On the morning of Sept. 30, 1999, at a nuclear fuel-processing plant in Tokaimura, Japan, 35-year-old Hisashi Ouchi and two other workers were purifying uranium oxide to make fuel rods for a research reactor.

As this account published a few months later in The Washington Post details, Ouchi was standing at a tank, holding a funnel, while a co-worker named Masato Shinohara poured a mixture of intermediate-enriched uranium oxide into it from a bucket.

Suddenly, they were startled by a flash of blue light, the first sign that something terrible was about to happen.

The workers, who had no previous experience in handling uranium with that level of enrichment, inadvertently had put too much of it in the tank, as this 2000 article in Bulletin of the Atomic Scientists details. As a result, they inadvertently triggered what's known in the nuclear industry as a criticality accident a release of radiation from an uncontrolled nuclear chain reaction.

Ouchi, who was closest to the nuclear reaction, received what probably was one of the biggest exposures to radiation in the history of nuclear accidents. He was about to suffer a horrifying fate that would become a cautionary lesson of the perils of the Atomic Age.

"The most obvious lesson is that when you're working with [fissile] materials, criticality limits are there for a reason," explains Edwin Lyman, a physicist and director of nuclear power safety for the Union of Concerned Scientists, and co-author, with his colleague Steven Dolley, of the article in Bulletin of the Atomic Scientists.

If safeguards aren't carefully taught and followed, there's potential for "a devastating type of accident," Lyman says.

It wasn't the first time it had happened. A 2000 U.S. Nuclear Regulatory Commission report noted that before Tokaimura, 21 previous criticality accidents had occurred between 1953 and 1997.

The two workers quickly left the room, according to The Post's account. But even so, the damage already had been done. Ouchi, who was closest to the reaction, had received a massive dose of radiation. There have been various estimates of the exact amount, but a 2010 presentation by Masashi Kanamori of the Japan Atomic Energy Agency put the amount at 16 to 25 gray equivalents (GyEq), while Shinohara, who was about 18 inches (46 centimeters) away, received a lesser but still extremely harmful dose of about 6 to 9 GyEq and a third man, who was further away, was exposed to less radiation.

Internet articles frequently describe Ouchi as 'the most radioactive man in history,' or words to that effect, but nuclear expert Lyman stops a bit short of that assessment.

"The estimated doses for Ouchi were among the highest known, though I'm not sure if it's the highest," explains Lyman. "These typically occur in these kinds of criticality accidents."

The radiation dose in a criticality accident can be even worse than in a catastrophic accident at a nuclear power plant, such as the 1986 reactor explosion at Chernobyl in Ukraine, then a part of the Soviet Union, where the radiation was dispersed. (Even so, 28 people eventually died from radiation exposure.)

"These criticality accidents present the potential for delivery of a large amount of radiation in a short period of time, though a burst of neutrons and gamma rays," Lyman says. "That one burst, if you're close enough, you can sustain more than a lethal dose of radiation in seconds. So that's the scary thing about it."

High doses of radiation damage the body, rendering it unable to make new cells, so that the bone marrow, for example, stops making the red blood cells that carry oxygen and the white blood cells that fight infection, according to Lyman. "Your fate is predetermined, even though there will be a delay," he says, "if you have a high enough dose of ionizing radiation that will kill cells, to the extent that your organs will not function."

According to an October 1999 account in medical journal BMJ, the irradiated workers were taken to the National Institute of Radiological Sciences in Chiba, just east of Tokyo. There, it was determined that their lymphatic blood count had dropped to almost zero. Their symptoms included nausea, dehydration and diarrhea. Three days later, they were transferred to University of Tokyo Hospital, where doctors tried various measures in a desperate effort to save their lives.

When Ouchi, a handsome, powerfully built, former high school rugby player who had a wife and young son, arrived at the hospital, he didn't yet look like a victim of intense radiation exposure, according to "A Slow Death: 83 Days of Radiation Sickness," a 2002 book by a team of journalists from Japan's NHK-TV, later translated into English by Maho Harada. His face was slightly red and swollen and his eyes were bloodshot, but he didn't have any blisters or burns, though he complained of pain in his ears and hand. The doctor who examined him even thought that it might be possible to save his life.

But within a day, Ouchi's condition got worse. He began to require oxygen, and his abdomen swelled, according to the book. Things continued downhill after he arrived at the University of Tokyo hospital. Six days after the accident, a specialist who looked at images of the chromosomes in Ouchi's bone marrow cells saw only scattered black dots, indicating that they were broken into pieces. Ouchi's body wouldn't be able to generate new cells. A week after the accident, Ouchi received a peripheral blood stem cell transplant, with his sister volunteering as a donor.

Nevertheless, Ouchi's condition continued to deteriorate, according to the book. He began to complain of thirst, and when medical tape was removed from his chest, his skin started coming off with it. He began developing blisters. Tests showed that the radiation had killed the chromosomes that normally would enable his skin to regenerate, so that his epidermis, the outer layer that protected his body, gradually vanished. The pain became intense. He began experiencing breathing problems as well. Two weeks after the accident, he was no longer able to eat, and had to be fed intravenously. Two months into his ordeal, his heart stopped, though doctors were able to revive him.

On Dec. 21, at 11:21 p.m., Ouchi's body finally gave out. According to Lyman's and Dolley's article, he died of multiple organ failure. Japan's Prime Minister at the time, Keizo Obuchi, issued a statement expressing his condolences to the worker's family and promised to improve nuclear safety measures, according to Japan Times.

Shinohara, Ouchi's co-worker, died in April 2000 of multiple organ failure as well, according to The Guardian.

The Japanese government's investigation concluded that the accident's main causes included inadequate regulatory oversight, lack of an appropriate safety culture, and inadequate worker training and qualification, according to this April 2000 report by the U.S. Nuclear Regulatory Commission. Six officials from the company that operated the plant were charged with professional negligence and violating nuclear safety laws. In 2003, a court gave them suspended prison terms, and the company and at least one of the officials also were assessed fines, according to the Sydney Morning Herald.

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Hisashi Ouchi Suffered an 83-day Death By Radiation Poisoning - HowStuffWorks