Over time, dozens of companies and government institutes were established nearby to provide nuclear research, experimentation, manufacturing, and fuel fabrication, enrichment and disposal facilities.

A gradual chemical reaction inside one fresh barrel ignited the already-hot contents at 10:00 a.m. and quickly spread to several others nearby.

At 8 p.m., just as people were preparing to reenter the building, built up flammable gases ignited and exploded, breaking windows and doors, which allowed smoke and radiation to escape into the surrounding area.

A week after the event, meteorological officials detected unusually high levels of cesium 40 km (25 miles) southwest of the plant.

[8] Aerial views over the nuclear processing plant building showed a damaged roof from the fire and explosion allowing continued external radiation exposure.

PNC management mandated two workers to falsely report the chronological events leading to the facility evacuation in order to cover-up lack of proper supervision.

This delay was due to their own internal investigation of the fire causing hampered immediate emergency response teams and prolonged radioactivity exposure.

[12] The second, more serious Tōkai nuclear accident (Japanese: 東海村JCO臨界事故, romanized: Tōkai-mura JCO-rinkai-jiko) occurred about four miles away from the PNC facility on 30 September 1999, at a fuel enrichment plant operated by JCO, a subsidiary of Sumitomo Metal Mining Company.

The steps included feeding small batches of uranium oxide powder into a designated dissolving tank in order to produce uranyl nitrate using nitric acid.

In the final process, uranium oxide is placed in the dissolving tanks until purified, without enriching the isotopes, in a wet-process technology specialized by Japan.

[10] This process inadvertently contributed to a critical mass level incident triggering uncontrolled nuclear chain reactions over the next several hours.

In addition to these three workers who immediately felt symptoms, 56 people at the JCO plant were reported to have been exposed to the gamma, neutron, and other irradiation.

[16] JCO facility technicians Hisashi Ouchi, Masato Shinohara, and Yutaka Yokokawa were speeding up the last few steps of the fuel/conversion process to meet shipping requirements.

It was JCO's first batch of fuel for the Jōyō experimental fast breeder reactor in three years; no proper qualification and training requirements were established to prepare for the process.

Uncontrolled nuclear fission (a self-sustaining chain reaction) began immediately, emitting intense gamma and neutron radiation.

[15] Upon the point of critical mass, large amounts of high-level gamma radiation set off alarms in the building, prompting the three technicians to evacuate.

A worker in the next building became aware of the injured employees and contacted emergency medical assistance; an ambulance escorted them to the nearest hospital.

[2] The next morning, workers ended the chain reaction by draining water from the surrounding cooling jacket installed on the precipitation tank.

A boric acid solution was added to the precipitation tank to reduce all contents to sub-critical levels; boron was selected for its neutron absorption properties.

Twelve hours after the incident, 300,000 surrounding residents of the nuclear facility were told to stay indoors and cease all agricultural production.

[25] Ouchi suffered serious radiation burns to most of his body, had severe damage to his internal organs, and had a near-zero white blood cell count.

[27] Doctors tried to restore some functionality to Ouchi's immune system by administering peripheral blood stem cell transplantation, which at the time was a new form of treatment.

[13] After receiving the transplant from his sister, Ouchi initially experienced increased white blood cell counts temporarily, but he began to succumb to his other injuries soon thereafter.

He underwent radical cancer treatments, numerous successful skin grafts, and a transfusion from congealed umbilical cord blood (to boost stem cell count).

Despite surviving for seven months, he was eventually unable to fight off radiation-exacerbated infections and internal bleeding, and succumbed to fatal lung and kidney failure.

The 1999 incident resulted from poor management of operation manuals, failure to qualify technicians and engineers, and improper procedures associated with handling nuclear chemicals.

In September 2000 JCO agreed to pay $121 million in compensation to settle 6,875 claims from people exposed to radiation and affected agricultural and service businesses.

In October, six officials from JCO were charged with professional negligence derived from failure to properly train technicians and knowingly subverting safety procedures.

A STA report indicated JCO management had permitted these hazardous practices beginning in 1993 to shortcut the conversion process, even though it was contrary to approved nuclear chemical handling procedures.

[13] As a response to the incidents, special laws were put in place stipulating operational safety procedures and quarterly inspection requirements.