The minimization of the amount of , as in weapons-grade plutonium (less than 7% ) 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. Plutonium from spent civilian power reactor fuel typically has under 70% and around 26% , the rest being made up of other plutonium isotopes, making it more difficult to use it for the manufacturing of nuclear weapons. For nuclear weapon designs introduced after the 1940s, however, there has been considerable debate over the degree to which poses a barrier for weapons construction; see the article Reactor-grade plutonium.