What is the process by which spent uranium can be reclaimed in fast-neutron reactors?
Spent uranium is usually reclaimed in thermal-neutron reactors.
This process is possible only in CANDU reactors and other similar types, which use heavey water as a moderator (a moderator slows neutrons to a speed at which they are more likely to be absorbed by a nuclei, as the neutrons impact the molecules of the moderator and are slowed).
Normal water (usually refered to as light water when dealing with nuclear reactors) is H20, H being a hydrogen atom, which does slow the neutrons, but it also sometimes absorbs neutrons that impact it. This means less neutrons are getting through to the reactor core, which means once a fuel bundle has less than a certain percentage of fissionable material left in it (idealy it should be about 5% for commercial reactors, that it, before it has been used. It can only continue to be productive in a light-water reactor above about 1- 2%.), not enough neutrons are getting through to keep the reactor critical (the point where enough neutrons are being released from fissions to sustain a chain reaction), and the fuel is discarded.
Heavy water, on the other hand, is D2O, with two deuterium atoms (an isotope of hydrogen which has 1 proton and 2 neutrons, instead of the regular 1 and 1). This means that the hydrogen atoms already have an extra neutron, making them less likely to absrob the neutrons they are supposed to be slowing down.
Thus more neutrons are getting through, and there are enough to cause fission in a significant portion of the remaining fissionable material, allowing such reactors to run on depleted uranium.