Exploring uranium minor isotopes (U-233, U-236) as a new tracer to highlight uranium contamination downstream former uranium mine sites
Résumé
Uranium (U) is naturally occurring in the environment and its concentration ranges between 1 to 10 mg.kg-1 in the earth’s crust. In the vicinity of former uranium mines, these concentrations can increase by several orders of magnitude, due to U remobilisation from mill tailings and waste rocks. Potential runoff and erosion after the rehabilitation of the mining sites could therefore explain this increase of U in the surrounding lands, watercourses and wetlands. However, high U concentrations have already been observed in sediments and soils in areas not influenced by U mining activities. In fact, organic matter and reducing conditions in sediments and soils can lead to an important U accumulation originating from the geochemical background. Therefore, identifying U origin (mine versus geochemical background) in the vicinity of former U mines is a key element for contribution to the assessment of uranium mining waste management strategies.
Natural processes like the alpha recoil effect and isotope fractionation limit the use of 234U/238U and 235U/238U isotope ratios as fingerprints to identify the U origin. In this work, the use of natural U minor isotopes was explored. The 236U/238U ratio of the geochemical background is estimated to be around 10-14. On the contrary, in U ore, the higher neutron flux from (α, n) reactions leads to a significant production of 236U by activation of 235U. Consequently, uranium ores show distinct isotope signatures with typical 236U/238U isotope ratios ranging between 10-12 and 10-10. However, 236U has also been released in the environment by global fallout from atmospheric nuclear weapon tests in the 1960s. This anthropogenic input of 236U in the environment increased the 236U/238U ratio of the geochemical background at levels close to U ore signature, thus limiting the use of this ratio as a fingerprint. Furthermore, natural 238U concentration variations can also induce fluctuation of the 236U/238U isotope ratios.
In order to overcome these limitations, we investigated the use of environmental 233U, for which detection became possible only recently. In nature, the main production process of 233U is by neutron activation of 232Th. In the geochemical background, the resultant concentration of 233U is very low. In various U ores, measurements showed 233U/236U isotope ratios 10-4. The main origin of 233U in the environment is global fallout from the atmospheric nuclear weapon tests with a typical 233U/236U isotope ratio of 10-2 (Hain et al., 2017) produced fusion neutrons via 235U(n,3n)233U. This isotopic “contrast” between global fallout, the geochemical background and U ores makes the 233U/236U ratio a potential fingerprint to highlight U contamination of the environment by mining and milling activities.
Measurements of 233U/236U ratios were performed by Accelerator Mass Spectrometry (AMS) at the Vienna Environmental Research Accelerator (VERA). First results of 233U/236U ratios in U ore samples, contaminated sediments from wetlands draining the former mine facilities and non-contaminated samples representing the geochemical background will be presented and discussed in this talk.
Domaines
Sciences de l'environnement
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