Study on Natural Convection Capability of Liquid Gallium for Advanced Passive Decay Heat Removal System

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Study on Natural Convection Capability of Liquid Gallium for Advanced Passive Decay Heat Removal System
Kang, Sarah
Bang, In Cheol
Issue Date
Graduate School of UNIST
Generation IV reactor design aims for sustainability, economics and safety, as well as non-proliferation. The promising, one of advanced reactor concepts is the sodium-cooled fast reactor or SFR. The safety issues of the SFR are important due to the fact that it uses sodium as a nuclear coolant, reacting vigorously with water and air. For that reason, there are efforts to seek for alternative candidates of liquid metal coolants having excellent heat transfer property and to adopt improved safety features to the SFR concepts. This study considers gallium as alternative liquid metal coolant applicable to safety features in terms of chemical activity issue of the sodium. As liquid metal coolant, liquid gallium has technical advantages such as the low melting point (29.76°C) in atmospheric pressure, the high boiling point (2,204°C), the low pressure of saturated vapor, a relatively low coefficient of volumetric thermal expansion, a chemical reaction safety, a low oxygen reactivity and a low toxicity of saturated vapor. The negative reactivity feedback induced by a relatively low thermal expansion of liquid gallium ensures the inherent safety of the core. However, it is excluded to consider gallium as a primary coolant of the reactor since gallium has a higher macroscopic absorption cross section than other liquid metals that adversely affects neutron economy. The attractive properties ensure that gallium can play an important role in nuclear safety as an alternative coolant of the decay heat removal system of the SFR. In particular, there is an emphasis to passive safety system in nuclear reactor design so that reactors can be made more tolerant of failing, and have less dependence on engineers. The passive safety natural convection systems for decay heat removal have several benefits such as simplicity, reduction of the cost and usage problems of pumps. They are contributed to set the power plant in a safe and stable condition. Therefore, this study aims to experimentally investigate the natural convection capability of gallium as a feasibility study for the development of gallium-based passive safety features. In this work, the design and construction of the liquid Gallium natural convection loops were carried out by using scaling law. Natural convection experiments were carried out in the ranges from 0 to 2.53kW and average heat transfer coefficients were obtained. By conducting the natural convection experiments, the performance of gallium can be confirmed and compared. The errors for the experimental flowrate data were found to be in the average values of 40% compared to CFX and 48% compared to calculated value respectively. The experimental results of heat transfer coefficient of liquid gallium were compared with existing correlations and they were much lower than the correlations. The heat removal capability of liquid gallium is better than other liquid metals except for sodium and the oxygen reactivity of liquid gallium is much lower than sodium. These advantages of liquid gallium can be more suitable to be substituted for sodium of PDRC (Passive Decay heat Removal Circuit). If so, liquid gallium will be contributed to safety of SFRs and to do that, it will be needed to conduct the natural convection experiment for liquid gallium in various experimental conditions.
Nuclear Science and Engineering
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