Flow oscillation fade-out and pool water level effect experiments on open loop passive cooling system

• Stable operation conditions for open loop heat removal system identified. • Flow oscillations disappear at saturation conditions if riser flooding is prevented. • If riser flooding is allowed, oscillations fade out when steam power is high enough for CCFL in riser. • Lowering pool water level reduces oscillation amplitude, no effect on heat transfer. • Heat transfer effective as long as sufficient water inventory in the loop remains. The stable operating conditions for an open loop passive containment heat removal system were identified through testing conducted with the PASI test facility, a half-height wall condenser model at LUT University, Finland. Previous tests have shown that open loop systems tend to operate in a quasi-steady oscillatory mode characterized by geysering and flashing. The cessation of flow oscillations depends on the sparger structure. When flooding of the riser pipeline is prevented, the oscillation fade-out and steady two-phase natural circulation is reached quickly after the system reaches saturation conditions. Conversely, if flooding is allowed, the oscillations disappear only at heating power large enough to meet the countercurrent flow limitation (CCFL) criterion in the riser. The impact of gravity head on the system behavior was also examined. The amplitude of two-phase flow oscillations decreased along the lowering of the pool water level. When the water level decreased below the pressure balancing hole, the flow behavior changed since the riser flooding ended. Additionally, the riser boil-out was tested. The results show that the open-loop natural circulation system can effectively remove heat as long as there is water inventory inside the loop, even if the pool is empty of water. The containment pressure rises only when boiling initiates in the heat exchanger.