On-line 60Co monitor for reactor recirculation system piping in primary containment vessel during reactor operation

Abstract

Water chemistry control during reactor operation and installation of temporary radiation shielding prior to scheduled outages are carried out in order to reduce workers’ dose exposure caused by 60Co which is the main radiation source during scheduled outages of boiling water reactor (BWR) power plants. It is necessary to monitor the deposited 60Co on inner surfaces of reactor recirculation system (RRS) piping to evaluate effects of water chemistry control. We have developed an on-line 60Co monitor (OLCM) for this purpose. The OLCM applies a pulse integral method as a new method to measure gamma-ray counts of more than 1.0×106 counts per second (cps) and a coincidence counting method to reduce an effect of background gamma rays caused by 16N in the measurement of 60Co cascade gamma rays. Energy resolution at 1.4×106cps is 33keV using the pulse integral method and single LaBr3:Ce scintillation detector. The energy resolutions of this detector using the fast response photo multiplier tube (PMT) are 50keV at 1.5×106cps and 59keV at 2.1×106cps. Furthermore, we measured the energy spectra using the pulse integral method, the coincidence counting method and two LaBr3:Ce scintillation detectors and examined the transition of coincidence counting for 60Co detection under high dose rate during reactor operation at the Kyoto University Research Reactor (KUR). The coincidence counting of 60Co cascade gamma rays could be detected, and the result was average values of 47.9 counts per 40min and standard deviations of 34%, 58.3 counts per 60min and 31%, and 77.3 counts per 80min and 19%. From the results and the environmental conditions in the PCV, it was confirmed that the lower detection limit of 60Co activity per unit area was 2.3×104Bq/cm2 when the measurement frequency was once a month. The OLCM performance can be adjusted based on the measurement frequency and the environmental conditions of the nuclear power plant in which it will be used.