Tritium breeding ratio
The number of new tritium atoms a reactor's blanket makes for each tritium atom burned. Unless it exceeds one, the reactor cannot supply its own fuel.
Each D-T reaction uses up one tritium atom and releases one neutron. That neutron is sent into lithium to make tritium again, but some neutrons are absorbed by structural materials or escape. Allowing also for tritium that is never recovered and for what decays while in storage, the breeding ratio has to be somewhat above one[1].
The difficulty is that nobody has yet measured this figure in a real reactor. Present estimates come from calculation and carry a significant uncertainty. Because the required margin is small, test facilities are being planned to measure breeding in a neutron environment close to that of real fusion.
Related terms
- Breeding blanketThe layer surrounding a fusion reactor's plasma. Neutrons striking lithium in it produce tritium fuel, and their energy is captured as heat.
- TritiumA heavy form of hydrogen with one proton and two neutrons. It is weakly radioactive, almost absent in nature, and serves as fusion fuel.
- Demonstration reactorThe step after an experimental reactor: a machine meant to show that fusion can actually generate electricity. It comes just before a commercial plant.