The D-3He Fuel Cycle

The burner fuses deuterium with helium-3, a reaction that releases its energy almost entirely as charged particles rather than neutrons.

Choosing an aneutronic-dominant fuel

The primary reaction is D + 3He, which yields a proton and a helium-4 nucleus (an alpha), both charged. The energy comes out as fast charged particles, which is why direct conversion is possible and why the machine can be nearly waterless. This is the choice that shapes the whole burner: aneutronic-dominant fuel enables the direct-conversion, low-shielding, load-adjacent design.

The neutron caveat

D-3He is not perfectly aneutronic in practice. Deuterium also fuses with itself (D-D), and one D-D branch produces a neutron and tritium; the tritium can then burn with deuterium to make a 14 MeV neutron. These side reactions are unavoidable whenever deuterium is present. In the burner design point they hold the neutron fraction to 5.44% of fusion power — small, but not zero, which is why modest shielding is still required.

Fuel logistics

• Deuterium is abundant and easily sourced from water
• Helium-3 is scarce on Earth — supply is a real constraint the program tracks honestly
• Side D-D reactions make some tritium, which is managed, not vented
• Fuel enters via pellet and gas injection and neutral beams

The helium-3 supply question is the burner's most significant external dependency, and Kronos treats it as strategic rather than assumed. What the fuel choice buys — direct conversion, low neutron load, near-zero water, and a machine that can sit beside a data center — is the reason the burner accepts that dependency.