Borated Shielding and Thermal Neutrons

Once fast neutrons are slowed, boron captures them cleanly — the finishing stage that stops thermal neutrons reaching the magnet.

Finishing the job

Dense shield material scatters and slows fast neutrons but leaves a population of low-energy, thermal neutrons that would otherwise drift onward. Boron, and especially boron-10, has a large appetite for thermal neutrons and captures them, largely converting them to easily-absorbed products. Borated material is the finishing layer of a layered shield.

Why order matters

Boron works on slow neutrons, so it belongs after the scatterer and moderator, not before. Placing absorber material where neutrons are still fast wastes it. The layered sequence — dense scatterer, moderator, boron absorber — is why shield layout is a design problem, not just a thickness number.

• Boron-10 has a large thermal-neutron capture appetite.
• It captures the neutrons the dense layer has already slowed.
• Effective only after moderation, so it is the last layer.
• Reduces the thermal-neutron dose reaching the coils.

Together with tungsten-carbide and moderating layers, borated shielding closes out the neutron spectrum so the magnet stays within its dose budget in the minimum inboard thickness.

Isotopic tailoring

Enriching the boron-10 fraction increases thermal-neutron capture per unit volume, valuable where inboard space is scarce. Like lithium-6 enrichment, it is a materials-supply choice that buys shielding effectiveness in less thickness, and it is weighed against availability and the activation products the captures leave behind in the shield.

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.