Neutron Fraction & Thermal Share

The ~5.44% neutron fraction fixes how much of the burner's power is unavoidably thermal, and therefore the size of the cooling job.

The one part DEC cannot touch

Direct conversion acts only on charged particles. The burner's D-3He primary is charged-dominant, but D-D side reactions produce neutrons, giving a design-point neutron fraction of about 5.44% of total fusion power. Neutrons carry no charge, ignore magnetic and electric fields, and deposit their energy as heat wherever they finally stop — in shielding and structure. That energy is the irreducible thermal share of the burner.

What the thermal share does

• Deposits heat in shielding and structure — removed by a closed cooling loop.
• Sets a materials requirement: components near the plasma must tolerate neutron flux.
• Bounds the water/cooling duty at a small fraction of total power.
• Is the reason the burner is not perfectly aneutronic despite its charged-dominant fuel.

Why 5.44% is a design point, not an accident

The neutron fraction depends on plasma conditions — temperature, density, and the balance of D-3He to D-D reactions. The burner's operating point is chosen to keep it near 5.44%: low enough that the great majority of power stays charged and directly convertible, while accepting that a fully aneutronic plasma is not physically available. It is a deliberate trade between conversion efficiency and neutron handling.

The link to water

Because only this ~5.44% plus converter losses must be rejected as heat, the cooling job is small in proportion to output. That is the quantitative bridge from the reaction physics to MetroVolt's near-zero-water claim: a small thermal share means a small cooling loop, which can be closed and dry-cooled rather than drawing on scarce local water.