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What is Heavy Water?

Brendan McGuigan
Brendan McGuigan

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.

About one in twenty-million water molecules are heavy water molecules.
About one in twenty-million water molecules are heavy water molecules.

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.

Discussion Comments


@ TheDoctor - Heavy water plants have been around a lot longer than people realize and they are really only coming to light a lot more now because of the web and more widespread news channels. It was actually isolated in the 1930s.

From what I've read, the Telemark heavy water plant was able to produce 12 tons per year as a by-product of fertilizer production. Once Germany got wind of heavy water's capabilities, they began to consider it for inclusion in their atomic bomb project.


So when exactly was heavy water created or isolated? Is this a relatively new compound?


Did you know that there are a number of heavy water uses that do not include necessarily nuclear applications? You can actually use heavy water to suspend an oral polio vaccine -- I learned this when I was delivering vaccines on a mission trip one time.

Normal polio vaccines lose their potency the moment they are exposed to ambient temperatures. By suspending the vaccine in heavy water they can create a vaccine that is less sensitive to temperature and retains its potency longer. This is really important for tropical countries such as India.


this stuff sounds as if it should have never been discovered. Even though there is a slight risk of poisoning can't if be used to power cars so there will be less pollution?

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    • About one in twenty-million water molecules are heavy water molecules.
      By: arbaes
      About one in twenty-million water molecules are heavy water molecules.