Beryllium's unique (n,2n) reaction doubles the effective neutron population in fusion blankets, making tritium self-sufficiency achievable in solid breeder designs — but toxicity, resource limits, and irradiation swelling constrain its use.
Each deuterium-tritium fusion reaction produces exactly one 14.1 MeV neutron, yet achieving a tritium breeding ratio (TBR) above unity — the minimum for fuel self-sufficiency — requires that some neutrons be multiplied before they are captured by lithium. Beryllium accomplishes this through the (n,2n) threshold reaction: a single fast neutron striking a beryllium-9 nucleus produces two lower-energy neutrons plus two helium-4 nuclei. No other light element matches beryllium's combination of high (n,2n) cross-section, low parasitic neutron absorption, and practical availability.1
In the European helium-cooled pebble-bed (HCPB) blanket concept for DEMO, beryllium appears as millimeter-scale pebbles packed in beds alternating with lithium ceramic (Li4SiO4 or Li2TiO3) breeder pebbles. The pebble geometry provides a high surface-to-volume ratio for helium cooling and accommodates irradiation-induced swelling without generating catastrophic stresses.2 An advanced variant uses beryllides — intermetallic compounds such as Be12Ti — which offer improved high-temperature stability and significantly reduced swelling compared to pure beryllium, at some cost in neutron multiplication efficiency.3
Beryllium dust is acutely toxic; inhalation can cause chronic beryllium disease, an incurable granulomatous lung condition. All fabrication, handling, and eventual disposal of beryllium blanket components require stringent industrial hygiene controls.1 Global beryllium production is modest — on the order of 300 tonnes per year — and a single DEMO-class reactor may require 400–600 tonnes. A fleet of fusion power plants relying on pure beryllium multipliers would strain or exceed known reserves, providing a strong motivation for beryllide development or alternative multiplier concepts such as lead in liquid PbLi breeders.3
Beyond its multiplier role, beryllium serves as the plasma-facing armor on ITER's first wall. Its low atomic number minimizes radiative power losses when beryllium atoms enter the plasma as impurities, and its oxygen-gettering properties help maintain plasma purity. However, beryllium's relatively low melting point (1287 degrees Celsius) limits its power-handling capability compared to tungsten, confining its use to areas of moderate heat flux.4