Molten (liquid) metals offer the possibility of much higher operating tempera­tures than can be obtained with water and have excellent heat transfer proper­ties. Only one metal, mercury, is liquid at room temperature, and in any case it has far too high a neutron absorption cross section for use in thermal reactors. It also has relatively high vapor pressures, and the vapor is toxic. However, it is interesting to note that mercury has been used in power generation cycles, and

several power stations in the United States were operated during the 1950s using mercury as a working fluid.

The only metals that combine the advantages of a relatively low melting point with low vapor pressure and low neutron absorption are sodium and potassium. Sodium and potassium are compatible with stainless steel at tem­peratures up to at least 800°C provided the liquid metal is kept free of oxygen. Sodium is the more abundant and cheaper to produce, and, furthermore, potas­sium can form compounds with oxygen that are explosive. In recent years, sodium has been the preferred liquid metal coolant, but in earlier reactors sodium-potassium mixtures (NaK) were frequently employed. The mixtures could be made liquid at room temperature and the pipework did not require heating in order to keep the coolant molten during periods of shutdown.

Sodium has been the primary choice as the coolant for fast breeder reactors. Referring to Table 3.1, we see that it has a higher thermal conductivity, though a lower specific heat, than water. For a given heat removal, the flow rate re­quired is five times higher for sodium than for water. However, the overriding advantage of sodium is its high boiling point, which allows sodium-cooled re­actors to operate near atmospheric pressure while maintaining a wide differ­ence between the operating temperature and the boiling point. The operating temperatures are sufficiently high for the sodium stream to evaporate water at high pressure to produce a high thermodynamic efficiency. However, there are a number of problems in using sodium as a coolant.

1. Because sodium is very reactive toward oxygen and water, contact must be avoided. An inactive cover gas such as argon is needed above all the sodium levels in the reactor system. This gas must be kept free of oxygen contami­nation. In the steam-generating system, the sodium heats tubes that contain the high-pressure evaporating water. These evaporators are one of the main sources of trouble in sodium-cooled reactors. Very small leaks can be toler­ated, but they cause contamination problems due to the formation of sodium hydroxide, which is corrosive. Large leaks can cause explosive interactions between the sodium and water, giving rise to hydrogen generation, and the hydrogen itself is explosive. Contamination of the reactor vessel can be min­imized by using an intermediate heat exchanger (Chapter 2); alternatively, some designs use double-walled tubes to enhance the separation between the two fluids in the steam generator. In the design of sodium-cooled reac­tors, provision is made to accommodate such sodium-water reactions safely. For instance, the steam generator can be isolated and its sodium content ejected through a special system that allows the hydrogen generated to be discharged through an outlet stack.

2. The primary coolant circuit becomes very radioactive through the formation of sodium-24, which has a 15-h half-life; the existence of this isotope in the primary coolant is another reason for having an intermediate exchanger be­tween the coolant and the steam generator. In practice, in view of the fairly short half-life, this isotope creates no particular difficulty associated with maintaining the reactor circuit, though sufficient time must be allowed for it to decay to a low level before the circuit is worked on.

To someone who has witnessed school chemistry laboratory experiments in which small pieces of sodium are dropped into water and has observed the dra­matic and explosive effects, the prospect of using this metal as a reactor coolant must seem rather horrifying. However, when contained within a reactor circuit, and with proper precautions taken to deal with any potential effects of its con­tact with the steam generator water, sodium is a surprisingly benign and ex­tremely efficient coolant.