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thermal reactor[′thər·məl rē′ak·tər]
a nuclear reactor in which the overwhelming majority of fissions of nuclei of the fissionable substance occur upon interaction with thermal neutrons.
A moderator—a substance that contains light nuclei and is a weak absorber of neutrons—is placed in the reactor core to slow neutrons to thermal energies. (The mean energy of fission neutrons is about 2 mega electron volts, or MeV.) Hydrogen (protium and deuterium), beryllium, carbon, and compounds thereof, such as ordinary and heavy water, hydrocarbons, or beryllium oxide, may be used as moderators. The most common moderators are water and graphite.
Such fissionable isotopes of uranium and plutonium as 233U 235U, 239Pu and 241 Pu, which have large capture cross sections for low-energy neutrons, are used as the fuel in a thermal reactor. This makes possible the construction of thermal reactors with relatively low critical mass and, consequently, with a relatively small charge of fissionable substance. The main type of nuclear fuel used in a thermal reactor is natural uranium or uranium slightly enriched with the isotope 235U. The fission of 235U releases ~2.5 neutrons per nucleus; when this occurs, an average of one neutron goes to sustain the nuclear reaction, and part of the remainder neutrons (up to 0.9 neutron) interacts with the 238U present in the fuel, sometimes called the fertile material, forming a secondary nuclear fuel, 239Pu. The fraction of neutrons interacting with the fertile material is determined by the choice of moderator and by the amount of fertile material actually in the core. In a thermal reactor with a uranium-thorium cycle, in which 233U is the nuclear fuel and 232Th is the fertile material, the number of such neutrons may exceed the number of split nuclei by a factor of 1.05–1.1. This makes possible the breeding of nuclear fuel.
The operation of a thermal reactor is usually regulated—that is, the rate of fission is slowed or stepped up when necessary—by a regulator rod: substances that intensively absorb neutrons are inserted into or withdrawn from the core. Cadmium, boron, and the rare-earth elements are good absorbers. Compounds of boron (such as boron carbide) or boron steel are most often used; in water-moderated water-cooled reactors, partial control is accomplished by changing the concentration of boron-containing substances, such as boric acid, in the coolant water. The operating state of a thermal reactor is characterized by the effective multiplication factor Keff, which is the ratio of the number of neutrons of one generation absorbed in the reactor to the number of absorbed neutrons of the previous generation. When Keff = 1 the reactor is in a critical steady state, when Keff > 1 the reactor power is increasing, and when Keff < 1 the power is decreasing.
Liquids and gases that are weak absorbers of neutrons and are capable of providing effective heat exchange—for example, ordinary and heavy water, organic liquids, carbon dioxide, and helium—are used as coolants to remove the heat of the fission process from the reactor. In some cases, liquid metals and salts are used. Water and organic liquids usually act simultaneously as moderator and coolant in a thermal reactor.
Structural materials for the core of a thermal reactor include Al for temperatures of 200°–250°C, Zr for 250°–400°C, and steel for temperatures above 400°C. Al and Zr have comparatively little effect on the rate of absorption of neutrons in the reactor; steel has a large neutron absorption cross section, and therefore enriched fuel must be used in thermal reactors with a steel core.
In modern nuclear engineering (mid-1970’s), thermal reactors are the most important and most widely used type of reactors. They are used for the generation of electric power, desalination of water, and production of artificial fissionable substances and radioactive isotopes. They are also used in technological tests of materials and structures, in the study of physical processes and phenomena, and in other applications.
S. A. SKVORTSOV