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.
Nuclear testing at Bikini Atoll
Six days after the Castle Bravo test, the U.S government set up a secret project to study the medical effects of the weapon on the residents of the Marshall Islands.