Steady-state characteristics of the open natural circulation for a new cooling water tank in secondary passive residual heat removal system

Abstract

The concept of open natural circulation has garnered considerable interest in the design of advanced nuclear reactors owing to its inherent safety features. The present study proposes an innovative configuration for the cooling water tank of a secondary passive residual heat removal system, which introduces an open natural circulation into the system. An experimental setup was meticulously built, and a comprehensive suite of experiments was carried out to research the steady-state characteristics of the natural circulation. The results reveal a stable natural circulation, with a consistent flashing flow observed across all experimental scenarios. This phenomenon is elucidated as the interplay between flashing and boiling processes. A reduction in the heating power of the residual heat removal heat exchanger and the liquid level of the recharge water tank leads to a decrease in mass flow rate of the experimental loop. Analogous trends are noted in the temperature and pressure distributions of the riser, coinciding with intensified flashing events. Furthermore, natural circulation and convection developed in the heat exchange booth markedly enhance fluid mixing, minimizing thermal stratification. Analysis of the condensation length in the heat transfer tubes indicates that the condensation section expands predominantly due to the reduced heating power. The shift fosters a more balanced distribution of wall heat flux and facilitates the downward propagation of high-temperature fluid domains in the heat exchange booth. These findings offer valuable insights into the optimal design of residual heat removal systems and their potential application in improving plant layouts of nuclear power plants.