- freely available
Geosciences 2017, 7(1), 5; https://doi.org/10.3390/geosciences7010005
- an appropriately large mass of bentonite is “in place”, rather than just a small plug of material within laboratory apparatus
- an appropriately dense overburden can settle into the bentonite in a manner similar to what would be expected in the repository EBS
- the settlement timescales are of much more relevance to a repository than accelerated laboratory and URL (underground research laboratory) studies
- potential chemical reaction between the overburden and the bentonite can be assessed. Generally, this is assumed to be diffusive transport of solutes (especially OH and Ca) into the body of the bentonite from the overburden/bentonite contact zone, again as would be expected in a repository where OH and Ca from the cementitious materials could interact with the bentonite
- the temperatures of reaction (ca. 10–30 ℃) are also repository-relevant.
2. Geological Setting of the Study Area
- pre-Quaternary sedimentary cover (Lefkara, Pahkna and Nicosia Formations)
- Quaternary Fanglomarate Group to the north of the KM area
- in the channel area of the Peristerona River, Holocene alluvium deposits
- as the Troodos Massif has been uplifted some 2000 m in ca. 2 Ma, the KM site has probably been exposed for around 0.5 Ma.
- average erosion rates of the limestone overburden could be in the order of 50 m in 1 Ma, suggesting local overburden loss at KM of some 25 m in that time.
- based on the likely main erosion processes onsite, it is suggested that the small valley in which the KM3 borehole was drilled was initiated at least 50 ka ago.
- it is also suggested that the valley sidewalls might retreat by some 6–12 m over a 10 ka time span, indicating that the valley bottom at the KM3 site has been exposed for a few ka at the very most, and possibly much less.
3. Materials and Methods
3.1. Field Sampling
3.2.1. X-ray Fluorescence Analysis
3.2.2. X-ray Diffraction Analysis
- Unoriented and untreated samples (all): To provide an overview of the bulk sample mineralogy, unoriented and untreated samples were measured initially. After air drying at room temperature, all samples were ground in an agate mortar until smooth to the fingertip. The powdered samples were then loaded into the sample holder (aluminium plate depth 1mm diameter 25 mm).
- Oriented and untreated samples (clay): In order to identify the clay mineral composition in detail, oriented and untreated samples were measured. The above powdered samples were diluted with distilled water in a 300 mL beaker and, after standing for 4 h, the upper 5 cm (which is presumed to contain clay particles of less than 2 μm) was collected, centrifuged (1900 rpm, 10 min) and the resulting precipitate placed on a glass slide (two per sample) and then air dried.
- Ethylene glycol treated samples (EG): If peaks were identified near 10 Å or 14 Å in the oriented and untreated sample, the oriented samples were treated with ethylene glycol to determine if they represented smectite or chlorite. One set of oriented and untreated samples had a drop of ethylene glycol added and allowed to stand for about 5 min.
- Hydrochloric acid treated samples (HCl): If peaks were identified near 7 Å or 10 Å in the oriented and untreated samples, the oriented samples were treated with hydrochloric acid to determine if they represented kaolinite or chlorite. The above powdered samples were added to HCl (10 mL of 6 N) in a test tube and placed in a hot water bath for 120 min. Thereafter, they were centrifuged (1900 rpm, 5 min), the supernatant discarded and resaturated with distilled water. This was repeated five times and the final precipitate was then air dried on a slide glass.
3.3. Optical Petrographic Examination
3.4. Bentonite Physical Properties
- Natural Moisture Content: All natural moisture content tests, in accordance with , were determined for all samples tested for Unconfined Compressive Strength, as well as for samples tested for Atterberg Limits.
- Specific Gravity/Particle Density: This test was carried out in accordance with .
- Bulk and Dry Density: These tests were carried out in accordance with .
- Atterberg Limits: Testing was carried out in accordance with the requirements of . The Cone penetration apparatus (method 2B) was adopted for the liquid limit tests. The results are presented graphically on individual test sheets and on the Casagrande Plasticity Classification Chart. All samples tested for Atterberg Limits were also tested for linear shrinkage.
- Unconfined Compression Tests: Eighteen Unconfined Compression tests were carried out on cored samples on the ELE-MULTIPLEX 50 triaxial machine. The treatment and testing of samples was in accordance with . The orientation of the samples when placed in the testing machine was as in situ and the rate of strain applied was 1.00 mm·min−1.
- Triaxial Strength Tests: The Unconsolidated Undrained Triaxial Tests (UU) were carried out at various cell pressures in accordance with the requirements of  in order to obtain values of the peak strength, cohesion and angle of shearing resistance (total values).
- Swelling Pressure: The testing was performed in accordance with the procedure given in . They are presented graphically with the cumulative weight vs. square root of time.
3.5. Cation Exchange Capacity (CEC) and Exchangeable Cation Composition (EC) Analysis
3.5.1. Ba—Mg Compulsive Exchange Method for CEC
3.5.2. EC Analysis
3.6. Stable Isotopes
3.7. Natural Decay Series (NDS)
3.7.1. Alpha Spectroscopy
3.7.2. Gamma Spectroscopy
3.8. Smectite Content Estimation Utilising the Methylene Blue Method
4.2. Optical Petrographic Examination
4.2.1. Petrography of Boreholes KM1, KM2 and KM3
4.2.2. Bentonite Texture
- if the sampling induced “drying”, pull-apart features are ignored, the pattern of faulting indicates that there has been some lateral shear along the bedding planes.
- in most cases, the sense of shear is unclear, so it is not possible to define if the shear is an accommodation of compressive stress by movement on foliation planes—or simply a response to gravitationally-induced movement towards the local valleys around the Kato Moni quarry.
- more importantly, there is evidence of some multi-directional shear in the bentonite. The network conjugate, steeply-inclined, micro-faults that tail-off into shear fabrics suggest movement in at least two directions—i.e., movement unlikely to be gravitationally induced.
- as in boreholes KM1 and KM2, there is evidence of multi-directional shear in the bentonite, only it is more clear in KM3. The S- and C-type shear fabrics present here clearly indicate movement in at least two directions, with the shear zone representing an accommodation of compressive stress by movement on foliation planes (cf. ).
- the “chicken wire” network has already been proposed as an analogue of industrial bentonite pellets  due to similarities in form between lapilli tuff sourced bentonite and the industrial bentonites. That the shear plane networks are of a different form to those propagated in the more homogenous bentonite of boreholes KM1 and KM2 suggests that these features would be worth further study to assess if the fundamental shear process is different in bentonite pellets. For example, is the load of the overlying limestone being dissipated in a different form due to the pellet structure? That the S- and C-type shear fabrics “reappear” in borehole KM3 at depth when the bentonite fabric is once more homogenous suggests this to be the case.
4.3. Bentonite Physical Properties
- Low hydraulic conductivity
- Sufficient swelling pressure
- Sufficient density
- Sufficient thermal conductivity
4.4. CEC and Methyl Blue Adsorption
4.4.2. Methyl Blue Adsorption
4.5. Stable Isotopes
- Normal marine seawater and limestone: δ13C PDB ~0‰ ± 2‰.
- Marine shells: δ13C PDB typically 0‰ ± 2‰.
- Bacterial sulphate-reduction and oxidadtion of sedimentary organic matter: δ13C PDB typically between −8‰ and −32‰.
- Methane oxidation: δ13C PDB typically less than −35‰.
- Fermentation: δ13C PDB around +5‰ to +10‰.
- Soil carbonates δ13C PDB are around −8‰ to −12‰.
- Meteoric cements δ13C PDB are around: −4‰ to −15‰.
4.6. Natural Decay Series (NDS)
5.1. Repository Relevance
- despite significant coupled uplift/erosion processes on the north side of the Troodos Massif producing massive sedimentary bodies and deep river valleys, the KM quarry ridge has likely been “protected” throughout much of this period (ca. 0.5 Ma) and has avoided catastrophic change.
- although the limestone overburden is an aquifer regionally, the isolated nature of the ridge suggests it has been uncoupled from the regional flow for some time, so minimising recent physico-chemical disturbance to the bentonite (something confirmed by the petrographic, stable isotope and NDS analyses).
- overall the climatic system in the area would tend to suggest that the site has been historically relatively dry, once again protecting the bentonite from significant external perturbations.
5.1.2. Relevance to Expected Repository Behaviour
- many of the reactions between the limestone and the bentonite (e.g., diffusion of Ca into the clay from the limestone and pillow lavas) would also be expected between the concrete waste packages and waste forms and the bentonite in a repository
- Further, the petrographic data indicate there has been little or no reaction along the older fractures which are present in the bentonite, meaning the site is repository representative insofar that solute transport would appear to be dominated by diffusion
- The smectite content is particularly applicable to repository designs utilising lower quality bentonite or bentonite sand mixtures.
5.2. Bentonite Stability at Kato Moni
5.3. Bentonite Behaviour
- mechanical integrity and confinement of radioactivity in case of a drop accident during the interim storage phase
- degradation of concrete due to carbonation or ground water access
- confinement of radioactivity in the post closure phase
- performance in case of gas generation inside containers
- available experience with container type and materials
- mechanical strength to:
- withstand stacking forces
- resist damage due to pressurisation by internally generated gases
- ensure that the specified impact accident performance can be achieved
- withstand other loads that may occur during the long-term management of the waste package, as required by the generic Environmental Safety Case
- radiation shielding to ensure that the external dose rate is minimised and that the limits specified for transport are not exceeded
- thermal properties to ensure that the required fire accident performance and other requirements for the thermal performance of the waste package will be achieved
- resistance to degradation to ensure the overall integrity of the waste container is maintained for an adequate period.
- swelling pressures/density/Pl, etc., from a range of natural bentonites should be examined to assess the true impact of these parameters on bentonite shear for both L/ILW and SF repository designs. If full saturation cannot be automatically assumed, then models of bentonite shear such as  will need to be modified and the potential impact on the bentonite performance re-assessed.
- the sedimentary fabric impacts on the deformation and fracturing behaviour of the bentonite as, where banding or lamination is present, the bentonite displays intense micro-fracturing oriented parallel to the lamination. In addition, the shearing tends to be better developed in the slightly coarser material. This has implications for repository performance as, for example, in those concepts where pre-compressed bentonite blocks are foreseen—is there a difference in the fabric when uniaxially or triaxially compressed materials are employed?
- similarly with the disseminated shear in the parts of core KM3 which contain the pelletised bentonite, what does this imply for those design concepts which utilise bentonite pellets around waste packages?
- all three cores display shearing throughout, but it would be worth assessing if the smectite content has influenced the degree of shearing to any extent. As higher smectite content generally means higher plasticity, can any difference be seen between the lowest smectite containing samples in core KM1 (20.6% smectite) and the highest in core KM3 (36.2%)?
- that shear planes have been found throughout the natural bentonites at KM indicates that the basic conceptual model is correct, but without any sense of direction of these shears, these observations are of only limited value. As such, it is recommended that additional samples be collected at the KM site with a directional coring tool which will allow the sense of movement of the shears to be ascertained.
- it would be also advisable to drill deeper samples, down to the underlying pillow lavas, to obtain a better spatial understanding of the change in the internal structures of the bentonite with depth above the fixed boundary at the base of the bentonite.
- further examination of the “pellet” bentonite in the KM3 borehole would be useful as the inherited structure appears to spread the shear through anastomising shear plane networks, potentially minimising disruption of the bentonite fabric. This could be an alternative bentonite design option, so avoiding a potential redesign of existing waste containers.
Conflicts of Interest
|AFM||triaxial octahedral Al—Fe—Mg cations data plot|
|APFU||atoms per functional unit|
|BSEM||backscattered scanning electron microscopy|
|CEC||cation exchange capacity|
|EBS||engineered barrier system|
|EDXA||energy-dispersive X-ray microanalysis|
|ESEM||environmental scanning electron microscopy|
|HLW||vitrified high-level waste|
|L/ILW||low- and intermediate-level waste|
|NDS||natural decay series|
|PDB||Pee Dee Belemnite|
|WOB||weight on bit|
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|Borehole ID||Eastings (°)||Northings (°)||Elevation (m.a.s.l.)|
|Atterberg Limits||UCS||Sp Grav.||Sw Press (kPa)||UU Triaxial|
|PI (%)||Shrinkage (%)||MC (%)||MC (%)||BD (g/cm3)||DD (g/cm3)||qu (kPa)||Cohesion (kPa)||φο|
|Parameter||Dry Density (kg·m−3)||Water Content (%)||Smectite Content (%)||Liquid Limit (%)||Plastic Limit (%)||Plasticity Index||CEC mEq (100 g−1)||Swelling Pressure (MPa)|
|KM1-B6-3||1410–1460||28.8||20.6–27.3 1||84||25||59||50.75 2||0.20|
|KM3a-B5-3||1230||34.2||26.6–36.2 4||112||32||80||55.95 5||0.34|
|T5 6 (Parsata)||1340–1390||17–23||24–28||106–131||39–49||67–95||34–43||0.12–0.17|
|Posiva SF rings 7,8||1752||17||>75||>80||>60|
|Posiva SF disc blocks 7,8||1701||17||>75||>80||>60|
|Posiva SF pellets 7,8||919||17||>75||>80||>60|
|Posiva buffer Ca-bentonite 9||13–15||65-≥75||60-≥80||50||10-≥30||50-≥60||≥2|
|Posiva buffer Na-bentonite 9||13–15||65-≥75||200-≥250||50||150-≥200||60-≥70||≥2|
|MX-80 10,11||2880||80||437.3||38.0||399.3||110.4||7.3 12|
|NEO-KUNIBOND 10,11 industrial Na-bentonite||2680||76||607.5||50.69||556.8||103.5|
|KUNIBOND 1,10 industrial Ca-bentonite||2.71||84||128.7||38.4||90.3||79.5|
|Bentonite||Kato Moni KM3a||Parsata||MX-80||MX-80||GEKO/QI||MX-80||GEKO/QI|
|Density (at water saturation) kg·m−3||1340–1700||1600–1680||2000||2100||2200||1950||2200|
|Swelling pressure (MPa in distilled water)||0.15–0.34||0.09–0.17||7.3||2.0||0.9||0.2||0.1|
|Sample||Depth (m)||CEC (meq 100 g−1)||Exchangable Cations (meq 100 g−1)|
|Ca||Mg||Na||K||Sr||Mn||Fe||Total Exchangeable Cations (Σ)|
|KM1 L1a2 (repeat analysis)||−7.71||−3.90|
|BDH calcium carbonate secondary standard (CCS)|
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