In addition to classifying nuclear reactors as thermal or fast, they may be characterized by their purpose, by the type of moderator used to slow down neutrons, by the type of coolant, or by the type of fuel. The principal purposes for which reactors may be used are for research, testing, production of materials such as radioisotopes or plutonium, or power generation. This text is concerned mainly with power reactors.

The most effective substances for slowing down neutrons are those elements of low molecular weight that have low probability of capturing neutrons, namely, hydrogen, deuterium (the hydrogen isotope of atomic mass 2, chemical symbol D), beryllium, or carbon. Examples of moderators containing these elements are light water (H20), heavy water (D20), beryllium oxide, and graphite.

In many types of thermal power reactors, moderator, fuel, and coolant are kept separate in the reactor. Figure 1.7 is a schematic diagram of a nuclear power plant utilizing such a reactor. Table 1.2 lists five examples of reactors with separate moderator, fuel, and coolant and gives references where more detailed information about these reactors may be obtained. In this type of reactor, fuel and moderator ordinarily remain in place in the reactor and only coolant flows through the reactor to remove the heat of fission. Hot coolant flows from the reactor to a steam generator, where it is cooled by heat exchange with feedwater. The feedwater is converted to steam, which drives a steam turbine. The steam then is condensed, preheated, and recirculated as feedwater to the steam generator. Coolant, after being cooled in the steam generator, is returned to the reactor by the coolant circulator. The steam turbine drives an electric generator.

When H20 is used as coolant, the same material serves also as moderator, so ihat the reactor structure can be simplified. Figure 1.8 is a schematic diagram of a pressurized-water reactor, in which the coolant and moderator consist of liquid water whose pressure of 150 bar (2200 lb/in2) is so high that it remains liquid at the highest temperature, around 300°C (572°F), to which it is heated in the reactor. The main difference in principle from Fig. 1.7 is

that there is no separation of coolant from moderator in the reactor. The pressurized-water reactor is one of the two types of power reactor in most common use in the United States. More information about it is given in Chap. 3.

The boiling-water reactor is the other type of power reactor in common use in the United States that uses H20 as coolant and moderator. In this type the water in the reactor is at a lower pressure, around 70 bar (1000 lb/in2), so that it boils and is partially converted to steam as it flows through the reactor. Coolant leaving the reactor is separated into water, which is recycled, and steam, which is sent directly to the turbine as illustrated in Fig. 1.9. Comparison with Fig. 1.8 shows that the boiling-water system differs from the pressurized-water system in having no external steam generator, the reactor itself providing this function.

In a fast-breeder reactor it is impractical to use water as coolant because it is too effective a moderator for neutrons. Liquid sodium is the coolant most extensively investigated for fast

reactors; helium gas has also been proposed. Fast reactors need a higher ratio of fissile to fertile isotopes than thermal reactors to support a chain reaction; a mixture of 20 percent plutonium and 80 percent 233 U is typical for a fast-reactor fuel. Mixed dioxides or mixed monocarbides are possible fuel materials. Although natural boron, which contains around 20 percent of the strong neutron-absorbing isotope 10 B, is satisfactory for control material in thermal reactors, concentrated 10 В is preferred for some fast reactors.

The molten-salt reactor differs from all reactors thus far described in that it uses a liquid solution of uranium as fuel and removes heat from the reactor by circulating hot fuel to an external heat exchanger. No reactor coolant is employed other than the fuel itself. The molten-salt breeder reactor (MSBR) uses as fuel a solution of UF4 in a solvent salt consisting of mixture of BeF2, 7LiF, and ThF4. Separated 7Li is required instead of natural lithium because the 7.5 percent of * Li in natural lithium would absorb so many neutrons as to make breeding impossible. The MSBR is a thermal reactor that breeds 233 U from thorium; neutrons are thermalized by means of graphite moderator blocks, fixed in the reactor, containing channels through which the molten salt flows.

Table 1.3 summarizes the materials used for the principal services in pressurized-water and boiling-water reactors, the high-temperature gas-cooled reactor, fast reactors, and the molten-salt reactor, and indicates which materials are fixed in each reactor and which flow through it.