Plutonium 239, 239Pu, is a fissile isotope, which means 239Pu is capable of undergoing fission reaction after absorbing thermal neutron. Moreover, 239Pu meets also 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. This isotope is the principal fissile isotope of plutonium in use.It is a man-made isotope and can be found in an irradiated uranium fuel or in a spent uranium fuel. Isotope 239Pu is formed in a nuclear reactor from fertile isotope 238U, which constitute more than 95% of uranium fuel (e.g. PWRs and BWRs require 3% – 5% of 235U). Absorption of a resonance or thermal neutron by the 238U nucleus yields 239U. The half-life of 239U is approximately 23.5 minutes. 239U decays (negative beta decay) to 239Np (neptunium), whose half-life is 2.36 days. 239Np decays (negative beta decay) to 239Pu. The transmutation and decay chain is shown below:239Pu itself decays via alpha decay into 235U with half-life of 24 100 years. 239Pu occasionally decays by spontaneous fission with very low rate of 0.00000000031%. On the other hand 239Pu has very high absorption cross-section for thermal neutrons. When loaded into the reactor core 239Pu can be easily fissioned by a neutron or can be transformed into the 240Pu via a radiative capture reaction.
Plutonium 239 Fission
Plutonium 239 is a fissile isotope and its fission cross-section for thermal neutrons is about 750 barns (for 0.025 eV neutron). For fast neutrons its fission cross-section is on the order of barns. Most of absorption reactions result in fission reaction, but a part of reactions result in radiative capture forming 240Pu. The cross-section for radiative capture for thermal neutrons is about 270 barns (for 0.025 eV neutron). Therefore about 27% of all absorption reactions result in radiative capture of incident neutron. About 73% of all absorption reactions result in fission.
Typically, when plutonium 239 nucleus undergoes fission, the nucleus splits into two smaller nuclei (triple fission can also rarely occur), along with a few neutrons (the average is 2.89 neutrons per fission by thermal neutron) and release of energy in the form of heat and gamma rays. The average of the fragment atomic mass is about 120, but very few fragments near that average are found. It is much more probable to break up into unequal fragments, and the most probable fragment masses are around mass numbers 103 and 134 (and around atomic numbers 40 – Zirconium and 54 – Xenon).
Most of these fission fragments are highly unstable (radioactive) and undergo further radioactive decays to stabilize itself, therefore part of the released energy is radiated away from the reactor. On the other hand most of the energy released by one fission (~175MeV of total ~207MeV) appears as kinetic energy of these fission fragments. The fission fragments interact strongly with the surrounding atoms or molecules traveling at high speed, causing them to ionize. Creation of ion pairs requires energy, which is lost from the kinetic energy of the charged fission fragment causing it to decelerate. The positive ions and free electrons created by the passage of the charged fission fragment will then reunite, releasing energy in the form of heat (e.g. vibrational energy or rotational energy of atoms). This is the principle how fission fragments heat up fuel in the reactor core.
Source: JANIS (Java-based Nuclear Data Information Software); ENDF/B-VII.1Neutron production per one fission of Plutonium 239.
Source: JANIS (Java-based Nuclear Data Information Software)
The JEFF-3.1.1 Nuclear Data LibraryFission fragment yield for different nuclei. The most probable fragment masses for 239Pu fission are around mass 103 (Zirconium) and 134 (Xenon).