Plutonium-239 (239Pu or Pu-239) is an isotope of plutonium. Plutonium-239 is the primary fissile isotope used for the production of nuclear weapons, although uranium-235 is also used for that purpose. Plutonium-239 is also one of the three main isotopes demonstrated usable as fuel in thermal spectrum nuclear reactors, along with uranium-235 and uranium-233. Plutonium-239 has a half-life of 24,110 years. Nuclear properties The nuclear properties of plutonium-239, as well as the ability to produce large amounts of nearly pure 239Pu more cheaply than highly enriched weapons-grade uranium-235, led to its use in nuclear weapons and nuclear power plants. The fissioning of an atom of uranium-235 in the reactor of a nuclear power plant produces two to three neutrons, and these neutrons can be absorbed by uranium-238 to produce plutonium-239 and other isotopes. Plutonium-239 can also absorb neutrons and fission along with the uranium-235 in a reactor. Of all the common nuclear fuels, 239Pu has the smallest critical mass. A spherical untamped critical mass is about 11 kg (24.2 lbs), 10.2 cm (4") in diameter. Using appropriate triggers, neutron reflectors, implosion geometry and tampers, the critical mass can be less than half of that. The fission of one atom of 239Pu generates 207.1 MeV = 3.318 × 10−11 J, i.e. 19.98 TJ/mol = 83.61 TJ/kg, or about 23 gigawatt hours/kg. Production Plutonium is made from uranium-238. 239Pu is normally created in nuclear reactors by transmutation of individual atoms of one of the isotopes of uranium present in the fuel rods. Occasionally, when an atom of 238U is exposed to neutron radiation, its nucleus will capture a neutron, changing it to 239U. This happens more easily with lower kinetic energy (as 238U fission activation is 6.6MeV). The 239U then rapidly undergoes two β− decays — an emission of an electron and an anti-neutrino (), leaving a proton — the first β− decay transforming the 239U into neptunium-239, and the second β− decay transforming the 239Np into 239Pu: {}^{238}_{92}U + {}^{1}_{0}n -> {}^{239}_{92}U ->[\beta^-][23.5\ \ce{min}] {}^{239}_{93}Np ->[\beta^-][2.356\ \ce{d}] {}^{239}_{94}Pu Fission activity is relatively rare, so even after significant exposure, the 239Pu is still mixed with a great deal of 238U (and possibly other isotopes of uranium), oxygen, other components of the original material, and fission products. Only if the fuel has been exposed for a few days in the reactor, can the 239Pu be chemically separated from the rest of the material to