beryllium is the light, stiff, gray metal used in 📝fusion designs as a neutron multiplier and as a plasma-facing material.
The element takes its name from beryl, the mineral in which it was first identified. In fusion its defining trick is easily confused with breeding, its near-neighbor function: beryllium does not produce tritium itself. Struck by a fast neutron, a beryllium nucleus undergoes an (n,2n) reaction — it absorbs one neutron and releases two — multiplying the neutron population so that more of them reach the 📝lithium in a 📝blanket and breed 📝tritium. Because each fusion reaction yields exactly one neutron while a self-sufficient plant must breed more than one tritium nucleus per burn, this multiplication supplies the margin the fuel cycle needs. The molten salt FLiBe carries beryllium for precisely this reason.
Beryllium's second role is at the wall. Its low atomic number means atoms sputtered into the 📝plasma radiate little energy away, which made it the 📝first wall armor of JET's ITER-like wall and the original specification for ITER's own first wall — though ITER moved to a 📝tungsten wall in its 2024 revised baseline. The metal's drawbacks are real: it is scarce, expensive, and its dust is toxic, demanding strict controls in manufacturing and maintenance.