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Nuclear fission products are the atomic fragments left after a large atomic nucleus undergoes nuclear fission. Typically, a large nucleus like that of uranium fissions by splitting into two smaller nuclei, along with a few neutrons, the release of heat energy (kinetic energy of the nuclei), and gamma rays. The two smaller nuclei are the ''fission products''. (See also Fission products (by element)). About 0.2% to 0.4% of fissions are ternary fissions, producing a third light nucleus such as helium-4 (90%) or tritium (7%). The fission products themselves are often unstable and radioactive, due to being relatively neutron-rich for their atomic number, and many of them quickly undergo beta decay. This releases additional energy in the form of beta particles, antineutrinos, and gamma rays. Thus, fission events normally result in beta radiation and antineutrinos, even though these particles are not produced directly by the fission event itself. Many of these isotopes have a very short half-life, and therefore give off huge amounts of radiation. For instance, strontium-90, strontium-89 and strontium-94 are all fission products, they are produced in similar quantities, and each nucleus decays by shooting off one beta particle (electron). But Sr-90 has a 30-year half-life, Sr-89 a 50.5-day half-life, and Sr-94 a 75-second half-life. When freshly created, Sr-89 will spray beta particles 10,600 times faster than Sr-90, and Sr-94 will do so 915 million times faster. It is these short-half-life isotopes that make spent fuel so dangerous, in addition to generating much heat, immediately after the reactor itself has been shut down. The good news is that the most dangerous fade quickly; after 50 days, Sr-94 has had 58,000 half-lives and is therefore 100% gone; Sr-89 is at half its original quantity, but Sr-90 is still 99.99% there. As there are hundreds of different isotopes created, the initial high radiation fades quickly, but never fades out completely.〔F. William Walker, Dr. George J. Kirouac, Francis M. Rourke. 1977. ''Chart of the Nuclides'', twelfth edition. Knolls Atomic Power Laboratory, General Electric Company.〕 == Formation and decay == The sum of the atomic weight of the two atoms produced by the fission of one fissile atom is always less than the atomic weight of the original atom. This is because some of the mass is lost as free neutrons, and once kinetic energy of the fission products has been removed (i.e., the products have been cooled to extract the heat provided by the reaction), then the mass associated with this energy is lost to the system also, and thus appears to be "missing" from the cooled fission products. Since the nuclei that can readily undergo fission are particularly neutron-rich (e.g. 61% of the nucleons in uranium-235 are neutrons), the initial fission products are almost always more neutron-rich than stable nuclei of the same mass as the fission product (e.g. stable ruthenium-100 is 56% neutrons; stable xenon-134 is 60%). The initial fission products therefore may be unstable and typically undergo beta decay towards stable nuclei, converting a neutron to a proton with each beta emission. (Fission products do not emit alpha particles.) A few neutron-rich and short-lived initial fission products decay by ordinary beta decay (this is the source of perceptible half life, typically a few tenths of a second to a few seconds), followed by immediate emission of a neutron by the excited daughter-product. This process is the source of so-called delayed neutrons, which play an important role in control of a nuclear reactor. The first beta decays are rapid and may release high energy beta particles or gamma radiation. However, as the fission products approach stable nuclear conditions, the last one or two decays may have a long half-life and release less energy. There are a few exceptions with relatively long half-lives and high decay energy, such as: *Strontium-90 (high energy beta, half-life 30 years) *Caesium-137 (high energy gamma, half-life 30 years) *Tin-126 (even higher energy gamma, but long half-life of 230,000 years means a slow rate of radiation release, and the yield of this nuclide per fission is very low) 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Nuclear fission product」の詳細全文を読む スポンサード リンク
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