.The moderator helps slow down the neutrons produced by fission to sustain the chain reaction. Control rods can then be inserted into the reactor core to reduce the reaction rate or withdrawn to increase it. The moderator of a nuclear reactor is a substance that slows neutrons down.  When fast neutrons strike the hydrogen atoms in H2O, they slow down a lot (like a billiard ball striking another). There are other good moderators like graphite, beryllium,