A fence line noble gas monitoring system for nuclear power plants

A noble gas monitoring system has been installed at Ontario Power Generation's Pickering Nuclear Generating Station (PNGS) near Toronto, Canada. This monitoring system allows a direct measure of air kerma from external radiation instead of calculating this based on plant emission data and meteorological models. This has resulted in a reduction in the reported effective dose from external radiation by a factor of at least ten. The system consists of nine self-contained units, each with a 7.6 cm X 7.6 cm (3 inch X 3 inch) NaI(TI) detector that is calibrated for air kerma. The 512-channel gamma ray spectral information is downloaded daily from each unit to a central computer where the data are stored and processed. A spectral stripping procedure is used to remove natural background variations from the spectral windows used to monitor xenon-133 (Xe-133), xenon-135 (Xe-135), argon-41 (Ar-41), and skyshine radiation from the use of radiography sources. Typical monthly minimum detection limits in air kerma are 0.3 nGy for Xe-133, 0.7 nGy for Xe-135, 3 nGy for Ar-41 and 2 nGy for skyshine radiation. Based on 9 months of continuous operation, the annualised air kerma due to Xe-133, Xe-135 and Ar-41 and skyshine radiation were 7 nGy, 8 nGy, 26 nGy and 107 nGy respectively.