Validation of RELAP5-3D thermal-hydraulic code against full-scale PERSEO Test 9

Passive systems are being implemented in several advanced reactors already constructed, under construction or under design. The purpose of these systems is to increase the inherent safety of the plant in transient conditions, so they can provide a higher safety level in case of an accident. Best estimate thermal-hydraulic system codes, widely adopted in the nuclear field to develop deterministic safety analyses, need to be validated against the phenomena typical of passive systems (e.g. natural circulation, thermal-hydraulic behavior of large pools, low pressure phenomena, etc.). An experimental Separate Effect Test Facility called PERSEO (in-Pool Energy Re-moval System for Emergency Operation) was built at SIET laboratory in Piacenza (Italy) modifying the existing PANTHERS IC-PCC facility. It is a full-scale test facility aimed at studying a new passive heat removal system operating in natural circulation and it can be used to analyze the general capability of system codes to simulate the related phenomena. Test 9 is an integral experiment at around 4.1 MPa performed in the PERSEO facility to study the activation and the heat removal capability of the proposed system. In the present study, Test 9 has been simulated with the RELAP5-3D code to assess its capability to predict the dominant phenomena typical of the transient (e.g. natural circulation, tube-side condensation, pool-side boiling, behavior of large pool of liquid, etc.).