Impact of a long-term thermal pulse on bentonite erosion rates in lab scale experiments: Initial results from samples of the LOT/ABM experiments

Due to their advantageous properties (i.e. swelling and sorption), bentonites are used as geotechnical barrier materials in the engineered barrier system (EBS) for radioactive waste disposal in crystalline host rocks [1]. To reduce the spatial footprint of the repository, these systems are being revised for thermal optimization [2]. A key aspect is determining the maximum thermal load at the canister-bentonite and bentonite-host rock interfaces under which bentonite maintains its functionality. Thermal loading affects both swelling [3] and possibly erosion behaviour, potentially compromising long-term integrity of the barrier.

 

Three samples were analysed to evaluate prolonged elevated thermal load effects on the erosion of bentonite: MX-80 Wyoming-type bentonite (Block 18) from the LOT S2, and MX-80 (Block 8) and FEBEX bentonite (Block 25) from the ABM 45:5 experiment, all conducted at Äspö HRL. Erosion experiments on untreated raw material from the same batches served as references.

 

 The LOT S2 experiment operated at 500 W over a 20-year period, resulting in a temperature gradient ranging from ~95 °C at the heater-bentonite interface to 60 °C at the bentonite-host rock interface [4]. The ABM 45:5 experiment applied a higher thermal load of 1800 W during the last year of its five-year runtime, producing a temperature gradient from ~250 °C at the heater interface to 170 °C at the interface with the host rock [5].

 

Plugs (13 mm diameter, 5 mm height) were placed in a 1 mm aperture artificial fracture within a PMMA flow cell to assess erosion [6].  Groundwater from the Grimsel Test Site, representing low-ionic-strength meteoric/glacial meltwater (0.2 mM), was used to simulate future repository scenarios. Swelling pressure, nanoparticle concentration and gel expansion were monitored to calculate mass loss and erosion rates. Erosion rates were determined to be 2.8 ± 1.2 kg/m²/a for raw MX-80 (runtime: 100 d) and 4.6 ± 1.7 kg/m²/a for thermally loaded MX-80 (runtime: 174 d) from the LOT experiment.

 

The results show increased erosion in thermally loaded bentonites, with higher erosion rates at the heater interface than at the host rock interface. Although erosion rates increase following thermal exposure, all values remain below the threshold of 6.5 kg/m²/a for 1 mm fractures deemed safe by Posiva [6].  Mineralogical analyses reveal no alterations in the 0.2-2 µm size fraction. However, distinct alterations were identified in the < 50 nm fraction via ATR-IR and XRD. These changes might have a high impact on the erosion behavior of the bentonites despite being not the major mass fraction. Based on these results, further mechanistic investigation is required.

 

[1] Wersin, P. et al. (2007) Phys. Chem. Earth 32:780–788

[2] Kim, J.-S. et al. (2019) J Korean Tunn. Undergr. Space 21(5): 587-609

[3] Kašpar, V. et al. (2021) Minerals 11(9):965

[4] Sandén, T. ; Nilsson, U. (2020) SKB TR-20-11

[5] Svensson, D. et al. (2023) SKB TR-23-25

[6] Schäfer, T., et al. (2024) NTB 23-10

[7] Hedström, M. et al. (2023) Appl. Clay Sci. 239: 106929