It is found in igneous rocks and in the water of numerous mineral sources. It is usually enriched in lithium-6 by chemical exchange.
Armaments manufacturers use lithium to produce tritium in two ways.
They create tritium at the industrial level in nuclear reactors by bombarding lithium targets with neutrons. They can also create tritium inside a nuclear warhead by bombarding with neutrons a compound containing lithium. The Americans use lithium hydride for boosted fission warheads and lithium deuteride principally for the fusion stage in thermonuclear warheads. The French military may use lithium deutero-tritiide. The use of a lithium compound to replace tritium in a warhead avoids the necessity of replacing tritium periodically because of its short period.