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Implications of between- and within-worker variations in radiation exposure on compliance at the Hope Creek Nuclear Generating Station
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Implications of between- and within-worker variations in radiation exposure on compliance at the Hope Creek Nuclear Generating Station

Kathryn McNamara
Master of Public Health (M.P.H.), Drexel University
27 May 2016
DOI:
https://doi.org/10.17918/etd-7613
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Abstract

Industrial hygiene Radiation Industrial safety Environmental Health Public Health
Objectives: To compare estimated annual doses from the Hope Creek Nuclear Generating Station with industry standards for radiation exposure, while accounting for between- and within-worker variability of monthly doses measurements, and to project compliance with limits on annual dose. Methods: Personal dosimeter data from Hope Creek Nuclear Generating Station was obtained for the period of January - December 2014, comprising 5,388 monthly dose measurements from 888 workers. 27% of measurements were below the limit of detection of 0.1 millirem (mrem). Compliance with the Nuclear Regulatory Commission's annual Occupational Exposure Limit (OEL) of 5,000 mrem was evaluated using three methods. First, a compliance estimate was calculated for the probability of any random worker exceeding the OEL. Second, the probability of overexposure on a monthly basis using 1/12 OEL and the plant's internal limit of 2,000 mrem was calculated for each group according to Rappaport et al (1994). To do so, we had to account for left censored data below the limit of detection using maximum likelihood estimation method of Jin et al (2010). Third, to account for truly unexposed months within a year for a worker, bootstrapped estimates of annual dose were computed; as part of this procedure, we also imputed non-detects based on the maximum likelihood estimates of mean and variances. We also compared projected doses with data for the industry as the whole, obtained from the Nuclear Regulatory Commission's Radiation Exposure Information and Reporting System (REIRS) and the Department of Energy's Radiation Exposure Monitoring System. Results: Using the compliance test, none of the workers exceeded the OEL. After considering between- and within-worker variances, 5 out of 14 groups had a >1% chance of exceeding the 1/12 OEL on a monthly basis. However, external contractors, maintenance, and radiation protection occupational titles had a greater than 5% chance of exceeding the lower internal limit on the monthly basis. Bootstrapped annual exposure doses revealed similar patterns, with very small chances of exceeding the OEL, but great potential for variability. Some workers may reach the 2,000 mrem internal limit if exposed at the 98th percentile of their projected annual dose. However, this scenario is unlikely given the presence of internal plant monitoring systems. Hope Creek is below the national average for boiling water nuclear reactors for total worker dose for 2013. Conclusions: Hope Creek Nuclear Generating Station appears to be typical of its industry peers in terms of annual ionizing radiation doses. Some variations exist in between- and within-worker exposure that are not captured in an aggregated annual dose estimate. These individual and occupational title-level variations are important to take into account when forecasting likelihood of exceeding OEL and suggesting preventative measures.

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