CANDU stands for Canada Deuterium Uranium. Deuterium is another name for heavy water, which occurs naturally in all bodies of water. In Lake Huron, it occurs one part in every 7,000. Once extracted, heavy water is 10% heavier than ordinary water due to an extra neutron in its nucleus giving it added weight.
Developed in Canada, the first CANDU reactor came on line in 1962. There are now 22 CANDU reactors in Canada and 17 abroad.