An investigation on flashing-induced natural circulation instabilities based on RELAP5 code

In this investigation, flow instability experiments carried out in a natural circulation loop at the low-pressure condition. Based on experiment data, the RELAP5 code was adopted in the analysis. The objective of this investigation is to get insight into the nature of flashing-induced natural circulation instability. Four typical oscillations observed, intermittent and periodical oscillation, double-peak oscillation, periodical sinusoidal oscillation, and staged irregular oscillation. Under low pressure and low heat flux condition, flashing plays a dominated role in natural circulation oscillation. It is suppressed in higher heat flux. The accumulation of bubbles and fluid inertia result in diverse nonlinear relationship between driving head and kinetic energy. The mechanism of flow instabilities is explored from different aspects. The simulation results of RELAP5 are presented and assessed against the experimental results. The deviation between the results for different circulation modes is discussed as well.