Nuclear Fuel Reprocessing

People who've paid any attention to the subject often have some misunderstandings about the relationship between spent fuel and the possibility of weapons proliferation. The source of the misunderstanding is that special production reactors were used in the weapons program and the material produced in them was used in making some kinds of atomic explosives. So we need to cover the difference between the weapons fuel cycle and the energy fuel cycle.

Wikipedia has some good information on reprocessing. For information on the difference between uranium and plutonium bombs, the US DOE has a good page on their history.

The first difference is the length of time the material is in the reactor. As the reaction goes on, some of the U238 turns into Pu239, going through some intermediate steps. If the material stays in much longer, some of the Pu239 turns into Pu240.

Pu240 makes the material unsuitable for weapons because it fissions spontaneously. When the bomb mechanism combines the fissile material into a critical mass, the fission of Pu240 causes it to pre-detonate, causing the material to separate, and the bomb only burbs instead of exploding.

So, in production reactors, the fuel has to be extracted in some short time, months instead of years, and sent to the separation facility. In power reactors, the fuel stays in for years and accumulates a high proportion of Pu240. To make the spent fuel into bomb material requires isotope separation on top of chemical separation. If a country has the resources to separate the plutonium isotopes, it would be better off separating uranium isotopes, because that would save it the trouble of operating a production reactor.

The second difference is the chemical process. For the weapons program, the whole point of processing was to separate out the plutonium. For commercial power, that's not necessary. It's cheaper and easier to keep the plutonium mixed with the leftover uranium.

Most (I think all) of the countries presently reprocessing spent fuel use the acid process, called PUREX, or something similar. In the US, a new facility is being developed at Savannah River, South Carolina, for an entirely different process based on molten salt instead of acid. This other process uses less energy and emits no significant amounts of greenhouse gases. In theory, it's less vulnerable to proliferation due to diversion of spent fuel; as we just saw, though, proliferation with spent fuel from commercial power plants isn't really a concern.