CBRN event in Japan on Mon 7th April 2008

On April 7, 2008, a licensee in Chiba prefecture notified the Ministry of Education, Culture, Sports, Science and Technology (MEXT) that one of their industrial gamma radiography exposure devices containing a sealed source of 370 GBq of iridium-192 was lost.

According to the report from the licensee, the device was confirmed its existence at 10:50 p.m. on April 4, in warehouse of the company, and an employee of the company realized the loss of the sealed source at around 07:00 a.m. on April 7. The door of the warehouse was locked. The device was supposed to be stolen and the event was reported to the local police, and national police agency.

The lost device is used for non-destructive inspection, and is 27cm in diameter, 40cm long and weighs 22kg. The dose rates at 1m from its surface are estimated to be maximally 0.007mSv/h when shielded in the device, and 50mSv/h when unshielded.

On May 8, a suspect in the theft of the radioactive source was arrested. The radioactive source was found in a river in Yokohama, Kanagawa prefecture, where the general public can not approach usually. The source holder was intact though the source was taken out from the outer shield.
The source was recovered by the national authorities on the same day.

The maximum exposure to the workers who searched and recovered the source was 0.5mSv. The exposures to the residents along the river were estimated to be far below the public dose limit. There was no information which suggests apparent excessive exposure to the suspect.