What Is Heavy Water and What Is It Used For?

Known chemically as deuterium oxide (D₂O), heavy water is a clear liquid identical in appearance to ordinary water. While it is 10% denser, this greater viscosity is not visible to the naked eye. Unlike ordinary water (H₂O), heavy water includes two deuterium atoms and one oxygen atom (D₂O) in its formula. In addition to making it slightly denser, these characteristics give heavy water a slightly higher boiling point.

There are also varying grades of heaviness in water. This can include semi-heavy water (HDO) which is more commonly occurring than heavy water. Deuterium-depleted water (DDW) is water that has a lower concentration of deuterium and is also known as protium water or light water. However, ‘light water’ more commonly a term for regular water, especially when used in comparison to heavy water.

You can discover more about the properties of water with our article asking what is water condensation?

This type of water is obtained through various processes, such as water distillation, hydrogen distillation, electrolysis or isotopic exchange using deuterium, which is naturally found in any hydrogen compound. Most of these processes are neither economical nor attractive, with isotopic exchange processes being the most widely accepted and proven methods for large-scale production.

Due to its different properties, heavy water has industrial uses which are not the same as regular water (aka ‘light water’ when being compared to heavy water). We look at some of these uses of heavy water in more detail below.

The properties of heavy water are harnessed for various industrial purposes, including the following:

• Neutron moderator in nuclear reactors: heavy water plays a key role in certain types of nuclear reactors, particularly those of the CANDU (Canadian Deuterium Uranium) type. Its main function is to slow down the neutrons released during the nuclear fission process, facilitating their capture by uranium nuclei. This is essential for sustaining a chain reaction, as it allows the use of natural uranium without the need for enrichment. One of the great advantages of heavy water over light water is that it absorbs far fewer neutrons. While reactors moderated with light water require the use of enriched uranium to reach critical mass, heavy water reactors can operate efficiently with natural uranium, making them more accessible to certain countries.
• Tritium production: heavy water also plays an important role in the generation of tritium, a radioactive isotope of hydrogen with multiple applications in the nuclear field. Reactions can occur in certain types of reactors that lead to the formation of tritium, such as those using heavy water as a moderator and coolant. This is a result the interaction between heavy water and uranium. It is this capability that makes heavy water a strategic input for the development of advanced and experimental nuclear technologies.
• Scientific research: heavy water is used as an isotopic marker in biology, chemistry and medicine. By replacing common hydrogen with deuterium, researchers can follow the path of molecules within living organisms or in chemical reactions. This better allows them to study metabolic processes, pharmacokinetics or reaction mechanisms, among other complex biological interactions.
• Use in hydrological studies: heavy water is used as an isotopic marker in hydrology to study the movement of water in the hydrological cycle. Since it contains deuterium instead of hydrogen, it allows the identification and tracking of water in different natural environments, such as aquifers, rivers, lakes or soils.

Learn more about how water is stored in natural environments with our article asking what is an aquifer?

Heavy water is heavy in comparison to regular water, commonly known as light water. The other differences between the two include:

• Composition: while ordinary water (H₂O) is made up of two hydrogen atoms and one oxygen atom, heavy water (D₂O) contains two deuterium atoms. Deuterium an isotope of hydrogen that has an extra neutron. Although small at the atomic level, this change significantly alters its physical properties.
• Density: heavy water is 10% denser than regular water. Objects that float in regular water can sink in heavy water.
• Freezing and boiling points: heavy water has a higher freezing point (around 3.8ºC compared to 0ºC for ordinary water), as well as a slightly higher boiling point (101.4ºC instead of 100ºC). This means that heavy water freezes faster than light water and boils at a slightly higher temperature.
• Biological properties: from a biological point of view, ordinary water is essential for life. Although heavy water is not toxic in small quantities, it can interfere with cellular processes if consumed in large volumes, affecting the speed of some biochemical reactions.
• Use: ordinary water is used daily for human consumption, sanitation, agriculture, industry and hygiene. Heavy water is used almost exclusively in specialized contexts, such as nuclear reactors, scientific research, hydrological studies and other industrial applications, due to its deuterium-derived properties.
• Availability: light water is naturally abundant while heavy water is rare and scarce and must be obtained through specific processes.
• Taste: although they taste very similar, it is said that heavy water has a slight sweetness to it when consumed.

Although there is a difference between light water and heavy water, not all light water is the same. We can see why with our article asking why is sea water salty?

Ingesting heavy water in small amounts does not pose a serious health risk. Deuterium is naturally occurring and non-radioactive. It is actually present in very low concentrations in the tap water we drink every day. Our bodies can tolerate these minimal amounts perfectly.

Heavy water would only be dangerous if a person completely replaces ordinary water with heavy water for an extended period, after which point negative biological effects could occur. This is because deuterium alters the rate and behavior of some chemical reactions within cells, interfering with essential metabolic processes such as DNA replication, energy production and enzyme functions.

Some studies indicate that if approximately 20 to 25% of a person's total body water were to be replaced with heavy water, noticeable physiological effects could occur. If 50% or more were replaced, the effects could become toxic or even lethal. These scenarios are extremely unlikely, as heavy water is expensive, unavailable for consumption and its taste is somewhat different, making it difficult to access its potential danger.

For those wanting to know more about water potability, take a look at our related guide on how to make a homemade water filter for drinking water.

If you want to read similar articles to What Is Heavy Water and What Is It Used For?, we recommend you visit our Environment (other) category.