Add a neutron to Thorium-232 and it becomes protactinium for 30 days which then decays to Uranium-233. Uranium-233 is great bomb material and perfect reactor fuel, although as mentioned it is quite hot and difficult to work with. The key with thorium reactors, is that apart from the U-233 seed to start the process of converting fertile thorium into fissile material, no more needs to be added.

Thorium is all around us. Adding a neutron to it to create uranium is no simple matter if one doesn't have a high flux neutron supply (such as a reactor).