Overexposure of employee in irradiation facility

Fleurus, Belgium is the site of a Sterigenics irradiation sterilization facility, performing principally medical device sterilization. The facility uses gamma radiation emitted from sealed cobalt 60 isotope source. The cobalt 60 source is strictly contained in a thick concrete containment vessel.
When not in operation, this source is stored in a water pool. A secured hydraulic system allows taking the source out of this pool in order to irradiate the products put in the cell. Safety locks prevent the system to take the source out of the pool when the door of the cell is open.
On Saturday March 11th, an employee went into the room where the cell is located and observed that the gamma monitor was in high level alarm. The door of the cell was open and the cell empty, no irradiation was performed at that time. He reset the monitor and verified that the alarm was not appearing again. He then decided to close the door of the irradiator. For safety rules, this requires that he has to enter the cell and to switch on a contact located in the back of the cell, proving that he verifies that nobody was inside before shutting the door. He remained about 20 s in the cell to perform this check. He did not notice any anomaly at this time, neither inside the cell nor outside. The gamma monitor did not actuate again.

Some while later he had nausea and vomited but did not consider that this had anything to do with his work. Nearly three weeks later, he observed that his hairs were massively falling down. He then went to the physician who decided to investigate his blood. This showed that he was severely exposed to high doses of radiations. Based on further biological analysis the received dose was estimated (on April 11th) to be 4,6 Gy (from 4,4 to 4,8 Gy). The employee has been hospitalized on March 31st in a French hospital highly specialized in treatment of radiation exposure. According to the fysicians, he seems now to recover from the exposure.
The accident was not reported to facility or company management until March 30th, 2006.

Computer records of the position of the source show that the "down" limit switch was actuated at several time in the period while the employee was inside the cell. It is provisionally assumed that during his short presence in the cell, due to a presently not yet identified defect of the hydraulic system, the source could have been slightly taken out of the water pool. Further investigation is performed in order to check the working of the hydraulic system and the electrical control system. Although not yet completly explained, some features of the hydraulic system could have caused the sources to move up.

The specific causes of the accident are still under further investigation.

We will communicate additional information as soon as it is available.

Location: STERIGENICS - Fleurus
Event date: Sat, 11-03-2006
Nuclear event report
Legenda & explanation