Experimental study of multi-mechanism flow instabilities in low-pressure natural circulation systems for pool type heating reactor

Abstract

In the two-phase thermal system, flow drift or self-sustaining oscillation is easy to occur, resulting in mechanical vibration or thermal stress damage, disturbing system control and even inducing boiling crisis in advance. The deep pool heating reactor is a complex low-pressure natural circulation system under the condition of loss of flow accident. Therefore, it is necessary to study the flow instability under such condition. In this paper, a series of transient performance data including inlet flow rate are obtained by building a simulation test bench. It is found that the oscillation characteristics of inlet flow change obviously with the small heating power increase (5 kW) when it is more than 120 kW. By distinguishing the causes of different flow oscillations, we found that there are many different flow instability mechanisms in this system, including pressure drop oscillations, geysering, flashing induced instability and density-wave oscillations. Among them, the longest pressure drop oscillations cycle reaches 449 s, and the relative amplitude reaches 33.9 %. The coupled density-wave oscillations phenomenon triggered by the lowest point of pressure drop oscillations causes the flow stagnation. Through the windowed fast Fourier transform and empirical mode decomposition method, we quantitatively and qualitatively analyze the effects of the interaction between different types of flow instability. we found that it is precisely because of such effects of the various parts above-mentioned that the strong nonlinear characteristics are caused. The experimental data show that the maximum heating power of the reactor during the accident should not exceed 120 kW (corresponding to 60.74 % of the design operating power). This study believes that adjusting the internal characteristic curve of the system will significantly improve the stability of such system.