The Burner's Residual Heat Loads

Direct conversion removes the steam cycle, but the burner still rejects heat from neutrons, magnets, and power electronics through closed loops.

A truthful water story names what still needs cooling. The burner (Aegis / MetroVolt) avoids the condenser, but it is not thermally inert. Four residual loads exist and are engineered deliberately.

The four residual loads

• Neutron heat: the 5.44% neutron fraction deposits energy in the first wall. Low-neutron is not no-neutron.
• Magnet cryogenics: superconducting coils at 26.49 T plug and 17 T throat must be held cold; the cryoplant rejects heat.
• Direct-conversion collectors: decelerating charged particles against electrodes dissipates resistive and secondary-emission losses.
• Balance of plant: power electronics, pumps, and building HVAC.

None of these requires a bulk condenser. Each is handled with a closed cooling loop sized to its duty, and the totals are small relative to a steam plant's heat-rejection load because the dominant energy path was collected electrically rather than thermalized.

Why this matters

Because these loads are modest and closed-loop, the burner can be air-cooled or use small makeup volumes rather than a river-scale withdrawal. That is the physical basis for the low-water claim. Stating the residual loads openly is what keeps the claim defensible: we are not asserting zero heat, we are asserting no steam cycle.

The engineering value of naming these loads is that each can be sized and cooled independently. A compact, well-characterized load is far easier to reject to air than a single large condenser duty, which is precisely why the burner can be water-light without heroics. There is no thermodynamic requirement that all this heat pass through a common water-cooled condenser, because the dominant energy path never became heat at all.

This is also why the low-neutron figure of 5.44% is stated so carefully: that neutron energy is a real first-wall heat load and a real activation source, small next to a D-T machine but not zero, and it is one of the residual loads engineered here rather than wished away.