scholarly journals Effects of calcium carbonate on hydro-mechanical properties of bentonite

2020 ◽  
Vol 198 ◽  
pp. 01053
Author(s):  
Xian-Lei Fu ◽  
Ai-Fang Qin ◽  
Wei-Fang Xu ◽  
Liang-Hua Jiang
Keyword(s):  
Hydraulic Conductivity ◽  
Compression Index ◽  
Scanning Electron Micrograph ◽  
Shear Tests ◽  
Direct Shear Tests ◽  
Peak Point ◽  
Radioactive Waste Repositories ◽  
Swelling Strain ◽  
High Level ◽  
Waste Repositories

To simulate characteristic changes of the bentonite mixing with CaCO3 crystals that precipitate from groundwater around the high-level radioactive waste repositories, different amounts of CaCO3 powders were mixed in the bentonite. In this study, four series of bentonite/CaCO3 mixtures were prepared with different contents of CaCO3 powders. A series of swelling, compression and direct shear tests were conducted on the bentonite/CaCO3 samples using deionized water. Terzaghi consolidation theory was adopted to compute the hydraulic conductivity of bentonite/CaCO3 mixtures. The results demonstrated that the final swelling strain decreased with increasing vertical stress and the content of CaCO3 powders. The compression index decreased slightly with increasing CaCO3 content. It was found that as the CaCO3 content increased, the hydraulic conductivity of bentonite/CaCO3 mixtures decreased. There was a peak point in the shear strength of unsaturated samples with the differences of the CaCO3 content. The cohesion of unsaturated samples increased first and then decreased, but saturated samples presented the opposite character. It could be seen from the scanning electron micrograph that the existence of CaCO3 powders filled pores of bentonite/CaCO3 mixtures and made the structure denser.

Appraisal of waveform repeatability for crosshole and hole-to-tunnel seismic monitoring of radioactive waste repositories

Geophysics ◽  
2010 ◽  
Vol 75 (5) ◽  
pp. Q21-Q34 ◽  
Author(s):  
Stefano Marelli ◽  
Edgar Manukyan ◽  
Hansruedi Maurer ◽  
Stewart A. Greenhalgh ◽  
Alan G. Green
Keyword(s):  
Radioactive Waste ◽  
Damage Zone ◽  
P Wave ◽  
Frequency Noise ◽  
Coherent Noise ◽  
High Level Radioactive Waste ◽  
Seismic Waveform ◽  
Radioactive Waste Repositories ◽  
High Level ◽  
Waste Repositories

Countries worldwide are seeking solutions for the permanent removal of high-level radioactive waste from the environment. Surrounding the waste with multiple engineered barriers and emplacement in deep geological repositories is widely accepted as a safe means of isolating it from the biosphere for the necessary [Formula: see text]. As a precautionary measure, society demands that repositories be monitored for [Formula: see text] after they are backfilled and sealed. Effective monitoring that does not compromise the engineered and natural barriers is challenging. To address this issue, we investigate the viability of crosshole and hole-to-tunnel seismic methods for remotely monitoring high level radioactive waste repositories. Measurements are made at two underground rock laboratories in Switzerland, one within granitic rock and one within clay-rich sediments. Numerical simulations demonstrate that temporal changes of the monitored features (i.e., bentonite plug, excavation damage zone, sand-filled microtunnel) should produce significant changes in the seismicwaveforms. Nevertheless, inversion for medium-property changes requires that true seismic waveform changes are not overwhelmed by recording variations. We find that a P-wave sparker source is highly repeatable up to frequencies of [Formula: see text] for propagation distances out to tens of meters involved in repository-scale monitoring. Hydrophone repeatability is limited by incoherent high frequency noise and variable hydrophone-borehole coupling conditions, but firmly grouted geophones within the tunnels yield consistent recordings. Three kinds of coherent noise contaminate the data: (1) mechanically induced electrical effects in the hydrophone chains; (2) high currents in the sparker cable, which cause it to oscillate radially as a line source; and (3) tube waves. Our investigations outline a quantitative methodology to assess data-quality requirements for successful monitoring. We suggest that full waveform seismic tomography can be used to monitor radioactive waste emplacement tunnels, provided that careful attention is paid to instrument fidelity and noise suppression.

scholarly journals Thermally Induced Bentonite Alterations in the SKB ABM5 Hot Bentonite Experiment

Minerals ◽  
2021 ◽  
Vol 11 (9) ◽  
pp. 1017
Author(s):  
Ritwick Sudheer Kumar ◽  
Carolin Podlech ◽  
Georg Grathoff ◽  
Laurence N. Warr ◽  
Daniel Svensson
Keyword(s):  
Thermal Conditions ◽  
Maximum Temperature ◽  
Thermally Induced ◽  
Calcite Dissolution ◽  
Mineral Stability ◽  
Radioactive Waste Repositories ◽  
High Level ◽  
Waste Repositories ◽  
Tetrahedral Substitution ◽  
Measuring Ph

Pilot sites are currently used to test the performance of bentonite barriers for sealing high-level radioactive waste repositories, but the degree of mineral stability under enhanced thermal conditions remains a topic of debate. This study focuses on the SKB ABM5 experiment, which ran for 5 years (2012 to 2017) and locally reached a maximum temperature of 250 °C. Five bentonites were investigated using XRD with Rietveld refinement, SEM-EDX and by measuring pH, CEC and EC. Samples extracted from bentonite blocks at 0.1, 1, 4 and 7 cm away from the heating pipe showed various stages of alteration related to the horizontal thermal gradient. Bentonites close to the contact with lower CEC values showed smectite alterations in the form of tetrahedral substitution of Si4+ by Al3+ and some octahedral metal substitutions, probably related to ferric/ferrous iron derived from corrosion of the heater during oxidative boiling, with pyrite dissolution and acidity occurring in some bentonite layers. This alteration was furthermore associated with higher amounts of hematite and minor calcite dissolution. However, as none of the bentonites showed any smectite loss and only displayed stronger alterations at the heater–bentonite contact, the sealants are considered to have remained largely intact.

Sign in / Sign up

Export Citation Format

Share Document