 INES 2 |
Potential Overexposures to Members of the Public |
On February 23, 2007, Oklahoma Department of Environmental Quality (ODEQ) notified the Nuclear Regulatory Commission (NRC) that personnel (non-occupational) employed by a well logging licensee may have received a radiation dose in excess of the regulatory limits for members of the public. The event involved a sealed well logging source containing approximately 33 gigabecquerel (GBq) [0.90 Ci] of Cs-137.
On February 22, 2007, at approximately 9:00 am, an individual working for the licensee noticed what appeared to be a component part laying on the ground at the licensee’s facility. The individual did not know the component was radioactive, so he picked it up and put it in his jacket pocket. Additionally, the individual visited several businesses and hung his jacket in an employee break room for the remainder of the day and overnight. On February 23, 2007, at approximately 6:00 am, the well logging crew unpacked their equipment and realized the source was missing. They immediately initiated a search and retrieved the source from the individual and concluded that the source was inadvertently dropped at approximately 6:00 am on February 22, 2007.
The ODEQ measured exposure rates at several distances from the source using an ion chamber. Next, the ODEQ used a dosimeter to measure the dose. Dosimeter results revealed an effective dose of 161 mSv (16.1 rem) and a shallow/extremity dose of 15.4 cGy (15.4 rad) over a period of 3 minutes and 35 seconds. Based on the dosimeter results, ODEQ estimates that the dose rate at one inch would be approximately 257 rem/hr.
The licensee estimated that the individual wore the jacket containing the source for approximately 5½ hours. In addition to the individual holding the source, several individuals were taken to the local hospital and examined. The attending physician noted that no radiation effects were observed. White blood cell cultures were taken from the individual that remained near the source on February 23-24, 2007, and were determined to be in the normal range.
On February 26, 2007, the physician interviewed the individual and conducted time motion studies. The physician stated (in part) in his report that: “... the individual received a calculated dose exposure over a period of 5½ hours of approximately 1120 R at the skin surface, 10 Gy of exposure to the skin of the abdominal wall [4 cm from source], a dose no more than 4.4 Gy to the abdominal structures [20 cm from the source], and a calculated dose (to the hand) of approximately 2000 R or approximately 18 Gy.” The physician further estimated the total body exposure to be less than 5 R (approximately 50 mSv) for the entire exposure period. At the request of ODEQ, NRC staff estimated that a co-worker may have received a dose of 140 mrem. NRC’s regulatory total effective dose equivalent limit for members of the public is 1 mSv (100 mrem).
As of March 1, 2007, the ODEQ is continuing its investigation of the event. The licensee sent the individual’s blood sample to the United Kingdom for cytogenetic evaluation (chromosome analysis) and the ODEQ expects to receive results the week of March 4, 2007.
Event date: Wed, 28-02-2007
