Analysis of Flow Excursion in a Natural Circulation System with Low Pressure and Low Steam Quality

A series of experments were perforned on the test loop (HRTL 5),which simulates the geometry and system design of the 5 MW nuclear heating reactor.The flow excursion accompanied with flow oscillation was observed under certain conditions.Some experimental results have been summarized in the natural circulation with low pressure and low steam quality.The mechanism of sub cooled boiling and flashing is analyzed,and an entire process of flow excursion is described too.The analysis shows:1)Sub cooled boiling and flashing play an important role on the flow instabilities in a natural circulation system.2)The static flow excursion in the natural circulation is a rather long thermo hydraulic process,which can be accompanied by the dynamic flow oscillation.3)In the course of the flow excursion's beginning,developing and gradually converting to the flow oscillation ,the flow instability is first dominated by sub cooled boiling ,then is dominated by void flashing,last is mainly the density wave oscillation.The investigation on flow excursion in the natural circulation is important for the design and accident analysis and upgrading development of the 5MW nuclear heating reactor.