A reactor coolant picks up the heat from fuel elements in the reactor core and carries it to the boilers where it gives up that heat to water carried in pipes to turn it into steam. The efficiency of this process depends upon the flow rate of the coolant, the flow cross-section of the fuel element, the difference in temperature between the cladding and the coolant and the choice of the material cladding the fuel. The fuel and its cladding are designed so they operate at the highest satisfactory and safe temperature. Similarly, the coolant is heated to the highest acceptable temperature to increase the plant's efficiency by providing steam to the turbines at high temperature and pressure. A special case is that in which the coolant is in the form of a boiling liquid. The vapor of the boiling fluid coolant is often used as the working fluid in the turbine [e.g., steam generated within a boiling water-cooled reactor is used directly in a steam turbine].
The general features that make a particular gas or liquid attractive as a reactor coolant are as follows:
Chemically compatible and noncorrosive to circuit materials;
Cheap and readily-available in pure state;
Good nuclear properties and stable under radiation;
Well-defined phase state and high boiling point;
High rates of heat transfer;
High specific heat and ease of pumping.
No practical fluids meet all these requirements. All known coolants have one or more disadvantages. The thermodynamic and heat transfer characteristics of a coolant can be compared conveniently by using a parameter called the figure of merit [F], which derives from the heat transfer process and the associated pumping power required
Table 1 lists possible reactor coolants and their figure of merit. Of those listed, water is the most common. It has three main disadvantages: its low boiling point [which requires operation at high pressure]; its neutron absorption [which requires enrichment of the fuel], and its corrosive nature [which requires specific steels and cladding]. Of the gases, carbon dioxide and helium have been widely-used. Liquid metals, whilst being excellent coolants, present a whole range of novel handling problems because of their chemical reactivity. Their use has been mainly restricted to fast reactors.
Collier, J. G. and Hewitt, G. F. (1987) Introduction to Nuclear Power, Hemisphere Publishing Corp. DOI: 10.1016/0142-727X(89)90061-1
Nuclear Power Technology, Volume 1, Ed. by W. Marshall (1983) Clarendon Press. Oxford.