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Evaluation of an Upward Trend in Background Count Rates from a Stack Particulate Continuous Air Monitor

Rishel, Jeremy P.; Barnett, J. Matthew; Mozhayev, Andrey V.; Dutcher, Elliott “Dutch”; Colby, Scott A.; Carter, Gregory L.; Patello, Gertrude K.; Steen, Franciska H.; Hanson, Eric M.1

doi: 10.1097/HP.0000000000000901

The Pacific Northwest National Laboratory operates the Radiochemical Processing Laboratory, which is a multi-purpose, non-reactor nuclear research facility. Regulations require both continuous sampling and monitoring of radioactive particulates and tritium gas in the exhaust from the main stack. Releases of other radioactive gases, including planned releases of radon, are tracked separately in a database and reported. During the 2015 calibration of the Radiochemical Processing Laboratory stack continuous air monitor, measured alpha and beta background count rates were much higher than expected, especially when compared to count rates from previous calibrations. The source of the higher background count rates was examined by trending of historical continuous air monitor measurements and a comparison to the sampler data. The analysis revealed that the sample results showed no increase in emissions, whereas the continuous air monitor showed a steady increase in count rates. Ultimately, the continuous air monitor filter was analyzed and found to contain higher-than-normal background levels of 220Rn progeny. Assessments were performed to determine the cause of the increased background values, including reviews of building research activities, radioactive material usage and storage, adequacy of procedures, and the potential for internal continuous air monitor contamination. Project reviews determined that a research activity involving 228Th was left in an unsealed state, resulting in 220Rn being released from a hot cell into the exhaust system. The 228Th source material was subsequently repackaged and contained, resulting in a decrease of continuous air monitor background count rates. An estimate of the 220Rn release was made and the contribution to the annual offsite dose from the facility was calculated. The released activity and reported dose results were well below the permit limits for the facility.

1Pacific Northwest National Laboratory, PO Box 999, Richland, WA 99352.

The authors declare no conflicts of interest.

Jeremy Rishel supports the Radioactive Air Task at PNNL. He is a meteorologist by training and graduated from the Pennsylvania State University with a BS (1996) and MS (1998) degree in atmospheric science. He also assists the U.S. Nuclear Regulatory Commission with ongoing development of atmospheric dispersion models (RASCAL, PAVAN, ARCON) and is committee chair for the Department of Energy’s (DOE’s) Consequence Assessment Modeling Working Group (CAMWG) under the Subcommittee on Consequence Assessment and Protective Actions (SCAPA).

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