Investigation on flow instability in a natural circulation loop with rod bundles

Highlights: • Flow instability is studied by experiment and RELAP5 with driving force method. • Pressure drop oscillation in natural circulation with rod bundles is analyzed. • Flow exchanges between pressurizer and heat section during flow oscillation. • Pressurizer downstream position is more stable than its upstream position. • Long horizontal pipe has impact on the second and third flow oscillation peak. - Abstract: Flow instability may threaten the nuclear safety and make the nuclear power plant more difficult to control. The objective of this research is to analyze the nature of flow instability in Natural Circulation (NC) with the rod bundles by the method of experiment and RELAP5 simulation. The Pressure Drop Oscillation (PDO) occurs when the pressurizer is located in the downstream of the test section. During the PDO, boiling, bubble expansion, flashing and condensation coexist, and there is fluid exchange between the pressurizer and the NC loop. No reverse flow occurs with the restriction of the experimental conditions. The system with the pressurizer in the downstream of the test section is more stable than that in the upstream of the test section. Increasing the inlet subcooling makes the system more stable. Based on the experimental data, RELAP5 was adapted to the analyses and was used to expand the research scope. With the increase of the heat flux, the single peak will turn to double peak and triple peak flow instability. The flow instability boundary is obtained by two methods. RELAP5 code applicability in the NC with low pressure condition has been validated.