Comment by adrian_b

Beryllium is very efficient as a neutron multiplier, but it is also extremely rare. It would not be acceptable as a consumable for energy production, as it is much more useful for other purposes.

In the Solar System, the abundance of beryllium is similar to that of gold and of the platinum-group metals. On Earth, the scarcity of beryllium is less obvious only because it is concentrated in the continental crust, where it is relatively easily accessible, even if its amount in the entire Earth is much smaller.

Lead neutron multipliers would be preferable, because they only inter-convert isotopes of lead, so it is not destroyed, like beryllium.

However lead used for this purpose becomes radioactive, with a very long lifetime, unless expensive isotope separation would be used for it.