Heavy liquid metals (HLMs) such as pure lead and its alloys are being studied worldwide as potential candidate coolants for future advanced nuclear systems due to their good neutronics and thermal-physical properties. Owing to the large thermal expansion coefficient, HLM-cooled nuclear systems have a significant natural circulation capability which is desired in reactor designs for improving the passive safety of reactor systems. Some advanced HLM-cooled reactor designs even apply natural circulation for primary cooling as normal operation mode. Moreover, natural circulation could be greatly enhanced by injecting an inert gas (e.g. Argon gas) into the hot leg of the coolant circuit. Although HLM-cooled reactors based on natural circulation or gas-lift mechanism have attractive advantages compared with traditional reactors utilizing mechanical pumps, some great challenges and uncertainties remain due to the lack of operation experiences. However, very few experiments have been carried out so far for HLM natural circulation or gas-lift studies. More efforts need to be carried out, especially on flow regime characterization, heat transfer coefficient and flow stability in natural circulation. For the purpose to study the basic mechanisms of HLM natural circulation as well as gas-lift pump, design activities of a lead-bismuth natural circulation loop were carried out in FDS team. In the present study, preliminary design and analysis of the loop were carried out. According to the present design, the maximum natural circulation flow rate in the loop was foreseen to reach 1.38kg/s with a limiting input power 30kW. The analysis results of natural circulation, gas-lift driven circulation as well as gas-injection enhanced circulation in the loop showed a similar trend to meet the power law with exponents differing from the buoyant mechanisms.

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