Experimental investigation on a passive injection and depressurization system for next generation of nuclear power plant

The new generation of Nuclear Power Plant (e.g. WESTINGHOUSE AP600, GE Simplified Boiling Water Reactor (SBWR), ABB Process Inherent Ultimate Safety (PIUS), etc.) have been designed in order to satisfy today's needs. They are simple, economic: midsize and fully standardized. Those plants enhance safety by extensively relying on passive safety systems that use natural forces and phenomena such as gravity, condensation and evaporation. Evolutionary plants rely on a full Reactor Coolant System (RCS) depressurization to allow gravity injection from an In Containment Tank and assure with its operation the long term core cooling. Depressurization is obtained by the opening of Motor Operated Valves or Squib valves actuated on a logic signal. The concept of depressurizing the primary system following a Small Break Loss of Coolant Accident (LOCA) signal was even developed in The past for the application to the Westinghouse Pressurizer Water Reactor (PWR) Italian Reference Plant. Studies performed to support the licensing process and design of both evolutionary and innovative reactors, have shown that cold water injection may, under particular plant conditions. induce a large plant depressurization. The conceptual design of a Passive Injection and Depressurization System (PIDS), based on the idea of depressurizing die Reactor Coolant System (RCS) by mixing cold water with die RCS hot water and inducing steam condensation in the primary system, has been pursued. While, die PIDS concept is applicable both to boiling and pressurized water reactor design, the PIDS so far developed has been conceived for the application to a PWR design. Due to die PIDS innovative concept: before proceeding to the actual system design, an experimental campaign has been undertaken. The aim was to explore the physical phenomena occurring during system actuation and operation and to demonstrate die concept viability. The first experimental step, devoted to asses siphon triggering phenomenologies and the components performances of the PIDS, has been performed at SIET laboratory in Piacenza, Italy. The test objectives were to: identify the limit conditions which would lead to an adverse mass distribution, then delaying the PIDS intervention (i.e. TMI Scenarios-Pressurizer filled by a Water/Steam mixture), confirm that the system will be actuated as soon as the reactor coolant system water level reaches the actuation level; confirm that spurious actuations are not foreseen; confirm that the injection flow is stable during PIDS operation: characterize the PIDS components performances: assess the PIDS performance in presence of non condensable gasses. The test facility design and scaling and the experimental activities have been supported by extensive RELAP5 Mod3 simulations, A visual tool (Nuclear Plant Analyzer, NPA) has been also used for the analysis of die results. The present paper. after a brief description of the PIDS concept, reports some of die more significant pretest analyses and experimental test results.