Geysering in parallel boiling channels

Abstract The authors have been investigating the fundamentals of thermo-hydraulic instabilities which may occur during the start-up in natural circulation BWRs in order to establish a rational start-up procedure and reactor configuration. In this paper, geysering is investigated experimentally in twin parallel channels in both natural and forced circulations under various conditions of heat input, inlet subcooling and non-heated riser length from the upper end of the heated section to the outlet plenum. The relationship between flow reversal and the onset of geysering is studied in particular. It makes clear that geysering in parallel boiling channels in natural circulation is fundamentally identical to that in forced circulation. Next, the effects of a large bubble covering the entire flow cross section and subcooling in the outlet plenum upon the occurrence of geysering are clarified. Finally, differences in behavior of geysering in a single channel as compared with that in parallel channels are discussed.