Description of the incident
A contractor, employed to replace level gauge mounting brackets, was exposed to gamma radiation when a gauge was removed from a vessel whilst the shutter remained open. The gauge contained a 111 GBq caesium-137 source.
Before the maintenance work, a site technician confirmed that he had closed the gauge shutter in accordance with the company’s permit to work scheme. However, due to an administrative error, the shutter of the wrong gauge was closed.
The contractor removed the correct gauge (ie with shutter still open) and put it on a walkway with the main radiation beam directed towards his lower legs. The contractor worked for approximately 30 minutes in close proximity to the gauge, before the incident was discovered.
The Radiation Protection Supervisor and subsequently the Radiation Protection Advisor were contacted and a dose investigation carried out.
Using pessimistic assumptions regarding the occupancy and location of the contractor, the estimated doses received by the contractor were 16 mSv to the lower legs, and 2 mSv to the whole body.
A “permit to work” system can provide an appropriate level of protection, especially in the case of occasional maintenance work. However, it needs to be supported with other safety measures. For example, there should always be an exchange of safety information between the site operator and contractors prior to the commencement of any work. This should include an agreed description of the work, and information on the device containing the radioactive source and the associated radiation hazards.
A radiation monitoring instrument should always be used to confirm that the shutter is closed whenever a gauge is removed from its mounting. (Source capsules have been known to become uncoupled from the external shutter mechanisms). The person carrying out the check should be trained in the use of the monitor.
It was recognised during the investigation that the identification system used for the gauges was ambiguous. Source containers should always be marked with a unique identification number to help prevent the type of error that occurred in this incident.