The five workers exposed to airborne plutonium at the Oarai Research & Development Center in Ibaraki Prefecture were quarantined for about three hours in the room where the accident occurred, a Japan Atomic Energy Agency official said Friday.
Although this action was taken to prevent the plutonium and other radioactive contaminants from spreading to other parts of the nuclear research facility, it probably worsened their internal exposure as they breathed the tainted air.
Internal radiation exposure has been confirmed in four of the five men.
Education and science minister Hirokazu Matsuno said at a news conference Friday that a specially appointed team in the ministry would question JAEA President Toshio Kodama about Tuesday’s accident in the coastal town of Oarai.
The accident occurred inside an analysis room at the facility’s fuel research building around 11:15 a.m. Tuesday when a worker in his 50s opened a sealed metal container that had a plastic container of plutonium and uranium powder samples inside that was double-bagged in plastic.
At some point, the bag ruptured, ejecting powder into the air.
JAEA says the tainted floor of the room is giving off 55 becquerels of radiation per square centimeter in the area in front of the apparatus — believed to be a fume hood — in which the container was placed before it was opened. The acceptable level is 4 becquerels, according to the Nuclear Regulation Authority.
The workers waited in the room for 3½ hours after reporting the accident. It was only at 2:44 p.m. that they started being checked for radioactive contamination, JAEA said.
When the accident occurred, the men were wearing masks that covered their noses and mouths, but the checks revealed radioactive material was in the nostrils of each one of them.
Subsequent lung checks showed that the man in his 50s had 22,000 becquerels worth of plutonium-239 in his system, compared with 5,600 to 14,000 becquerels in three of the other four. Four of the five were thus diagnosed with internal radiation exposure.
The metal container, which had not been opened once since it was sealed in 1991, was being checked on the instructions of the NRA.
Experts evaluating the accident say it is possible that helium had accumulated in the bag over the years, raising the pressure in the container.
The NRA, the government’s nuclear watchdog, plans to look into the accident, including the manner in which the workers wore their masks.