Not the best direct source, but this is asserted to be from Nuclear Engineering International magazine in November 2009 http://www.greenpeace.org/international

Not the best direct source, but this is asserted to be from Nuclear Engineering International magazine in November 2009 http://www.greenpeace.org/international/community_images/88/2288/98676_156425.jpg http://www.greenpeace.org/international/community_images/88/... via http://www.greenpeace.org/international/en/news/Blogs/nuclear-reaction/the-mythologies-of-thorium-and-uranium/blog/48625/ http://www.greenpeace.org/international/en/news/Blogs/nuclea... Thorium waste appears substantially less radioactive at shorter timescales we care about. On really long (10^4 - 10^6 years) timescales the proactinium-231 becomes an issue and uranium waste pulls ahead of thorium waste slightly. As we don't really have any idea how to plan on those timescales anyway (it's not like they're still going to be sitting in casks in a pool of water in New Jersey in 10^4 years), I think thorium is a lot better candidate from this perspective.