The CASTLE BRAVO nuclear test of March 1954 was the largest nuclear-explosion ever detonated by the United States, and a radiological disaster. To the surprise of the scientists and military observers who set up the test, BRAVO’s 15 million tons of TNT equivalent (15 megaton) explosion generated high-levels of radioactive fallout downwind of the Bikini atoll (Marshall Islands) where it was set off.
The native peoples on the inhabited atolls of Rongerik, Rongelap, and Uterik were all forcible evacuated for their own safety, American observers on boats were themselves exposed by the test, and the crew of the Japanese fishing boat Daigo Fukuryū Maru, who had strayed into the “danger area,” were exposed to levels of radiation high-enough to make them sick and to cause one of them to eventually die.
The Armed Forces Special Weapons Group assembled one of the earliest reconstructions of the fallout plume, based on a loose model they developed on the basis of radiation readings from other atolls:
The units of the map above are of the H+1 dose rate, in R/hr, which is a standard approach in fallout modeling that can be a bit unintuitive for the average person. You can roughly consider the values to be somewhat on par with what the dose would be (in Roentgens) were you to be exposed to the fallout for a month or so. 100 R absorbed over a short period of time results in radiation sickness and an increased lifetime fatal cancer risk; 400-600 R usually results in death. So these are very high values.
The above model was later re-rendered as the H+96 total dose contour printed in Samuel Glasstone and Philip J. Dolan’s The Effects of Nuclear Weapons, 3rd edn. (U.S. Atomic Energy Commission and U.S. Department of Defense, 1977), which people are more familiar with:
“Local” fallout contours on this level had not been contemplated prior to the BRAVO test. (Why not? That’s a longer question, one I will be writing more on in the near future…) The prior experience with nuclear fallout for kiloton-range weapons had led scientific and military planners to more or less consider this kind of downwind fallout to be a nuisance, not a serious threat, at least not compared to the kinds of threats you get from the immediate effects of a detonating atomic bomb.
Immediately, they began to ask: What are the implications of something like BRAVO for the future of nuclear war? What would happen if this weapon had gone off over an American city, as opposed to over a coral atoll in the Pacific Ocean? Which led to diagrams like the below, which transplant BRAVO’s detonation point from Bikini to Washington, DC:
