Assessing RELAP5 performances for the simulation of Containment Wall Condenser

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Abstract The new designs of Nuclear Power Plants are increasingly emphasizing the implementation of passive systems to fulfill several safety functions. Such systems operate with low driving forces, and often employ components that require a “change state” signal for actuation. These features may lead to complete or partial actuation failures, altering the driving force conditions and consequently reducing the operational ranges and reliability of the systems themselves. However, even though the operational modes differ from those of active systems, the safety standards remain equivalent. To study these possible situations, the computer system codes must be able to simulate the phenomena related to the specific system, and they should be assessed using data both from separate and integral experimental facilities. Within the framework of the EU2020 PASTELS (Passive Systems: Simulating the Thermal hydraulics with Experimental Studies) project, a series of natural circulation experiments have been conducted in the PASI facility, an open loop Containment Wall Condenser (CWC), varying initial and boundary conditions to support the validation of system codes. As project partner, ENEA developed the PASI input deck for RELAP5/mod3.3 to assess the code’s capability in simulating complex loop components, such as the sparger, and related thermohydraulic phenomena like water flashing and geysering within the open loop. This paper presents the post-test calculations conducted with RELAP5 for test 2 (quasi-static test) and test 6 (replicating the drain phase of the water storage tank).