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The thermal neutron absorption cross section of Be9 is only 10 mbarns, a rather small value, so beryllium makes a good neutron moderator in fission reactors. It is lighter than C (9 vs. 12), so a neutron can lose more energy in one collision with beryllium that with carbon. Neutron absorption reactions are Be9(n,α)He6 (winding up as Li6), and Be9(n,2n)Be8 (winding up as 2α). Beryllium is not only a moderator, but also a source of neutrons. Beryllium was considered a promising material for high-temperature nuclear reactors (carbon, of course, cannot be used). Beryllium was used as a neutron reflector to reduce the size of reactor cores. It is used in nuclear weapons for the same purpose.
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