In geological disposal, the buffer material must be designed to maintain the required thickness and density for tens of thousands of years, even when subjected to various loads, such as the weight of the waste package and creep deformation of rocks.

Regarding the settlement (strain) of the buffer material over time, after primary consolidation had progressed, secondary consolidation progresses almost linearly with respect to the logarithm of elapsed time during the measurement period

In the geological disposal of high-level radioactive waste, the buffer material (bentonite) is placed around the waste package, which is then disposed of within the rock mass (Fig. 1). The buffer material constitutes the engineered barrier system and is expected to have important functions for ensuring the safety of the repository after closure, such as low permeability and sorption of radionuclide materials. In addition, the buffer material is subjected to various forces (loads) due to the weight of the waste package and deformation of the rock mass (Fig. 1). This force will slowly consolidate the buffer material (increase strain), and as the waste package sinks, the thickness of the buffer material may decrease. Data acquisition and model development are therefore necessary to evaluate the long-term consolidation behavior of the buffer material.

Conventional test apparatus could only last for a few months. Consequently, a new consolidation test apparatus was constructed to enable continuous measurements without being affected by the influence of external factors such as power outages and seismic activity. To date, consolidation tests have continued for 2.7 to 4 years without any problems, and it has been confirmed that secondary consolidation continues as shown by the strain increases linearly (sinking) when plotted against the logarithm of elapsed time (Fig. 2).

The tests will continue running and provide continuously measured consolidation data in excess of 10 years.

This study was part of the results of a joint research project with NUMO, "Study to evaluate the long-term behavior of barrier materials and nuclides during state transitions in near-field systems.”

Takayama et al., Secondary Consolidation Characteristic of Bentonite by Long-Term Consolidation Tests of 2.7, 3.7 and 4 Years, Japanese Geotechnical Journal, vol.18, 2023, p.317–330 (in Japanese).

Paper URL: https://doi.org/10.3208/jgs.18.317

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