It’s rare that you design something that works not only better than you expected, but several times better that physics 9as you understood it) said it should work. One of those lucky times was the Castle Bravo hydrogen bomb. It was Americas first “dry” H-bomb, set off March 1, 1954. Prior to this, the “hydrogen” in an H-Bomb was cryogenic liquid deuterium; this was not practical for deliverable bombs. But with the introduction of lithium deuterium, a room temperature solid, H-bombs became practical weapons.
The Castle Bravo device, set off on Bikini Atoll, was expected to have a yield of 5 megatons. While certainly a respectable yield, it turned out to actually have a yield of 15 megatons. This was not due to a design flaw, per se, but due to a misunderstanding of physics. The lithium in the lithium deuteride was 40% Li-6 and 60% Li-7, The Li-7 was thought to be inert, from a nuclear point of view. But… this was wrong.
When nuked by fissioning plutonium in the core, the Li-6 isotope was expected to absorb a neutron and spit out an alpha particle and a tritium atom. The tritium would then do a little nuclear dance with the nearby deuterium and undergo fusion, boosting the yield. The Li-7 was expected to more or less just sit there. But as it turns out, Li-7 decided to join in. When it absorbed an energetic neutron, it, too spat out an alpha particle and tritium… and a neutron. This not only fed more tritium into the reaction, but also more neutrons. The end result was three times the yield that had been expected. Woo!
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