[nickparker]

Huh, I suppose you do! I had a naive short-circuit in my head that "neutron flux is bad because it destroys your your materials. If your material is molten lead anyway who cares?"

I also didn't know lead has such a low neutron absorption cross section.

For what it's worth lead with extra neutrons seems a lot less scary than eg uranium fission waste. Stable isotopes 206, 207, and 208 represent 98% by abundance, 208 has the lowest cross section, and 209 has a 3 hour half-life into Bismuth-209 which is nearly stable (2e19 year half-life). So it seems almost all of your neutron captures just make other stable lead isotopes or briefly-terrifying 209 that's totally safe after a couple days. You only get real scary stuff if the trace amounts of undecayed 209 manage a second capture.

Edit: I should add they want to mix lithium into the lead to absorb neutrons and regenerate fuel

[pfdietz]

Lead has a low neutron absorption cross section, but it has a fairly hefty inelastic scattering cross section for fusion neutrons (for fission neutrons, which are less energetic, it just elastically scatters them, with very little moderation). It also causes some (n,2n) reactions with fusion neutrons, which would be nice to make up for tritium losses.

[the8472]

What about 204? If that produces 205 you'd have some long-lived waste.

[grogers]

10 million year half life is long enough that it's not very dangerous anymore.