While almost all plutonium is manufactured synthetically, extremely tiny trace amounts are found naturally in uranium ores. These come about by a process of neutron capture by 238
U nuclei, initially forming 239
U; two subsequent beta decays then form 239
Pu (with a 239
Np intermediary), which has a half-life of 24,110 years. This is also the process used to manufacture 239
Pu in nuclear reactors. Some traces of 244
Pu remain from the birth of the solar system from the waste of supernovae, because its half-life of 80 million years is fairly long.
A relatively high concentration of plutonium was discovered at the natural nuclear fission reactor in Oklo, Gabon in 1972. Since 1945, approximately 7700 kg has been released onto Earth through nuclear explosions.
Manufacture of Pu-240, Pu-241 and Pu-242
The activation cross section for 239
Pu is 270 barns while the fission cross section is 747 barns for thermal neutrons. The higher plutonium isotopes are created when the uranium fuel is used for a long time. It is the case that for high burnup used fuel that the concentrations of the higher plutonium isotopes will be higher than the low burnup fuel which is reprocessed to obtain bomb grade plutonium.
Manufacture of Pu-239
Plutonium-239 is one of the three fissile materials used for the production of nuclear weapons and in some nuclear reactors as a source of energy. The other fissile materials are uranium-235 and uranium-233. Plutonium-239 is virtually nonexistent in nature. It is made by bombarding uranium-238 with neutrons in a nuclear reactor. Uranium-238 is present in quantity in most reactor fuel; hence plutonium-239 is continuously made in these reactors. Since plutonium-239 can itself be split by neutrons to release energy, plutonium-239 provides a portion of the energy generation in a nuclear reactor.
Manufacture of Pu-238
There are small amounts of Pu-238 in the plutonium of usual plutonium-producing reactors. However, isotopic separation would be quite expensive compared to another method: when a U-235 atom captures a neutron, it is converted to an excited state of U-236. Some of the excited U-236 nuclei undergo fission, but some decay to the ground state of U-236 by emitting gamma radiation. Further neutron capture creates U-237 which has a half-life of 7 days and thus quickly decays to Np-237. Since nearly all neptunium is produced in this way or consists of isotopes which decay quickly, one gets nearly pure Np-237 by chemical separation of neptunium. After this chemical separation, Np-237 is again irradiated by reactor neutrons to be converted to Np-238 which decays to Pu-238 with a half-life of 2 days.
Isotopes of Plutonium
238Pu [144 neutrons]
Half life: 88 years [ Alpha Decay ]
Decay Energy: 5.5 MeV
Decays to 234
239Pu [145 neutrons]
Half life: 24.1 x 103
years [ Alpha Decay ]
Decay Energy: 5.245 MeV
Decays to 235
Pu can undergo nuclear fission if its nucleus is struck by a neutron, particularly a thermal neutron. The fission of 239
Pu itself releases neutrons that bombard other 239
Pu atoms, which fission and release more neutrons and so on in a nuclear chain reaction. This isotope has a positive multiplication factor (k), which means that if the metal is present in sufficient mass and with an appropriate geometry (e.g., a compressed sphere), it can form a critical mass. During fission, a tiny fraction of the nuclear material (i.e., the mass defect) is converted directly into a large amount of energy; a kilogram of 239
Pu can produce an explosion equivalent to 20,000 tons of TNT. It is this energy that makes 239
Pu useful in nuclear weapons and reactors.
240Pu [146 neutrons]
Half life: 6.5 x 103
Decay Energy: 0.005 MeV
Decays to 240
The presence of the isotope 240
Pu in a material limits its nuclear bomb potential since it emits neutrons randomly, increasing the difficulty of accurately initiating the chain reaction at the desired instant and thus reducing the bomb’s reliability and power.
Plutonium is identified as either weapon grade, fuel grade, or power reactor grade based on the percentage of 240Pu that is contained in the plutonium. Weapons grade plutonium contains less than 7% 240Pu. Fuel grade plutonium contains from 7 to less than 19% percent, and power reactor grade contains from 19% and greater 240Pu. The isotope 238Pu is not capable of undergoing nuclear fission.
241Pu [147 neutrons]
Half life: 14 years [ Alpha Decay ]
Decay Energy: 4.9 MeV
Decays to 237
242Pu [148 neutrons]
Half life: 3.73x 105
years [ Alpha Decay ]
Decay Energy: 4.984 MeV
Decays to 238U.
244Pu [150 neutrons]
Half life: 8.08 x 107
years [ Alpha Decay ]