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Flow oscillations during two-phase natural circulation experiments for pressurised water reactors (PWR) with inverted U-tube steam generators occur at high pressure and at a primary inventory range between two-phase circulation and re-flux heat removal. These oscillations, originating in the steam generator U-tubes, consist of alternating phases of interrupted (or stalled) flow and rapid two-phase circulation. Using integral system data from LOBI-MOD2 experiments it is demonstrated that the predominant mechanism which 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 counter-current flow (CCFL). The paper also discusses the possible mechanisms of liquid hold-up in the vertical U-tubes 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, a discussion of the influence of pressure shows that the phenomenon can only be truly observed in a high pressure facility.

Additional information

Authors: DE SANTI G F, JRC Ispra (IT);MAYINGER F, Institute of Thermodynamics, Technical University of Munich (DE)
Bibliographic Reference: Paper presented: ANS Winter Meeting, Washington (US), Nov. 11-16, 1990
Availability: Available from (1) as Paper EN 35657 ORA
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