Study On Minimum Detectable Activity Concentration For NH-UAV Unmanned Aerial Vehicle Aviation Radiation Monitoring System

Nuclear safety is the precondition of the sustainable development of nuclear power. Using UAV to perform environmental monitoring under nuclear accidents is a fast and effective method for nuclear emergency. In order to predict the suitability and evaluate the detection performance of the NH-UAV unmanned aerial vehicle aviation radiation monitoring system in major nuclear accidents, in this study,considering the Fukushima nuclear accident as the research background, the efficiency calibration and minimum detectable activity concentration of this system in different stages of a nuclear accident were studied in this paper.Three monitoring models of the NH-UAV system were established for radioactivity measurements in the the early, middle and late nuclear accident. 1) In the the early stage of nuclear accident,radioactive contamination was considered only from the radioactive plume in the atmospheric environment, the efficiency calibration and MDAC were performed in different nuclear accidents,non-uniform source term, different horizontal distances from the detection position to source term center, different source term sizes and different environmental activity concentrations. Besides, the measures to improve the detection ability of double detector system; 2) In the the middle stage of nuclear accident, radioactive contamination was considered from both the radioactive plume in the atmospheric environment and radioactive fallout on the ground, the efficiency calibration and MDAC were performed in different flight altitudes, different horizontal distances from the detection position to source term center, different source term sizes; 3) In the the late stage of nuclear accident,radioactive contamination was considered only from the radioactive fallout on the ground, the efficiency calibration, the minimum detection limit of the HPGe detector and the MDAC of the LaBr₃ detector were performed in different flight altitudes, different horizontal distances from the detection position to source term centera and different source term sizes.The results showed that the NH-UAV system was able to work in major nuclear accidents, such as the Fukushima nuclear accident and the Chernobyl nuclear accident; the NH-UAV system was able to use in non-uniform radiation source; HPGe detector possessed measurement thresholds, the measurement thresholds for 364 ke V?131I? and 662 ke V?137Cs? were 236 and 609 Bq m-3, respectively,while the MDAC of the LaBr₃ detector would not change with the variation in activity concentration;The results showed that the variation in the detection performance of the HPGe detector increased with the flight altitude, the whereas the detection performance of the LaBr₃ detector decreased with the flight altitude; In the progress of the NH-UAV system closing to the source term center, the detection capability of the HPGe detector decreased gradually, while the LaBr₃ detector had the opposite results; For the HPGe detector, a larger detection radius in the air causes weak detection sensitivity, by contrast, for the LaBr₃ detector, a smaller detection radius in the air indicates weak detection sensitivity; Prolonging the system acquisition time was an effective measure to enhance system detection performance, meanwhile, adding the shielding thickness of the HPGe detector was also a good way to improve the detection limit.The research results of this paper may provide the data support for the calculation of activity concentration and the detection performance evaluation of the NH-UAV system in the environment of major nuclear accidents, and provide a reference for in-situ radioactivity measurement of the NH-UAV system.