heavy water 
noun [mass noun]
Water in which the hydrogen in the molecules is partly or wholly replaced by the isotope deuterium, used especially as a moderator in nuclear reactors.The most common moderators are substances of low atomic weight such as heavy water (deuterium oxide) or graphite....
- Because of their heavier weight, heavy water molecules evaporate less readily than light water molecules.
- In heavy water the hydrogen bonds are slightly stronger, and this changes the properties of the liquid sufficiently to disrupt the lubricating effect water has on biochemical processes.