INES-event
INES 2

Overexposure of worker by unshielded source

A 185 GBq (5Ci) Am-241/Be (Gulf Nuclear Inc. 71-1) sealed source used for well logging was unintentionally disconnected from its holder and attached on the outside of the storage/transport container (neutron/gamma shield). It remained unshielded for a period of 52 days.
Following a well logging operation on October 31, 2005, the operator was unable to insert the connected source and source holder into the transport/storage container. The operator therefore examined the device. He found that the base of the source capsule was irregular and therefore mistook it for a mounting device that had broken off the interior of the container. He disconnected the source from the holder with his fingers and subsequently placed the empty holder in the container. The Operater then attached the source to the exterior of the container with adhesive tape. Upon his return to the headquarters he returned the container to the storage room. He reported to his superiors that a mounting device had broken off the interior of the container. They failed to react and the source remained unshielded in the storage room.
48 days later, the container with the unshielded source still attached on the outside, was brought on a 4-day operation. This operation was carried out by a different operator, who was informed about the seemingly broken off mounting device, but told not to take notice. Deployment of the neutron probe was not due until the end of the 4th day. During the 4-day use of various types of non-radioactive probes, the operator repeat-edly rested on top of the container only a few inches from the unshielded source. From this position he could operate the winch and the computer simultaneously. Apparently he rested approximately 3-6 inches from the source in 8-9 hours integrated over the 4-day period of operation. After failure of the neutron logging operation the operator became aware that the source was disconnected from the holder and eventually found that it was attached to the shield exterior. The source was returned to a safe position in the container on December 22, 2005.
Although the operators were instructed in the use of the neutron log, none of them were familiar with the design of the source, source holder and container in detail. None of them were equipped with a real time dosimeter. The first operator carried a personal dosimeter, which reveals that he received an effective dose of 6.6 mSv. The second operator did not carry a personal dosimeter as he should according to Danish regulations. His dose was therefore assessed from biological dosimetry as well as log-timer readouts and distance estimates. The dose was below the LLD of biological dosimetry using standard analysis of dicentric chromosomes. This is consistent with preliminary maximal dose estimates of 100-150 mSv. These are calculated from a simple time-distance estimate. A more accurate calculation of doses to the involved logging operators will be made with Monte Carlo calculations.

Location: Zealand
Event date: Thu, 22-12-2005
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