You have to understand a bit about the subject. 1. Current reactors have U-238 and U-235. U-238 turns into Plutonium and some of that fissions but some is turn

You have to understand a bit about the subject. 1. Current reactors have U-238 and U-235. U-238 turns into Plutonium and some of that fissions but some is turned into heavier long lived transuranics. U-235 is very rare so we can't run just on that: this is what is enriched with centrifuges (no chemical separation possible as it's chemically similar to U-238) and what people mean when they say we will run out of uranium. Most of spent nuclear fuel is still just U-238. Maybe one could simplify and say that the spent fuel is not "used up" : instead it is "poisoned". 2. Thorium reactors have Th-232 and U-233. Simplified, the Th-232 turns into U-233 when it absorbs a neutron. U-233 fissions very well and releases more than two neutrons when it captures a neutron. There's quite many steps up to transuranics from that where fission can still happen, so you get very little transuranics. 3. There is the "breeder reactor" that would use just U-238 by making plutonium out of that. But you need fast spectrum and it's hard to control. It would have plenty of fuel though since there's lots of U-238. 4. What's more significant is that since U-233 is so good, you only need very little. AND you can use thermal spectrum making control easy. The thorium system is very clever in so many ways but it only works well if you design the whole reactor concept understanding what you have - neutronics and chemistry. It's not very complex at all, just read up on it, the basics can be understood in a couple of hours. Then you understand why it produces less waste, consumes less fuel, why putting thorium rods to solid fuel reactors doesn't make sense (protactinium) etc etc...