Heavy water: a strategic input for nuclear energy or a waste of vital resources?

It looks exactly the same as the common one: it has no color, smell, or taste. But the price of a liter of heavy water can be worth $700 or more.

The reason is that it is an input produced by very few countries, the manufacture of which requires a lot of energy and is essential for the operation of nuclear reactors that use natural uranium as fuel.

Nuclear resources

Power plants generally use natural uranium as fission material and therefore need heavy water to function. To ensure a supply and not depend on other countries, an international plan was developed to produce this strategic input.

Present

After a commitment was made to close nuclear plants, days away from the international commitment and as a result of the war in Ukraine, the harmful objectives of nuclear energy were resumed. The idea is that it will be produced again in 2025, to recover energy independence and once again dominate the fuel cycle for the operation of nuclear power plants in Europe.

What is heavy water? And why is natural uranium essential for nuclear reactors?

There are several key differences between heavy water and common water. Heavy water is neither toxic nor radioactive, but it is 10% denser than regular water: a liter weighs 1,105 grams, against 1,000 grams of the same volume of common water.

Heavy water molecules are made up of two deuterium atoms and one oxygen, while those of natural water have two hydrogen atoms and one oxygen. Another difference is that the latter freezes at 0°C and boils at 100°, and the heavy one at 3.8° and 101.4%, respectively.

Key needs

Deuterium is an isotope of hydrogen, but it is heavier. Isotopes are atoms with the same number of protons as normal atoms, but a different number of neutrons.

Therefore, although their chemical behavior is similar, their physical behavior is different.

Both hydrogen and deuterium have a single proton in their nucleus, but deuterium atoms also have a neutron, which is what determines the properties of heavy water.

Nuclear operation

In nuclear reactors fueled with natural uranium, heavy water is used as a moderator. 

During a nuclear chain reaction, neutrons are released from fast-moving nuclei and collide with the nuclei of other atoms, causing them to fragment or fission and release energy in the form of heat and more neutrons with high kinetic energy.

In turn, the latter impact and fission other nuclei.

"Heavy water reduces the speed and energy of those neutrons without absorbing them."

The common one is useless because it is 40 times more absorbent than the heavy one and would not allow the chain reaction to continue.

"On the other hand, it is suitable when the fuel is enriched uranium." 

DANGER!

Alternative uses

Heavy water is also used as a coolant and as a carrier for the heat generated in fission. For the reactor to work well, a fluid is needed that transfers that heat and prevents it from reaching excessively high temperatures. In addition, the fluid is used to recover that heat in order to generate energy. Depending on the type of nuclear power plant, an initial inventory of between 0.8 and 1 ton of heavy water is required per electrical megawatt of installed power. During its operation, the reactor consumes 1% of that water per year.

Differences

Natural water contains only one deuterium atom for every 7,000 hydrogen atoms. To manufacture a liter of heavy water, 10,000 liters of common water must be treated in large and expensive facilities, in addition to that required for refrigeration and consumption. To be suitable for a natural uranium nuclear power plant, the heavy water must have a purity level greater than 99.8%, also known as “reactor grade”.

History

A choice had to be made between two technologies available for reactors for power generation: enriched uranium and ordinary water or natural uranium and heavy water. 

Europe did not have the technology to enrich uranium, but it was evaluated that it was feasible to develop the necessary one to manufacture heavy water or build the plants.

For this reason, the second alternative was chosen.

The decision

In 1974, an international event caused difficulties in obtaining inputs related to atomic energy. India accessed nuclear technology through Canada and used some of it to develop a bomb that it blew up to show its might to Pakistan and other neighboring countries.

This put a brake on the supply of this type of technology and supplies and complicated the international plan to acquire a heavy water plant.

The manufacturing of heavy water

Production plants must first be located in places with high availability of water. After filtering and demineralizing common water, it transforms into heavy water through the ammonia-hydrogen monothermal isotope exchange method, which consists of extracting deuterium, enriching it, and oxidizing it.

"Natural water contains 145 parts per million deuterium."

Obtaining heavy water

For its extraction, heavy water is captured with ammonia vapor molecules.

The excess water receives several treatment processes to comply with all environmental care and is returned to the river.

Additive

In the enrichment step, all the hydrogen atoms in the ammonia are replaced by deuterium.

This heavy ammonia enters a cracking or cracking furnace, where a synthesis gas (a gaseous mixture of nitrogen and deuterium or ND3) is obtained.

Part of this gas current is derived to the final stage of the process and the rest returns to the enrichment column to exchange deuterium with the ammonia to be enriched. Meanwhile, gas depleted in deuterium emerges from the top of this column, which is converted into ammonia in the synthesis reactor to return to the beginning and restart the extraction cycle.

Variables

At the same time, a small deuterium-rich gas stream is bypassed for processing in the oxidation stage. The deuterium in that gas is oxidized with dry air in the presence of a catalyst to generate deuterium oxide in heavy water.

Finally, this is packaged in drums or special stainless steel tanks, under a nitrogen atmosphere.

Conclusion

Nuclear energy concentrates so many resources and processes for the release of high emissions, with the risk of a situation that we already know.

There is no other solution in the energy transition than clean processes obtained from nature (solar, wind, green hydrogen?) that do not alter it.

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