How does a thorium reactor compare with a modular pebble bed reactor?
The pebble bed reactor uses uranium that's encapsulated in "pebbles." I'm not sure about the exact size, but think of them as tennis balls or softballs. In operation, an inert gas such as helium is passed through the "bed" to transfer the heat to a turbine or heat exchanger. I don't know whether thorium can be used in place of uranium. In any case, the "pebble bed" refers to a structure, not to the fissile material used.
All the discussion of thorium reactors revolves around liquid thorium fluoride. That's probably not the only way to utilize thorium in a reactor, but that's the way it's been looked at in the past. This approach seems to have many advantages over the way we now build uranium-fueled reactors, so should be investigated.