Heavy water: production and function in a nuclear reactor

Heavy water: production and function in a nuclear reactor

Heavy water is a form of water that contains a higher amount of deuterium, an isotope of hydrogen that has an extra neutron in its nucleus. While common water (H2O) contains two hydrogen atoms and one oxygen atom, heavy water (D2O) contains two deuterium atoms and one oxygen atom.

Heavy water is slightly denser than regular water and has some different chemical and physical properties.

What is heavy water used for?

Heavy water has various applications in industry and science due to its unique physical and chemical properties. Some of the most common applications are:

  1. Nuclear Reactor Neutron Moderator: Heavy water is used as a neutron moderator in some nuclear reactors to slow down the neutrons released in nuclear fission and increase the likelihood of further nuclear reactions.

  2. Tritium production: Tritium is a radioactive isotope of hydrogen used in the production of nuclear weapons and in some lighting and medical devices. This type of water is used in the production of tritium through a process called radiolysis, in which tritium is formed from the exposure of heavy water to radiation.

  3. Scientific research: In some scientific experiments it is used as a tracer to study the dynamics of water in different biological and chemical systems.

  4. Polymer production: in this case it is used as a reagent in the production of some polymers, such as nylon, polyester and polyurethane.

  5. Chemical analysis: for example, to determine the isotopic composition of certain compounds.

How do you get heavy water?

Heavy water (D2O) can be obtained through various processes, but the most common methods are through fractional distillation of water and hydrolysis of ammonia water.

The production of heavy water requires a very careful purification process, since even small impurities can significantly affect its physical and chemical properties.

Fractional distillation

The fractional distillation process involves heating water until it evaporates and then condensing the steam to collect the distilled water. Because hydrogen isotopes have different boiling points, the water vapor that is collected will be deuterium-enriched.

This steam is then passed through isotope exchange columns, where heavy water is separated from normal water.

Hydrolysis of ammoniacal water

Another method of obtaining heavy water is through the hydrolysis of ammonia water, where the deuterium in the ammonia is exchanged with hydrogen in the water to produce heavy water.

It is also possible to produce heavy water using nuclear reactors, although this method is more expensive and is mainly used for large-scale production.

What role does heavy water serve in a nuclear reactor?

Heavy water has a crucial role in some nuclear reactors as a neutron moderator. In CANDU (Canada Deuterium Uranium) nuclear reactors, for example, heavy water is used as a moderator to slow down the neutrons released in nuclear fission.

Fast neutrons generated in fission are less likely to cause additional nuclear reactions than slow neutrons. By slowing down these neutrons using heavy water, the likelihood of them colliding with uranium nuclei is increased and further nuclear reactions occur.

In addition to being a neutron moderator, CANDU reactors also use heavy water as a coolant. In these reactors, it is used to transfer the heat generated by nuclear fission from the reactor core to a heat exchanger, where it is used to generate steam that will drive a turbine to produce electricity.

Advantages and disadvantages of heavy water in nuclear reactors

The use of heavy water in nuclear reactors has both advantages and disadvantages, which are described below:


  1. Higher Efficiency: Heavy water has a larger neutron absorption cross section than ordinary water, which means it can better slow down the neutrons released in nuclear fission. This increases the probability that the neutrons will cause more nuclear reactions and thus increases the efficiency of the reactor.

  2. Greater flexibility in fuel selection: In addition to natural uranium, these reactors can use depleted uranium, thorium, and other fuels, allowing them to adapt to different fuel supply conditions.

  3. Less nuclear waste production: Reactors that use heavy water as a moderator have a lower rate of nuclear waste production than reactors that use ordinary water. This is because the increased efficiency of neutron moderation in heavy water allows for greater utilization of nuclear fuel and therefore reduces the amount of unused nuclear material that becomes waste.

  4. Better safety: Heavy water is less flammable than ordinary water, which can improve safety in the event of a reactor accident.


  1. Higher cost: Heavy water is more expensive than ordinary water due to the more complicated production process and the high degree of purity required for its use in nuclear reactors.

  2. Increased Nuclear Proliferation Risk: This type of water can be used in the production of plutonium for nuclear weapons, increasing the risk of nuclear proliferation.

  3. Increased risk of accident: Reactors that use heavy water may be more prone to nuclear accidents due to the greater amount of radioactive material present in the system.

Publication Date: May 11, 2023
Last Revision: May 11, 2023