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
The United States was in a Cold War Nuclear arms race with the Soviet Union to build bigger and better bombs. The next series of tests over Bikini Atoll was code named Operation Castle. The first test of that series was Castle Bravo, a new design utilizing a dry fuel thermonuclear hydrogen bomb. It was detonated at dawn on March 1, 1954.