The thorium is secretly burned in small 'research' reactors to produce unregistered uranium for the doomsday machine. 
Except you can't make a bomb from U233.You can, it's just much more difficult than doing the same thing with plutonium, for several technical reasons. Not the least of which is that U233 has a quantum "delay" from the time it absorbs an energetic free neutron to the time it actually splits, and that delay can vary within a known time period. It's very short, but it adds a variable that 'traditional' nuclear fuels do not. This delay is not an issue in a reactor, because the goal is to not produce a cascading reaction that expands at a logarithimic rate, power reactors just need to keep the reaction between the navigational beacons. The difficulty in making a weapon, combined with it's relative ease of making it into fuel components, plus it's relative abundance all make it an ideal nuclear fuel for peaceful power. Add to that, the gamma signature of U233 can be detected & triangulated from orbit, and a U233 weapon is extremely impractical.
