The function of heavy water in a nuclear reactor is to

1. In a nuclear reactor, nuclear fission produces fast-moving neutrons. 2. For certain types of reactors (especially those using natural uranium), these fast neutrons need to be slowed down to thermal energies to efficiently cause further fission reactions. 3. Heavy water (Deuterium oxide, D2O) is an excellent moderator, meaning its primary function in these reactors is to slow down the speed of neutrons without absorbing them excessively. 4. While heavy water can also be used as a coolant (option C), its key role that distinguishes it, particularly compared to ordinary water, is its moderating property. Therefore, slowing down neutrons is its defining function as a moderator. 5. Option (B) is incorrect; moderators slow down neutrons. Option (D) is the function of control rods, not the moderator.