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From Colin Bruce

Clive Semmens’s fear that a deuterium-tritium fusion reactor can’t produce enough tritium for its own needs is unjustified (15 March, p 32).

Lithium comprises two isotopes. Lithium-6 has an enormous cross-section to capture slow neutrons, fissioning to tritium with a large energy yield. Even better, lithium-7 absorbs fast neutrons, fissioning to tritium plus an extra slow neutron for the lithium-6. This neutron-multiplying ability of lithium was discovered in 1954, when the Castle Bravo H-bomb test on Bikini Atoll produced twice the expected yield, with consequences for the islanders of nearby Rongelap among others.

By juggling the ratio of the two lithium isotopes, a fusion reactor can be made that produces either exactly enough tritium to keep itself going, or even a surplus if desired. This is almost spookily fortunate: lithium is the only light element capable of fission. Liquid lithium has a large heat capacity, low melting point, high boiling point and a low vapour pressure. It conducts, so can be pumped electromagnetically. It is light enough to act as a moderator, slowing rebounding neutrons. It is the perfect fluid-come-shield for a fusion reactor.

Physicists sometimes grumble that the universe seems designed to make fusion hard. These improbably helpful properties of lithium are more like a hint: if you can’t do fusion when it’s this easy, frankly you don’t deserve it!
Oxford, UK