Neutron Energy Spectra of the Two Machines

Neutron damage depends on energy: the breeder's 14.1 MeV neutrons and the burner's mixed 2.45/14.1 MeV output behave very differently.

How much a neutron activates or damages a material depends strongly on its energy. The two Kronos machines produce different neutron spectra, and the difference shapes their material and waste engineering.

The breeder spectrum

Hyperion's D–T reaction produces a nearly monoenergetic 14.1 MeV neutron. In practice the blanket softens this: elastic and inelastic scattering, plus (n,2n) multiplication in beryllium or lead, spreads the population down toward thermal energies, where ⁶Li captures neutrons to breed tritium. So the first wall sees a hard 14 MeV spectrum while the outer blanket sees a moderated one.

The burner spectrum

The burner's small neutron output is a two-line spectrum: 2.45 MeV neutrons from the D–D branch and 14.1 MeV neutrons from the D–T secondary reaction. Because the total is only 5.44% of power, the fluence on any surface is far lower than in the breeder, but the 14.1 MeV component still drives the same kinds of damage, just at a much smaller rate.

Spectrum also determines which activation nuclides form, because reaction cross-sections vary strongly with neutron energy. A 14 MeV neutron opens (n,2n) and (n,p) channels that a 2.45 MeV neutron cannot, so the harder breeder spectrum both damages more and creates a broader set of activation products than the burner's softer, sparser flux.

The engineering takeaway is that spectrum, not just count, sets activation. A materials choice that tolerates 14 MeV neutrons in the breeder is more than adequate for the burner's rarer, partly softer flux. All spectra here are simulation outputs for unbuilt machines and are used as design inputs, not measurements.