WATER DISTRIBUTION IN A NORDIC BWR CONTAINMENT DURING A LOCA

摘要

During a main steam line break in a Boiling Water Reactor (B WR) the pressure suppression pool is used as a water source for the Emergency Core Cooling System (ECCS) and the Containment Spray (CS). These systems drain water from the pool through strainers, which are long perforated plates or cylinders submerged to a certain depth. Proper functioning of the ECCS and the CS must be ensured to maintain the water inventory in the vessel and to limit the containment pressure. However, if the liquid level in the suppression pool goes below the level of the strainers intake, the operators would be forced to stop their pumps. The liquid level in the suppression pool can be reduced when a significant fraction of ECCS and CS flow is relocated to the lower drywell. In this work, we use the thermal-hydraulic code GOTHIC to simulate the containment evolution during a main steam line break inside the biological shield. The containment volumes and their connections were modeled with 2D and 3D volumes. With this model, scenarios considering different operational conditions were assessed: (ⅰ) full capacity of all the safety systems, (ⅱ) half capacity of all the safety systems, (ⅲ) ECCS stops injecting water after a certain liquid level is restored in the vessel, and (ⅳ) the pipes used to drain water from the suppression pool and flood the lower drywell are partially or totally clogged in different directions. The results showed that there is a risk of an early shut down of the ECCS and CS systems in the case of main steam line break inside the biological shield. It was observed that when the ECCS provided a continuous water injection into the vessel, the water spilled through the break into the biological shield flowed downwards driven by gravity and went directly into the lower drywell. This caused a fast decrease in the liquid level of the suppression pool, which led to an uncovery of the ECCS and CS strainers about 2000 s after the break. The activation at 1800 s of the flooding of the lower drywell led to a backward flow, from the lower drywell to the suppression pool, since at that time the liquid level in the suppression pool was lower than in the lower drywell. However, this backward flow was not enough to maintain the liquid level in the suppression pool, which continued to decrease. In the case where the pipes used for the flooding were clogged in the direction of the suppression pool, uncovery of the strainers was observed even earlier.