Wide-Range Krypton Gas Monitor Using a Plastic Scintillation Detector Operated in Current Mode with a Gate Circuit

The radioisotopic gas 85 Kr is discharged from the stack of nuclear fuel reprocessing plants and the 85 Kr concentration varies widely from 10 −3 to 10 2 Bq/cm 3 according to the reprocessing procedure. A krypton gas monitor consisting of a single detector was devised to monitor 85 Kr gas over this wide range. A plastic scintillation detector was operated in current mode in combination with a gate circuit. The discrimination level of the gate circuit was adjusted to suppress the effect of the dark current in the photomultiplier. The effects of temperature and humidity were found to be insignificant under the working conditions. The detector was calibrated using 85 Krgas samples of known concentrations. The calibration showed that the krypton gas monitor gave valid results in a measurement range as wide as 10 −3 to 10 3 Bq/cm 3 . The detection limit at low concentration was 5:6 × 10 −3 Bq/cm3 for a 1-minute measurement. The detector was examined in an actual reprocessing laboratory and the 85 Kr effluent was measured simultaneously with a GM detector. The measured concentrations obtained using both detectors were confirmed to be in good agreement.