Daigo Fukuryū Maru (第五福竜丸?, Lucky Dragon 5) was a Japanese tuna fishing boat, which was exposed to and contaminated by nuclear fallout from the United States’ Castle Bravo thermonuclear device test on Bikini Atoll, on March 1, 1954.
Aikichi Kuboyama, the boat’s chief radioman, died less than seven months later, on September 23, 1954, suffering from acute radiation syndrome. He is considered the first victim of the hydrogen bomb of Operation Castle Bravo.
In 1947, the fishing boat was launched from Koza, Wakayama, named Dainana Kotoshiro Maru (第七事代丸?, Kotoshiro Maru No. 7). Later it became a tuna fishing boat in Yaizu, Shizuoka, where it was renamed the Daigo Fukuryū Maru.
The Daigo Fukuryū Maru encountered the fallout from the U.S. Castle Bravo nuclear test at Bikini Atoll, near the Marshall Islands, on March 1, 1954. The boat, along with its 23 fishermen aboard, as well as their catch of fish, were contaminated. They returned to Yaizu, Japan on March 14. The crew members, suffering from nausea, headaches, burns, pain in the eyes, bleeding from the gums, and other symptoms, were diagnosed with acute radiation syndrome and admitted to two Tokyo hospitals. On September 23, chief radio operator Mr. Aikichi Kuboyama, 40, died — the first Japanese victim of a hydrogen bomb. He left these words: “I pray that I am the last victim of an atomic or hydrogen bomb.”
The sky in the west lit up like a sunrise. Eight minutes later the sound of the explosion arrived, with fallout several hours later. The fallout, fine white flaky dust of calcinated Bikini Island coral, had absorbed highly radioactive fission products, and fell on the ship for three hours. The fishermen scooped it into bags with their bare hands. With one fisherman, Matashichi Oishi, reporting that he “took a lick” of the dust that fell on his ship, describing it as gritty but with no taste. The dust stuck to surfaces, bodies and hair; after the radiation sickness symptoms appeared, the fishermen called it shi no hai (死の灰?, death ash).
The US government refused to disclose the fallout’s composition due to “national security”, as the isotopic ratios, namely a percentage of uranium-237, could, through a radio-chemical analysis of the fallout, reveal the nature of the device to the Soviet Union, which had, as of 1954 not been successful with thermonuclear staging. Lewis Strauss, the head of the Atomic Energy Commission (AEC), issued a series of denials; he also hypothesized that the lesions on the fishermen’s bodies were not caused by radiation but by the chemical action of the caustic burnt lime that is produced when coral is calcined, that they were inside the danger zone (while they were 40 miles away), and told President Eisenhower’s press secretary that the Lucky Dragon #5 may have been a “Red spy outfit”, commanded by a Soviet agent intentionally exposing the ship’s crew and catch to embarrass the USA and gain intelligence on the tests device. He also denied the extent of the claimed contamination of the fish caught by Daigo Fukuryu Maru and other ships. The FDA however later imposed rigid restrictions on tuna imports. The United States dispatched two medical scientists to Japan to study the effects of fallout on the ship’s crew and to assist their doctors.
Publications of the fallout analysis was a militarily sensitive issue, with Joseph Rotblat possibly deducing the staging nature of the Castle Bravo device by studying the ratio and presence of tell-tale isotopes present in the fallout. With this information having a history of being regarded as potentially revealing the means by which megaton yield nuclear devices achieve their yield. In the History of the Teller–Ulam design, Soviet scientist Sakharov hit upon his “Sakharov’s third idea” during the month after the Castle Bravo test, the final piece of the puzzle being landing upon the notion that the compression of the secondary by the primary’s x-rays before fusion began.
When the test was held, the Daigo Fukuryū Maru was catching fish outside the danger zone which the U.S. government had declared in advance. However, the test was over twice as powerful as it was predicted to be, and changes in weather patterns blew nuclear fallout, in the form of a fine ash, outside the danger zone. The fishermen realized the danger, and attempted to escape from the area, but they took time to retrieve fishing gear from the sea, exposing themselves to radioactive fallout for several hours.
Later, the United States expanded the danger zone and it was revealed that in addition to the Daigo Fukuryū Maru, many other fishing boats were in the expanded zone at the time. It is estimated that about one-hundred fishing boats were contaminated to some degree by fallout from the test. Many hundreds of inhabitants of the Marshall Islands were also exposed, and a number of islands had to be evacuated entirely.
After Daigo Fukuryū Maru returned to its homeport of Yaizu, Shizuoka on March 14, 1954, Japanese biophysicist Yasushi Nishiwaki immediately traveled from Osaka to Yaizu to examine the crew and their boat. He quickly concluded that they had been exposed to radioactive fallout and wrote a letter to the chief of the AEC asking for more information on how to treat the crew. The US did not respond to Nishiwaki’s letter or to letters from other Japanese scientists requesting information and help. Contrary to these claims, The United States dispatched two medical scientists to Japan to study the effects of fallout on the ship’s crew and to assist their doctors.
The US at first tried to cover up the Lucky Dragon incident, sequestering the victims and declaring them off limits. Later the United States paid Kuboyama’s widow and children the equivalent in yen of about $2,500 ($22,000 in 2014).
The tragedy of the Daigo Fukuryū Maru gave rise to a fierce anti-nuclear movement in Japan, rising especially from the fear that the contaminated fish had entered the market. The U.S. government feared this movement would lead to an anti-American movement, and attempted to quickly negotiate a settlement with the Japanese government (led at the time by the Prime Minister Shigeru Yoshida, who was considered to be a pro-U.S. politician[by whom?]). The Japanese and U.S. governments reached a compensation settlement, with the transfer to Japan of a compensation of US $15,300,000, of which it is reported that the fishery received a compensation of US$2 million, with the surviving crew receiving about ¥ 2 million each, ($5,550 in 1954, $48,700 in 2014). It was also agreed that the victims would not be given hibakusha status. The Japanese government also acknowledged that it would not pursue further reparations from the U.S. government.
The Daigo Fukuryū Maru was preserved in 1976 and is now on display in Tokyo at the Tokyo Metropolitan Daigo Fukuryū Maru Exhibition Hall.