Plutonium-240

Plutonium-240 (Pu-240) is an isotope of the metal plutonium formed when plutonium-239 captures a neutron. About 62% to 73% of the time when Pu-239 captures a neutron it undergoes fission; the rest of the time it forms Pu-240. The longer a nuclear fuel element remains in a nuclear reactor the greater the relative percentage of Pu-240 in the fuel becomes. For weapons use, the fuel needs to be as low in Pu-240 as possible, usually around 7% of the total plutonium, but this is achieved by reprocessing the fuel after just 90 days of use. Such rapid fuel cycles are highly impractical for civilian power reactors and are normally only carried out with dedicated weapons plutonium production reactors. Spent civilian power reactor fuel typically has under 70% Pu-239 and around 26% Pu-240, the rest being made up of other plutonium isotopes, making it extremely difficult but not impossible to use it for manufacturing nuclear weapons.

240Pu has only about 1/3 as large a neutron absorption cross section as 239Pu, and nearly always becomes plutonium-241 rather than fissioning. In general, isotopes of odd mass number are both more likely to absorb a neutron, and more likely to fission on neutron absorption, than isotopes of even mass number. Thus, even mass isotopes tend to accumulate, especially in a thermal reactor.