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野外试验场核素迁移试验通常采用的方法是在示踪核素投放后不同时间在试验坑取土样 ,测定不同深度处的核素比活度分布。本文介绍了为中国辐射防护研究院野外试验场包气带土壤中核素迁移而开发的直接测定方法 ,主要介绍测量原理、装置、刻度及部分测定结果。试验所用的示踪核素源层为布放在地表下 1m深处体积为 55mm× 6 mm的85Sr、13 4Cs、60 Co与黄土的混合物。直接测量装置包括测井用探头 [ 50 mm× 30 mm的 Na I(Tl)探测器及其铅屏蔽体 ]、探头升降及深度测读部分、数据获取系统和测井四部分。文中给出了在天然条件 (C坑 )和人工喷淋条件 (F坑 )下示踪核素投放后不同时间的85Sr的垂向活度分布曲线。试验结果表明 ,F坑中示踪核素投放后第 2 6 4天 85Sr峰位向下迁移了 5cm左右 ,第 4 30天已向下迁移了约 8cm;而在 C坑 ,在约 2年试验期内85Sr的峰位几乎仍在原处。本方法所得结果基本上与采集土芯样测量的结果相符。最后分析了本方法尚待探讨的问题
The field test method for radionuclide migration usually adopts the method of taking soil samples in the test pit at different times after the tracer nuclide is put into operation and measuring the radionuclide specific activity distribution at different depths. This paper presents a direct method for the determination of radionuclide migration in soils with aeration zone in the field test facility of China Radiation Protection Research Institute. The measuring principle, device, scale and some measurement results are introduced. The source layer of the tracer nuclide used in the experiment is a mixture of 85Sr, 134Cs, 60Co and loess with a volume of 55 mm × 6 mm and 1 m depth below ground surface. Direct measurement devices include logging probes [I 50 mm × 30 mm Na I (Tl) detectors and their lead shields], probe lift and depth gauging, data acquisition systems, and logging. In the paper, the vertical activity distribution curve of 85Sr at different time after the radionuclides were placed under natural conditions (C pit) and artificial sprinkler conditions (F pit) is given. The experimental results show that the 85Sr peak shifts downward about 5cm on the 264th day after the tracer nuclide is put into operation and about 8cm downward on the 30th day. In C pit, at about 2 years 85Sr during the peak position is still almost the same place. The results obtained by this method are basically consistent with the results of soil-based measurements. Finally, the method to be discussed is still to be explored