Lithium hydride
Before the Castle Bravo nuclear weapons test in 1954, it was thought that only the less common isotope lithium-6 would breed tritium when struck with fast neutrons. The Castle Bravo test showed (accidentally) that the more plentiful lithium-7 also does so under extreme conditions, albeit by an endothermic reaction.
Castle Bravo
The device was mounted in a "shot cab" on an artificial island built on a reef off Namu Island, in the Bikini Atoll. A sizable array of diagnostic instruments was trained on it, including high-speed cameras trained through an arc of mirror towers around the shot cab.