Plutonium: Encyclopedia II - Plutonium - Occurrence

Plutonium - Occurrence

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 ²³⁸U nuclei, initially forming ²³⁹U; two subsequent beta decays then form ²³⁹Pu (with a ²³⁹Np intermediary), which has a half-life of 24,100 years. This is also the process used to manufacture ²³⁹Pu in nuclear reactors. Some traces of ²⁴⁴Pu remain from the birth of the solar system from waste of supernovae, because its half-life (80 million yrs) is fairly long.

A relatively high concentration of plutonium was discovered at the Natural nuclear fission reactor in Oklo, Gabon in 1972. Since 1945, about 10 tons of plutonium have been released onto Earth through nuclear explosions.

Plutonium - Manufacture

The isotope Pu-239 is the key ingredient to most nuclear weapons. Its manufacture is therefore important to nuclear weapon states. Controlling or preventing the manufacture of refined Pu-239 is also important in preventing nuclear proliferation.

Pu-239 is normally manufactured in nuclear reactors. If U-238 is exposed to neutron radiation, the nuclei will occasionally capture a neutron, becoming U-239. This happens more easily with fast neutrons than with slow neutrons, although both can be used. The U-239 rapidly undergoes beta decay to give Np-239, which rapidly undergoes a second beta decay, giving Pu-239. Fission activity is relatively rare, so even after significant exposure, the Pu-239 is still mixed with a great deal of U-238 (and possibly other isotopes of uranium, oxygen, other components of the original material, and fission products). The Pu-239 can then be chemically separated from the rest of the material to give high-purity Pu-239 metal.

If Pu-239 captures a neutron, it becomes Pu-240. Pu-240 undergoes spontaneous fission at a relatively high rate. As a result, plutonium containing a significant fraction of Pu-240 is not well-suited to use in nuclear weapons; it emits neutron radiation, making handling more difficult, and its presence can lead to a "fizzle" in which a small explosion occurs, destroying the weapon but not causing fission of a significant fraction of the fuel. (The US has constructed a single experimental bomb using only reactor-grade plutonium.) Moreover, Pu-239 and Pu-240 cannot be chemically distinguished, so expensive and difficult isotope separation would be necessary to build a nuclear weapon using such a mix. Thus for the purposes of plutonium production, it is necessary to remove the produced Pu-239 frequently, before significant amounts of Pu-239 can be converted into Pu-240.

A nuclear reactor that is used to produce plutonium must therefore have a means for exposing U-238 to neutron radiation, and for frequently rotating this U-238. A reactor running on unenriched or moderately enriched uranium naturally contains a great deal of U-238. However, most commercial power reactor designs require the entire reactor to shut down, often for weeks, in order to change the fuel elements. They therefore produce plutonium in a mix of isotopes that is not well-suited to weapon construction. Such a reactor could have machinery added that would permit U-238 slugs to be placed near the core and changed frequently, or it could be shut down frequently, so proliferation is a concern; for this reason, the IAEA inspects licensed reactors frequently. A few commercial power reactor designs, RBMK and CANDU, do permit refueling without shutdowns, and they therefore pose a proliferation risk. (In fact, the RBMK was built by the Soviet Union during the cold war, so despite their ostensibly peaceful purpose, it is likely that plutonium production was a design criterion.)

Most plutonium is produced in research reactors or plutonium production reactors. Some production reactors are called breeder reactors because they produce more plutonium than they consume fuel; in principle, such reactors make extremely efficient use of natural uranium. In practice, their construction and operation is sufficiently difficult, and proliferation is a serious enough concern, that they are generally only used to produce plutonium. Plutonium reactors are generally (but not always) fast reactors, since fast neutrons are somewhat more efficient at plutonium production.

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 an 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 got 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.

Adapted from the Wikipedia article "Occurrence", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki