Heavy water, known chemically as deuterium oxide, is a specialized form of water distinct from ordinary water. This difference lies in the hydrogen atoms: normal water contains hydrogen (protium), while heavy water contains the heavier stable isotope, deuterium. The deuterium nucleus has one proton and one neutron, making the water molecule approximately 10% heavier than protium oxide. This isotopic substitution alters the physical and nuclear properties of the water. The unique nuclear characteristics of deuterium make its oxide indispensable for certain industrial applications, especially within the nuclear power sector, which necessitates its large-scale manufacture.
The Scarcity of Deuterium in Natural Water
The necessity for a complex industrial manufacturing process stems directly from the low natural concentration of deuterium. Deuterium is naturally present in all water sources, but only about one in every 6,400 to 6,760 hydrogen atoms is deuterium. This translates to an atomic abundance of roughly 0.015%, or 150 parts per million (ppm).
The challenge of extracting heavy water is compounded because isotopes share nearly identical chemical properties. Standard chemical separation methods, such as filtration, cannot efficiently distinguish between protium and deuterium atoms. Only slight differences in mass and bond strength can be exploited, requiring highly specialized, large-scale, and energy-intensive industrial methods to concentrate the isotope.
The Girdler Sulfide Process
The most successful industrial method for bulk separation is the Girdler Sulfide (GS) process, which relies on a chemical exchange reaction. This method exploits the temperature-dependent equilibrium of deuterium exchange between water and hydrogen sulfide gas within a series of massive, dual-temperature exchange towers. The process uses two distinct temperature zones to create a concentration gradient for deuterium atoms.
Cold Tower Enrichment
In the cold tower, maintained at approximately \(30^\circ\text{C}\), deuterium atoms preferentially transfer from the gaseous hydrogen sulfide to the liquid water. The chemical equilibrium at this lower temperature favors the formation of deuterated water molecules. This slightly enriched water is then drawn off from the cold section.
Hot Tower Depletion
Conversely, the deuterium-depleted water from the cold tower is fed into a hot tower, operating between \(120^\circ\text{C}\) and \(140^\circ\text{C}\). At this elevated temperature, the chemical equilibrium shifts, causing deuterium atoms to preferentially transfer back from the water to the circulating hydrogen sulfide gas. This enriched gas is then cycled back into the cold tower, while the depleted water is discarded.
This continuous, countercurrent flow system constitutes a “cascade,” where enriched water from one stage feeds into the next, gradually increasing the deuterium concentration. The GS process is highly energy-intensive and requires a large infrastructure. This initial bulk enrichment typically raises the deuterium content to \(15-20\%\), requiring further purification to reach the reactor-grade purity of over \(99.8\%\).
Alternative Enrichment and Separation Techniques
While the Girdler Sulfide process is the primary method for initial bulk enrichment, other techniques are employed for secondary purification or smaller-scale production. These alternative methods often rely on physical properties rather than chemical exchange.
Distillation
Distillation exploits the minor difference in boiling points between heavy water (\(101.42^\circ\text{C}\)) and ordinary water (\(100^\circ\text{C}\)). Vacuum distillation involves boiling and condensation in a multi-stage column, where the less volatile heavy water concentrates in the liquid residue. Cryogenic distillation of liquid hydrogen can also be used, but both methods require significant energy inputs and are generally less economical than the GS process for initial large-scale separation.
Electrolysis
Electrolysis, the process of splitting water into hydrogen and oxygen gas using electricity, provides another pathway for enrichment. When water is electrolyzed, protium-containing molecules decompose slightly faster than deuterium-containing molecules. As the process continues, the remaining water becomes progressively enriched in heavy water. Electrolysis is slow and energy-consuming, and is now primarily used for the final purification of already enriched water to achieve the high purity required for nuclear applications.
Essential Applications and Handling Requirements
The primary need for manufactured heavy water is its use in the nuclear power industry, where it functions as both a moderator and a coolant in specific reactor designs. Deuterium is highly effective as a neutron moderator because it is significantly less likely to absorb neutrons than ordinary hydrogen. This property allows heavy water reactors, such as the Canadian CANDU reactor, to operate efficiently using natural, unenriched uranium fuel, which simplifies the fuel cycle.
Beyond power generation, heavy water is a valuable tool in scientific and medical research. It is used in Nuclear Magnetic Resonance (NMR) spectroscopy as a solvent, and it serves as a tracer in metabolic studies to track chemical reactions. Handling this high-value material requires strict protocols to prevent contamination. Heavy water has a high affinity for ordinary water, which quickly degrades its purity. Therefore, it must be stored in containers designed to prevent contact with moisture in the air.