Reactor-grade plutonium is found in spent nuclear fuel that a nuclear reactor has irradiated (burnup/burnt up) for years before removal from the reactor, in contrast to the low burnup of weeks or months that is commonly required to produce weapons-grade plutonium, with the high time in the reactor(high burnup) of reactor-grade plutonium leading to transmutation of much of the fissile, relatively long half-life isotope ²³⁹Pu into a number of other isotopes of plutonium that are less fissile or more radioactive.