Twice the mass, and it shows
Isotopes of the same element are usually interchangeable. Carbon 13 behaves like carbon 12 in every reaction that matters, differing by 8 % in mass.
Deuterium is 100 % heavier than protium. Nothing else in the periodic table shows that proportion, and it is why the hydrogen isotopes have names rather than numbers.
The consequences are measurable. Heavy water boils at 101.4 °C rather than 100, freezes at 3.8 °C rather than 0, and is 11 % denser — ice made from it sinks in ordinary water.
⚠️ More useful to chemists: a bond to deuterium breaks more slowly than a bond to protium, by a factor of six or seven. If replacing a hydrogen with deuterium slows a reaction down, that hydrogen was involved in the slow step. This kinetic isotope effect is one of the few tools that reveals a reaction mechanism directly.
Harold Urey identified deuterium in 1931 and received the Nobel Prize in Chemistry three years later.
Moderating without absorbing
A reactor needs to slow its neutrons. Fast neutrons pass through uranium 235 without causing fission; slowing them to thermal energies increases the chance of fission enormously.
Anything light will slow them, but the moderator must also not absorb them. Ordinary water is a good slower and a mediocre absorber — enough that a reactor using it needs uranium enriched to 3 to 5 %.
Heavy water absorbs far fewer neutrons, because deuterium already has the neutron that protium would capture. That margin is enough for a reactor to run on natural uranium, with no enrichment at all.
⚠️ Which is the whole strategic point. A country with heavy water can build a reactor without building an enrichment plant, and it can produce plutonium in that reactor. This is why heavy water is a controlled export and why heavy-water reactor programmes attract attention.
Canada built its power programme this way. So did India, which produced its first weapon plutonium in a heavy water research reactor supplied for civil use.
Vemork
The Norsk Hydro plant at Vemork, near Rjukan, made hydrogen by electrolysis for fertiliser. Heavy water accumulated in the cells as a by-product, because the lighter isotope electrolyses slightly faster and the residue enriches in deuterium.
By 1940 it was the only significant producer in Europe, and Germany’s reactor programme needed it.
Norwegian commandos parachuted in, skied across the Hardanger plateau in winter, climbed a gorge the Germans considered impassable, and destroyed the production cells in February 1943 without a shot fired. Allied bombing followed. In 1944 saboteurs sank a ferry carrying the remaining stock across Lake Tinn.
⚠️ How much difference it made is debated — the German programme had other problems, including a decision to abandon graphite as a moderator based on measurements made with contaminated material. But the operation removed the only European supply of a material the programme had committed to.
Tracing without radiation
Because deuterium is stable, it can be given to people.
Doubly labelled water — containing deuterium and oxygen 18 — is the standard method for measuring energy expenditure in free-living humans. The subject drinks a dose and provides urine samples over the following days; the two isotopes leave the body at different rates, and the difference gives carbon dioxide production and so energy use.
It is how the field learned that human energy expenditure is far less variable across populations and lifestyles than had been assumed.
⚠️ Deuterium is also used in drug design. Replacing a hydrogen at the site where an enzyme attacks a drug slows the breakdown, extending how long the drug lasts. The first deuterated drug was approved in 2017, and it is a real design strategy rather than a curiosity.
Drinking it
Heavy water is not radioactive and a glass of it is harmless.
Replacing a substantial fraction of the body’s water is another matter. Cell division depends on reactions involving hydrogen bonds, and slowing them by a factor of six disrupts mitosis. Mammals fed heavy water as their sole source die when body water reaches roughly 25 % deuterium.
⚠️ Reaching that requires drinking almost nothing else for days, and heavy water costs enough to make it an expensive way to come to harm. The occasional sensational framing of heavy water as poisonous is technically true and practically irrelevant.