60 Years Ago Today on Bikini Atoll

March 01, 2014

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Image: Earth Observatory, Landsat OLI
Summary Author: David K. Lynch; Dave's Web site

Bikini_detail copyOn March 1, 1954 the United States detonated a thermonuclear hydrogen bomb on Bikini Atoll (above) in the Marshall Islands. Code-named Castle Bravo, the 15 megaton (MT) explosion blew a crater (left) in the island’s coral roughly 6500 ft in diameter and 250 ft deep (2000 m by 76 m). The resulting mushroom cloud ultimately reached a height of 130,000 ft (40,000 m), well into the stratosphere. Castle Bravo was the largest above-ground H-bomb test ever conducted by the US.

H-bomb energy is obtained by fusion, where two isotopes combine (fuse) to form a third, and in the process release energy. In the case of Castle Bravo, tritium released as lithium fissions (splits) combines with deuterium and produces energetic, fast-moving atomic particles and X-rays that are immediately absorbed by surrounding material, thereby heating it. Rapid expansion of the multimillion degree detonation products is the source of the blast wave. All stars including our Sun are powered by fusion reactions in their cores.

Castle Bravo was originally intended to be 4-6 MT but owing to incomplete (at the time) knowledge about how lithium isotopes absorb neutrons, the blast was much larger than anticipated. The unexpectedly large neutron flux that occurred when lithium-7 decayed caused uranium-235, one of the bomb’s components, to fission (split) and release additional energy.

The surprisingly large detonation and unanticipated winds spread radioactive fallout over a much larger area than expected. A number of Marshallese, Japanese and American people received high radiation doses and one died soon after exposure. It seems likely that more delayed deaths and health problems have occurred as a result of fallout from Castle Bravo. The incident provoked an international outcry, and in 1963 the US, Great Britain and the Soviet Union signed the Nuclear Test Ban Treaty prohibiting surface tests in the future.