Decay Heat and Passive Cooling
After shutdown the residual heat comes from activated structure, is small, and is removed without powered cooling.
Decay heat is the power released by radioactive decay after the reaction stops. It sets the demand on post-shutdown cooling and, in the worst case, whether a core can melt. In fusion there is no fuel core to decay — the residual source is neutron-activated structure, which produces a small, rapidly falling heat load.
Where it comes from
14 MeV neutrons from D–T fusion transmute atoms in the first wall, blanket, and vessel into short- and medium-lived radioisotopes. Their decay releases heat. Kronos favors low-activation materials so that both the activity and the decay heat are minimized and decline quickly after shutdown.
Because the source is distributed in solid structure rather than concentrated in fuel, and because the total is small, natural conduction to surrounding components and radiation to cooler surfaces keep temperatures below damage limits. No powered pumps or assured makeup water are required to reach a safe state.
The burner advantage
The burner (Aegis / MetroVolt), being D–³He with a 5.44% neutron fraction, activates far less material and therefore has an even smaller decay-heat inventory than the breeder. This is one reason the burner's severe-accident consequences are especially benign.
- Residual heat source is activated structure, not fuel — small and short-lived.
- Passive conduction and radiation suffice; no assured active cooling credited.
- Low-activation material choices shrink the source at the design stage.
See neutron activation and low-activation materials for the material basis.