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Production

Because plutonium occurs in nature in only trace amounts, it cannot be mined. Large amounts of plutonium can, however, be produced in nuclear reactors by transmutation from uranium-238, by far the most common isotope of that rather abundant element. A reactor rated at 100 megawatts thermal can produce 100 grams (3.5 oz) of plutonium a day. When an atom of uranium-238 is struck by a neutron, it becomes uranium-239. Uranium 239 decays, with a half-life of 23 minutes, into neptunium-239, which then itself decays, with a half-life of 56 hours, into plutonium-239—the fissile, stable isotope sought. An estimated 500 metric tons of plutonium had been produced by 2008.

The several "grades" of plutonium produced are defined by the balance of isotopes they contain, mostly by the proportion of Pu-239 and Pu-240. The different grades of plutonium reflect mainly how long each stayed in the reactor. Pu-240 is produced readily from Pu-239 by its capturing a neutron, and the proportion of Pu-240 rises over time as long as the plutonium remains in the reactor, so it is the most troublesome isotope for the producer. The amount of Pu-240 present is important because it, unlike its well-behaved cousin Pu-239, fissions spontaneously, producing high-energy neutrons and heat. It also causes a nuclear explosion to "fizzle" by fissioning before the pit has had time to be compressed to criticality. Super-grade plutonium contains less than 3 percent Pu-240, making it the most suitable for nuclear weapons, owing to the relatively small size of the pit that can be made with it. Weapon-grade contains less than 7 percent Pu-240, making it acceptable for nuclear weapons. Fuel-grade contains between 7 and 18 percent Pu-240 and is sufficiently pure to be used in propulsion systems. Reactor-grade contains more than 18 percent Pu-240 and additionally contains a significant amount of other isotopes, rendering it unstable and dangerous to handle.

Plutonium is produced whenever U-238 is bombarded with neutrons, so all electricity-generating nuclear reactors produce plutonium, because the nuclear fuel in their core is about 95 percent U-238 when fresh. The plutonium thus produced is, however, of very low quality and would require extensive and costly processing to purify it. Also, much of the plutonium produced as a by-product of power generation is itself consumed by fission before the fuel is swapped out. If the fuel is to be removed while the plutonium is still there in the fuel and has not yet degraded, it has to be done every few weeks, necessitating shutting down the reactor. These factors make the production of plutonium in the core of ordinary power-generating reactors so inefficient as to be impractical. Certain power reactors, such as the CANDU, can be refueled without shutting down, and these could be used to produce plutonium from their fuel.

Any reactor can be modified for relatively efficient plutonium production by providing for the exposing of peripheral U-238 to neutrons from the core. The U-238 thus bombarded is in the form of an array of pellets or a jacket or blanket around the core. Some reactors specially designed for plutonium production use this method, such as breeder reactors.

Production reactors are specially designed to produce high-quality plutonium in their fuel.