Neutron Shield Materials
Between blanket and magnet sits the shield: dense, hydrogenous, and neutron-absorbing materials that protect the coils' dose budget.
Protecting the magnet
After the blanket has bred tritium and taken most of the neutron energy, a shield attenuates what remains before it reaches the magnets. The shield's job is to keep the integrated neutron and gamma dose to the coil insulation and superconductor within their end-of-life budget, using the least radial space possible.
How shielding works
Effective shielding combines three functions: heavy elements such as tungsten to slow and scatter fast neutrons and absorb gammas, hydrogenous material to moderate neutrons to lower energy, and absorbers such as boron to capture the slowed neutrons. A good shield layers these so each energy range is handled by the right material.
- Dense high-Z material (e.g. tungsten) attenuates fast neutrons and gammas.
- Hydrogenous material moderates neutrons to capturable energies.
- Boron-bearing material absorbs thermal neutrons.
- Layer order matters as much as total thickness.
The compact-machine squeeze
In a spherical tokamak the inboard shield shares the narrow center stack with the TF leg and center-post. There is little room, so the inboard shield must be exceptionally effective per millimeter. This is one of the fundamental tensions of a compact breeder: shielding wants thickness, the compact design cannot spare it, and the center-post accepts a short life partly as a result.
Two limits, one thickness
The same inboard shield must meet two different requirements at once: keep integrated dose within the magnet's life budget, and keep instantaneous nuclear heating within the cryoplant's capacity. Because both are satisfied from the same scarce thickness, shield-material effectiveness per millimeter is what makes a compact machine feasible at all.
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.