Heavy Water (D2O) - Density, Properties & Production

Deuterium Oxide (D₂O) — "Heavy Water"

Heavy water is water where both hydrogen atoms are replaced by deuterium (²H or D), a stable isotope of hydrogen with one proton and one neutron. While chemically almost identical to normal water, its extra mass gives it distinctly different physical properties.

Why "Heavy"?

• Molecular weight: D₂O = 20.028 g/mol vs H₂O = 18.015 g/mol (+11.1%)
• Density at 25°C: D₂O = 1,104.4 kg/m³ vs H₂O = 997.0 kg/m³
• Freezing point: 3.82°C (ice cubes sink in normal water!)
• Boiling point: 101.4°C
• Maximum density at: 11.6°C (vs 4°C for H₂O)

Natural Abundance

Deuterium occurs naturally at about 156 parts per million (0.0156%) of all hydrogen in ocean water. This means roughly 1 in every 6,400 hydrogen atoms is deuterium. One ton of seawater contains about 33 grams of D₂O.

Applications

• Nuclear reactors: Neutron moderator in CANDU reactors (slows neutrons without absorbing them)
• NMR spectroscopy: Deuterated solvents (CDCl₃, D₂O) for clean spectral backgrounds
• Isotope tracing: Track metabolic pathways in biology and medicine
• Neutron scattering: Research into material structures

How is Heavy Water Produced?

Because deuterium is so rare (156 ppm), separating D₂O from regular water requires enormous effort. Three main industrial methods exist:

1. Girdler-Sulfide (GS) Process

The most cost-effective method for large-scale production, used by AECL in Canada.

• Principle: Chemical exchange between water and hydrogen sulfide gas (H₂S) at two different temperatures
• Hot tower (130°C): H₂S + HDO ⇄ HDS + H₂O — deuterium moves to H₂S
• Cold tower (30°C): HDS + H₂O ⇄ H₂S + HDO — deuterium moves back to water
• Cascading hundreds of stages enriches D concentration from 156 ppm → ~30% D₂O
• Final enrichment to 99.8% by vacuum distillation or electrolysis
• Separation factor: α ≈ 1.3 per stage

2. Water Electrolysis

The oldest and most intuitive method, first used by Urey in 1932.

• Principle: When water is electrolyzed, H₂ is released preferentially over HD or D₂
• The remaining water becomes progressively enriched in D₂O
• Separation factor: α ≈ 3–6 per stage (much higher than GS!)
• Downside: Extremely energy-intensive (850 kWh/kg D₂O)
• Used for final enrichment: takes 30% D₂O from GS → 99.97% pure reactor-grade

3. Water Distillation

The simplest conceptually, but least efficient.

• Principle: D₂O has a slightly higher boiling point (101.4°C vs 100°C)
• In a distillation column, D₂O concentrates in the bottom (liquid phase)
• Separation factor: α ≈ 1.05 (very low — requires thousands of stages!)
• Only viable for upgrading already-enriched D₂O, never from natural water

Global Production

Annual world production is estimated at ~400-500 tonnes. Canada historically produced the most (for CANDU reactors), followed by India and Argentina. Current market price ranges from $600–$800 per kilogram.