You know what caused Chernobyl? People, not graphite moderators.
I'm not talking about the cause, I'm talking about the risk. Pebbles aren't any safer because the modelling shows that over time, they can crack and set the stage for the same kind of fallout. You have to once again count on people to be vigilant and replace the pebbles before it happens. That still doesn't does address the vast amount of waste they produce.