Breeder vs Burner: The Neutron Comparison
Two Kronos machines sit at opposite ends of the neutron scale: the breeder is neutron-rich by design, the burner is low-neutron.
Kronos runs two fuel cycles for two purposes, and their neutron budgets could hardly be more different. The breeder (Hyperion) is a D–T spherical tokamak that puts about 80% of its energy into 14.1 MeV neutrons on purpose, because those neutrons breed tritium and are themselves a product. The burner (Aegis / MetroVolt) is a D–³He tandem-mirror generator whose neutron output is only 5.44%.
Why two answers is the right answer
The breeder's neutrons are not a problem to be minimized — they are the mechanism that makes tritium and helium-3 available at all. Without a neutron-rich breeder, there is no domestic helium-3 to feed the low-neutron burner. The burner, in turn, needs very few neutrons because its job is direct electrical conversion, where every neutron is energy that bypasses the converter and lands in the wall instead.
- Breeder: high 14 MeV flux → tritium breeding, isotope production, materials testing.
- Burner: low 5.44% flux → minimal activation, direct energy conversion, compact shielding.
- Together: the neutron-rich machine supplies the fuel the low-neutron machine burns.
The pairing is deliberate at the fleet level as well: a neutron-rich breeder foundry supplies the helium-3 that a low-neutron burner fleet consumes, so the two neutron budgets are complementary by design. Neither number is a target to be pushed toward the other; each is optimal for its own machine's job.
Both figures are design-and-simulation results for machines not yet built. Stating them side by side is deliberate: it prevents the burner's low-neutron nature from being read as a claim that all fusion is neutron-free, which the breeder plainly is not.