Glaciation cycle from the Pleistocene epoch to present and its affects on crystalline bedrock. Credit: American Chemical Society
What must nations consider when managing spent nuclear fuel with the utmost safety? The geological composition of the storage site is one critical aspect. Glacial meltwater loaded with oxygen may be introduced deep into bedrock fractures during deglaciations. The potential intrusion of dissolved oxygen is a risk factor for corrosion of nuclear waste canisters, and therein a potential cause for leakage of radionuclides. Research at Linnaeus University in Sweden has found that iron-rich fractured crystalline bedrock exposed to several deglaciation events retains significant oxygen consumption capacity.
“The data we produced not only points to a huge reducing capacity of iron minerals for safe storage of nuclear waste, but also advances our understanding of redox fluctuations in the deep fracture network over geological timescales,” said Changxun Yu, Researcher in the Department of Biology and Environmental Sciences at Linnaeus University and Principal Investigator of the study.
The study found that iron (Fe II) in crystalline rock fractures provides a natural ‘sieve’ for seeping oxygenated meltwater that could potentially reach canister repositories at roughly 500 meters depth. Minerogenic iron acts as an electron donor which reduces oxygen molecules in water as it makes contact with the bedrock. The premise is that oxygen would be substantially consumed before reaching significant depths.
Locked inside the rocks
The global team of researchers analysed Paleoproterozoic-era bedrock fractures and altered rock from two Baltic Shield sites in Sweden; Forsmark, chosen as a site for the spent nuclear fuel repository, and Laxemar, further south. Mineral composition and iron valence of the samples were determined using Fe K-edge X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) with beamline I811 at MAX-lab in Lund, in combination with Mössbauer spectroscopy.
The study looked at how recurrent Pleistocene glaciations have affected the geochemistry of minerogenic iron (II). Results revealed no notable oxidation signatures or loss of iron (II) concentrations due to deglaciation events. In essence, repositories built on these sites have bedrock with extensive capacity for oxygen consumption and inherent resistance to geological weathering, according to the scientists.
“This is a detailed and thorough study on natural Fe minerals in fractured crystalline bedrock for redox purposes. The oxidation of Fe(II) in silicate minerals is more complicated [than in other Fe(II)-bearing minerals] and involves many bacterially-mediated and surface-catalysed processes. Our data show that the fractured bedrock still sustains a huge capacity to consume oxygen,” said Yu.
The team encountered several challenges, among them to acquire intact mineral assemblages, which are typically thin and fragile. A triple-tube drilling technique developed by the Swedish nuclear fuel and waste management company (SKB), gave them access to many well-preserved drill cores from which to identify and sample large fractures with relatively abundant mineral assemblages.
Another challenge was to obtain a thorough and quantitative assessment of Fe(II) and Fe(II)-bearing minerals throughout the two fracture systems. “We are very grateful to the strong support from the MAX-lab and the XAS beamline I811. Without the support, we would not have been able to obtain high-quality XAS data for such a large number of mineral assemblages at different depths,” explained Yu.
The data may aid toward improvements in future modelling and risk assessment for current or planned deep nuclear waste repositories in the Nordics and elsewhere. The work also holds potential to enhance parallel studies of nuclear fuel containment which look at different aspects such as canister durability.
Yu and colleagues are planning for more synchrotron-based studies on fracture systems pertaining to the chemical and redox behaviour of uranium and rare earth elements. The results may increase our understanding of actinide contaminants in the deep subsurface.
A Combined X-ray Absorption and Mössbauer Spectroscopy Study on Fe Valence and Secondary Mineralogy in Granitoid Fracture Networks: Implications for Geological Disposal of Spent Nuclear Fuels. Changxun Yu, Henrik Drake, Knud Dideriksen, Mikael Tillberg, Zhaoliang Song, Steen Mørup, and Mats. E. Åström. Environmental Science & Technology 2020 54 (5), 2832-2842. DOI: 10.1021/acs.est.9b07064