Why The Neutron Fraction Matters For Water

The lower the neutron fraction, the smaller the unavoidable heat load and the easier it is to cool without water; D-3He's 5.44% keeps that load small.

Neutrons set the heat floor

In any fusion device, the fraction of energy carried by neutrons sets a floor on how much energy must be handled as heat, because neutrons cannot be captured by direct energy conversion. A D-T reaction carries most of its energy in neutrons; the D-3He reaction used by the burner carries only 5.44% as neutrons, leaving the rest as charged particles suited to direct conversion.

Fewer neutrons, less heat to reject

• Neutron energy becomes structural heat that must be cooled
• A low neutron fraction means a small mandatory heat load
• A small load is dry-rejectable, keeping water consumption near zero

The chain to the water story

The water story depends on keeping the thermal load small enough to reject to air. The choice of D-3He fuel, with its low neutron fraction, is what makes that possible: it maximizes the charged-particle energy that direct conversion can capture and minimizes the neutron energy that must be handled as heat. Fuel choice and water footprint are directly linked.

Honest scope

5.44% is not zero; the neutrons are real and require blanket and structural cooling, and they carry engineering implications beyond water. The water-relevant point is narrow and firm: a low neutron fraction keeps the mandatory heat load small, which is why the burner can be cooled without evaporation.