On March 1, 1954, the United States Atomic Energy Commission (AEC) set off a thermonuclear bomb on the Bikini Atoll, part of the Marshall Islands in the equatorial Pacific. The test, called Castle Bravo, was the first of a hydrogen bomb and proved the largest nuclear explosion ever initiated by the United States.
In fact, it was much more powerful than American nuclear scientists had predicted. They expected a four- to six-megaton explosion, but it had an actual yield equivalent to more than 15 megatons of TNT. As a result, the effects were much more widespread than predicted.
Castle Bravo blew an enormous crater into the Bikini Atoll, still clearly visible in the northwest corner of the atoll on satellite images. It also sprayed radioactive contamination across an enormous area of the Marshall Islands and the Pacific Ocean downwind from the detonation site, as the fallout map indicated. The AEC had created an exclusion perimeter of 30 nautical miles for U.S. Navy vessels, but the radioactive fallout was dangerously high as far out as 200 miles.
The AEC had not warned vessels from other nations to stay out of the exclusion area. Even if it had, that would not have helped the Japanese tuna fishing boat Daigo Fukuryu Maru, or Lucky Dragon 5, which was 90 miles from Bikini at the time of the test. It was the Lucky Dragon's very bad fortune on that day to be directly downwind from Castle Bravo.
Fallout on the Lucky Dragon
At 6:45 a.m. on March 1, the 23 men aboard the Lucky Dragon had their nets deployed and were fishing for tuna. Suddenly, the western sky lit up as a fireball seven kilometers (4.5 miles) in diameter shot up from Bikini Atoll. At 6:53 a.m., the roar of the thermonuclear explosion rocked the Lucky Dragon. Unsure what was happening, the crew from Japan decided to continue fishing.
Around 10 a.m., highly radioactive particles of pulverized coral dust began to rain down on the boat. Realizing their peril, the fishermen began to pull in the nets, a process that took several hours. By the time they were ready to leave the area, the Lucky Dragon's deck was covered with a thick layer of fallout, which the men cleared away with their bare hands.
The Lucky Dragon quickly set off for its home port of Yaizu, Japan. Almost immediately, the crew began to suffer from nausea, headaches, bleeding gums, and eye pain, symptoms of acute radiation poisoning. The fishermen, their catch of tuna, and the Lucky Dragon 5 herself were all severely contaminated.
When the crew reached Japan, two top hospitals in Tokyo quickly admitted them for treatment. Japan's government contacted the AEC for more information about the test and the fallout, to help with treatment of the poisoned fishermen, but the AEC stonewalled them. In fact, the U.S. government initially denied that the crew had radiation poisoning - a very insulting response to Japan's doctors, who knew better than anyone on Earth how radiation poisoning presented in patients, following their experiences with the Hiroshima and Nagasaki atomic bombings less than a decade earlier.
On September 23, 1954, after six months of agonizing illness, the Lucky Dragon's radio operator Aikichi Kuboyama died at the age of 40. The U.S. government would later pay his widow approximately $2,500 in restitution.
The Lucky Dragon Incident, coupled with the atomic bombings of Japan's cities in the closing days of World War II, led to a powerful anti-nuclear movement in Japan. Citizens opposed the weapons not only for their capacity to destroy cities but also for smaller dangers such as the threat of radioactively contaminated fish entering the food market.
In the decades since, Japan has been a world leader in calls for disarmament and nuclear non-proliferation, and Japanese citizens turn out in large numbers for memorials and rallies against nuclear weapons to this day. The 2011 Fukushima Daiichi nuclear power plant meltdown has re-energized the movement and helped expand anti-nuclear sentiment against peacetime applications as well as weaponry.