First report on global fallout 236U and uranium atom ratios in soils from Hunan Province, China

More nuclear power plants continue to be built in China. Due to its long half-life, radiotoxicity and potential application as an environmental tracer, U-236 is one of the most important artificial radionuclides deserving more study since activity data are important for risk assessment. However, the ultra-trace activity of U-236 and its dilution by natural uranium isotopes make it difficult to distinguish its sources and there are only limited global fallout U-236 data for present in Chinese environmental samples. In order to understand the background levels for uranium isotopes, especially U-236, and clarify their sources, inductively coupled plasma tandem mass spectrometry (ICP-MS/MS) was applied to analyze uranium isotopes in 48 soil samples from Hunan Province, China. The U-234, U-235, U-238 and U-236 concentrations were measured as 9.91-33.7, 0.312-1.43, 6.63-28.7 Bq kg(-1) and (1.61-21.3) x 10(7) atoms g(-1), while, the U-236/U-238, U-234/U-238 and U-235/U-238 atom ratios were (0.470-4.91) x 10(-8), (5.10-9.31) x 10(-5), and (7.11-7.82) x 10(-3), respectively. The uranium isotopic fractionation may be due to irrigation of the agricultural lands where the samples were collected. Considering the facts that neither previous nuclear tests nor nuclear accidents had occurred in Hunan Province and the present U-236/U-238 atom ratios were included in the range of global fallout values in other areas, it may be concluded that U-236 in soils from Hunan Province is mainly from global fallout. To the best of the authors' knowledge, the presence of global fallout U-236 in soil samples from China has been confirmed for the first time, and these values may be useful as background data for risk assessment in the future.