Plutonium-239

Plutonium-239 is an isotope of plutonium. Plutonium-239 is one of the three fissile isotopes used for the production of nuclear weapons and in nuclear reactors as a source of energy. Other fissile isotopes used are uranium-235 and uranium-233.

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Plutonium-239 has a half-life of 24,110 years. The nuclear properties of plutonium-239, as well as the ability to produce large amounts of nearly pure plutonium-239, led to its use in nuclear weapons and nuclear power. 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. Plutonium fissions provide about one-third of the total energy produced in a typical commercial nuclear power plant. The use of plutonium-239 in power plants occurs without it ever being removed from the nuclear reactor fuel, i.e., it is fissioned in the same fuel rods in which it is produced.

Manufacture

Pu-239 is normally manufactured 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 fast neutrons than with slow neutrons, although both can be used. The U-239 rapidly undergoes beta decay to produce Np-239, which rapidly undergoes a second beta decay, producing 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 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 supercritical mass before predetonation.

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. 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 acquire enough fuel to build a nuclear weapon using such a mix. Weapons-grade plutonium must contain 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.

Plutonium is classified according to the percentage of the contaminant plutonium-240 that it contains: Super grade 2-3%; Military grade less than 7%; Fuel grade 7-18%; Reactor grade 18% or more.

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 the fuel. A reactor running on unenriched or moderately enriched uranium naturally 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 RBMK and 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.)

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.

See also

• Teller-Ulam design
• Ching, Janice (1999). Half-Life of Plutonium-239. The Physics Factbook.