More than six decades after the events, the full extent of radiation exposure is gradually coming to light. Between 1946 and 1958, the United States conducted a series of nuclear tests in the Pacific, including at Bikini Atoll. For years, it was widely believed that only the crew of the Daigo Fukuryu Maru—a tuna fishing boat that suffered immediate casualties—had been affected. However, recent scientific research has uncovered traces of radiation exposure in the teeth and blood of crew members from other vessels.
Meanwhile, diplomatic researchers have brought new evidence to light through declassified U.S. and Japanese government documents. Despite the passage of more than half a century, some researchers remain committed to uncovering the truth. But why do they continue to focus on Bikini?
( written by Yasuhito Sasajima, 2017 )
The Kochi City Central Community Center is located on the upper floors of the Karupoto cultural complex. On a humid July 17, 2016, a large lecture hall on the 11th floor hosted a "Health Consultation on the Bikini Hydrogen Bomb Tests," featuring three radiation exposure experts.
In the 1950s, Kochi Prefecture was a major hub for deep-sea tuna fishing, and even today, many former fishermen and their families live there. The lingering concerns—"Did I suffer radiation exposure?" "Could it have affected my children or grandchildren?"—remain unresolved. This health consultation, organized by the prefectural government, was an attempt to address these fears.
One of the invited scientists, Kunio Tanaka, took the microphone before an audience of about 60 former crew members and their families. Tanaka, 65, was formerly the head of the Biological Effects Research Division at the Institute for Environmental Sciences in Rokkasho, Aomori Prefecture. He now serves as a senior advisor.
It was during this event that Tanaka first spoke publicly about chromosomal damage as evidence of past radiation exposure.
“These are the chromosomes of individuals exposed to radiation from the Bikini nuclear tests,” Tanaka explained as a magnified image appeared on the screen. The chromosomes were fragmented and abnormally fused—damage caused by radiation.
Radiation can break chromosomes, causing them to attach incorrectly to other chromosomes, leading to lasting abnormalities. Some of these changes remain in the body permanently, making it possible to estimate past radiation exposure levels based on the frequency of such abnormalities.
Since 2013, Tanaka had traveled to Kochi, Miyagi, and Kanagawa prefectures to collect blood samples from former fishermen. His study included 19 participants, all of whom were confirmed—through ship logs and official records—to have been operating in the vicinity of the nuclear tests between March and May 1954.
The youngest participant, who had been a teenager at the time, was now 76 years old. The oldest was already 90. More than anything, these men feared not only for their own health but also for the possibility that their exposure might have affected their descendants.
What did the analysis reveal?
“Based on chromosomal abnormalities detected in their blood, we estimate that the average radiation exposure among these 19 former crew members was 91 millisieverts. The highest recorded dose was 295 millisieverts," Tanaka announced.
This individual had been fishing 420 kilometers from the test site. According to the International Commission on Radiological Protection (ICRP) and Japan’s Ministry of Health, Labour and Welfare, radiation exposure exceeding 100 millisieverts is linked to an increased risk of cancer.
“But there may have been individuals with even higher exposure,” Tanaka added. “These estimates are based only on those who are still alive today and whom we were able to visit.”
Between 1946 and 1958, the United States conducted over 100 nuclear tests in the Pacific, continuing until 1962 at locations like Christmas Island. Britain, France, and the Soviet Union also conducted numerous nuclear tests during this period.
The most infamous incident occurred on March 1, 1954, when the Japanese tuna fishing boat Daigo Fukuryu Maru was exposed to fallout from the Bravo hydrogen bomb test at Bikini Atoll. The bomb was 1,000 times more powerful than the Hiroshima atomic bomb.
The fallout—pulverized coral turned to radioactive ash—descended on the Daigo Fukuryu Maru, which was operating 160 kilometers east of the test site. Six months later, the ship’s radio operator, Aikichi Kuboyama, died from radiation sickness. His death, along with reports of "radioactive tuna" contaminating the domestic fish market, sparked outrage across Japan.
By the end of 1954, 992 fishing vessels had brought contaminated tuna into Japanese ports. The term "Bikini" became synonymous with radioactive contamination.
Yet, by December of that year, the Japanese government suddenly halted all portside radiation inspections. As a result, the full extent of radiation exposure among other fishing crews was never properly documented.
Why was the Bikini incident never fully investigated? According to researchers, the answer lies in a political settlement between Japan and the United States.
Hiroko Takahashi, a researcher at the Meiji Gakuin University Institute for International Peace Research, has been analyzing newly uncovered classified U.S. government documents.
“In 2014, we finally obtained declassified materials that had previously been withheld,” she explains. Among them was a diplomatic telegram sent on December 27, 1954, by John M. Allison, then U.S. Ambassador to Japan, detailing a meeting with Japanese Foreign Minister Mamoru Shigemitsu.
In this meeting, Shigemitsu presented a six-point list of urgent bilateral issues, with the first item being a resolution to the Bikini compensation problem. The last item was the release of Japanese Class A war criminals still held in Sugamo Prison.
The timing is revealing. The very next day, the Japanese government ordered an end to radiation inspections at fishing ports. A week later, Japan and the U.S. signed an agreement declaring the Bikini issue “completely resolved.”
Under the agreement, the United States provided Japan with $2 million in "sympathy payments," and Japan waived all future compensation claims related to the tests. No further investigations were conducted. By 1958, all remaining war criminals had been released.
“Individual fishermen’s suffering was erased in the name of diplomacy,” Takahashi states. “Even though multiple ships had been affected, the Bikini issue was deliberately reduced to just the Daigo Fukuryu Maru.”
Masatoshi Yamashita, 72, a former high school teacher in Kochi Prefecture, has been investigating the Bikini exposure cases for over three decades.
His efforts began in 1985 when he and his students met a woman who claimed her son, a fisherman, had been exposed to both the Nagasaki atomic bombing and the Bikini nuclear tests. The young man had suffered from illness for years before taking his own life at 27.
Shocked by the story, Yamashita and his students began interviewing former fishermen. Many described seeing a fireball in the distance or white ash falling from the sky—all while fishing in waters near Bikini Atoll.
Many of them later developed leukemia or cancer. Some collapsed after vomiting large amounts of blood. Others continued working despite their illness, trying to provide for their families.
For decades, the Japanese government dismissed concerns, claiming "no evidence" of exposure beyond the Daigo Fukuryu Maru.
Then, in 2014, Yamashita’s requests for information finally yielded results. The Ministry of Health, Labour and Welfare released 1,900 pages of previously undisclosed documents—including radiation exposure data for 556 vessels and their crew members.
In 2016, 45 former fishermen and their families filed a lawsuit against the Japanese government, arguing that critical health data had been hidden for decades, depriving them of the chance to seek compensation.
Among them is Kazuma Masumoto, 82, who has attended every hearing. “Many of my fellow fishermen died without knowing the truth,” he says.
“I am here to fight for them.”
The lawsuit continues.
