Heavy water

Heavy water differs from ordinary water because its hydrogen atoms are deuterium atoms, each containing one proton and one neutron. The oxygen atom is normally the common isotope oxygen-16, so the increased mass comes chiefly from the hydrogen isotope substitution.¹ Pure D₂O is colorless and chemically similar to H₂O, but it is denser, has somewhat higher melting and boiling points, and displays isotope effects in reaction rates and molecular motion.²

Natural water contains only a small proportion of deuterium, roughly one deuterium atom for several thousand hydrogen atoms. Heavy water is therefore made by concentrating an isotope that is naturally present rather than by creating deuterium through nuclear reactions. It is not radioactive: deuterium is a stable isotope, unlike tritium, which is radioactive.

Industrial heavy-water production separates deuterium-containing molecules from ordinary water through differences in chemical equilibrium and physical behavior. Processes have included electrolysis, distillation, and chemical exchange systems such as hydrogen sulfide–water exchange; large plants commonly combine stages to improve efficiency.³

Deuterium was discovered in 1931 by Harold Urey, Ferdinand Brickwedde, and George Murphy, work recognized with the 1934 Nobel Prize in Chemistry.⁴ During the Second World War, heavy water became strategically important because Germany investigated it as a neutron moderator. Allied concern focused on the Norsk Hydro plant at Vemork in Norway, where sabotage and military action disrupted production. The episode became one of the war’s best-known scientific-industrial campaigns.

Heavy water is valuable in certain nuclear reactors because it slows neutrons efficiently while absorbing relatively few of them. This allows a reactor to sustain a chain reaction using natural uranium, which contains only a small fraction of the fissile isotope uranium-235.⁵ Canada’s CANDU reactor design is the best-known heavy-water power-reactor system; heavy water serves as both moderator and, in many designs, coolant.

Its advantages come with engineering and security costs. Heavy water is expensive to produce and must be recovered from reactor systems to limit losses. Reactors using natural uranium can also produce plutonium in spent fuel, so international safeguards monitor related facilities, material flows, and reactor operations. Heavy water itself is not a fuel and does not make a reactor radioactive, although it becomes contaminated during service.

Heavy water has measurable biological effects because replacing ordinary body water with D₂O alters hydrogen bonding, enzyme kinetics, and cell division. Small exposures are generally handled by the body, but sufficiently high replacement levels are harmful; laboratory studies therefore use controlled concentrations rather than treating D₂O as biologically interchangeable with H₂O.²

Its isotope effects also make heavy water useful beyond power generation. Deuterated compounds help researchers trace metabolic pathways, study reaction mechanisms, and distinguish newly synthesized molecules from pre-existing ones. Heavy water has been used in stable-isotope techniques for estimating total body water and body composition, while deuterium-labeled molecules support research in chemistry, medicine, and pharmacology.⁶ In ordinary natural waters, deuterium abundance varies with evaporation, condensation, latitude, and altitude, making isotope ratios useful in hydrology and climate research.

Deuterium

A stable isotope of hydrogen whose nucleus contains one proton and one neutron; it is often written as ²H or D.

Deuterium oxide

The chemical compound D₂O, commonly called pure heavy water.

Neutron moderator

A material that slows fast neutrons, increasing the likelihood that they will cause fission in suitable nuclear fuel.

Isotope effect

A change in physical, chemical, or biological behavior caused by substituting one isotope for another.