Mechanism and evaluation of flashing-induced density wave oscillations in natural circulation BWR

Abstract

Experiments were conducted to investigete two-phase flow stability of a natural circulation BWR due to flashing at low pressure. The facility used in the experiment was designed to have non-dimensional values which are nearly equal to those of typical natural circulation BWR. The observed instability is suggested to be the flashing induced density wave oscillations, since the oscillation period was nearly one and a half to two times the passing time in the chimney section, and correlated well with a single line regardless of system pressure, heat flux, and inlet subcooling. Stability maps were obtained in reference to the core inlet subcooling and the heat flux at the system pressures of 0.1, 0.2, 0.35, and 0.5 MPa. The flow became stable below a certain heat flux regardless of the channel inlet subcooling. The stable region enlarged with increasing system pressure. According to the stability map, the stability margin becomes larger in a startup process of a reactor by pressurizing the reactor sufficiently before withdrawing control rods.