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)