Reactor-grade plutonium

Reactor-grade plutonium (RGPu) is the isotopic grade of plutonium that is found in spent nuclear fuel after the uranium-235 primary fuel that a nuclear power reactor uses has burnt up. The uranium-238 from which most of the plutonium isotopes derive by neutron capture is found along with the U-235 in the low enriched uranium fuel of civilian reactors. In contrast to the low burnup of weeks or months that is commonly required to produce weapons-grade plutonium (WGPu/239Pu), the long time in the reactor that produces reactor-grade plutonium leads 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. When 239Pu absorbs a neutron, it does not always undergo nuclear fission. Sometimes neutron absorption will instead produce 240Pu at the neutron temperatures and fuel compositions present in typical light water reactors, with the concentration of 240Pu steadily rising with longer irradiation, producing lower and lower grade plutonium as time goes on. Generation II thermal-neutron reactors (today's most numerous nuclear power stations) can reuse reactor-grade plutonium only to a limited degree as MOX fuel, and only for a second cycle. Fast-neutron reactors, of which there are a handful operating today with a half dozen under construction, can use reactor-grade plutonium fuel as a means to reduce the transuranium content of spent nuclear fuel/nuclear waste. Russia has also produced a new type of Remix fuel that directly recycles reactor grade plutonium at 1% or less concentration into fresh or re-enriched uranium fuel imitating the 1% plutonium level of high-burnup fuel. At the beginning of the industrial scale production of plutonium-239 in war era production reactors, trace contamination or co-production with plutonium-240 was initially observed, with these trace amounts resulting in the dropping of the Thin Man weapon-design as unworkable.