Note:
Tritium breeding blankets are layers of lithium placed behind the Fusion First Wall Materials in a reactor. The first wall slows neutrons from the fusion reaction and those neutrons will hit the lithium. If you hit a Lithium nucleus with a neutron it will split into two nuclei, Helium-4 and Tritium.
Because the fusion reaction does not produce enough neutrons to sustainably breed tritium a neutron multiplier is needed, this is a layer of material which amplifies the neutron flux.