Manufacturing
Pu-239 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 U-238 is exposed to neutron radiation, its nucleus will capture a neutron, changing it to U-239. This happens more easily with lower Kinetic Energy (as U-238 fission activation is 6.6MeV). The U-239 then rapidly undergoes two beta decays. After the 238U absorbs a neutron to become 239U it then emits an electron and an anti-neutrino by β− decay to become Neptunium-239 (239Np) and then emits another electron and anti-neutrino by a second β− decay to become 239Pu:
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. Only if the fuel has been exposed for a few days in the reactor, can the Pu-239 be chemically separated from the rest of the material to yield high-purity Pu-239 metal.
Pu-239 has a higher probability for fission than U-235 and a larger number of neutrons produced per fission event, so it has a smaller critical mass. Pure Pu-239 also has a reasonably low rate of neutron emission due to spontaneous fission (10 fission/s-kg), making it feasible to assemble a mass that is highly supercritical before a detonation chain reaction begins.
In practice, however, reactor-bred plutonium produced will invariably contain a certain amount of Pu-240 due to the tendency of Pu-239 to absorb an additional neutron during production. Pu-240 has a high rate of spontaneous fission events (415,000 fission/s-kg), making it an undesirable contaminant. 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. (However, in modern nuclear weapons using neutron generators for initiation and fusion boosting to supply extra neutrons, fizzling is not an issue.) It is because of this limitation that plutonium-based weapons must be implosion-type, rather than gun-type. (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 separate them. Weapons-grade plutonium is defined as containing no more than 7% Pu-240; this is achieved by only exposing U-238 to neutron sources for short periods of time to minimize the Pu-240 produced. Pu-240 exposed to alpha particles will incite a nuclear fission.
Plutonium is classified according to the percentage of the contaminant plutonium-240 that it contains:
- Supergrade 2–3%
- Weapons grade less than 7%
- Fuel grade 7–18%
- Reactor grade 18% or more
A nuclear reactor that is used to produce plutonium for weapons therefore generally has a means for exposing U-238 to neutron radiation and for frequently replacing the irradiated U-238 with new U-238. A reactor running on unenriched or moderately enriched uranium contains a great deal of U-238. However, most commercial nuclear 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 International Atomic Energy Agency inspects licensed reactors often. A few commercial power reactor designs, such as the reaktor bolshoy moshchnosti kanalniy (RBMK) and pressurized heavy water reactor (PHWR), do permit refueling without shutdowns, and they may 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.) By contrast, the Canadian CANDU heavy-water moderated natural-uranium fueled reactor can also be refueled while operating, but it normally consumes most of the Pu-239 it produces in situ; thus, it is not only inherently less proliferative than most reactors, but can even be operated as an "actinide incinerator." The American IFR (Integral Fast Reactor) can also be operated in an "incineration mode," having some advantages in not building up the Pu-242 isotope or the long-lived actinides, either of which cannot be easily burned except in a fast reactor. Also IFR fuel has a high proportion of burnable isotopes, while in CANDU an inert material is needed to dilute the fuel; this means the IFR can burn a higher fraction of its fuel before needing reprocessing. Most plutonium is produced in research reactors or plutonium production reactors 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 that they are generally only used to produce plutonium. Breeder reactors are generally (but not always) fast reactors, since fast neutrons are somewhat more efficient at plutonium production.
Read more about this topic: Plutonium-239