FUNDAMENTAL-STUDY ON THERMOHYDRAULICS DURING START-UP IN NATURAL CIRCULATION BOILING WATER-REACTORS .3. EFFECTS OF SYSTEM PRESSURE ON GEYSERING AND NATURAL CIRCULATION OSCILLATION

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, based on our proposed driving mechanisms of geysering and natural circulation oscillation, the effects of system pressure on their occurrences and features are investigated experimentally. It is made clear that an increase in the system pressure suppresses their occurrences. From the results, a consideration is drawn to understand the differences in the start-ups between thermal natural circulation boilers using fossil fuel and the Dodewaard reactor. Lastly, a rational start-up procedure to prevent their instabilities from occurring is recommended and a new idea of separator is proposed.