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.

Ouchi was standing at a tank, holding a funnel, as this story that appeared in The Washington Post a few months later describes. Masato Shinohara, one of Ouchi's coworkers, was pouring a mixture of intermediate-enriched uranium oxide into the tank from a bucket.

A flash of blue light suddenly surprised them, the first indication that something horrible was about to happen.

This 2000 article in the Bulletin of the Atomic Scientists describes how the workers unintentionally put too much uranium in the tank since they had never handled uranium with that level of enrichment before. Consequently, they unintentionally set off what is referred to in the nuclear business as a criticality accident, which is the emission of radiation as a result of an uncontrollably large nuclear chain reaction.

According to this April 2000 study by the U.S. Nuclear Regulatory Commission, the Japanese government's inquiry determined that the accident's primary causes included poor worker training and certification, a lack of a suitable safety culture, and inadequate regulatory monitoring. Six employees of the business running the facility were accused of breaking nuclear safety regulations and exhibiting professional negligence. According to the Sydney Morning Herald, in 2003, a court sentenced them to suspended prison terms and fined the corporation and one of the officials.