Neutron Shielding Basics

Neutrons carry no charge, so fields cannot deflect them; they are slowed and absorbed by mass, in layers chosen to minimize activation.

Unlike charged particles, neutrons cannot be turned by magnetic or electric fields. The only way to stop them is with material: first slow the fast neutrons through collisions, then absorb the slowed neutrons in a nucleus that captures them cleanly. Good shield design does both while avoiding materials that become long-lived waste.

Two stages

• Moderation: light nuclei (hydrogen in water or hydrides, beryllium, carbon) take large energy bites out of fast neutrons per collision.
• Absorption: boron, lithium-6, and similar nuclei capture slow neutrons, ideally through reactions that emit only charged particles or short-lived products.

In the breeder the blanket is itself the first shield: it moderates 14 MeV neutrons and captures them in lithium-6 to breed tritium, so shielding and fuel production happen in the same structure. Outboard of the blanket, additional shield protects the magnets and reduces activation of the building. The burner needs far less neutron shielding overall because its neutron output is only 5.44%.

A further design goal is to keep the shield's own activation low, since a poorly chosen absorber can itself become the dominant waste. Boron and lithium-6 are favored because their capture reactions release charged particles rather than seeding long-lived nuclides, so the shield decays quickly and can often be recycled with the rest of the structure.

Shielding is where waste minimization begins: choosing moderators and absorbers that either do not activate or activate only to short-lived nuclides keeps the shield itself out of the long-lived category. These are design-and-simulation choices for machines not yet constructed.