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.
Operation Castle
"Operation Castle" was organized into seven experiments, all but one of which were to take place at Bikini Atoll. Below is the original test schedule (as of February 1954).