Deep Dive: The p–¹¹B Endpoint

The truly aneutronic proton–boron-11 reaction is the far end of the Kronos fuel roadmap — a gated research destination that closes as a driven system, not the commercial baseline.

Reaction

p + ¹¹B → 3 α (aneutronic)

Difficulty

Highest temperature; strong radiation

Closure

Driven: Q_plasma ≈ 2.4–2.9 (S73)

Conversion

η ≈ 0.52 (below ³He-rich 0.60)

The proton–boron-11 (p–¹¹B) reaction is the only genuinely aneutronic fuel in the Kronos roadmap — its primary reaction produces three alpha particles and no neutrons. It's the reason "aneutronic" is reserved for this endpoint and never used for the commercial machine.

But it's the hardest reaction to run: it needs even higher temperatures than D–³He, and radiation losses are severe. The deposited endpoint analysis (S73) finds it closes only as a driven system, with a plasma gain of Q_plasma ≈ 2.4–2.9 — net-positive, but sustained by continuous heating, not ignited. And counterintuitively, its direct-conversion optimum (η ≈ 0.52) is lower than the ³He-rich D–³He regime (η ≈ 0.60, S71).

So Kronos treats p–¹¹B as a gated research destination at the end of the staged fuel cycle — the cleanest reaction, pursued after the economically optimal D–³He stages, not the commercial baseline. It's an aspiration held honestly, not a headline claim.