A low-atomic-number plasma-facing material and essential neutron multiplier, beryllium plays dual roles in fusion reactor design despite significant handling and lifetime challenges.
Beryllium (Be, Z=4) occupies a unique niche in fusion technology. It serves simultaneously as a low-Z first-wall armor that minimizes core-plasma radiative losses and as a neutron multiplier in tritium-breeding blanket designs. ITER's first wall will be clad with approximately 700 m2 of beryllium tiles, and multiple DEMO blanket concepts rely on beryllium or beryllium-containing pebble beds for neutron economy.
Unlike the divertor, the tokamak first wall receives comparatively moderate heat fluxes (0.5--2 MW/m2) but faces a large plasma-wetted area. Because radiative power loss from impurities scales roughly as Z2 to Z4, minimizing the atomic number of eroded wall atoms is essential to maintaining core plasma performance. Beryllium, with Z=4, radiates far less efficiently than carbon (Z=6) or tungsten (Z=74) if it reaches the core, making it the preferred first-wall cladding for ITER.1
Beryllium's most important nuclear property for fusion is its large (n,2n) cross-section above ~1.9 MeV. When a 14.1 MeV fusion neutron strikes a beryllium nucleus, the reaction 9Be(n,2n)24He produces two lower-energy neutrons from each incident neutron. This multiplication is essential for closing the tritium fuel cycle: because not every neutron born in the plasma can be captured by lithium in the breeding blanket (some escape, some are parasitically absorbed), the blanket must contain a neutron multiplier to achieve a tritium breeding ratio (TBR) above unity. Beryllium and lead are the two principal multiplier candidates, with beryllium preferred in solid breeder blanket concepts such as the European Helium-Cooled Pebble Bed (HCPB) design.2
Beryllium's relatively low melting point (1,287 °C) limits its use to moderate heat flux regions; it cannot survive divertor conditions. Under neutron irradiation, beryllium swells significantly due to helium gas generation from (n,α) transmutation reactions, and its mechanical properties degrade with dose. Pebble beds used in breeding blankets must accommodate volumetric swelling of several percent over their service life.3
The material also presents a serious occupational health hazard. Beryllium dust is acutely toxic and can cause chronic beryllium disease (CBD), a granulomatous lung condition, upon inhalation. Stringent workplace exposure controls, including engineered enclosures and air monitoring, are mandatory during fabrication and maintenance operations. The combination of toxicity, resource scarcity, and limited geological reserves has motivated research into beryllium-titanium intermetallic compounds (Be12Ti) and beryllium-vanadium alloys that may offer comparable neutron multiplication with improved irradiation stability.4
The Joint European Torus (JET) operated with a beryllium/tungsten (Be/W) wall configuration under the ITER-Like Wall (ILW) project from 2011 onward, providing the most reactor-relevant dataset on beryllium erosion, fuel retention, and dust production in a tokamak environment. JET demonstrated that the Be/W combination dramatically reduced long-term fuel retention compared to the prior all-carbon wall, validating the material choice for ITER.