Is there a trend in pebble bed reactors towards smaller pebble sizes? The smallest conceivable would be a carbon buckyball encasing a (single?) uranium atom. This Wikipedia article (https://en.wikipedia.org/wiki/Endohedral_fullerene) says two cerium atoms have been encapsulated in a c80 buckyball. So I guess there's enough room inside an arbitrarily large carbon fullerene for a uranium atom. I have no idea if this is actually possible. If fission were to occur would the fullerene necessarily split apart? I believe they are quite stretchy, of course fission is an enormous energy release. I just don't know the strength of c80 vs the energy of fission. I can't envisage buckynuke fuel being at all practical, most likely too expensive to fabricate. You'd need have some separator (piping) for coolant. Most likely the fullerene wouldn't survive fission so graphene fragments and loose fission products making mischief. But as a thought experiment would the carbon shell act well as a moderator? Or would the carbon shell act to protect the uranium from thermal neutrons, by bouncing neutrons away from the encapsulated uranium?