福岛核事故后上海气溶胶中~(131)I、~(134)Cs和~(137)Cs的来源途径分析

利用对气溶胶中典型放射性核素(~(131)I和134,~(137)Cs)的分析,可以评估福岛核事故产生的放射性物质对上海及全球的大气放射性本底水平造成的影响。本工作结合核事故释放过程、核素的天然衰变以及气象条件等因素,获得核事故期间上海的气溶胶中~(131)I和134,~(137)Cs活度浓度及其比值的分布特征:~(131)I被检出的时间(2011-03-27)早于~(134)Cs(2011-04-06)和~(137)Cs(2011-04-08),~(131)I的活度浓度(0.01~1.20 mBq/m3)比~(134)Cs(0.01~0.58mBq/m3)和~(137)Cs(0.01~0.65mBq/m3)大2~10倍,而且在不同的时间段出现相应的多峰值现象;~(131)I/~(137)Cs活度浓度比值(1.3~10.6)在2011年4月5日之后呈递减趋势,但是~(134)Cs/~(137)Cs活度浓度比值(0.8~2.9)则一直在1.1左右波动。利用HYSPLIT模型模拟放射性气团运移轨迹的分析方法,表明在核事故期间输入到上海的放射性气溶胶的途径有东北和西北两条主要迁移路径。同时通过结合国内相关城市核事故期间大气放射性监测数据,证实了东北路径在中国境内的控制地位。另外,通过总结和分析北半球大气监测数据中~(131)I/~(137)Cs和~(134)Cs/~(137)Cs活度浓度比值最大值的分布特征,验证了日本核事故产生的放射性气溶胶在北半球的传输过程。

Source and Pathway of 131I, 134Cs and 137CS in Aerosols of Shanghai After the Fukushima Nuclear Accident

The typical radionuclides (131I and 134,137Cs) in aerosols could be efficiently used to evaluate the impacts of radioactive materials from the Fukushima nuclear accident on the radioactive background level of the atmosphere in Shanghai and the world.In the present work,combining impact factors,such as the nuclear accident release process,natural radioactive decay and the meteorological conditions,the distribution patterns of 131I and 134,137Cs activities and their ratios in Shanghai aerosols were obtained.The date when 131I (2011-03-27) could be detectable is earlier than 134Cs (2011-04-06) and 137Cs (2011-04-08).The 131I activity (0.01-1.20 mBq/m3) is 2-10 times higher than those of 134Cs (0.01-0.58 mBq/m3) and 137Cs (0.01-0.65 mBq/m3).Moreover,multiple peak values of 131I are found with time series.The activity ratios of 131I/137Cs (1.3-10.6) decrease after the date of April 5th,2011,however,the activity ratios of 134Cs/137Cs (0.8-2.9) are constant to be around 1.1.Using the analytical method of HYSPLIT model,the primary air mass migration pathways of radioaerosols in Shanghai during this accident can be qualitatively inferred as the northeast and northwest paths.Meanwhile,the northeast pathway of radio-aerosols in Shanghai is further confirmed to be dominant by analysis of the reported radio-aerosol data in the concerned cities of China during that time period.In addition,the results of maximum values of 131I/137Cs and 134Cs/137Cs activity ratios are summarized and analyzed to reveal the spreading pathways of radio-aerosols in the northern hemisphere.