Some differences may exist between french and english regulations. Lessons learned are only a translation of those of french incident and are not adapt to english regulation.
Description of incident
A food processing plant had a level gauge containing a 37 MBq caesium-137 source. The gauge was not working, and a newly qualified engineer from the company that supplied the equipment (the only company authorised to work on this gauge), sawed through the pencil containing the radioactive source believing that he was shortening the shutter. He was not familiar with the type of equipment he was working on.
After the maintenance work, the system was still not operational which led the engineer to suspect an electronic board failure. After the installation of the new electronic board (2 days later), the test of the control device was not conclusive. The supplier of the source was alerted to this situation during a telephone conversation with the engineer. During this conversation, the company realized that their employee was working on the gauge thinking that it was a different model.
The same evening, the establishment called in a specialized company to check for radioactive contamination.
Three contaminated areas were identified in the establishment:
- A mechanical workshop in which the source had been placed in a vice and sawed with a hacksaw. The dose rate in contact with the floor was between 40 and 50 µSv/hr.
- In a foreman’s office, the contamination of two carpets was identified (probably transported by the soles of shoes).
- The gauge area had a few spots of contamination.
It was suspected that the engineer and two other employees who worked directly on the source received an external radiation exposure, as well as internal and external contamination. However, subsequent measurements revealed no internal contamination.
The activity level of the damaged source was 37 MBq. The radiological consequences could have been more serious had the activity been higher : some gauges on the market contain sources up to several hundred GBq.
Lessons to be learned from the incident
The source was notified to, and authorized by, the regulatory athorities, and the relevant periodic safety checks had been performed. Consequently, no failure to comply with the regulations was assigned to the operator. Instead, the equipment supplier (and the only body authorised to maintain the gauge) was deemed to have sole responsibility for the incident.
The lack of proper training of the engineering staff within the supplier company is thought to be the main cause of the incident. Various measures, that would have helped avoid the incident, are listed below:
- An instruction manual for engineers, clearly identifying the different parts of the equipment, should have been prepared. In the incident, the engineer confused the shutter with the source holder.
- Practical modules on equipment maintenance should be included in the radiation protection training provided.
- Engineers should always have a suitable radiation monitoring instrument. In this case, the use of a dose rate monitor would have allowed the operator to quickly identify the precise location of the source.