A rapid warning and activity concentration estimates method for online water γ-spectrometry systems

The increasing scale of nuclear energy and the extensive application of non-power nuclear technology have propelled the radioactivity levels of waters in the proximity of nuclear facilities to the front line of radiation environment monitoring. However, online γ-spectrometry systems for inland waters, most of which extract samples in situ and in real time, are able to produce reliable activity concentration measurements for waterborne radionuclides only when they are distributed relatively uniformly and enter into a steady-state (SS) diffusion regime in the measurement chamber. Better timeliness is required for this measurement method to protect the health of residents and to ensure the safety of the living environment. To address this issue, this study establishes a mathematical model for an online water γ-spectrometry system to give rapid warning and activity estimates for water in non-steady-state (NSS) conditions. In addition, dynamic detection efficiencies for radionuclides during the NSS diffusion process are determined for the measurement system through the calculation of the diffusion of the radionuclide-containing fluid inside the measurement chamber in conjunction with Monte Carlo simulations of the efficiencies of the detector. On this basis, a method is developed for the online water γ-spectrometry system to give rapid warning and activity concentration estimates for radionuclides in water. Subsequent analysis of the NSS-mode measurements of 40K radioactive solutions with different activity concentrations determines an optimum warning threshold for the measurement system as well as an optimum measurement time for producing accurate activity concentration estimates for radionuclides (40K) in water. The experimental results show that the NSS measurement method developed in this study is able to give warning and to yield accurate activity concentration estimates for radionuclides 55.42 and 69.42 min after the entry of a 10-Bq/L 40K radioactive solution into the measurement chamber, respectively. These times are much shorter than the 90 min required by the conventional measurement method. Furthermore, the NSS measurement method allows the measurement system to give rapid (within approximately 15 min) warning when the activity concentrations of some radionuclides reach their respective limits stipulated in the Guidelines for Drinking-water Quality of the World Health Organization, suggesting that this method considerably enhances the warning capacity of in situ online water γ-spectrometry systems.