Flow instability analysis of natural circulation under ERVC condition for SMR

Abstract

In this work, the internal natural circulation flow of ERVC system in SMR is calculated by the thermal-hydraulic code RELAP5. Both the transient characteristics and the flow instability are analyzed in detail to reveal the flow mechanism. Firstly, to verify the applicability of RELAP5 models, multiple cases of steady-state natural circulation with low heating power are simulated by comparing with the REPEC experiment. Next, a sliced model of ERVC loop is built to obtain the transient phenomena of natural circulation. As the circulation loop has no cold source, the inlet temperature of the heating section will keep rising to transform the flow patterns into stable-oscillating-stable in turns. During this transition, the flow is dominated by the subcooled boiling of heating section, and the mass flux is about 60–160 kg/m2·s. Based on the features of flow oscillation, the unstable flow patterns can be further divided into low-frequency and high-frequency oscillations. Both of them belong to the density wave oscillation, and their trajectory diagrams can present the attributes of the limit cycle. Finally, in the plane formed with the inlet subcooling (horizontal axis) and heating power (vertical axis), the boundaries of flow instability are divided. The instability domain change with the effects of heating power, back pressure, inlet resistance coefficient, and rising section height is discussed. The results indicate that increasing the back pressure will compress the instability domain. Reducing the inlet resistance coefficient will shift the instability boundaries towards the direction of power increase. When lengthening the height of the rising section, the flow can be gradually dominated by the flashing with the decrease of inlet subcooling, which may also introduce another instability.