A Study on Critical Water Level during Mid-Loop Operation

A significant phenomenon associated with the midloop operation is air or gas entrainment into the suction line of the shutdown cooling system (SCS) due to free surface vortex. To address this problem, NRC presents acceptance criteria based on Froude Number. Also Westinghouse did some experimental works and established a correlation between critical water level and vortex occurrence. This paper proposes acceptance criterion on the critical water level to vortex formation in the SCS suction line based on Vortex-Froude number correlation derived from Computational Fluid Dynamic (CFD) analyses. An analysis tool is STAR CCM+. Several correlations have been proposed for a critical water level where vortex could be expected to start and grow. Those are based on experimental and theoretical analysis results, which are basically represented using Froude number and H/d parameters. The used Froude number is based on process average values regardless of vortex formation. The proposed acceptance curve for critical water level based on Vortex-Froude number can be used as a preliminary guidance for air intrusion into the suction pipe. Also it is noted that the proposed curve shows a little more conservative limit for a critical water level to be on a safer operation during the mid-loop operation.