information about Fukushima published in English in Japanese media info publiée en anglais dans la presse japonaise
25 Mai 2013
Scientists at a nuclear physics laboratory in Tokai, Ibaraki Prefecture, ignored the ringing of an alarm that radioactive substances were leaking and continued with the experiment that triggered the problem for more than four hours, the Japan Atomic Energy Agency said May 25.
The government and the public were kept in the dark for about 30 hours after the accident occurred. At least four scientists, all male, received internal doses of up to 1.6 millisieverts, about the same level that constitutes the annual background dose for anybody living in Japan.
The leak occurred at the Hadron Experimental Facility on the grounds of the Japan Proton Accelerator Research Complex (J-PARC), which is operated jointly by the JAEA and the High Energy Accelerator Research Organization.
About 20 scientists were in the laboratory at the time. The experiment involved bombarding gold with proton beams to generate elementary particles.
The alarm sounded at 11:55 a.m. on May 23.
Officials said 55 individuals entered the facility and were in close proximity to the experiment equipment on that date. All of them will be screened for internal exposure to radiation.
According to JAEA officials, the equipment halted operations after the beam intensity had risen to 400 times normal levels, likely due to a glitch in the power supply system. But the scientists stopped the alarm and went on with their experiment 13 minutes later.
"Equipment stoppages due to malfunctions are not uncommon," Naohito Saito, a deputy director of the J-PARC Center, told a news conference on May 25.
The accident occurred because the gold, which had been irradiated with the unusually intense proton beams, evaporated. The scientists inhaled sodium and other radioactive substances generated during the process.
Radiation detectors showed enhanced dose rates inside the facility around 12:30 p.m. Readings of radiation monitors, also inside the facility, soared to about 10 times normal levels by around 1:30 p.m. The scientists closed down the experiment at 2:26 p.m.
Radiation levels subsided temporarily after a ventilator was turned on at 3:15 p.m., prompting the scientists to resume the experiment at 3:32 p.m. The ventilation was causing radioactive substances to leak outside.
No filter or other safety tool was installed in the ventilator to remove radioactive substances. That is because nobody had expected that radioactive materials could leak from the facility.
"The facility is designed to generate radiation," Saito explained. "We thought it was probably OK to go on with the experiment as long as the dose rates stayed below the standards stipulated for radiation control zones. We thought the case was less than report-worthy."
Radiation levels inside the facility surged again around 4 p.m., so the scientists stopped the experiment at around 4:15 p.m.
A radioactive contamination reading of 30 becquerels per square centimeter registered on the floor at around 5 p.m., so the facility was designated off-limits.
The ventilator was reactivated around 5:30 p.m. The scientists left the lab between that time and 11 p.m. and were screened for contamination on their skin and clothes. They were all allowed to go home after being told that the readings were below levels that should not be exceeded when radiation workers leave a control zone.
"We ventilated the air out due to concerns about the safety of our scientists," Saito said. "It was thoughtless of us to have released polluted air out of the facility."
Overnight on May 24, the JAEA administered a whole body counter to measure doses of internal exposure to radiation with scientists who asked to be screened. Officials checked out radiation monitor data outside the facility from earlier that evening and learned for the first time that the readings had shot up around the time the ventilators were switched on the previous day.
But it still took the JAEA several more hours to notify the relevant authorities. Officials had apparently convinced themselves that the estimated dose of external exposure to radiation stayed below the facility's control target level. The dose estimate, in fact, exceeded the control target level.
The JAEA only notified the Nuclear Regulation Authority at 9:20 p.m. on May 24 that radioactive substances had leaked outside radiation control zones. The prefectural and village governments were notified at 9:40 p.m.
"We were too indulgent with ourselves," Saito repeated to persistent questions from reporters about the reason the JAEA was so late in notifying the authorities.
The latest accident came on the heels of another scandal concerning lax safety oversight at the JAEA.
On May 15, the Nuclear Regulation Authority decided to order a suspension of operations at Monju, the JAEA's prototype fast-breeder reactor in Tsuruga, Fukui Prefecture, after it learned that the JAEA had skipped inspections of nearly 10,000 pieces of equipment at Monju since 2010. The NRA said the suspension would remain in effect until the agency rectified its safety control system.
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