Numerical Calculations on Countercurrent Air-Water Flow in Small-Scale Models of a PWR Hot Leg Using a VOF Model

In the case of loss of the residual heat removal (RHR) systems under mid-loop operation during shutdown of a PWR plant, reflux cooling by a steam generator (SG) is expected, and the generated steam in a reactor core and the condensed water in the SG form a countercurrent flow in a hot leg, which consists of a horizontal pipe, an elbow and an inclined pipe. In order to improve a countercurrent flow model of a transient analysis code, countercurrent air-water tests were conducted using the 1/15^th scale model of the PWR hot leg at Kobe University and the authors conducted numerical calculations of the 1/15^th scale tests using the thermal-hydraulic analysis code FLUENT6.3.26 and an Euler-Euler model or a VOF model. In the tests and calculations, however, the expansion of the inclined pipe in the PWR hot leg was not simulated. In this study, using the VOF model, the authors conducted numerical calculations for a 1/15^th scale model of the PWR hot leg with the expansion of the inclined pipe, which mitigates CCFL (countercurrent flow limitation) there. The calculated flow patterns in the hot leg using the VOF model were quite different with the data for the 1/15^th tests without the expansion of the inclined pipe due to underestimation of CCFL characteristics at the upper end of the inclined pipe, but became similar with the observed results for the 1/15^th scale model with the expansion of the inclined pipe due to the mitigation of CCFL at the inclined pipe. The results indicate that the VOF model could not correctly calculate air-water two-phase flows at the upper part of the inclined pipe but could calculate two-phase flows in the horizontal pipe.