REMIX-Fuel (REgenerated MIXture of U, Pu oxides) was developed in Russia to simplify the reprocessing process, reuse spent fuel, reduce the consumption of natural uranium and to enable multi-recycling.
Approximately half of the plutonium-239 "bred" during the fuel cycle is fissioned and another 25% is transmuted through additional neutron capture into other plutonium isotopes, primarily Pu-240.
The longer the fuel remains in the reactor undergoing fission the more the uranium percentage decreases while the other materials increase.
Russia spent nearly a decade developing techniques similar to nuclear pyroprocessing that allows them to reprocess spent nuclear fuel without separating the recycled uranium and plutonium as is done in the PUREX chemical reprocessing system used to manufacture MOX fuel.
In December 2024 the third final 18-month phase of the program has started with the goal to achieve closed nuclear cycle for VVER reactors.