A moderator is not used to control the reaction, but to make it possible. The neutrons produced by fission, have high kinetic energy, and fuel has a small cross section for neutrons at such high energy. A neutron moderator slows down the neutrons, making them more likely to interact with the uranium and split them. typically water is a good moderator, but here they use graphite for obvious reason (high temperature salts doesn't play well with water)

That makes a lot of sense... thanks!

As for molten salts and water... oh yeah. That gets into very dangerous territory.[1] A very similar reaction seems to happen even with just molten table salt, where it "shouldn't".

  1 - https://www.youtube.com/watch?v=LmlAYnFF_s8

One of the great practical challenges with molten salt is to design a reliable valve. At temperatures this high and with the high corrosive properties of the medium, this is not trivial.

I didn't realize that the viscosity stays high for molten salt[1], that makes everything abrasive, along with dealing with possible phase change if the salt solidifies... yikes!

So, what we're dealing with is highly radioactive corrosive molasses with a constant stream of radioactive decay product gasses bubbling out of it, like Xenon and Iodine. Yeah, that seems more challenging that I first thought.

  1 - https://www.powermotiontech.com/home/article/21144640/armoring-a-valve-against-molten-salt