239Pu is normally created in nuclear reactors by transmutation of individual atoms of one of the isotopes of uranium present in the fuel rods.

), leaving a proton in the nucleus — the first β− decay transforming the 239U into neptunium-239, and the second β− decay transforming the 239Np into 239Pu: 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.

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.

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.

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.

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.

Such plutonium is produced from fuel rods that have been irradiated a very short time as measured in MW-day/ton burnup.

Such low irradiation times limit the amount of additional neutron capture and therefore buildup of alternate isotope products such as 240Pu in the rod, and also by consequence is considerably more expensive to produce, needing far more rods irradiated and processed for a given amount of plutonium.

The need to reduce radiation exposure justifies the additional costs of the premium supergrade alloy used on many naval nuclear weapons.

Fissioning of plutonium-239 provides more than one-third of the total energy produced in a typical commercial nuclear power plant.

[7] However, ingested plutonium is by far less dangerous as only a tiny fraction is absorbed in gastrointestinal tract;[8][9] 800 mg would be unlikely to cause a major health risk as far as radiation is concerned.

Lower proportions of 239Pu would make a reliable weapon design difficult or impossible; this is due to the spontaneous fission (and thus neutron production) of the undesirable 240Pu.