The core of most Generation II nuclear reactors contains a set of hollow metal rods, usually made of zirconium alloys, filled with solid nuclear fuel pellets – mostly oxide, carbide, nitride or monosulfide of uranium, plutonium or thorium, or their mixture (the so-called MOX fuel). The most common fuel is oxide of uranium-235.
Nuclear reactor scheme
Fast neutrons are slowed by moderators, which contain water, carbon, deuterium, or beryllium, as thermal neutrons to increase the efficiency of their interaction with uranium-235. The rate of nuclear reaction is controlled by introducing additional rods made of boron or cadmium or a liquid absorbent, usually boric acid. Reactors for plutonium production are called breeder reactor or breeders; they have a different design and use fast neutrons.
Emission of neutrons during the fission of uranium is important not only for maintaining the nuclear chain reaction, but also for the synthesis of the heavier actinides. Uranium-239 converts via β-decay into plutonium-239, which, like uranium-235, is capable of spontaneous fission. The world's first nuclear reactors were built not for energy, but for producing plutonium-239 for nuclear weapons.
The "胖子原子彈"-type plutonium bombs produced during the 曼哈頓計劃 used explosive compression of plutonium to obtain significantly higher densities than normal, combined with a central neutron source to begin the reaction and increase efficiency. Thus only 6.2 kg of plutonium was needed for an explosive yield equivalent to 20 kilotons of TNT.[28] (See also Nuclear weapon design.) Hypothetically, as little as 4 kg of plutonium—and maybe even less—could be used to make a single atomic bomb using very sophisticated assembly designs.[29]
Plutonium-238 is potentially more efficient isotope for nuclear reactors, since it has smaller critical mass than uranium-235, but it continues to release much thermal energy (0.56 W/g)[21][30] by decay even when the fission chain reaction is stopped by control rods. Its application is limited by the high price (about US$1000/g). This isotope has been used in thermopiles and water distillation systems of some space satellites and stations. So Galileo and Apollo spacecraft (e.g. Apollo 14[31]) had heaters powered by kilogram quantities of plutonium-238 oxide; this heat is also transformed into electricity with thermopiles. The decay of plutonium-238 produces relatively harmless alpha particles and is not accompanied by gamma-irradiation. Therefore, this isotope (~160 mg) is used as the energy source in heart pacemakers where it lasts about 5 times longer than conventional batteries.[21]
锕-227被用作中子源。 Its high specific energy (14.5 W/g) and the possibility of obtaining significant quantities of thermally stable compounds are attractive for use in long-lasting thermoelectric generators for remote use. 228Ac is used as an indicator of radioactivity in chemical research, as it emits high-energy electrons (2.18 MeV) that can be easily detected. 228Ac-228Ra mixtures are widely used as an intense gamma-source in industry and medicine.[16]
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Scerri, Eric. Provisional Report on Discussions on Group 3 of the Periodic Table. Chemistry International. 18 January 2021, 43 (1): 31–34. S2CID 231694898. doi:10.1515/ci-2021-0115.
^Although "actinoid"（rather than "actinide"）means "actinium-like" and therefore should exclude actinium, that element is usually included in the series.
^ 13.013.113.2Yu.D. Tretyakov (编). Non-organic chemistry in three volumes. Chemistry of transition elements 3. Moscow: Academy. 2007. ISBN 978-5-7695-2533-9.
^G. G. Bartolomei; V. D. Baybakov; M. S. Alkhutov; G. A. Bach. Basic theories and methods of calculation of nuclear reactors. Moscow: Energoatomizdat. 1982.
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