Then there is the safety problem with all graphite moderated reactors.
Graphite moderated reactors are less stable than water moderated reactors. You might recall that there was a slight problem when a graphite moderated reactor at Chernobyl overheated and caught fire. The very thing that the engineers said was impossible actually happened, so when engineers say that it is impossible to have a catastrophic accident with a graphite pebble reactor, I am skeptical.