Lithium-6 Versus Lithium-7
The two natural lithium isotopes breed tritium by different reactions, and their balance is a design lever for the breeding blanket.
Two isotopes, two reactions
Natural lithium is a mix of two stable isotopes: lithium-6 (about 7.5%) and lithium-7 (about 92.5%). Both breed tritium, but differently. Lithium-6 captures a slowed neutron and reliably yields tritium plus helium. Lithium-7 reacts with fast neutrons, producing tritium, helium, and — importantly — an additional neutron, which can help sustain the breeding chain.
| Isotope | Neutron energy | Tritium? | Extra neutron? |
|---|---|---|---|
| Lithium-6 | slow / thermal | yes | no |
| Lithium-7 | fast (14 MeV) | yes | yes |
Why enrichment is a lever
The proportion of lithium-6 in the blanket is a design choice. Enriching lithium-6 raises the reliable, high-yield breeding channel, while retaining lithium-7 preserves the neutron-multiplying reaction that feeds the neutron economy. Tuning this balance is one of the levers that sets the achievable tritium breeding ratio.
Coupled to the neutron economy
- Lithium-6 dominates thermal-neutron breeding
- Lithium-7 adds a neutron via a fast reaction
- Enrichment level trades reliability against neutron multiplication
- The choice interacts with multipliers and blanket geometry
The isotope balance is set together with the rest of the neutron budget. See the neutron economy and the breeding-ratio lever.
Two channels, one tunable blanket
Having two breeding channels in the same element gives designers a genuine lever rather than a fixed constraint. By choosing how much lithium-6 to enrich, a blanket can favor reliable thermal capture or preserve the fast reaction that returns an extra neutron to the budget. The natural isotope mix is only a starting point; the working blanket is tuned deliberately to hit the target breeding ratio.