Does Kronos depend on a breakthrough?

It depends on what you mean. No new physics is required. But the burner rests on one hard, honestly-named engineering requirement — and the breeder on a demanding confinement number. Those are the bets.

New physics required?

No

Breeder's main bet

A high confinement multiplier (H₉₈ ≈ 2.28)

Burner's main bet

End-plug density ≈ 16× central cell (largest open item)

Everything else

Demonstrated or reproducible

Kronos separates "needs a miracle" from "needs hard engineering," and places itself firmly in the second. The reactions, the reactivities, and the power balances are standard and reproduce from the deposited code.

• The breeder needs a confinement multiplier of about H₉₈ = 2.28 — demanding, and not yet confirmed by nonlinear gyrokinetics, but within the space of measured tokamak performance rather than beyond it.
• The burner needs one big thing: an end-plug density about 16× the central cell — far above any mirror yet built. Kronos calls this "the largest open item" and labels the whole burner result requirement-class because of it.

So the honest answer is: no new physics, but a genuine engineering leap for the burner and a demanding confinement target for the breeder. Both are named, quantified, and falsifiable.