I'm not a nuke physicist, but I recall LFTR presentations on the "safety plug" to stop meltdown: basically a plug melts and the fluid flows into a shallow pool.

Since the fluid is "thinned", neutron economy plummets and chain reactions stop.

In addition, I though the fluid would expand a bit under high heat in the reactor, which would drop neutron economy as well, so a fluid can somewhat self-moderate.

So unless the fluid is in a ball or tank where the neutron economy is maintained in three dimensions, if you drain it out it no longer has the neutron economy.

So what they probably are saying is that since the fuel is fluid, and if you need to replace the moderator, you drain the fuel (and as stated above, that stops the reaction as well) and replace the moderator.

If the fuel is designed such that it needs a graphite moderator to sustain a reaction, then if the fuel is removed from the reactor it's less likely to go critical than if the fuel was "higher-grade". It's less likely to have a criticality accident.

A moderator is a material that slows neutrons down. Slow neutrons are more likely to initiate fission in a Uranium (or other fissile) nucleus that they hit, so a moderator increases the reactivity of the reactor.

Thanks, that’s exactly the detail I was misunderstanding.