neutron multiplier is a material, such as beryllium or lead, that releases more neutrons than it absorbs when struck by a fast 📝fusion neutron, boosting the tritium breeding of a fusion blanket.
The term is easily confused with its near-neighbor, the moderator: a moderator slows neutrons down without changing their number, while a multiplier increases their number outright. The multiplying mechanism is the (n,2n) reaction — an incoming fast neutron knocks a second neutron loose from the target nucleus, so one neutron enters and two leave. 📝Beryllium is the classic multiplier because its loosely bound nucleus gives up a neutron readily; lead is the leading alternative, favored in liquid-metal designs where it doubles as coolant.
Multiplication exists to solve an arithmetic problem at the heart of the fuel cycle. Each 📝D-T reaction consumes one 📝tritium nucleus and yields exactly one neutron, yet some neutrons are inevitably lost to structure, ports, and absorption before they ever reach the 📝lithium in the 📝blanket. Without a multiplier, breeding one new tritium per burn would be arithmetically out of reach; with one, the neutron population gains the surplus that lets a plant's tritium breeding ratio exceed one and its fuel cycle close.