Experimental investigation of stratified two-phase flows in the hot leg of a PWR for CFD validation

摘要

In the event of hypothetical accident scenarios in a pressurised water reactor (PWR), emergency strategies have to be mapped out, in order to guarantee the reliable removal of the decay heat from the reactor core, also in case of component breakdown. Therefore, the primary circuit is designed to favour a natural circulation which allows releasing the heat to the secondary circuit over the steam generators (SG). One of the most important passive heat removal mechanisms is the reflux cooling mode, which can be operated as long as the secondary side of the steam generators is filled with water. In the reflux condenser mode [19], the water level in the reactor pressure vessel (RPV) is already reduced to the level of the hot leg nozzle or even below, consequently, only steam flows to the steam generator. The steam arriving from the RPV condenses in the vertical U-tubes of the steam generator and, in each half of the steam generator, the condensate flows down the tube in which it has been formed. Therefore, about one half of the condensate flows as usual over the pump to the downcomer, whereas the other part flows over the hot leg back to the upper plenum. In the hot leg, the condensate has to flow in counter-current to the steam. However, the stratified counter-current flow of condensate and steam is only stable for a certain range of flow rates [1]. In fact, if the steam flow increases too much, the condensate is clogged in the hot leg. This is the onset of the counter-current flow limitation (CCFL): the liquid is carried over by the steam and partially entrained in opposite direction to the steam generator. As a consequence, the hot leg and steam generator are flooded, which decreases the water level in the RPV and reduces core cooling. In case of an additional increase of the steam flow, the condensate is completely blocked and the reflux cooling mode ends. In this situation, cooling of the reactor core from the hot leg is impossible, but may be continued by coolant drained through the cold leg to the downcomer.