Experimental study on flashing concerned instability in a natural circulation system at nuclear heating reactor conditions / Experimentelle Studie zu Instabilitäten bei Siedeverzug in einem nuklearen Heizreaktor mit Naturumlauf

Abstract

The experiment was performed on the test loop (HRTL-5), which simulates the geometry and system design of the 5 MW nuclear heating reactor. The flow behavior for a wide range of inlet subcoolings, in which the flow experience from single to two phase, is described in a natural circulation system at low pressure (p = 0.1, 0.24 MPa). Several kinds of flow instability are investigated, including subcooled boiling instability, subcooled boiling induced flashing instability, pure flashing instability as well as flashing coupled density wave instability and high frequency flow oscillation. The mechanism of flashing and flashing with regard to flow instability, which has never been studied well in this field, is particularly interpreted. The experimental results show that, firstly, for a low system pressure natural circulation the two phase flow is unstable in most inlet subcooling conditions, the two phase stable flow can only be achieved with very low inlet subcoolings; secondly, at high inlet subcoolings the flow instability is dominated by subcooled boiling in the heated section, and at intermediate inlet subcoolings it is dominated by void flashing in the adiabatic long riser; thirdly, in two phase stable flow region the condition for boiling out of the core, namely, single phase flow in the heated section, two phase flow in the riser due to vapor flashing, can be realized. The experimental results are very important for the design and accident analysis of the vessel and swimming pool type natural circulation nuclear heating reactor.