heavy water

American  

noun

1. water in which hydrogen atoms have been replaced by deuterium, used chiefly as a coolant in nuclear reactors.

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heavy water British  

noun

1. water that has been electrolytically decomposed to enrich it in the deuterium isotope in the form HDO or D 2 O

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heavy water Scientific  

1. Water in which deuterium, a heavy isotope of hydrogen, takes the place of hydrogen. Heavy water has physical and chemical properties that are like those of ordinary water, but heavy water is 10 percent heavier and has higher freezing and boiling points. Also called deuterium oxide. Chemical formula: D ₂ O.

2. ◆ Semiheavy water is similar to heavy water, but only one of the two hydrogen atoms in each molecule is replaced with deuterium. Chemical formula: DHO.

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heavy water Cultural  

1. Water with a higher average molecular weight than ordinary water. In a molecule of heavy water, the hydrogen atoms are isotopes in which the nuclei each contain a proton and a neutron, and hence are twice as heavy as a normal hydrogen atom.

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Closer Look

The nucleus of most hydrogen atoms consists of a single proton, but in one isotope of hydrogen, called deuterium or heavy hydrogen, the nucleus also contains a neutron and thus weighs nearly twice as much as standard hydrogen. The substance called heavy water is chemically identical to ordinary water (H₂O), except that the hydrogen atoms in the molecule are the deuterium isotopes (D₂O). Heavy water makes up a small percentage (0.02%) of water naturally occurring on Earth. It is an excellent moderator for nuclear reactions, slowing down the fast neutrons produced in a nuclear fission reaction, increasing the likelihood that the neutrons will successfully collide with heavy nuclei to cause further fission. Although heavy water is chemically nearly identical with ordinary water, it is about ten percent heavier and interferes with cell mitosis if consumed in place of normal water. Ice cubes made of heavy water are denser than ordinary liquid water and will sink to the bottom of a glass of cola.

Etymology

Origin of heavy water

First recorded in 1930–35

Example Sentences

Unlike Iran's Natanz uranium enrichment facility or the future Arak heavy water nuclear power plant, the Bushehr facility is not considered a proliferation risk.

From Barron's • Apr. 4, 2026

Maxar also provided imagery from two other key Iranian nuclear sites which showed no visible evidence of damage, specficially the the Arak heavy water reactor or the Fordow enrichment facility.

From BBC • Jun. 15, 2025

Near a destroyed boat and melted furniture hung three heavy water hoses that could have been used by firefighters.

From Los Angeles Times • Sep. 14, 2024

The heavy water grabs onto the ultrafine cement powder and starts to transport it down into the mix.

From Seattle Times • Mar. 18, 2024

Lewis soon was providing heavy water in quantity to Ernest Lawrence, who vaporized it into gas to pump into the cyclotron.

From "Big Science" by Michael Hiltzik