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.

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