Reactor-grade plutonium is found in spent nuclear fuel that a nuclear reactor has irradiated for years instead of weeks or months, leading to transmutation of much of the fissile, relatively long half-life isotope Pu into other isotopes of plutonium that are less fissile or more radioactive. Thermal-neutron reactors can reuse reactor-grade plutonium only to a limited degree as MOX fuel, and only for a second cycle; fast-neutron reactors, which are uncommon today, can use this or any other actinide material indefinitely. The degree to which reactor-grade plutonium is less useful than... weapons-grade plutonium for building nuclear weapons is debated, with many sources saying it is difficult or impossible, and others saying it is relatively easy with modern technologies like fusion boosting to overcome predetonation, remote manipulation for assembly of highly radioactive components, and cooling of the pit during storage to offset accumulation of decay heat. The difference is important in assessing significance in the context of nuclear proliferation.
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