Development of ~(60)Co Monitoring System for RRS Piping during Plant Operation

摘要

On-line ~(60)Co monitoring system has been developed to measure dose rate of RRS piping during operation of a BWR plant. This monitoring system will contribute to not only a more precise shielding plan in advance of outage but optimization of water chemistry control such as Zn injection. These are expected to reduce radiation exposure of workers in a PCV during outage. ~(60)Co, which is a main source of dose rate of RRS piping, deposits on an inner surface of RRS piping during plant operation. Direct measurement of dose rate caused by this ~(60)Co during plant operation had not been achieved so far due to high radiation background of ~(13)N and ~(16)N which are produced in reactor water by reactions of ~(16)O (p, α) and ~(16)O (n, p). Then a coincidence method is applied to discriminate ~(60)Co from an gamma ray energy spectrum obtained under such a high background above 10 mSv/h. In decay process of ~(60)Co to ~(60)Ni, two cascade gamma rays are released simultaneously. The gamma rays from ~(60)Co can be discriminated from back ground radiations by simultaneous detection with two detectors of LaBr_3:Ce scintillators, which has high energy resolution of 3.7% and high count rate of 1.2×10~6 cps. Continuous 80 hours performance test of this monitoring system was conducted in KUR (Kyoto University Research Reactor). It was confirmed that this system had the performance of ~(60)Co monitoring with the error range of below approximately ±20% under high radiation background of 5.4 mSv/h, and with the lower limit detectability of 1.9 × 10~4 Bq/cm~2 under once-a-month output frequency. Besides, a practical system was proposed for actual use in a nuclear power plant.