Siphon Breaker Design on Research Reactor with Real-Scale Experiment

Abstract

Siphons are effective ways to move fluids without any power source except gravity. However, a siphon causes continuous flow of liquid, so it sometimes must be blocked. Pipe rupture in a research reactor could cause a loss-of-coolant accident (LOCA) due to the siphon effect. Therefore, siphon breakers are required, especially as a passively-operated device for safety in nuclear reactors that have lost power. However, the siphon breaking phenomenon is water-air two-phase, transient, complex flow and there are no available analytic researches. In this study, experiments on siphon breaker were conducted in a real-scale facility. The size and type of siphon breaker, size and position of pipe rupture and consideration of existence of reactor core pressure drop were investigated experimentally, the siphon breaking phenomenon was observed and analyzed. The main focus was to predict the amount of coolant loss during siphon breaking. The siphon breaking phenomenon is a combination of water and air flow, so a correlation to predict final water level was developed using the momentum ratio of water and air. Finally, the correlation was refined using the design parameters of the piping system and it can be used during the design stage of nuclear reactors. During siphon breaking, the air swept is out with water flow in some cases. When air sweep-out occurs, it causes larger error and less safety than non-sweep cases. Therefore, the air sweep-out conditions were separated from non-sweep-out conditions using vertical downward co-current two-phase flow regime map. A criterion between dispersed flow and separated flow was suggested, then improved.