Light water is simply ordinary water that does not contain large amounts of deuterium, making it distinct from heavy water. Although this water does contain small numbers of heavy water molecules, it isn't enough to make any significant changes in its properties. Light water plays an important role in the generation of electricity from nuclear energy, as it can serve both as a moderator and a coolant to carry away the energy generated by nuclear fission.
In nuclear fission reactors, the neutrons must be slowed down to ensure an effective fission chain reaction occurs. This process of slowing neutrons down is known as moderation, and the material that slows down these neutrons is known as a neutron moderator. Light water can be used as a moderator in certain reactors, mainly pressurized water reactors and boiling water reactors. Light water can only work as a moderator in certain situations, as it absorbs too many neutrons to be used with uranium that is unenriched, so uranium enrichment is necessary to operate reactors that use light water as the moderator. This increases the overall cost of the operation, but makes light water reactors cheaper to build.
The simplest of these light water reactors is the boiling water reactor. More than 80% of the world's nuclear power plants use these light water reactors, with light water as their moderator. In light water reactors, there exists a thick-walled pressure vessel that contains the nuclear fuel and the moderator and coolant water circulates among the fuel rods to slow neutrons and carry away thermal energy.
There are several safety benefits that come from using light water as a moderator. First, the loss of any coolant deprives the reactor of its moderator, stopping the nuclear chain reaction. However, radioactive decay still continues to produce energy. As well, any increase in the temperature of the reactor causes the water to become less dense, reducing the level of moderation that the light water supplies and reducing the activity in the reactor. This means that if the reactivity increases too much, there will be less moderation to slow the nuclear reaction.