Norway and Sweden to Cooperate on Development of Lead Cooled Small Nuclear Reactors.

[NNadir]

2. 4He has a capture cross section of zero, as isotopes with mass numbers 5 (and 8) do not exist. The scattering...

...cross section is significant, making it a moderator, but it does not absorb neutrons. This is a function of ⁴He position on the binding energy curve.



Neither ⁵He nor ⁵Li exist, nor does ⁸Be. It is the reason that stars upon exhaustion of ¹H and ²H must be dense enough to cause 3 atoms of ⁴He to fuse to form a single ¹²C.

²⁰⁸Pb does have a low capture cross section, but it is non-zero, with resonances, albeit not particularly strong, appearing near the energy of fission neutrons.



In a fast reactor, small amounts of ²⁰⁸Pb are thus transmuted thus into ²⁰⁹Bi.

But yes, you're right in the sense that isotopically pure or highly enriched ²⁰⁸Pb as a coolant offers high neutron economy.

Thanks for your note.