The Canadian designed CANDU reactor can use unenriched uranium fuel. Is this a big advantage? I guess these reactors must operate less efficiently than reactors using enriched uranium fuel. At any rate, are they being built today? What does the spent fuel turn into-can it be reprocessed?
CANDU reactors are no less efficient than enriched uranium designs, but they require the use of heavy water (D[sub]2[/sub]O) as a moderator. A great deal of Canada’s electricity is generated by CANDU reactors. Several other countries have purchased them as well. That link will also tell you a bit about how the system works.
We have a lot of uranium when compared to most nations. Unenriched uranium is easy and “relataively” safe to handle.
Processing uranium to enrich it is expensive. Heavy water (which CANDUs use) is a terrific moderator that is very efficient in dealng with the neutrons, so instead of enriching uranium, we process heavy water.
Running on unenriched uranium allows for a low pressure system, which is a lot less expensive to build, and which also makes it fairly simple to switch fuel bundles on the fly without costing down time. The lower intensity does not lead to metal fatigue of the reactor as quickly, although it is still a major problem that significantly reduces the life of our reactors.
In Ontario, spent uranium bundles are kept on-site in swimming pools for the first decade, and then stored in drums. Off-site storage has yet to be determined, but burial in the Canadian Shield is often suggested.
Theoretically, used uranium from a CANDU could be reprocessed, but it isn’t economical.
Of interest is that a CANDU can burn all sorts of crap, including spent fuel from an enriched uranium reactor, or even plutonium from surplus bombs, At the other end of the scale, a CANDU can burn thorium in the mix (which is a more common element than uranium).
Here is a nice summary on CANDU:
http://www.nuclearfaq.ca/brat_fuel.htm
One important and good thing about CANDU reactors is that the D[sub]2[/sub]O moderator is required for the reaction to take place. It also cools the reactor. If it leaks out, the reaction simply stops. No meltdown. 
I can’t believe this doesn’t get mentioned more.