Smectite alteration mechanisms in geotechnical barrier systems for high-level radioactive waste
Country / Region: Germany
Begin of project: September 1, 2014
End of project: December 31, 2029
Status of project: November 1, 2023
Fig. 1: BGR uses an end-over-end shaking-oven to investigate the reaction of bentonites in different solutions at temperatures up to 300 °C (from Kaufhold et al., 2019)
Source: BGR
According to the German Site Selection Act (StandAG), high-level radioactive waste must be concentrated and safely isolated in deep geological formations in a future geologic repository. Pursuant to Section 23 (1) StandAG, the host rocks considered suitable for a geologic repository for high-level radioactive waste are rock salt, claystone and crystalline rocks. In the case of the crystalline rock type, Paragraph 4 StandAG defines that an alternative concept to an effective isolating rock mass is possible to achieve safe isolation. However, this stipulates much higher specifications for the long-term integrity of the technical barriers and/or the geotechnical barriers (the swelling clay bentonite).
The concept for crystalline rocks and eventually claystones presupposes the installation of an additional geotechnical safety barrier between the metal canisters which contain the high-level radioactive waste, and the host rock itself. This becomes relevant if a geologic repository is to be constructed in a fractured rock, which is often the case within granitic bodies for instance. The results of numerous tests have already revealed that a special strongly swelling clay can be used to create such a barrier (the clay in question is bentonite). Bentonite has been used for many decades to seal landfills, and to create sealing walls to isolate sites with contaminated soil. Special raw material products were developed for this purpose. Because of its swelling properties, bentonite has a very low permeability. Its pore structure and its surface charge, also mean that bentonite has a certain capacity for retaining toxic substances.
Fig. 2: Surface analysis from the FEBEX project (from Kaufhold et al., 2018)
Source: BGR
BGR has already spent many years investigating the properties and applications of bentonites in geologic repository barrier systems for claystone and granite host rocks. In the project, "Clay-mineralogical-organic processes in barrier systems“, criteria were defined to enable the differentiation between bentonites which are less suitable and more suitable for use in a geologic repository (Kaufhold & Dohrmann, 2016). These investigations revealed that the mechanisms behind the observed reactions of bentonite are still not understood well enough to make reliable long-term safety assessments feasible.
For this reason, an additional job package “Smectite alteration mechanisms in geotechnical barrier systems for HLRW” was launched in 2014. In this job package, investigations are carried out to elucidate the reactions in the proximity of barrier systems with steel canisters (claystone, crystalline rocks), which are poorly understood but important in assessing the long-term stability of a geologic repository incorporating bentonite: iron corrosion, reaction with cement, and the magnesium enrichment / solubility of smectites. Many of the laboratory tests have already been run for many months. However, some of the results revealed that some of the reactions do not proceed continuously, so that the length of the test phase had to be increased to at least 4 years. The importance of investigations in the form of large-scale tests was highlighted in this context, some of which have currently been running continuously for much longer than 4 years. It also highlights the need for national and international networking.
Fig. 3: Test set-up for carrying out corrosion tests (here copper (Cu); (Kaufhold et al., 2017)
Source: BGR
As part of the project, samples are investigated derived from the SKB tests: LOT (long-term tests), ABM (alternative buffer material test), and PTR (prototype repository test, all Äspö, Sweden); as well as FEBEX (full scale engineered barrier experiment), and the HotBENT project (current full-scale test running in Grimsel, Switzerland). The ABM-5 and HotBent tests are particularly interesting in this context because the high temperatures used in these tests may significantly accelerate some of the processes. The samples from ABM-5 are investigated in cooperation with SKB as part of the European Joint Programme on Radioactive Waste Management (EJP/Eurad) (Kaufhold et al, 2021). The combination of laboratory and field tests with various boundary conditions provide the information required to understand the mechanisms behind smectite alteration and the various boundary surfaces. The results achieved so far as part of the ongoing and earlier projects, have been summarised in numerous publications (see references). Another priority is research into the corrosion of iron (potential canister material) in contact with bentonite. The results to date have revealed that the corrosion is influenced by the charge density of smectites. It is now assumed that the charge density correlates with the redox-buffer potential of the bentonites, and that this influences the corrosion. Long-term tests revealed that the corrosion rate lies between 2-5 µm/a in the long term. These results are compatible with the results achieved by other research groups. As part of the joint UMB-2 project (Conversion mechanisms bentonite) further investigations are carried out into the corrosion mechanism, amongst other aspects, whilst the IMKORB project (Implementation of a monitoring system for the evaluation of corrosion processes on canister materials in bentonite-based geologic repository concepts), includes investigations into various Fe alloys to study their corrosion rates. Against this background, a much better understanding of the processes taking place is expected in a few years time.
Explanation of Fig. 2:
The differently coloured materials, which consist of altered bentonite and corroded iron, indicate the various reactions taking place in the iron in contact with bentonite. The perforated metal shown (liner) comes from the pipes into which the canister was installed in in the FEBEX experiment.
Explanation of Fig. 3:
20 to 30 metal pellets (diameter approx. 3 mm), are mixed into a clay gel (“slurry”), and then heated up in an oven under anaerobic conditions (in a glove box). After a defined period of time, the clay and the pellets are separated, and the mass loss of the pellets as a result of the reaction is determined using a balance. In addition, X-ray fluorescence (XRF) is used to determine the increase in copper concentration (Cu-increase).
Literature:
Kaufhold, S., Dohrmann, R., Ufer, K., Svensson, D., Sellin, P. (2021) Mineralogical Analysis of Bentonite from the ABM5 Heater Experiment at Äspö Hard Rock Laboratory, Sweden. - Minerals 2021, 11, 669. https://doi.org/10.3390/min11070669.