Types of Nuclear Power Reactor

Several types of nuclear fission reactor have been used as heat sources to generate steam in power plants. A power reactor may be identified in different ways. For example, it may be a thermal or a fast reactor, depending on the energy of the neutrons causing fission (see Chapter 13, Section 13.7.3). A thermal reactor is usually identified according to the neutron moderator used. If the moderator and coolant are different, both are specified. Table 14-8 lists characteristics used to identify power reactors.

Almost all nuclear power reactors in operation worldwide at the time of writing are thermal reactors; that is, fission is induced when 235U captures a thermal rather than a fast neutron. Thermal power reactors using H2O as both

TABLE 14-8

Characteristics Commonly Used to Identify Nuclear Power Reactors

TABLE 14-8

Characteristics Commonly Used to Identify Nuclear Power Reactors

Characteristic

Examples

Fissile material

233u 235U 239Pu

Moderator

H20, D20, C (graphite)

Coolant

H20, D20, molten Na, C02, He

Neutron spectrum

Slow (thermal), intermediate, fast

Fertile material

238U, 232Th

moderator and coolant are known as LWRs (light water reactors), those using D20 as HWRs (heavy water reactors). LWRs, the most popular type in most countries, are further subdivided into PWRs (pressurized water reactors) and BWRs (boiling water reactors).

Nuclear reactors are also classified in terms of the way fissile material is used and formed. A reactor that uses highly enriched uranium as fuel without fertile material is a "burner." A nonmilitary research reactor is an example, although most of these devices have been modified to use fuel with no more than 20% 235U. A "converter" is a reactor that burns one type of fissile nuclide (e.g., 235U) and makes another (e.g., 239Pu) but makes less fuel than it burns. LWRs are converters. Breeder reactors produce more of a given fissile material than they fission.

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