The byproducts of the operation of fission reactors can be managed in two ways: without treatment that supports recycle or with the recycle option enabled. In the former approach (once-through or open cycle), the fissile resource is the 235U taken from the ground (enriched to 3-5% to allow light water moderation of neutrons) and the several percent of 238U that is trans­muted to 239Pu and fissioned during normal reactor operations. The used fuel once removed from the reactor is considered as waste and is managed accordingly. The latter option (closed loop) can be pursued with varying degrees of processing, though at present only plutonium isotopes are recy­cled in mixed oxide (MOX) fuel (while the residual 235 236 238u is stored for future use).

Though plutonium production reactors and reprocessing facilities oper­ated within the nuclear weapons complex in the US between 1944 and the early 1990s, the reprocessing of spent fuel from commercial reactors was practiced for only a very brief period during the 1970s. A de facto morato­rium was placed on reprocessing of commercial spent nuclear fuel in the US in 1977; this ban was lifted in 1981, but no attempts were made to revisit this option until recently. A 2005 energy bill has again allowed consideration of more complete used fuel management and has spurred a modest revival of research into reprocessing (and transmutation) options. However, current US nuclear fuel management policy remains the once-through option with direct disposal in a deep geological repository.

Most nuclear power producing nations practice the once-through option; France, Japan, the UK and Russia operate at least partially closed fuel cycles in which fuel grade Pu is recovered for recycle. At present, no country operates a more complete recycling program, though research exploring options is in its third decade. It can easily be understood that the waste management issues associated with these options are markedly different (although ultimately it is generally accepted that every option will require a geological repository for the residual radioactive materials).