Center-Post Shielding

The narrow inboard region leaves little room to shield the center post, the core reason its lifetime is limited in a spherical tokamak.

The tightest shielding budget in the machine

Shielding works by putting enough material between the neutron source and the component to slow and absorb the flux. That takes space. The center post sits at the smallest radius, wrapped closely by the plasma, in the one place a spherical tokamak has almost no space to give. Its shielding budget is therefore the tightest in the machine, and often nearly zero.

The consequence

With little shielding, the center post absorbs a large share of the 14 MeV neutron flux directly. This is what drives its limited lifetime, noted at roughly 0.01 fpy: radiation damage accumulates fast because the protection that shields outboard components is not available inboard. It is a physics-and-geometry fact of the configuration, not an oversight in the design.

What can be done

The available responses are material selection to make the post more damage-tolerant, using whatever thin shielding the geometry permits, and, decisively, designing the post as a replaceable module so its finite life becomes a maintenance schedule rather than a failure. How much inboard shielding is feasible without giving up too much field or breeding coverage is an active design-and-simulation trade, and the lifetime figure is expected to sharpen as that work matures.

• Inboard geometry allows minimal center-post shielding
• Direct flux exposure drives the ~0.01 fpy lifetime
• Managed by tolerant materials and replaceable-module design

This page describes a design and simulation study, not a built machine. Construction begins Q2 2027; first-of-a-kind first tritium is targeted near 2030.