Moderator is required in a thermal reactor to slow down the neutrons produced in the fission reaction to .025 ev so that the chain reaction can be sustained. Different moderators normally in use are Heavy Water, Graphite , Beryllium and Light water. Heavy Water is an excellent moderator. A good moderator should have excellent slowing down power and low absorption cross section for neutrons. Heat energy produced in the fission reactor will be removed by coolant.
Water is an excellent coolant that can remove the heat from the feed.
Heavy Water is used as a primary coolant to transport heat generated by the fission reaction to secondary coolant, light water. In Gas cooled reactors carbon di-oxide gas is used as coolant. Coolant transports heat to secondary coolant, i.e water for generation of steam at an appropriate pressure for running steam turbines. Steam turbines drive generators to generate Electricity. It is not.