![]() Most absorption reactions result in fission reaction, but a part of reactions result in radiative capture forming 240Pu. For fast neutrons, its fission cross-section is on the order of barns. Plutonium 239 is a fissile isotope, and its fission cross-section for thermal neutrons is about 750 barns (for 0.025 eV neutron). When loaded into the reactor core, 239Pu can be easily fissioned by a neutron or transformed into the 240Pu via a radiative capture reaction. On the other hand, 239Pu has a very high absorption cross-section for thermal neutrons. 239Pu occasionally decays by spontaneous fission with a very low rate of 0.00000000031%. The transmutation and decay chain is shown below: 239Pu itself decays via alpha decay into 235U with a half-life of 24 100 years. 239Np decays (negative beta decay) to 239Pu. 239U decays (negative beta decay) to 239Np (neptunium), whose half-life is 2.36 days. The half-life of 239U is approximately 23.5 minutes. Absorption of resonance or thermal neutron by the 238U nucleus yields 239U. ![]() Isotope 239Pu is formed in a nuclear reactor from fertile isotope 238U, constituting more than 95% of uranium fuel (e.g., PWRs and BWRs require 3% – 5% of 235U). It is a manufactured isotope and can be found in irradiated uranium fuel or spent uranium fuel. This isotope is the principal fissile isotope of plutonium in use. Moreover, 239Pu also meets the alternative requirement that the amount ( ~2.88 per one fission by thermal neutron) of neutrons produced by fission of 239Pu is sufficient to sustain a nuclear fission chain reaction. Plutonium 239, 239Pu, is a fissile isotope, which means 239Pu can undergo a fission reaction after absorbing a thermal neutron. ![]()
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