Numerical analysis on restoring stable status in natural circulation loop

Abstract

Natural circulation flow instability is a common phenomenon in nuclear reactor systems, especially in components such as passive safety systems, reactor vessel downcomers, and steam generators. In general, this kind of instability is undesirable as it can jeopardize nuclear system safety, leading to fatigue damage, problems of system control, and heat transfer deterioration. It is very crucial to evaluate impact factors of restoring the stable status of natural circulation since reducing the duration of instability or restoring the stable status at the early stage of instability will prevent reactor systems from potential failures and risks. Despite the significance of system stability, the majority of the literature has focused on different impact factors of instability onset, while few has discussed the restoration conditions. This manuscript investigates conditions to restore stable status of natural circulation, including various axial power factor distributions, multiple parallel channel types, and inlet subcooling. The working condition of the studied natural circulation loop is under 10 MPa. Insights and suggestions are provided in this manuscript on enhancing the safety and reliability of nuclear reactors by optimizing operating conditions and designs for two-phase natural circulation systems. Numerical analysis employs the RELAP5 system code to model natural circulation loop with two types of channels, one with a single channel and the other with parallel channels, based on a well-validated natural circulation test facility. Criteria of instability onset and restoring stable status are defined by the amplitude and period of mass flow rate. Key responses, including mass flow rate, time of restoring stable status, duration of instability, and flow regime are examined. Results indicate that the uniform power distribution in parallel channels with high inlet subcooling will postpone the instability onset and shorten the duration of instability, with which condition will effectively help loop to restore stable status.