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Efficacy of Personal Air Samplers and Exit Portal Monitors for the Daily Detection of Significant Intakes of a Mixture of Radionuclides Present in a Contamination Event at a Nuclear Power Plant in the Presence of Radon Progeny

Abstract
In α or αß counting of a filter from a Personal Air Sampler (PAS), the interference from radon progeny can be significant. This interference limits the timely assessment of the count from long lived α and ß emitters. Based on a contamination event at a nuclear power plant, the implied Committed Effective Dose (CED) for the radionuclide mixture is calculated from either the total predicted α or αß count from radon progeny. The nuclear power plant was being refurbished at the time of the contamination event after a 20 year operating history, and then after not having operated for more than 10 years.  The operating history included experience with fuel cladding failures. Long lived α emitters comprise a fraction Fα of 0.0314 of the total αß radioactivity of the mixture, which corresponds to a very low ß to α activity ratio of 30.8. A CED to α count ratio of 0.863 µSv per α count and a CED to αß count ratio of 0.0271 µSv per αß count for the mixture of radionuclides in the contamination event are used to convert the total α count, Cα, or total (α + ß) count, Cαß, from progeny to their implied CEDs.  Counting intervals from 30 to 600 seconds are used for counting of the PAS filter on a counter assumed for practicality and simplicity to have a zero background. Thus, the critical decision count is zero, and the required mean count µ is 3 for a 95% chance of obtaining a count above zero. A count of 3 from progeny thus corresponds to an implied CED of only 2.59 µSv for the α counting mode and an even much smaller value of 0.0813 µSv for the αß counting mode. When it is considered to be present, concentrations of Rn-220 are assumed to be 10% of those assumed for Rn-222. Because of the interference from radon progeny, alpha screening counts of a PAS filter do not have the capability for the timely detection of intakes corresponding to an investigation level of 1 mSv when workers exit a Radiological Control Area (RCA). Measurements of a PAS filter with a pancake GM detector-counting rate meter are expected to yield an αß counting rate of 20.5 cps per CED of 1 mSv from the mixture of radionuclides in the contamination event. For a 30 second αß count made two minutes after an 8 hour collection interval when the Rn-222 concentration is 4 pCi per liter, the implied CED from radon progeny αß counts is 0.515 mSv or 51.5% of an investigation level of 1 mSv. When the Rn-222concentration is 1 pCi L-1, the implied CED from radon progeny counts is 0.129 mSv or 12.9% of an investigation level of 1 mSv. Recommendations are included in this poster for establishing screening guides for daily measurements of the PAS filter that would trigger followup investigations, including corrective actions in the workplace, bioassay procedures, and special analyses of radioactive aerosols collected on the PAS filter. Also included is a comparison of the efficacy of an exit portal monitor used by workers and of the PAS αß daily screening method.  The 95% confident Reliable Detectable-Committed Effective Dose (RD-CED) of the exit portal monitor is 8.1 mSv from the mixture of radionuclides present in the contamination event. This RD-CED is 8.1 times an investigation level of 1 mSv. When potential exposures of workers to mixtures of radionuclides that have a ß to α activity ratio of 300 or less or when exit portal monitors do not have the capability of detecting an exposure corresponding to a CED equal to an investigation level of 1 mSv, then it is recommended that a daily PAS αß screening procedure be used for the daily detection of significant exposures of workers at nuclear power plants when they exit the RCA.

by
K. W. Skrable, G. E. Chabot, C. S. French, and M.A. Tries
University of Massachusetts Lowell
and
E. L. Darois, J. P. Tarzia, and F. P. Straccia
Radiation Safety and Control Services

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