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.
[1] https://www.sciencedirect.com/science/article/abs/pii/S00223...
[2] https://scipub.euro-fusion.org/wp-content/uploads/eurofusion...
[3] https://publikationen.bibliothek.kit.edu/1000160103