Historical research guide · 1158 · 2024
Castle BRAVO at 70: The Worst Nuclear Test in U.S. History
National Security Archive, The George Washington University
In one minute
On 1 March 1954 the United States detonated a thermonuclear device code-named Shrimp at Bikini Atoll, the Bravo shot of Operation Castle. It yielded 15 megatons, nearly three times the six megatons its planners had estimated, and its fallout reached inhabited atolls, United States personnel and the crew of the Japanese fishing boat Lucky Dragon. This National Security Archive briefing book, published for the seventieth anniversary, gathers declassified records on the test and its consequences, and its essay states how the yield was later understood. The fuel included lithium deuteride, and part of the lithium was lithium-7, which the designers had treated as essentially inert. It was not: when ignited it produced tritium, itself a fusion fuel, and that raised the yield. Los Alamos designer Carson Mark explained in July 1954 that lithium-7 had been assumed to be a much less good fuel than lithium-6. An ingredient assumed to sit out the reaction had taken part.
Connection
Chapter 18 is about machines whose output is decided by what their fuel actually does. Bravo is the sharpest historical lesson in measuring every channel: a component the design treated as a bystander carried a reaction nobody had put in the model, and the measured output was the only thing that revealed it. Every energy result on this site, conventional or frontier, earns trust the same way.
From the source
The yield. The Bravo detonation in the Castle series had an explosive yield of 15 megatons, about a thousand times that of the Hiroshima weapon and nearly three times the six megatons its planners estimated; it was the largest nuclear test in United States history.Introductory essay, opening paragraph
The prediction, as recorded. The July 1954 record of the Atomic Energy Commission's General Advisory Committee meeting gives the predicted yield of the detonation as four to eight megatons, and records Carson Mark's explanation of why the yields of the Castle shots were substantially higher than predicted in most cases.Archive's description of Document 35, General Advisory Committee meeting of 12 to 15 July 1954, page 11
The cause. The designers included lithium deuteride in the fuel, part of it as lithium-7, which they had assumed to be inert; when ignited it produced tritium, a fusion fuel, which greatly increased the yield. Carson Mark's words were that lithium-7 had been assumed to be a much less good fuel than lithium-6 or liquid deuterium, and that the higher yields were now understood in terms of its nuclear reactions.Introductory essay, paragraph beginning How and why Bravo produced; archive's description of Document 35
What was anticipated. According to the archive, the Los Alamos designers understood that the yield could be far greater than the estimated six megatons, possibly as high as 15, and advised the Joint Task Force to take measures to protect service personnel; the possibility that fallout could reach atolls a hundred miles away was not seriously considered, and no evacuation arrangements were made in advance.Introductory essay, same paragraph and the next; archive's reference to Document 44
The way in
https://nsarchive.gwu.edu/briefing-book/nuclear-vault/2024-02-29/castle-bravo-70-worst-nuclear-test-us-historyHow to cite it
National Security Archive, The George Washington University (2024) Castle BRAVO at 70: The Worst Nuclear Test in U.S. History https://nsarchive.gwu.edu/briefing-book/nuclear-vault/2024-02-29/castle-bravo-70-worst-nuclear-test-us-history