 INES 3 |
Potential Overexposure During Co-60 Source Exchange |
On July 21, 2008, two radiation workers, A and B, one American and one Brazilian, respectively, had received a potential overexposure. While in a hospital in the State of São Paulo, Brazil, they were in the process of exchanging an expended cobalt-60 source with a new cobalt-60 source. After engaging the source holder with the removal tool, worker A transferred the source holder from the unit head into the transfer cask. Once the source holder was in the transfer cask, worker A continued making preparations to complete the removal sequence. He had completed all the steps of the removal sequence and was in the process of removing the removal tool from the transfer cask, when it was determined by audible alarms and before the tool was completely out of the cask, that the cobalt-60 source was still connected to the end of the removal tool. At that point, worker A reinserted the removal tool in the cask and repeated the steps to make sure that the source was disengaged from the removal tool. Once he was sure that the source was no longer attached to the removal tool, it was removed from the cask, and all the covers installed. After that, the transfer cask contained both the expended and the new source.
Immediately after the container was secured, both workers read their SRD's and reported their dose as follows:
• Worker A: Whole Body dose: 52 mSv (5.2 rem), Left Wrist: 3 mSv (300 mrem), and Right Wrist: Off Scale (10R dosimeter);
• Worker B: Whole Body dose: 5.3 mSv (533 mrem), Left Wrist: Off scale (10R dosimeter), and Right Wrist: 41 mSv (4.1rem)
The TLD's were sent via FED-EX to Global Dosimetry Solutions in Irvine, CA, for emergency reading and the results are as follows:
• Worker A: Monthly Whole Body (WB) Deep Dose Equivalent (DDE)- 135 mSv (13.5 rem), Quarterly WB DDE: 111 mSv (11.1 rem),Left Wrist Shallow Dose quivalent (SDE): 733.4 mSv (73.4 rem),
Right Wrist SDE: 7.5 mSv (0.75 rem).
• Worker B: Monthly WB DDE: 7.02 mSv (0.7 rem), Quarterly WB DDE: 15.6 mSv (1.56 rem), Left Wrist SDE: 30.3 mSv (3.03 rem),
Right Wrist SDE: 85.4 mSv (8.5 rem)
Two inspectors from the National Regulatory Body were also onsite, but were following the operation remotely and therefore, did not receive relevant doses.
Worker A has returned to the United States (U.S.). On August 6, 2008, staff from the U.S. regulatory authorities met with the individual and observed a re-enactment of the event. At that time, the individual was exhibiting reddening of the skin on his left forefinger and the dose to the finger was estimated to be at least 5 Sv (500 rem). On August 15, 2008, U.S. regulatory authorities learned that the individual’s finger was blistering. Based on the physical response, the company’s medical consultant now estimates the extremity dose to be approximately 25 Sv (2,500 rem).
Event date: Mon, 21-07-2008
