How to Breed U-238 Into Plutonium

The breeder reactor being developed at the Oak Ridge Laboratory was molten salt-cooled and didn't use a water moderator or coolant. Instead, it was fueled by thorium, another element which is four times more plentiful in Earth's crust than uranium. A distinct advantage of thorium as a fertile fuel source is that it can be bred into U-233 using slow neutrons.