Fundamental Study on Thermo-Hydraulics during Start-Up in Natural Circulation Boiling Water Reactors, (II)

Natural Circulation Oscillation Induced by Hydrostatic Head Fluctuation

Abstract

The authors have been investigating the fundamentals of thermo-hydraulic instabilities which may appear during the start-up in natural circulation BWRs in order to understand their driving mechanisms and to examine the methods preventing their occurrence with an aim of establishing a rational start-up procedure and reactor configuration. In this paper, 'Natural Circulation Oscillation' was investigated experimentally under various conditions of heat input, inlet subcooling and non-heated riser length to reveal the driving mechanism and its feature. As a result, it was made clear that the in-phase natural circulation oscillation was induced in parallel channels by hydrostatic head fluctuation in a long vertical non-heated channel such as steam separators and a divided chimney in natural circulation BWRs due to alternate flow of vapor and liquid therein while the vapor generation rate is insufficient.