The core of mostGeneration II nuclear reactors contains a set of hollow metal rods, usually made ofzirconium alloys, filled with solidnuclear fuel pellets – mostly oxide, carbide, nitride or monosulfide of uranium, plutonium or thorium, or their mixture (the so-calledMOX fuel). The most common fuel is oxide of uranium-235.Nuclear reactor scheme
Fast neutrons are slowed bymoderators, which contain water,carbon,deuterium, orberyllium, asthermal neutrons to increase the efficiency of their interaction with uranium-235. The rate of nuclear reaction is controlled by introducing additional rods made ofboron orcadmium or a liquid absorbent, usuallyboric acid. Reactors for plutonium production are calledbreeder 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 viaβ-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.
胖子原子彈(the fat guy)是鈽原子彈，以曼哈頓計劃中所生產的高濃度濃縮鈽為材料，以中子撞擊鈽原子啟動整個鏈式反應 Thus only 6.2 kg of plutonium was needed for anexplosive yield equivalent to 20 kilotons ofTNT.[28] (See alsoNuclear 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 inthermopiles and waterdistillation systems of some space satellites and stations. SoGalileo andApollo 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 ofradioactivity 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]
^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.
^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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