Countercurrent Air-Water Flow in a Scale-Down Model of a Pressurizer Surge Line

Steam generated in a reactor core and water condensed in a pressurizer form a countercurrent flow in a surge line between a hot leg and the pressurizer during reflux cooling. Characteristics of countercurrent flow limitation (CCFL) in a 1/10-scale model of the surge line were measured using air and water at atmospheric pressure and room temperature. The experimental results show that CCFL takes place at three different locations, that is, at the upper junction, in the surge line, and at the lower junction, and its characteristics are governed by the most dominating flow limitation among the three. Effects of inclination angle and elbows of the surge line on CCFL characteristics were also investigated experimentally. The effects of inclination angle on CCFL depend on the flow direction, that is, the effect is large for the nearly horizontal flow and small for the vertical flow at the upper junction. The presence of elbows increases the flow limitation in the surge line, whereas the flow limitations at the upper and lower junctions do not depend on the presence of elbows. 1. Introduction The mid-loop operation is to be conducted during plant refueling and maintenance of a PWR (Pressurized Water Reactor). In this operation, the reactor coolant level is kept around the primary loop center, and decay heat is removed by RHR (Residual Heat Removal) systems. If the loss of cooling systems such as RHR and/or other cooling systems takes place, cooling water in the reactor core may be heated up to boil and the top of the fuel assembly can be exposed to the air. In such an event, reflux cooling by the steam generators (SG) is regarded as one of the possible and effective core cooling methods. The reflux cooling is a way of core cooling by making use of water condensed in SGs. The steam generated in the reactor core and water condensed in the SG form a countercurrent flow in the hot leg. The authors therefore measured CCFL (Countercurrent Flow Limitation) characteristics in a scale-down model of a hot leg using air and water [1] and reported that CCFL can be accurately evaluated based on a one dimensional momentum balance for air-water two-phase flow [2]. In addition to this CCFL, the steam generated in the reactor core and water condensed in the pressurizer due to heat transfer to the vessel wall may also form a countercurrent flow in a surge line which connects the hot leg and the pressurizer. The ROSA-IV/LSTF (Rig-of-Safety-Assessment No. 4/Large Scale Test Facility) experiment [3], which simulated the loss of RHR systems during mid-loop operation, reported that