 INES 2 |
Overexposure to Worker |
Note: the below updates the initial report language in its entirety.
On February 12, 2009, the licensee employee was sent for a lung count at the Battelle (Pacific Northwest National Laboratory) for a bioassay measurement of the lung. The licensee employer ordered the lung bioassay because airborne contamination levels exceeded action levels. The employee’s first lung count detected an average of approximately 14.8 Bq (0.4 nCi) of Am-241. Assuming the exposure was from 10 days prior (based on post exposure investigation findings), the intake was approximately 70 Bq (1.9 nCi) Am-241. In the U.S., the annual limit on intake for Am-241 is 222 Bq (6 nCi) (1 micron Activity Median Aerodynamic Diameter particle size). The estimated dose was about 1/3 of the annual limit, or 0.16 Sv (16 rem) Committed Dose Equivalent, which exceeded the statutory annual limit of 0.5 Sv (50 rem). The worker had previous whole body exposure, but this added amount did not cause the statutory limit to be exceeded.
On March 25, 2009 the licensee employer informed the Washington State Department of Health that further testing by Battelle necessitated a revision to the original calculated dose and the new calculated dose would exceed the 0.5 Sv (50 rem) Committed Dose Equivalent limit. The licensee employer assumed that the date of exposure was February 3, 2009, Fecal bioassay results from one other employee who was also in the containment showed a small amount of activity, and a dose was assigned to this second worker that did not exceed regulatory limits. The second employee’s lung bioassay was less than detection limits.
On June 22, 2009 the licensee informed the Washington State Department of Health that the Committed
Effective Dose Equivalent for the employee was 68 mSv (6.8 rem) and the Committed Dose Equivalent was 1.2 Sv (120 rem) to the bone surface. The employee’s Deep Dose Equivalent from his dosimetry for the first quarter of 2009 was 0.3mSv (30 mrem). Intake was calculated using methodology of ICRP 30, modified for clearance function. Intake for Am-241 was calculated from lung deposition and calculated clearance rates. Intake of Plutonium (Pu) was inferred from excreta bioassay results and assumed ratios of Am-241 to Pu. Dose was calculated using CINDY code version 1.2. The particle size was considered, and a 1 micron Activity Median Aerodynamic Diameter was chosen as the most appropriate particle size.
The exact cause of the incident is unknown, but the assumed cause is a failure of the respiratory protection system.
The licensee’s corrective actions to prevent recurrence include: testing each worker with a challenge gas prior to high risk work; implementing increased engineering controls to mitigate airborne contaminants; providing specific training using phosphorescent powder and black lights for workers; performing more frequent bioassay samples; inclusion of nasal smears for immediate detection of intakes; the use of supplied air respirators for high risk work, and training for workers, managers and health physics staff. Work was resumed in the area, and no further exposures have occurred.
Initial event description posted on 2009/02/14:
On February 3, a worker was in a containment in which air sample results were about
3.7E-07 kBq/ml (1E-08 microCi/ml) gross Am-241 alpha concentration for several hours
and was wearing a Powered Air Purifying Respirator (PAPR, with a protection factor of
1000). On February 9, 2009, this worker was sent for a bioassay measurement (lung
count). On March 25, 2009 the licensee informed the Washington State Department of
Health that further analysis of the original calculations revealed a dose to the worker that
exceeded the 0.5 Sv (50 rem) Committed Dose Equivalent (CDE) statutory annual limit to
the lungs. The estimated dose to the worker is about 1 Sv (100 rem) CDE and 0.05 Sv (5
rem) Committed Effective Dose Equivalent (CEDE).
Event date: Tue, 03-02-2009
