Isotopic evidence for long-term behaviour of fuel-derived uranium in soils of the Chornobyl Exclusion Zone

Abstract

The accident at the Chornobyl Nuclear Power Plant (Ukraine) resulted in extremely high contamination in adjacent areas and radioactive plumes transported further afield. A distinctive feature was the direct release of uranium-rich reactor fuel fragments i.e. ‘hot particles’ to the environment. However, the fate of uranium in terrestrial ecosystems is poorly known in relation to short-lived radionuclides. We investigated the long-term behaviour of nuclear reactor particles across a range of soils and land-use types in the Chornobyl Exclusion Zone, a unique natural laboratory, following a well-defined pulse injection that can be precisely dated to the accident in 1986. We present autoradiographic evidence of the remains of fuel fragments in soils from moderate-to-highly contaminated areas. These discrete particles are still present after decades of weathering. Fuel particles have undergone limited vertical redistribution and are primarily located in topsoils, acting as non-uniformly distributed point sources of radioactive contamination. We also present data on ²³⁴U, ²³⁵U, ²³⁶U and ²³⁸U in topsoils and subsoils. Their concentrations were in general slightly higher in topsoils, particularly in soil profiles closer to the reactor; however the spatial distribution was extremely heterogeneous. A clear preponderance of ²³⁵U/²³⁸U ratios above natural values indicated the presence of fuel-derived uranium in the majority of topsoils. This was further confirmed by higher ²³⁴U/²³⁸U ratios in the most contaminated sites. The strongest evidence was provided by ²³⁶U/²³⁸U ratios, which were several orders of magnitude higher than native values in a number of soils. Differences in the isotopic composition of different solid fractionation extractions suggest that full equilibration between native and reactor-derived uranium has not been achieved on a decadal time-scale due to slow hot particle weathering rates. Estimations from ²³⁶U/²³⁸U ratios suggest that 7–77 % of the readily exchangeable uranium in soils was derived from spent fuel, whilst this source only accounted for 3–52 % of the total uranium in soil. Thus, isotopically enriched and irradiated uranium retains a greater potential to migrate, enter the trophic chain and interact with the ecosystem in the long-term than native uranium.