Return momentum effect on water level distribution during mid-loop operations

Abstract

An accurate prediction of the Reactor Coolant System (RCS) water level is of importance in the determination of the allowable operating range to ensure safety during mid-loop operations. However, complex hydraulic phenomena induced by the Shutdown Cooling System (SCS) return momentum causes different water levels from those in the loop where the water level indicators are located. This was apparently observed at the pre-core cold hydro test of the Yonggwang Nuclear Unit 3 (YGN 3) in South Korea. In this study, in order to analytically understand the effect of the SCS return momentum on the RCS water level distribution, a model using a one-dimensional momentum and energy conservation for cylindrical channel, hydraulic jump, and total water volume conservation has been developed. The model predicts the RCS water levels at various RCS locations during the mid-loop conditions and the calculation results were compared with the test data. The analysis shows that the hydraulic jump in the operating cold legs, in conjunction with the pressure drop throughout the RCS, is the main cause creating the water level differences at various RCS locations. The prediction results provide a good explanation for the test data and show the significant effect of the SCS return momentum on the RCS water levels.