Proton–Boron-11 (p–¹¹B) Fusion

p–¹¹B is the truly aneutronic fusion reaction — the gated research endpoint of the Kronos fuel roadmap, not its commercial baseline.

Reaction

p + ¹¹B → 3 α (three helium nuclei)

Neutrons

None from the primary reaction — truly aneutronic

Difficulty

Very high ignition temperature

Kronos status

Gated endpoint, not the commercial baseline

Proton–boron-11 (p–¹¹B) fusion is the reaction that turns a proton and a boron-11 nucleus into three helium nuclei (alpha particles) — and releases no neutrons from the primary reaction. It is the only fuel in the Kronos roadmap that is genuinely aneutronic, which is why the term "aneutronic" is reserved for it and never used for the commercial machine.

The catch is that p–¹¹B demands even higher temperatures and better confinement than D–³He, and its direct-conversion optimum (η ≈ 0.52) is actually lower than the ³He-rich regime (η ≈ 0.60). Kronos therefore treats p–¹¹B as a gated research endpoint at the far end of the staged fuel schedule — a destination, not the commercial baseline, which is low-neutron D–³He.