EXPERIMENTAL-STUDY ON FLASHING CONCERNED INSTABILITY IN A NATURAL CIRCULATION SYSTEM AT NUCLEAR HEATING REACTOR CONDITIONS

The experiment was performed on the test loop (HRTL-5). which simulate s the geometry and system design of the 5 MW nuclear heating reactor T he flow behavior Sar a wide range of inlet subcoolings, in which the f low experience from single to two phase is described in a natural circ ulation system at low pressure (p = 0.1, 0.24 MPa). Several kinds of f low instability are investigated, including subcooled boiling instabil ity, subcooled boiling induced flashing instability, pure flashing ins tability as well as flashing coupled density wave instability and high frequency flow oscillation. The mechanism of flashing and flashing wi th regard to flow instability which has never been studied well in thi s field, is particularly interpreted The experimental results show tha t, firstly, for a low system pressure natural circulation the two phas e flow is unstable in mast inlet subcooling conditions, the two phase stable flow can only be achieved with very low inlet subcoolings; seco ndly, at high inlet subcoolings the flow instability is dominated by s ubcooled boiling in the heated section, and at intermediate inlet subc oolings it is dominated by void flashing in rite adiabatic long riser; thirdly, in two phase stable flow region the condition for boiling ou t of the core, namely, single phase flow in the heated section, two ph ase flow in the riser due to vapor flashing can be realized The experi mental results are very important for the design and accident analysis of the vessel and swimming pool type natural circulation nuclear heat ing reactor.