The general features that make a particular fluid (gas or liquid) attractive as a re­actor coolant are as follows.

1. High specific heat. Suppose we have a nuclear reactor that is generating heat at a rate of Q watts. Coolant at a flow rate W (kilograms per second) is passed to the reactor, entering the core at temperature 7 and leaving the core at temperature Tut — From the first law of thermodynamics (see Section 1.1.1), these quantities are related by the equation Q = WCp (Tout — !Tn),where Cp is the specific heat or specific heat capacity of the fluid. The specific heat is the amount of heat required to heat 1 kg of a substance by 1 K (1°C) and thus has the units joules per kilogram per kelvin. In designing reactors it is important to prevent excessive temperatures within the core, in order to avoid damaging the fuel and the core construction materials. The above equation indicates that this can be accomplished in two ways for a given inlet temperature of the coolant. First, the flow rate Wcan be so high that the outlet temperature is not too much higher than the inlet temperature, irre­spective of the value of Cp. Second, a fluid can be chosen that has a high value of Cp, which will also limit the outlet temperature. Of course, the out­let temperatures cannot be too low, or the reactor will not be able to gener­ate steam efficiently, as explained in Chapter 1. Also, with high flow rates

significant amounts of power are needed to pump the coolant, and this is power that is not available as electricity to the customer.

A special case is that in which the coolant is in the form of a boiling liquid. Here, heat can be absorbed by the coolant at its boiling point with no change in temperature and can be used to convert the liquid into vapor. The amount of heat required to convert one unit mass of liquid to vapor is called the latent beat of vaporization (joules per kilogram). The boiling-fluid coolant is often also used as the working fluid in the turbine (e. g., steam generated from a boiling-water coolant in a reactor is used in a steam tur­bine). For the reasons discussed in Chapter 1, the higher the boiling point of the fluid the higher the thermodynamic efficiency. Since boiling point in­creases with pressure, the boiling-coolant system should be operated at the highest practicable pressure. However, the higher the pressure, the more ex­pensive the system, and there is a trade-off between increased capital cost and increased thermodynamic efficiency.

2. High rates of heat transfer. The rate at which heat can be transferred from the fuel elements to the coolant is determined by a number of factors, which are discussed in more detail in Section 33. One of the parameters is the thermal conductivity of the fluid, which is the constant of proportionality between the rate at which heat is transferred through a static volume of fluid and the temperature gradient, i. e., the rate at which temperature is changing per unit length. Liquid metal coolants have high thermal conductivity, whereas gaseous coolants have relatively low thermal conductivity.

3. Good nuclear properties. For all reactors, it is important that the coolants should have low neutron absorption. As explained in Chapter 2, any neutron absorption by the coolant and structure reduces the number of neutrons available for the fission reaction. The neutrons should not react appreciably with the coolant to form radioactive isotopes. Excess radioactivity in the cir­culating system increases operational difficulties, as mentioned in Chapter 2. If the coolant is also acting as the moderator, good moderation properties are required (the processes of moderation were explained in Chapter 1). In fast reactors, of course, it is important that the coolant not moderate the neu­trons, since unmoderated (fast) neutrons are required in the reaction.

4. Well-defined phase state. It is preferable for the coolant to have the same phase state (i. e., liquids remain as liquids and gases remain as gases) during both normal and accident conditions. To achieve this in the case of liquids, a high boiling point is desirable to avoid changes of phase if the liquid is over­heated. A high boiling point also has the advantage of minimizing the pres­sure required to operate at a certain temperature level and of achieving high thermodynamic efficiency.

5. Cost and availability. Since the inventory of coolant in typical reactor sys­tems is quite high (hundreds of tons), it is important that the cost be mini­mized. Also, coolants may leak from reactor circuits, and this can be a significant cost in some cases. The ideal coolant should also be freely avail­able in a sufficiently pure form for use in the reactor circuit.

6. Compatibility. It is obviously axiomatic that the coolant should be compati­ble with the reactor circuit and not corrode it, even under the conditions of high radiation flux that occur in the core.

7. Ease of pumping. Fluids of low viscosity require much less pumping power to circulate them around the reactor circuit than do fluids of high viscosity. The viscosity of a fluid is related to its temperature, that of liquids decreasing with increasing temperature and that of gases increasing with increasing temperature. The viscosity of a fluid is indicated by the symbol Jl.

No practical fluid meets all of 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, which derives from the heat transfer processes and the associ­ated pumping power required. The figure of merit F is defined as

where Cp is the specific heat, Q the fluid density (kilograms per cubic meter), and Jl the viscosity. The rather peculiar-looking powers appearing in this equa­tion result from the empirical correlations used to predict the pumping power and the heat transfer rates.

There are relatively few practical choices for reactor coolants. The ones mainly used are listed in Table 3.1, which shows their density, viscosity, specific heat, thermal conductivity, and figure of merit value. In terms of figure of merit, ordinary water is outstanding. However, it has three main disadvantages: its low boiling point, which requires operation at high pressure in order to reach even moderate thermodynamic efficiencies; its neutron absorption; and its corrosion properties. The latter two disadvantages require enrichment of the fuel and spe­cial containment materials, respectively.