The 5.44% Neutron Fraction and Materials

D-³He is mostly aneutronic, but a neutron fraction near 5.44% still loads the first wall — material life at that flux is a qualification gate.

Mostly, not entirely, aneutronic

D-³He fusion is chosen partly because it produces far fewer neutrons than D-T. But it is not neutron-free: side reactions (notably D-D) give the burner a neutron fraction near 5.44%. That fraction is small relative to a breeder, yet large enough to activate and damage first-wall and structural materials over time.

The gate is qualifying materials that survive the burner’s neutron spectrum and heat loads for a useful service life. This is partial: the breeder program and the wider field advance fusion materials, and the burner’s lower fraction eases the problem — but burner-specific qualification is not complete.

What qualification requires

It requires material data under the relevant fluence, models of damage and activation, and test-unit exposure to confirm predicted life. The lower fraction is an advantage, not a pass — the wall still needs a demonstrated service life.

• D-³He is low-neutron, not neutron-free (~5.44%).
• Wall and structure still activate and degrade over time.
• Qualification uses breeder and field data plus test-unit exposure.