My two cents (in relation to the OP on Gen IV reactors), but several of the Gen IV reactors designs actually can't be used for enriching to weapon levels. Yes the same idea of reprocessing spent fuel exists with Gen IV reactors, but the process used to recycle the fuel is a totally different one, one that actually makes it harder to obtain the weapon fuels out of the recycled material due to the way that they are bound up in the fuel. (I mean it would seriously be easier to just process the fuel to weapons-grade in the first place if you wanted to rather then running it through this type of reprocessing and then trying to extract it).
The basic problems the U.S. has with Gen IV reactors are these:
1)They are expensive to build and make, and since Uranium is so cheap right now nobody wants to spend money on them, regardless of the fact that we have tons of nuclear waste piling up without a place to put it. (Of course currently the U.S. government is losing millions of dollars in lawsuits every year because they promised the nuclear companies that they would have a place to store their waste (by 2010 IIRC), and now that they've canceled Yucca Mountain they don't have one.)
2)When the laws were made "reprocessing" equaled PUREX (the process that generates weapon-grade materials). Since then several new types of reprocessing have been invented that don't allow for the creation of weapons-grade materials, but since the law says "reprocessing is banned" then all of these are, and nobody wants to take the trouble to go back and get it changed to say "PUREX is banned (excluding military use)".
Personally I've got to go with the SFR Reactor (Gen IV) for the win though. I mean it runs on spent fuels up to and including weapons-grade material. It only requires highly fissile stuff (that could potentially be transformed into weapons-grade) to start it up in the first place, after which it will run on old fuels that normally would last millions of years. It can potentially burn up to 99% of the fuel put into it, assuming you just keep reprocessing it over and over again. The wastes that it does have generally only last for centuries as opposed to millenniums. And lastly some of the designs are immune to power-failure induced meltdowns like we almost saw in Japan since they are designed in such a way that if power is cut it stops the reaction from continuing rather then the reaction continuing but the cooling system stopping. I mean sure if they leak they might leak Sodium (highly reactive with water) and Argon (asphyxiant), but personally I would rather have a fire and suffocation problem at the site then a nuclear meltdown that could potentially kill everything in the area for decades.