Qualification of RELAP5-3D Code Against the In-pool Passive Energy Removal System PERSEO Data

Passive safety systems are implemented or envisaged in several nuclear plant designs. However, it should be demonstrated that they provide a higher reliability with respect to active systems. Best Estimate thermal-hydraulic system codes are adopted in the methodologies developed to evaluate the reliability of passive systems. Therefore, the qualification of the codes against the phenomena typical of passive systems is a fundamental step to assess their reliability. In the framework of the OECD/NEA/CSNI/WGAMA, a "Status report on thermal-hydraulic passive systems design and safety assessment" is currently ongoing, and an open benchmark has been conducted. The benchmark was performed on Test 7 conducted in the PERSEO (in-Pool Energy Removal System for Emergency Operation) facility at SIET laboratory (Piacenza, Italy). Politecnico di Torino participated to the benchmark adopting the RELAP5-3D code developed by INL. The results show that the code is in general able to qualitatively predict the main phenomena occurred in the test. However, the heat transfer from the heat exchanger to the pool was significantly underestimated. This has been attributed to the underestimation of the condensation heat transfer coefficient within the heat exchanger tubes. Alternative correlations for the condensation heat transfer coefficient have been investigated, showing higher values with respect to RELAP5-3D. A correction factor for the heat transfer coefficient has been derived, which allowed to quantitatively better reproduce the experimental transient behavior.