Experiments on open loop passive cooling of nuclear reactor containment: Quasi-steady-state heat removal, dynamic loads, and feasibility of practical periodic testing

Post-accident heat removal from nuclear containments is an important safety function in nuclear reactors. Passive systems have been proposed to increase the reliability of the function, but owing to small driving forces, concerns exist on the performance and testability of such systems. The basic functioning of an open loop gravity-driven containment passive heat removal system was studied with the PASI test facility, a half-height wall condenser model at LUT University, Finland. The quasi-steady-state behavior of the natural circulation flow was observed in both single-phase and two-phase flow modes. A wide range of heating powers was tested, and robust heat transfer performance was observed throughout. Two-phase flow was found intermittent with flashing-induced sharp flow peaks separated by slow steady flow. Piping dynamic loads due to flow peaks were found to be small, although one weak indication of a possible flashing instability induced water hammer (FIIWH) was observed. In addition to accident operating conditions, moderate conditions that could be reproduced during an actual power plant outage were also tested, showing that periodic testing of these passive systems is practicable.