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'The Second Hiroshima': The Largest U.S. Nuclear Test Ever Hit a Japanese Fishing Vessel

On March 1, 1954, the United States tested what was the most powerful thermonuclear device in U.S. history. Castle Bravo was the first test of a dry-fuel version of the hydrogen bomb developed by Edward Teller. Bravo was unfathomably powerful, yielding...
March 7, 2012, 7:45pm

On March 1, 1954, the United States tested what was the most powerful thermonuclear device in U.S. history. Castle Bravo was the first test of a dry-fuel version of the hydrogen bomb developed by Edward Teller. Bravo was unfathomably powerful, yielding 15 megatons, which was triple what designers had expected. That incredible difference between calculated and actual yield, the result of a calculation error, helped Bravo produce what was the worst radiation contamination ever caused by the Americans.


In the middle of that fallout was the Daigo Fukuryū Maru (Lucky Dragon 5), a Japanese tuna boat. The ship and its 23 man crew were near Bikini Atoll when the bomb went off, and all were contaminated with radioactive fallout. Two weeks after the test, the crew returned to Yaizu, Japan, suffering from nausea, burning eyes, bleeding gums, and headaches. All 23 men were diagnosed with acute radiation syndrome.

That September 23, the boat’s chief radioman, Aikichi Kobayama, died at the age of 40. Kobayama became the first Japanese victim of a hydrogen bomb. Over time, 11 of the crew died of radiation poisoning, which, combined with the massive fallout from Bravo, was cause for international uproar. Anti-nuclear sentiments exploded in Japan, with some referring to it as “the second Hiroshima”.

Still, Teller, the man behind the Marshall Islands tests, was concerned less about lives than further development of the States’ thermonuclear capabilities, having once said “It’s unreasonable to make such a big deal over the death of a fisherman.”

Image via Rhee and Rhee, which has more photos of the Daigo Fukuryū Maru in Tokyo today.

The Castle Bravo test featured “Shrimp,” a 23,500 pound “light body” device that was an evolution of Ivy Mike, the first American thermonuclear hydrogen bomb that was the product of Teller. Shrimp used solid lithium deuteride fuel, as opposed to the liquid deuterium-tritium used in Ivy Mike. Shrimp’s massive yield — estimates were in the 4-6 megaton range — was due to a miscalculation about the theoretical energy of the lithium fuel. Two isotopes, lithium-6 and lithium-7, were present, and designers at Los Alamos thought that lithium-7, which made up 60% of the fuel, would be inert in the reaction. Unforeseen mechanisms proved that incorrect; the lithium-7 boosted the fusion reaction, sending the yield far beyond estimates.

That huge yield is the cause of the massive fallout, that spread over an area (pictured above) over 100 miles long within 7 hours. Now, it wasn’t just the Daigo Fukuryū Maru that was in the area; a number of atolls were also contaminated, including the Rongelap and Rongerik atolls, which were both inhabited. The islanders were evacuated late, and experienced a number of health issues and birth defects, outlined in a medical study called Project 4.1. When the islanders were later moved back, high levels of radioactive cesium were found in indigenous coconut milk, among other thing, and the islanders were evacuated a second time.

Traces of the fallout were found in Australia, Japan, and India, and as far as the U.S. and Europe. This was due to a number of factors. The yield was massive, and the natural uranium used in the fission reactions produced a large amount of fallout. Mixed with a wind shift from the day before the test meant the fallout spread farther and in a worse direction than expected. The man who decided to run the test was Dr. Alvin Graves, the Scientific Director of Operation Castle. He had the sole decision to fire the bomb, with authority even over the military command.

Teller comments on the early H-bomb tests amid key archival footage from on-site.

Oddly, Graves had already been the recipient of a 400 röntgen dose of radiation (500 röntgens in five hours is considered lethal) at a 1946 Los Alamos test that killed his friend, Dr. Louis Slotin. (It’s important to note that a number of the test personnel were also poisoned by the fallout from Castle Bravo.) He still chose to fire Castle Bravo despite less than ideal wind conditions that increased the fallout cloud would float over the populated atolls.

Castle Bravo was a secret test, but the worldwide spread of fallout quickly turned the test into a certified international incident. The U.S. initially tried to play down the incident. Absurdly, the official U.S. line at the time was that an increase in the power of a nuclear bomb did not produce an equivalent increase in fallout, a notion proved wrong by London-based doctor Sir Joseph Rotblat.

After getting caught trying to cover up the incident, the Americans came to a quick settlement with the Japanese. The U.S. put up $2 million towards damages, with each surviving fisherman getting about $48,000 in 2012 dollars. (Meanwhile, the Marshallese are still fighting for reparations.) With the settlement not fully appeasing anti-nuclear sentiments in Japan, the Daigo Fukuryū Maru was preserved in the mid-70s and is currently on display in Tokyo. It stands as a testament to the single-minded pursuit of more powerful weapons embodied by Teller that considered the consequences of such developments trivial in the name of progress.