Yuanzineng kexue jishu (Feb 2024)
Research on Calibration Method of 18F Aerosol Monitor
Abstract
To prevent severe safety accidents caused by primary coolant leakage in pressurized water reactors (PWRs), it is necessary to monitor the leakage rate of key parts of the reactor coolant pressure boundary (RCPB) in order to take timely countermeasures. The primary coolant of PWRs is usually high-pressure supercritical water containing boron, which can produce 18F in the high neutron flux environment of the reactor. The leak rate of the RCPB can be inferred by measuring the activity concentration of the radioactive 18F aerosol produced by the coolant leaking into the containment. A measurable sample source is usually obtained by extracting sample gas containing 18F aerosol from the containment and filtering it in the monitor. The measurement method used by the monitor is to screen the γ signal by using the β signal as the gate signal, which is actually a γ relative measurement method using the coincidence method to reduce the background. The electronic design of the monitor prevents the β-channel signal from being recorded, which makes it impossible to achieve the absolute measurement as with conventional coincidence methods. To ensure the accuracy of the monitoring results, the 18F aerosol monitor needs to be calibrated. In this paper the monitor was calibrated using a standard 18F surface source produced by the intermittent spot method on a filter paper base. By means of the decay of the 18F source and the automatic recording function of the monitor, the indication number of the monitor was measured at six source activities of different orders of magnitude, i.e. the count of the energy spectrum at the position of the 511 keV γ-window. The data were used to obtain the activity response and uncertainty of the monitor's readings and to verify the feasibility of such a calibration scheme on the monitor. Two standard point sources of 22Na and 68Ge/68Ga were also used to calibrate the monitor to verify the feasibility of using long-lived nuclides to make alternative sources for calibration. Sets of 511 keV γ-window count data from γ and coincidence channels were recorded to investigate the effect of the γ coincidence additive effect and different β emission spectra on the activity response, and to determine which long-lived positron nuclides should be used in the field calibration. Calibration with a standard 18F surface source shows that the monitor can give stable and reliable readings of this type of standard source over different orders of magnitude within the range. The extended uncertainty (k=2) of the displayed activity response is 6.0%, which meets manufacturing requirements. And the calibration of the long-lived positron nuclide standard point sources shows that the positron spectrum has a great influence on the activity response of the monitor, and the γ coincidence additive effect is not very significant. Therefore, it is recommended to use 22Na with β energy close to 18F to make point sources or surface sources for field calibration.
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