Thermal-hydraulic experiments and code validation (TOKE): Condensation pool experiments with non-condensable gas and Fluent 5 simulations

The formation, size and distribution of non-condensable gas bubbles in the condensation pool of the Olkiluoto nuclear power plant (NPP) in a conceivable loss-of-coolant accident (LOCA) was studied experimentally with a scaled down condensation pool test rig. Particularly, it was important to find out if any air bubbles flowed inside the emergency core cooling system (ECCS) strainer close to the pool wall and bottom. The effect of non-condensable gas on the performance of an ECCS pump was also examined. Computational fluid dynamics (CFD) calculations with the Fluent 5 code were made to support the design of the test rig and the planning of the experiments. Compressed air was blown to the test pool through blowdown pipes or, alternatively, air was injected directly into the intake pipe of the ECCS pump. The first large air bubbles forming at the blowdown pipe outlet touched the ECCS strainer. When two blowdown pipes were used simultaneously, a lot of air bubbles were detected inside the strainer during the first 30 seconds. A 3-7 % volume fraction of air injected directly into the pump intake pipe was enough to make the pump head and flow collapse.