To answer Jay's question about why people haven't built thorium reactors before now, there are a few reasons:
- Cost. Building a uranium reactor of known design is already very expensive (billions of dollars), and some of the proposed improvements on uranium technology (like gas-cooled pebble bed reactors or supercritical steam reactors) would cost tens of billions to develop. The Liquid-Fluoride Thorium reactor would have challenges regarding new materials and on-line reprocessing, and so be even more expensive.
- Politics. There is NIMBY-ism, environmental concern, proliferation concern, and so on. Even fairly sensible (technical) solutions like Yucca mountain are defeated by politics.
- Short term thinking. Like other breeder reactors, thorium can get 100x or more energy out of a certain amount of fuel, but it takes about 100x as long to do it. Thorium and uranium breeders can support civilization for hundreds of millennia, but our outlook isn't that long. The bigger benefit is that it produces 100x less nuclear waste, which may win the day.
- Cheap fuel. The price of uranium mining and enrichment is low enough that we can afford to waste 99% of it by using a non-breeder reactor.
- American and Soviet weapons. It's true what Steve said, people in the 1950s realized the benefit of thorium for power generation, but it was more important to make lots of warheads, so they pursued uranium reactors instead.
- Non-American weapons. Proposed designs require fuel reprocessing, which has proliferation concerns. It's harder to make weapons from U-233 (the fissile output of a thorium breeder) than U-235 or Pu-239, but not impossible. Also, any radioactive material can be used to make a dirty bomb.
- Competitors. There are other interesting proposals, like the uranium-burning Traveling Wave breeder supported by Bill Gates, that show promise, but also face many of the same roadblocks. Of course, the major competitor isn't nuclear at all. The fossil-fuel lobby is powerful, and it doesn't want to be put out of business.