Countercurrent Flow in a PWR Hot Leg under Reflux Condensation

TLDR: In this article, the authors investigated the countercurrent flow in the hot leg of a PWR during a mid-loop operation and developed a flow model to improve the reliability of the transient analysis.

Abstract: Nuclear power plants periodically shut down for plant maintenance and refueling. During a pressurized water reactor (PWR) plant outage, decay heat is removed by residual heat removal (RHR) systems. The reactor coolant level temporarily needs to be kept around the middle of the primary loop to inspect the steam generator (SG) tubes and so on. This operation is called “mid-loop operation”. In some plants, a loss of RHR event occurred during the mid-loop operation (USNRC, 1987, USNRC, 1990). Probabilistic safety assessment (PSA) studies under plant shutdown conditions have been performed and they confirmed that the loss of RHR cooling during the mid-loop operation is a relatively high risk event for PWR plants. One of the effective methods to cool the reactor core in this event is reflux condensation, in which water condensed in a SG flows into the reactor core through a hot leg and cools down the reactor core as shown in Fig. 1. In the reflux condensation, steam generated in the core and water condensed in the SG form a countercurrent flow in the hot leg. This phenomenon limits water into the reactor vessel and affects the performance of core cooling. System computer codes, as typified by RELAP, employ a simplified model to a certain extent to calculate efficiency, however, the hot leg consists of a horizontal section, elbow, and inclined section. Hence it is unclear whether the countercurrent flow in the hot leg can be well predicted or not. To improve reliability of the transient analysis, we need to understand and model the countercurrent flow in the hot leg. A number of experiments have been made about the countercurrent flow in the hot leg (Richter et al., 1978, Ohnuki, 1986, Ohnuki et al., 1988, Mayinger et al., 1993, Wongwises, 1996, Navarro, 2005), observations of detailed flow patterns in the hot leg, however, have not been reported. On the other hand, numerical simulations are a realistic way to evaluate the flow, because it is difficult to check the flow pattern in the actual PWR hot leg. Few examples of numerical simulations have been reported, however. Our objectives in this study were to clarify the flow pattern and dominant factors of the countercurrent flow and to develop the flow model which improves the reliability of the transient analysis. At first, we carried out air-water experiments with a scale model of the actual PWR hot leg. We used two types of small scale PWR hot legs. One was a 1/5th scale