Prediction of Flow Rate in a Passive Residual Heat Removal System with Various Water Levels

A passive residual heat removal system (PRHRS) is one of passive safety systems that have been adopted in SMART. In the case of an emergency such as an unavailability of the secondary side feedwater supply or a station blackout, the PRHRS passively removes the core decay heat and sensible heat through a two-phase natural circulation, and thus maintains the reactor in a stable condition without any AC power or operator actions. The PRHRS consists of an emergency cool-down tank (ECT), a condensing heat exchanger (HX), a makeup tank (MT), valves, pipes, and monitoring instruments. Its conceptual diagram is given in Fig. 1. If the passive residual heat removal actuation signal is generated, the PRHRS starts running. Subcooled water in the HX flows into the secondary side of the SG due to the difference in the water level. The feedwater is evaporated by residual heat, and exits the SG cassette nozzle header at a two-phase flow or superheated steam condition. Then, as it flows into the HX submerged in the ECT, the steam is condensed into subcooled water by emitting the residual heat into the cool-down water. Thus, continuous coolant circulation occurs in the PRHRS. Such a natural circulation becomes weakened, however, as the water level and density differences between the HX and the secondary side of the SG dwindle due to the decrease of residual heat. In this study, therefore, the effects of water level in the PRHRS on the flow rate are theoretically examined. To obtain the flow rate variation, the natural circulation in PRHRS is modeled with basic hydraulic theory. The effect of the water level of the SG, HX and MT on the natural circulation in the PRHRS has been investigated. The HX flow rate also increases with the decrease in the SG water level. It is noted that a natural circulation in PRHRS mainly occurs through the flow path of the HX because the flow path configuration through the MT gives an inherently high hydraulic resistance. Thus, the total flow rate has a similar value as the HX flow rate. The highest HX water level yields 0.6%, 13.4% and 25.0% augmented total flow rates compared to the design flow rate when the SG water level is high, middle, and low, respectively. However, the low water level in the MT provides an extremely low total flow rate owing to the decline in the water level difference