Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

The world’s first permanent nuclear-waste repository for high-level waste (HLW) will be the underground rock characterization facility tunnel ONKALO, situated near the nuclear power plants of Olkiluoto, approximately 300 km northwest of Helsinki (Finland). Geological mapping showed that the bedrock of the tunnel is mainly composed of veined gneisses with a migmatic appearance, which are interspersed by numerous fractures. Massive 5–10-mm thick biofilms (microbial communities) are observed attached to the tunnel walls where groundwater is seeping from bedrock fractures feeding the biofilm. Groundwater and biofilm samples were taken during the EURATOM collaborative project ReCosy for laboratory experiments in flow cells, where uranium(VI) was added to the circulating groundwater with a final uranium concentration (4.25×10–5 M), which was expected from a damaged and leaking waste canister in the far-field of a nuclear-waste repository. The aim of our studies was to simulate the fate and behaviour of uranium(VI) in the presence of microorganisms using different methods to find evidence of the possibility of uranium retention.

In our experiments, aqueous uranium percolated the biofilm and induced interaction between biofilm microorganisms and uranium(VI), forming an intracellular U-phosphate mineral similar to autunite (Ca[UO2]2[PO4]2 • 2-6H2O or meta-autunite Ca[UO2]2[PO4]2 • 10-12H2O) as determined by means of EF-TEM/EELS. The potential toxicity of uranyl was consequently avoided by the uranium forming these strong complexes with phosphate in the cytoplasm of these microorganisms. Inorganic phosphate (H2PO4–) was released from the cellular polyphosphate as a cells’ response to the heavy metal stress. In contrast, aqueous uranium carbonate species, likely calcium uranyl carbonates species (Ca2UO2[CO3]3), were formed using the large available amount of carbonate in the uranium-contaminated circulating groundwater. The results of the laser-induced fluorescence spectroscopy studies performed at 283 K are in excellent agreement with the thermodynamic calculations of the theoretical predominance fields of the uranium species formed in the uranium-contaminated circulating groundwater. These complexes consequently influence the speciation of uranium, contributing to the transport and migration of uranium(VI).

Our studies aimed to improve our understanding of the mechanisms by which biofilms respond to the exposure to radionuclide with respect to safety assessments in the far-field of nuclear-waste repositories. In this environment, microorganisms must be considered, along with minerals, as an important factor influencing radionuclide transport.