Reactor-grade plutonium

Reactor-grade plutonium (RGPu)^[1][2] 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 ²³⁹
Pu absorbs a neutron, it does not always undergo nuclear fission. Sometimes neutron absorption will instead produce ²⁴⁰
Pu at the neutron temperatures and fuel compositions present in typical light water reactors, with the concentration of ²⁴⁰
Pu 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.