Supergrade Plutonium

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 ($\bar_e$), leaving a proton — the first β⁻ decay transforming the ²³⁹U into neptunium-239, and the second β⁻ decay transforming the ²³⁹Np into ²³⁹Pu:

:^_U + ^_n -> ^_U -> beta^-23.5\ \ce] ^_Np -> beta^-2.356\ \ce] ^_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 yield high-purity ²³⁹Pu metal.

²³⁹Pu has a higher probability for fission than ²³⁵U and a larger number of neutrons produced per fission event, so it has a smaller critical mass. Pure ²³⁹Pu 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 will invariably contain a certain amount of ²⁴⁰Pu due to the tendency of ²³⁹Pu to absorb an additional neutron during production. ²⁴⁰Pu 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 ²⁴⁰Pu 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. Moreover, ²³⁹Pu and ²⁴⁰Pu 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% ²⁴⁰Pu; this is achieved by only exposing ²³⁸U to neutron sources for short periods of time to minimize the ²⁴⁰Pu produced.

Plutonium is classified according to the percentage of the contaminant plutonium-240 that it contains:

* Supergrade 2–3%
* Weapons grade 3–7%
* 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 ²³⁸U to neutron radiation and for frequently replacing the irradiated ²³⁸U with new ²³⁸U. A reactor running on unenriched or moderately enriched uranium contains a great deal of ²³⁸U. 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 ²³⁸U 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 ²³⁹Pu 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 accumulating the plutonium-242 isotope or the long-lived actinides, 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.

Plutonium-239 is more frequently used in nuclear weapons than uranium-235, as it is easier to obtain in a quantity of critical mass. Both plutonium-239 and uranium-235 are obtained from Natural uranium, which primarily consists of uranium-238 but contains traces of other isotopes of uranium such as uranium-235. The process of enriching uranium, i.e. increasing the ratio of ²³⁵U to ²³⁸U to weapons grade, is generally a more lengthy and costly process than the production of plutonium-239 from ²³⁸U and subsequent reprocessing.

Supergrade plutonium

The "supergrade" fission fuel, which has less radioactivity, is used in the primary stage of US Navy nuclear weapons in place of the conventional plutonium used in the Air Force's versions. "Supergrade" is industry parlance

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