Existing reactor designs and new concepts rely to varying degrees on heat removal processes driven by natural convection as a potentially important design feature or ultimate heat removal mechanism. This is independent of whether the nuclear core is cooled by water, gas or liquid metal, since in many shut down or emergency conditions forced cooling is assumed or predicted to be lost. However, using natural convection to advantage is possible, since it can provide significant cost-savings by the elimination of pumps and ancillary equipment and also can result in simplified and hence higher reliability safety systems. It is highly desirable to build on the inherent or existing heat removal processes than to graft design or add them on afterwards. The limits to the heat removal are set by the natural circulation flow and heat removal capability, so these need to be predicted with accuracy. The capability limit is determined by well-known physically linked parameters, including the flow rates, driving heads, heat sinks, fluid thermal expansion, and flow thermal and hydraulic stability, hi natural convection plants, there are opportunities for the limits to be set by the absolute power output available from naturally convective flow, and the onset of instability in that flow. We are interested in the ultimate or maximum power output both in order to minimize power generation costs, and to determine how far the natural circulation designs can be developed. This paper reviews some of the fundamental equations and analytical solutions for natural convection flows, and examines their application to determine the limits of heat removal as a means of establishing simple criteria and fundamental design limits. This type of physical analysis can be used to investigate the flow and stability limits for a thermally expandable fluid, which encompasses the extremes of both low and supercritical pressure applications. To illustrate the approach, simple analytical expressions are derived for the ultimate or maximum heat removal. We can then relate the maximum thermal hydraulic limits to hypothetical reactor power output. The relationship between some of the various enhanced design features is then clear when seeking the ultimate or maximum safe power output at least cost. Hypothetical natural circulation designs are discussed as a basis.
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