CFD simulation of boiling flows inside fuel rod bundle of a natural circulation BWR during SBO

Abstract

Abstract AHWR, designed in India, is a natural circulation based BWR and equipped with many passive features for safety. The reactor utilizes an isolation condenser system for decay heat removal passively in case of SBO. Earlier, it has been established by analysis and experimentally in the scaled test facility that decay heat can be removed for more than 7 days without any safety implications. However, there remains a possibility of hot spots occurring inside the fuel rod cluster of the reactor while removing heat in sub-cooled or single phase operating zones. In this work, CFD simulations are carried out to predict the void and temperature distribution inside the rod bundle to identify the growth of any hot spots during the removal of decay heat. The modeling framework is developed by validating with the results of high pressure flow boiling experiments cited in the literature. The boiling two phase flow is simulated using an Euler-Euler approach with the incorporation of the RPI wall heat flux partitioning model. Performance of various sub models for interfacial momentum and energy transfer has been studied for development of the boiling frame work. The manuscript includes intrinsic details of the CFD model utilized for the simulation of boiling two phase flows inside a complex geometry of rod bundles. The steady state profiles of the complete flow field inside the rod bundle is estimated at two instances of the transient calling as pseudo steady state instances. Predictions of the CFD are also compared with RELAP5 results. RELAP5 under predicts the void fraction at any axial location. Potential sites for hot spots have been observed by the CFD predictions. CFD is able to capture the boiling phenomena occurring inside the complex geometry of rod bundle. The detailed flow field map including the contours of void fraction and temperature at various axial and radial planes shows potential locations for occurrence of hot spot inside the bundle during decay heat removal.