Method for rapid warning and activity concentration estimates in online water γ-spectrometry systems

摘要: 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. To protect residents’ health and ensure the safety of the living environment, better timeliness is required for this measurement method. To address this issue, this study established a mathematical model of the online water γ-spectrometry system so that rapid warning and activity estimates can be obtained for water under non-steady-state (NSS) conditions. In addition, the detection efficiency of the detector for radionuclides during the NSS diffusion process was determined by applying the computational fluid dynamics (CFD) technique in conjunction with Monte Carlo simulations. On this basis, a method was developed that allowed the online water -spectrometry system to provide 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 determined the optimum warning threshold and measurement time for producing accurate activity concentration estimates for radionuclides. The experimental results show that the proposed NSS measurement method is able to give warning and 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 WHO, suggesting that this method considerably enhances the warning capacity of in situ online water γ-spectrometry systems.