Is proton–boron-11 the endpoint?

Not on the current roadmap. Kronos's burner runs D–³He, a low-neutron fuel it can actually close. Proton–boron-11 is worth understanding as an aneutronic ideal — but it is not where Kronos is headed.

Kronos burner fuel

D–³He (low-neutron)

p–¹¹B role

Educational reference, not a Kronos endpoint

Why not p–¹¹B

Extreme temperature; does not close on this design

Frozen wording

Low-neutron, not aneutronic

An earlier Kronos narrative treated proton–boron-11 (p–¹¹B) as a long-run endpoint. The frozen strategy does not pursue it. The burner's fuel is D–³He, chosen because it is low-neutron and reaches a closing operating point Kronos can reproduce.

p–¹¹B is genuinely attractive in principle — it is aneutronic, releasing its energy as charged alpha particles. But it only becomes reactive at temperatures far beyond even D–³He, and it does not close on the Kronos tandem-mirror design. Treating it as a near-term endpoint would overstate the roadmap.

So p–¹¹B stays on these pages as an educational reference for what "aneutronic" really demands — and as a reminder of why Kronos describes its own machines as low-neutron, not aneutronic.