On-site monitoring the activity concentration of nuclide ⁵⁸Co in coolant during PWR oxygenation shutdown

[Background] During the oxidation operation of a reactor, the max active concentration of ⁵⁸Co in the coolant and the time to the end of the oxidation operation (activity concentration < 25 GBq · t^-1) are compulsory. The current chemical sampling method produces large amounts of radioactive waste liquid. [Purpose] This study aims to reduce the current radioactive waste liquid by establishing an external γ spectrum monitoring system. [Methods] The HPGe γ measurement system is used to measure the γ spectrum of the coolant outside the pipeline. The net count rates of the ⁵⁸Co characteristic peak variation with time were obtained after reactor shutdown. These data can be used to predict the time to reach the maximum active concentration of ⁵⁸Co in the coolant and the time to reach the end of the oxidation operation, and then performed a sample analysis. [Results] During the oxidation operation of a nuclear power plant, this monitoring system was used to continuously measure the γ spectrum outside the pipeline. The time of the maximum activity concentration of ⁵⁸Co was predicted to be 33 h and 39 min after shutdown, and the oxidation operation end time predicted to be 57 h after shutdown. By continuously coolant sampling data, the time of the maximum activity concentration of ⁵⁸Co was 33 h and 34 min after shutdown, and the oxidation operation end time was 57 h and 9 min after shutdown. Two time errors are in normal range (less than 10 min). [Conclusions] The external monitoring method can reduce the sampling frequency significantly and reduce the amount of radioactive waste generated by sampling analysis. (authors)