Experimental Measurement and Analysis of Natural Circulation Flow Oscillations in a Vertically Heated Channel

Abstract

Fuel utilization and passive safety are some of the important design aspects of advanced reactors. Thorium due to its abundance and importance in providing sustainable energy has been considered as future fuel for advanced reactors in India such as advanced heavy water reactor (AHWR) and compact high-temperature reactor. AHWR uses thorium-based fuel and has passive natural circulation of water in parallel channels in its primary heat transport and decay heat transport systems. The two-phase boiling heat transport system exhibits oscillations and flow instabilities in varying scales in operating and accident conditions and they influence the thermal and reactivity margins in design. Experimental studies on natural circulation at low pressures assume importance in understanding the heat transport mechanisms and system behavior in case of a loss of coolant accident or a pressure boundary failure. The present research work highlights the multimodal oscillations, measured in a boiling natural circulation loop and experimental studies on the physics behind the components contributing to the flow oscillations. The boiling parallel channels of the facility are vertical with central heater simulating a typical vertical coolant channel with fuel in a nuclear reactor. The bubbles and slugs in the flow channel have been observed to form and coalesce around the length of the heater and flashing has been observed at a higher elevation. The sequence of events has been nearly similar, but appeared faster at increased heating rates. The overlapping temporal and spatial events in the channel lead to new nonlinearities and multimodal chaotic oscillations. Advanced signal analysis methods have been applied to the signals to study the time-evolving characteristics of flow oscillations at different channel heating rates.