Heavy water is the common term for water which includes the deuterium isotope of hydrogen, 2H2O or D2O. The term can refer to water which contains any amount of deuterium, but often refers to water which has been enriched to contain at or near 100%.
The deuterium in heavy water is much the same as normal hydrogen, which is also known as protium, except that it contains an extra neutron. So a deuterium atom contains one neutron, one proton, and one electron. The extra neutron adds a small amount of weight, about 10% of the total weight, to the atom, which is what makes it heavy.
This type of water is most known for its role in regulating the processes of nuclear reactors that don’t use enriched uranium. As nuclear reactors engage in fission, they release neutrons. These neutrons move incredibly quickly. In order to have a more controlled chain reaction with unenriched uranium, the neutrons need to be slowed down. So to do that, a regulator of some sort is used. Beryllium, graphite, and normal water are all used as regulators, and all have their own benefits and limitations. Heavy water is another regulator, with the added neutron making it more stable to regulate neutrons passing through it.
In nature, deuterium occurs once for roughly every 4000-7000 normal hydrogen atoms. This means that about one in twenty-million water molecules are heavy water molecules. This material isn’t really created, as such, but it is refined. Normal water is drawn out of large reservoirs, and the small amount of heavy water is extracted from the lighter water. Heavy water is stockpiled by various nations and corporations, and since it isn’t really expended in its use as a nuclear regulator, these stockpiles grow over time.
There is an even heavier isotope of hydrogen, called tritium, but it isn’t used for heavy water in nuclear reactors. Tritium is radioactive, and very rarely occurs in nature, instead occurring most often as a byproduct of nuclear events. The water itself, thankfully, isn’t radioactive, but it is slightly toxic to humans. Even though it is largely the same as normal water, its increased weight does affect the speed of certain important chemical reactions, including cell division. The level at which heavy water becomes toxic can range from roughly 10% to 50% of total water in the body, depending on a number of factors. In nature, however, these levels will never be reached, so poisoning isn’t a real concern.
Because of its central role in regulating the fission process of breeder reactors, which can be used to produce weapon-grade plutonium, this substance is tightly regulated. The international community keeps a close eye on nations producing or procuring large amounts of it, as this can be a sign that the nation is moving towards generating nuclear weapons.