Neutron Shielding

With only 5.44% of power in neutrons the shield is modest, but it still protects magnets, electronics, and workers and recovers neutron heat.

A thin shield, not a blanket

A D-T plant surrounds its plasma with a thick breeding blanket and shield because roughly 80% of its power is 2.45 and 14 MeV neutrons. The burner's 5.44% neutron fraction changes the problem entirely: the shield is a protective layer, not a massive energy-capture structure. That reduction in mass and complexity is central to the compact footprint.

What the shield must do

• Protect the REBCO magnets from neutron-induced degradation
• Limit activation of surrounding structure so maintenance stays hands-on where possible
• Attenuate radiation at the site boundary for the safety case
• Capture neutron heat for recovery in the thermal system

Where the neutrons come from

The neutron source is concentrated where D-D reactions occur — chiefly the high-density central cell. Shielding is therefore heaviest around the central cell first wall and the nearest magnets, and lighter toward the ends. The linear geometry helps: shielding a straight cylinder is simpler than shielding a torus with ports on every side.

Activation analysis for the burner is part of the design study rather than a measured result, since the machine is not yet built. The modeling uses the 5.44% fraction and the D-D neutron spectrum to size the shield and to bound the activated inventory the safety case must account for.

Getting the shield right is a balance: enough to protect the magnets and the site boundary, but no more, since excess shield adds mass and bulk that work against the compact footprint. The linear geometry and low neutron fraction together keep that balance far easier to strike than in a D-T machine.