The Effect of Spontaneous Fission of Plutonium-240 on the Energy Release in a Nuclear Explosive

Plutonium-240

• Plutonium-240 is a radioactive isotope of plutonium. The number 240 refers to the total number of particles in its nucleus. Plutonium has the atomic number 94, which means it has 94 protons.

  A different isotope of plutonium, plutonium-239, is the main isotope used in the production of nuclear explosives. However, plutonium-240 is present in nuclear explosives in small amounts. Plutonium-240 can be split by nuclear fission, which involves firing a neutron at it, and spontaneous fission.

Spontaneous Fission

• A heavy isotope that breaks down into smaller nuclei and single particles without any outside influence is said to have undergone spontaneous fission. Spontaneous fission only occurs in isotopes with a mass greater than about 60 atomic mass units, 60 times the mass of a single proton.

  As the nucleus of plutonium-240 has a mass approximately 240 times that of a single proton, it can undergo spontaneous fission and does this with a relatively high rate. Spontaneous fission of plutonium-240 in an explosive can affect the explosive's eventual energy release.

Reduction in Energy Release

• The presence of plutonium-240 in a mass made mostly of other isotopes of plutonium reduces the usefulness of the mass as an explosive. This is because of plutonium-240's high rate of spontaneous fission. When the plutonium-240 isotope breaks down by spontaneous fission, neutrons are released. Each of these neutrons could potentially set off a chain reaction, prematurely using some of the plutonium and reducing the amount available for a final explosion. This would reduce the energy release of the explosion.

Consequences

• Because plutonium-240 reduces the eventual energy release, the amount of plutonium-240 in a sample of mainly plutonium-239 determines its grade. Almost pure plutonium-239 with up to 7% plutonium-240 is classed as "weapons grade." Samples with higher plutonium-240 percentages are either "fuel grade" (between 7 and 20% plutonium-240) or "reactor grade" (more than 20% plutonium-240). Any of these grades can be made into nuclear weapons, but using "weapons grade" plutonium is more convenient because the more plutonium-240 a sample has, the less energy is released.