Why didnt they build Thorium reactors instead of Uranium reactors in the first place, since they knew about this since the '50s?
"Benefits and challenges
A 2005 report by the International Atomic Energy Agency discusses potential benefits along with the challenges of thorium reactors. According to Australian science writer Tim Dean, "thorium promises what uranium never delivered: abundant, safe and clean energy - and a way to burn up old radioactive waste." With a thorium nuclear reactor, Dean stresses a number of added benefits: there is no possibility of a meltdown, it generates power inexpensively, it does not produce weapons-grade by-products, and will burn up existing high-level waste as well as nuclear weapon stockpiles. Ambrose Evans-Pritchard, of the British Daily Telegraph, suggests that "Obama could kill fossil fuels overnight with a nuclear dash for thorium," and could put "an end to our dependence on fossil fuels within three to five years." He also points out that "China is leading the way" with its own "dash for thorium," which it announced in March 2011.
Turkish nuclear expert Ayhan Demirbaş has summarized some of the benefits of thorium when compared with uranium as fuel:
Weapons-grade fissionable material (U-233) is harder to retrieve safely and clandestinely from a thorium reactor;
Thorium produces 10 to 10,000 times less long-lived radioactive waste;
Thorium comes out of the ground as a 100% pure, usable isotope, which does not require enrichment, whereas natural uranium contains only 0.7% fissionable U-235;
Thorium cannot sustain a nuclear chain reaction without priming, so fission stops by default.
However, unlike uranium-based breeder reactors, thorium requires a start-up by neutrons from a uranium reactor. But experts note that "the second thorium reactor may activate a third thorium reactor. This could continue in a chain of reactors for a millennium if we so choose." They add that because of thorium's abundance, it will not be exhaused in 1,000 years.
The Thorium Energy Alliance (TEA), an educational advocacy organization, emphasizes that "there is enough thorium in the United States alone to power the country at its current energy level for over 1,000 years."  Reducing coal as an energy source, according to science expert Lester R. Brown of The Earth Policy Institute in Washington DC, would significantly reduce medical costs from breathing coal pollutants. Brown estimates that coal-related deaths and diseases are currently costing the U.S. up to $160 billion annually.""http://en.wikipedia.org/wiki/Thorium#Benefits_and_challenges