A review of anthropogenic radionuclide 236U: Environmental application and analytical advances.

236U is an anthropogenic radionuclide that is produced from nuclear reactions of 235U(n, γ) and 238U(n, 3n). It has gained extensive attention in the field of environment, geology, nuclear emergency, and nuclear forensics. Due to the unique physical and chemical character and the distinct fingerprint character from different sources, 236U has been successfully applied in the environmental tracer, nuclear material source appointment, and environmental assessment. Until now, few reviews were published about the database, application, and the latest analytical technology development of 236U. In this review, the 236U concentration and 236U/238U isotope ratio were summarized, and the data were classified into four categories, including soil and seawater samples affected by global fallout and nuclear incidents. Furthermore, the development of environmental application and pretreatment methods were also summarized. The advanced pretreatment technology using alkali fusion and flow injection was especially discussed to introduce the development of a rapid analytical method. Finally, the research challenge and direction of 236U were proposed for further research, such as the tracer application combining 236U with other radionuclides in the terrestrial environment and the precise analysis of minor isotopes in ultra-trace uranium samples. We hope this review will help scholars to have a deep research on the analysis and application of 236U.

Background and fingerprint characteristics of anthropogenic 236U and137Cs in soil and road dust samples collected from Beijing and Zhangjiakou, China.

236U has attracted more attention as an environmental tracer in recent years. However, in-depth study of 236U in terrestrial environments is still rare in China. Data on 236U and 137Cs concentrations in soil and road dust samples collected from Beijing and Zhangjiakou, China were obtained to demonstrate the background and distinct characteristics of anthropogenic 236U and 137Cs. 236U and 137Cs were detected in the range of (1.10-7.90) × 107 atoms g-1 and below the method limits of detection to 5.30 Bq kg-1. A clear characteristic was observed in road dust, where 236U concentrations increased with decreasing of sample particle size. Soil samples showed an irregular characteristic, but the highest 236U concentrations were observed in particle size fraction of <0.053 mm in both samples. This phenomenon was caused by U chemical properties, higher specific surface areas and organic compounds in fine particles. Anthropogenic radionuclides fingerprint characteristics in <0.053 mm samples were specially discussed. 236U/238U atom ratios were detected in the range of (0.627-3.38) × 10-8. A weak correlation between anthropogenic 236U and natural U isotopes were observed. The intermediate correlation between 236U and 137Cs indicated somewhat distinct migration behavior of these two radionuclides in soil after release to the environment. The released amount of 236U from global fallout during the period of atmospheric nuclear weapons testing was roughly estimated to be 1300 ± 448 kg. These results could be used as fingerprint information for anthropogenic 236U migration behavior and tracer application in environment.

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, 236U 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 236U and its dilution by natural uranium isotopes make it difficult to distinguish its sources and there are only limited global fallout 236U data for present in Chinese environmental samples. In order to understand the background levels for uranium isotopes, especially 236U, 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 234U, 235U, 238U and 236U concentrations were measured as 9.91-33.7, 0.312-1.43, 6.63-28.7 Bq kg-1 and (1.61-21.3) × 107 atoms g-1, while, the 236U/238U, 234U/238U and 235U/238U atom ratios were (0.470-4.91) × 10-8, (5.10-9.31) × 10-5, and (7.11-7.82) × 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 236U/238U atom ratios were included in the range of global fallout values in other areas, it may be concluded that 236U in soils from Hunan Province is mainly from global fallout. To the best of the authors' knowledge, the presence of global fallout 236U 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.