STEAM CONDENSATION AND LIQUID HOLDUP IN STEAM GENERATOR U-TUBES DURING OSCILLATORY NATURAL CIRCULATION

Abstract

Flow oscillations during two-phase natural-circulation experiments for pressurized water reactors (PWRs) with inverted U-tube steam generators occur at high pressure and at a primary inventory range between two-phase circulation and reflux heat removal. These oscillations, originating in the steam generator U-tubes, consist of alternating phases of (1) interrupted (or stalled) flow and (2) rapid two-phase circulation. Using integral system data from LOBI-MOD2 experiments, it is demonstrated that the predominant mechanism that governs the duration of the interrupted flow phase is steam condensation at the top of the U-tubes. The frequency is not governed by mechanisms related to limitation of countercur-rent flow (CCFL). The article also discusses the possible mechanisms of liquid holdup in the vertical U-tube region, necessary to have a closed condensation space. This may be CCFL but could also be simply related to the water inventory in the circuit. Furthermore, the influence of pressure is discussed, showing that the phenomenon can truly be observed only in a high-pressure facility.

Additional information

Notes on contributors

G. F. De Santi

Address correspondence to Dr. G. F. De Santi, Commission of the European Communities, Joint Research Centre-Ispra Site, Institute of Safety Technology, 1-21020 Ispra (VA), Italy.