All of the stages
The problems with this? You've got a whole lot of highly radioactive, very dangerous stuff being shipped around the planet -- the very dangerous spent fuel from reactor to reprocessing plant, and the plutonium heading back out to reactors. Unlike uranium, which requires elaborate technology to enrich from reactor to weapon grade, plutonium can be separated out chemically, by a smart college kid. All of the stages in this cycle make excellent targets for attacks by depraved maniacs, whether to cause a disaster on the spot or to steal material for use in weapons. Terrorists aside, plain old accidents would be unacceptable. Furthermore the process generates a stream of highly radioactive liquid waste which is hard to dispose of. Reprocessing and other technologies to extend the nuclear fuel resource may become part of the global energy mix in the future, but that won't make sense until we have nuclear disarmament -- so the plutonium is not subject to diversion -- and a technological regime which can implement these processes safely and securely. We are nowhere close to that yet.
The problems with this? You've got a whole lot of highly radioactive, very dangerous stuff being shipped around the planet -- the very dangerous spent fuel from reactor to reprocessing plant, and the plutonium heading back out to reactors. Unlike uranium, which requires elaborate technology to enrich from reactor to weapon grade, plutonium can be separated out chemically, by a smart college kid. All of the stages in this cycle make excellent targets for attacks by depraved maniacs, whether to cause a disaster on the spot or to steal material for use in weapons. Terrorists aside, plain old accidents would be unacceptable. Furthermore the process generates a stream of highly radioactive liquid waste which is hard to dispose of. Reprocessing and other technologies to extend the nuclear fuel resource may become part of the global energy mix in the future, but that won't make sense until we have nuclear disarmament -- so the plutonium is not subject to diversion -- and a technological regime which can implement these processes safely and securely. We are nowhere close to that yet.