Why is heavy water used instead of water in a nuclear fission reaction?
Heavy water is water that has the mundane hydrogen atoms (protium) replaced by deuterium (hydrogen that has a neutron the protium does not have). The deuterium has the right mass to effectively slow down neutrons coming at high speed (by having them bounce off) so that they are at a speed that is ideal to break the next uranium nucleus. Light water can also be used (and it is in some reactors), but it is less effective because the protium have a tendency to absorb the neutron and turn into deuterium, taking them off from the game, so to speak. Heavy water is better than light water as a moderator, but it is not the only stuff that would work, if someone accepts the trade off.
