Computational study of instabilities in a rectangular natural circulation loop using 3D CFD simulation

•3D transient CFD simulations of single-phase NCL are carried out.•Steady state and transient characteristics are computed and compared with experimental data.•Instability in a NCL is explained by using temperature fields of flow initiation transients.•Power raising and power step back transients are carried out.

Steady state and transient characteristics of a natural circulation loop working with water are obtained. For this purpose, 3D steady state and transient CFD simulations are performed. The CFD model includes pipe thickness as well as secondary side coolant passage apart from primary side. Steady state and transient characteristics are computed for various configurations i.e. Vertical Heater Vertical Cooler (VHVC), Horizontal Heater Horizontal Cooler (HHHC), etc. Steady state data was compared with available correlations. Flow initiation transients were compared with experimental data. Both the steady state and transient results are found to be in good agreement with previously published data. The reason for formation of unidirectional and bi-directional pulsing in HHHC configuration at different powers is explained with the help of temperature fields at different instants of time. Effect of sudden power rise/power step back on instability in HHHC configuration is estimated using CFD simulations.