When we talk about nuclear energy, one of the terms that often comes up is heavy water . Although its name sounds exotic, it's simply a variant of common water, with a fundamental difference in its atomic composition that makes it key in certain scientific and industrial applications.
What is heavy water?
Heavy water, known in English as heavy water and whose chemical formula is D₂O, is similar to the water we use every day (H₂O), but with one difference: instead of containing "light" hydrogen atoms (protons), it contains deuterium , an isotope of hydrogen that has an extra neutron in its nucleus.
This small but significant change makes heavy water approximately 10% denser than ordinary water and gives it slightly different physical properties. Although visually indistinguishable, its behavior in certain nuclear reactions makes it an essential tool in the field of nuclear energy.
Uses of heavy water
Thanks to its unique properties, heavy water has a variety of applications, especially in the fields of nuclear energy and scientific research.
1. Neutron moderator in nuclear reactors
Its best-known use is as a neutron moderator in some types of nuclear reactors, especially CANDU (Canada Deuterium Uranium) reactors. In a fission reaction, the released neutrons have a lot of energy (they are "fast"), and to trigger further fissions more efficiently, they must be slowed down.
Heavy water slows these neutrons without absorbing them significantly—something that ordinary water does to a greater extent. This allows CANDU reactors to operate on natural uranium without the need for enrichment, which is a strategic and economic advantage.
In addition to its role as a moderator, heavy water can also function as a coolant, extracting the heat generated in the reactor and allowing its conversion into electricity using steam turbines.
2. Tritium production
In some reactors, heavy water can serve as a basis for the production of tritium (³H), a radioactive isotope of hydrogen with applications in weapons, scientific research, and certain lighting devices. This process is not trivial and involves specific nuclear reactions within the reactor, not simply the radiation of the water.
3. Scientific research and medicine
In science, heavy water is used as an isotopic tracer in biological and chemical studies. By following the path of deuterium in cellular processes or chemical reactions, scientists can gain valuable information without significantly altering the system they are studying.
It is also used as a solvent in techniques such as nuclear magnetic resonance (NMR), where its presence prevents interference in the detection of normal hydrogen.
How is heavy water produced?
Deuterium, although naturally occurring, is very rare: it only represents about 0.015% of the hydrogen found in water. Therefore, obtaining heavy water requires highly specialized isotopic separation processes. The main methods are:
Fractional distillation
It takes advantage of the difference in boiling point between H₂O and D₂O. Through multiple distillation cycles, it is possible to progressively concentrate the deuterium.
Isotopic chemical exchange
In this method, chemical reactions that exchange deuterium between different compounds are promoted. The Girdler sulfide process, which uses hydrogen sulfide, has been widely used industrially.
Electrolysis
When electric current is applied to water, light hydrogen is released more easily than deuterium. After successive steps, a solution enriched with D₂O is obtained.
Advantages and disadvantages in nuclear reactors
Advantages:
- Use of natural uranium : No fuel enrichment is required, simplifying the uranium cycle.
- High moderation efficiency : Allows for a higher fission rate with lower neutron absorption.
- Fuel flexibility : Can use not only uranium, but also thorium or other variants.
- Less waste production : Improved efficiency can translate into less radioactive waste per unit of energy generated.
Disadvantages:
- High cost : Heavy water is expensive to produce and maintain due to the complexity of the process and purity requirements.
- Proliferation risk : Can facilitate plutonium production if not properly controlled.
- Delicate handling : Although it is not radioactive itself, it can be activated by contact with radiation in the reactor, so it requires careful handling.