Buckyball encased uranium - the smallest possible pebble bed

[Jeremy Thomson]

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?

 

[Astronuc]

Encasing one U atom in a shell of ~80 C atoms would make for a low fuel (U) to moderator (C) ratio.

Atomic bonds have energies on the order of eV, whereas the combined kinetic energy of the fission products is on the order of 168 MeV.

 

[raining Pete]

It's difficult to say for sure if there is a trend towards smaller pebble sizes in pebble bed reactors, as there are many different designs and approaches being explored. However, there are certainly advantages to using smaller pebbles, such as increased surface area for heat transfer and improved fuel utilization.

As for the idea of using a carbon buckyball to encapsulate a single uranium atom, it's an interesting concept but it's unlikely to be practical or feasible. The energy release from fission is indeed enormous, and it's difficult to imagine a carbon shell being able to contain it without breaking apart. Additionally, the cost of fabricating such a fuel would likely be prohibitively expensive.

It's also worth considering the potential safety concerns of using a buckyball as a fuel. As you mentioned, if fission were to occur, the buckyball would likely break apart and release fission products, which could be dangerous. It's also possible that the carbon shell could act as a moderator, but it's difficult to say for sure without more research and testing.

Overall, while the idea is intriguing, it's unlikely that we will see buckyball fuel being used in practical pebble bed reactors any time soon. There are many other promising designs and materials being explored that are more likely to be viable and safe options for nuclear energy production.