Isotope Separation

Separating helium-3 from helium-4 relies on their small mass difference, using cryogenic distillation and related physical methods rather than chemistry.

Same element, different mass

Helium-3 and helium-4 are the same element, so no chemical reaction can tell them apart. They differ only in mass — three versus four nucleons — and separation exploits that difference physically. The principal method is cryogenic distillation: at temperatures near the boiling point of helium, the lighter helium-3 is slightly more volatile and concentrates in the vapor. Repeated stages enrich it to fusion grade.

Other handles

Besides distillation, the mass difference can be exploited by superfluid and phase-separation effects at very low temperature, and by diffusion and thermal-gradient methods. In practice a fusion fuel plant combines coarse bulk separation with fine polishing stages to reach the required helium-3 purity. The energy and hardware cost of separation is why purity specifications are set no tighter than the plasma actually needs.

• No chemical route — separation is purely physical
• Cryogenic distillation is the primary method
• Superfluid, diffusion, and thermal methods supplement it
• Multi-stage cascades reach fusion-grade purity

Separation appears twice in the fuel cycle: upstream, when decay-born helium-3 is cleaned of helium-4 before delivery, and downstream, when helium-3 is recovered from burner exhaust that is full of helium-4 ash.