Magnet Nuclear-Heating Budget
Neutrons and gammas that reach the coils deposit heat at cryogenic temperature, where every watt is expensive to remove.
Heat where you least want it
The radiation that penetrates the shield deposits energy as heat in the cold magnet. Heating a cryogenic magnet is costly because refrigeration at low temperature is inefficient: a watt deposited at magnet temperature costs many watts of room-temperature power to remove. The shield is sized partly to keep this nuclear heating within the cryogenic plant's capacity.
Two shield objectives at once
The inboard shield must satisfy two limits simultaneously: keep integrated dose within the magnet's end-of-life budget, and keep instantaneous nuclear heating within what the cryogenic plant can remove during operation. These are different requirements — one integrates over life, one is a steady rate — and both squeeze the same scarce inboard thickness.
- Nuclear heating is deposited at cryogenic temperature.
- Low-temperature refrigeration multiplies the room-temperature cost of each watt.
- Shield thickness trades against both dose and heating.
- Center-stack space limits how much shielding is possible.
Nuclear heating ties the shield, cryogenics, and magnet together and is one more reason the compact center stack is the breeder's most contested volume.
Margin for off-normal
The heating budget carries margin for off-normal conditions, not just nominal operation, because a transient that briefly raises flux still deposits heat in a cold mass with limited thermal buffer. Sizing the shield and cryoplant to the steady case alone would leave no headroom, so the budget is set with realistic operational variation included.
This page documents a design and simulation study, not a built machine. Construction begins Q2 2027; first-of-a-kind first tritium is targeted near 2030. Figures are computed, reproducible targets, not measurements.