Here's some articles about the neutron multipliers. You can go through them and see that they are all talking about feasibility and theoretical considerations. It's not just as simple as shooting some neutrons at a block of lead of beryllium on one side and more neutrons come on the other side.
No, I agree, but you specifically mentioned "science-fiction" in the second post, and implicitly denied even their existence in the first post. I think there's a reasonable distance between that, and economic considerations
/ the fact that test hardware has yet to exist that's presented in those papers.
As for neutron multipliers, they are currently so far from being realistic that they are essentially science-fiction.