive system integrated in the plant design, increase the safety margin in accidental scenarios. Within the framework of the EU2020 PASTELS (Passive Systems: Simulating the Thermal-hydraulics with ExperimentaL Studies) project, a series of test comprising the natural circulation phenomena in a low pressure open loop CWC has been performed. The PASI facility, developed by LUT University, has been used in order to identify the physical phenomena and investigate upon the mechanisms to assure the preservation of the safety function of the entire system and further, to support the system codes validation. As partner of the project, ENEA developed the RELAP5/mod3.3 input deck and performed the whole PASI test matrix, to verify the capability of the code to simulate complex loop component, e.g. the sparger, and related thermohydraulic phenomena such as the water flashing and geysering inside the open loop. In the present paper will be shown the post-test calculations performed with RELAP5 of test 1 (quasi-static test) and test 5-10 (reproducing the drain and refill phase of the water storage tank).