Numerical study on flow instability development of natural circulation in narrow channel under medium-to-high pressure

Natural circulation flow instability is an important phenomenon for nuclear system safety, especially in narrow channels which have a broad application prospect. However, the small gap of narrow channels can easily lead to the narrow-space effect, resulting in mechanisms and performance of instability different from those in conventional channels. Medium-to-high pressure is a common condition in nuclear systems. Therefore, it is necessary to study flow instability in narrow channels under medium-to-high pressure. The ultimate goal is to effectively avoid the instability and improve the safety of nuclear systems. This manuscript investigates the characteristics of natural circulation instability in a narrow channel with deionized water under medium-to-high pressure (7.0 MPa-15.0 MPa). Results provide new insights into the safety and reliability of nuclear reactor cooling systems, supporting the establishment of more accurate and reliable natural circulation systems. A numerical study establishes a model of natural circulation loop including a narrow channel via system code RELAP5 based on a well-validated natural circulation test facility. The instability development is divided into three stages, natural circulation stable flow, instability flow (no reverse flow), and instability flow (reverse flow) or periodic dryout instability, according to the amplitudes and periods of mass flow rate. The proposed RELAP5 model demonstrates the relationship between mass flow rate and pressure drop on a time scale of a few seconds in every stage. Mass flow rate, exit quality, two-phase length, and flow regime are examined throughout all stages. Results indicate that increasing system pressure and inlet subcooling have a significant inhibitory effect on instability oscillation.