The following information was obtained from the State of
Minnesota via email:
[On June 18, 2009, at a temporary job site,] the licensee was conducting radiography of circumferential welds. At approximately 1220 hrs, the radiographers heard a 'bang' from inside the horizontal heavy wall vessel. The licensee was using a 42 Curie Cobalt-60 source in an AEA 680 exposure device. The lead radiographer immediately attempted to retract the source but could not move the control handle. Thinking that the problem was the result of a tight radius, the radiographer attempted to withdraw the guide tube from the tank. That effort was abandoned when the guide tube began to slide out of the vessel. However, the radiographer identified a dent in the guide tube that was approximately 18 inches from the far end. The source was shielded with 3/8 inch lead plates.
The radiographer conducted a survey and calculated the exposure to hammer out the crimp in the guide tube. After consultation with the Radiation Safety Officer, a radiographer approached the guide tube, turned it 1/4 of a turn, and hit it once with a hammer. The source was then successfully retracted.
Total doses for the retrieval were 190 mrem to the lead radiographer and 20 mrem to the second radiographer. The damaged guide tube has been removed from service.
The root cause of the problem has been determined to be that the guide tube was extended to its fullest length; therefore, the tension and/or weight of the tube caused the stand to fall over and crimp the guide tube. The corrective action was to add an additional guide tube and to secure the stand with weights to prevent tipping.