Rapid Detection and Estimation of an Unexpected Atmospheric Release of 129I

A sampling system was designed to rapidly detect an unexpected release of 129I during a weekly batch-sampling of a stack gas. The system consisted mainly of a sampling unit and an iodine monitor with an NaI(T1) scintillation detector. A relationship between in situ count rate of the monitor and 129I activity collected in the sampling unit was derived from several sets of data obtained with the system at the Tokai reprocessing plant. The system and relationship allowed us to estimate the amount of 129I release at an arbitrary time during the batch-sampling. A different iodine monitor with a Ge detector was also tested to overcome a difficulty in selective measurement of 129I with the NaI(T1) scintillation detector in the presence of 85Kr in the target gas. The test demonstrated a feasibility of the real-time, 85Kr-interference-free monitoring of atmospheric 129I release.