Abstract
The removal of potentially harmful radioactive waste from the anthroposphere will require disposal in geological repositories, the designs of which often favour the inclusion of a clay backfill or engineered barrier around the waste. Bentonite is often proposed as this engineered barrier and understanding its long-term performance and behaviour is vital in establishing the safety case for its usage. There are many different compositions of bentonite that exist and much research has focussed on the properties and behaviour of both sodium (Na) and calcium (Ca) bentonites. This study focusses on the results of a swelling test on Bulgarian Ca bentonite that showed an unusual gel formation at the expanding front, unobserved in previous tests of this type using the sodium bentonite MX80. The Bulgarian Ca bentonite was able to swell to completely fill an internal void space over the duration of the test, with a thin gel layer present on one end of the sample. The properties of the gel, along with the rest of the bulk sample, have been investigated using ESEM, EXDA and XRD analyses and the formation mechanism has been attributed to the migration of nanoparticulate smectite through a more silica-rich matrix of the bentonite substrate. The migration of smectite clay out of the bulk of the sample has important implications for bentonite erosion where this engineered barrier interacts with flowing groundwater in repository host rocks.
Highlights
Geological disposal of intermediate and high level hazardous waste in an especially constructed repository is the favoured choice for the long-term removal of radioactive material from the anthroposphere [1,2,3]
Load cell R3 was located on the sample midplane, and as such, it was in contact with the sample, it was close to the starting internal void space
As a result that the radial R3 load cell on the vessel midplane was closer to the right-hand end of the sample than the radial R2 sensor was to the left-hand end, swelling pressures were seen on the R3 sensor before the R2 sensor (Figure 1B)
Summary
Geological disposal of intermediate and high level hazardous waste in an especially constructed repository is the favoured choice for the long-term removal of radioactive material from the anthroposphere [1,2,3]. The repository designs often include a number of different natural and engineered barriers to prevent the waste from contaminating the environment (e.g., [1,4,5]). Bentonite is commonly included as the clay backfill or engineered barrier in these designs [6,7,8] because of its low permeability, high swelling capacity and self-sealing properties [9,10,11,12]. Numerous nations [22,23,24] are still considering the use of a Ca bentonite at different dry densities in their radioactive waste disposal concepts, because of a natural availability (e.g., Fourges Clay, Cerný Vrch deposit)
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