Compacted bentonites are proposed to be used as the barrier and backfilling materials in underground disposal of high-level nuclear waste in many countries. Due to the high swelling and low permeability characteristics, the bentonite buffer surrounding the waste canisters is expected to retard the flow of radionuclide to the natural barrier systems. The in situ boundary conditions dictate exposure of compacted unsaturated bentonites to elevated temperature from the waste canisters and hydration upon fluid uptake from the surrounding host rock. Bentonites exhibit swelling during the hydration process, whereas the drying process is accompanied by a decrease in the volume. Under confined condition and upon exposure to a hydration source, the development of swelling pressure is expected to provide stability to the waste disposal repositories [1
Several studies have been carried out in the past that focused in investigating the coupled thermo-hydro-mechanical behaviour of compacted bentonites and bentonite–sand mixtures when subjected to thermal, hydraulic, and thermo-hydraulic gradients [5
]. The reported laboratory tests have been conducted at various temperatures (60 to 140 °C) and water injection pressures (5 kPa to 2 MPa). A choice of water injection pressure depends upon the pore water pressure conditions in the repository. The variations of the temperature and relative humidity within compacted bentonites were monitored in several investigations. Similarly, the changes in the dry density, the water content, the degree of saturation and the concentration of anions and cations were measured after the termination of the tests in various studies [9
]. A variation of the axial stress during the tests was also monitored in some cases (see for example, [17
]). This information is crucial in order to understand the coupled thermo-hydraulic-mechanical-chemical response of the engineered barriers, both under short and long-term in situ conditions involving anticipated temperatures and water pressures.
Studies concerning the thermal and thermo-hydraulic response of compacted bentonites at high temperatures are of significant interest, particularly for understanding the early-life phase of deep geological disposal facilities. Very high temperature of spent fuel, radioactive decay of wastes, and formation of high thermal zones at and near the interface of waste canisters and the buffer are some of the in situ conditions which demand detailed investigations concerning the response of compacted bentonites under non-isothermal and non-isothermal hydraulic boundary conditions involving high temperatures. A detailed review of the literature suggested that a majority of the laboratory tests in the past have been conducted by considering an applied temperature of less than 100 °C [5
]. A thermal limit of about 100 °C or lower is preferable in order to eliminate the risks associated with chemical alteration of the buffer and backfilling materials and the subsequent changes in swelling and hydraulic properties [14
]. Horseman and McEwen [23
] stated that the magnitude of temperature in the waste repositories influences the decisions concerning the spacing between waste package, the distance between disposal galleries, and therefore the overall size of the repository. Sellin and Leupin [22
] stated that the heat generation from the waste decreases relatively rapidly with time, and montmorillonite alteration is a slow process, any temperature criteria will always be somewhat arbitrary. Studies in the past have shown that the hydraulic properties of initially saturated bentonites did not deteriorate at temperatures up to 120 °C and hence it may be anticipated that unsaturated bentonite would remain stable even at high very temperatures [20
]. Zheng et al. [24
] stated that an applied temperature of 200 °C may cause a decrease in the swelling pressure, a reduction in the smectite volume fraction, and an increase in the total stress due to thermal pressurization.
During the lifetime of the repositories the heat originated from the waste may dissipate due to various processes, such as radiation, conduction, and convection. The thermal properties of the host rock, the magnitudes of temperature and water pressure, and the thickness of compacted bentonite may influence the temperature rise within the buffer, which may in turn influence the distribution of water and development of stress within the engineered barrier systems. The main objective of this paper is to bring out the influence of thermal and thermo-hydraulic gradients involving high temperatures and high water injection pressure on the behaviour of compacted bentonites.
2. Materials and Methods
MX80 bentonite was procured from TOLSA UK Ltd (Scunthorpe, UK; www.tolsa.com
) for this research work. The initial water content, specific gravity, liquid limit and plastic limit of the bentonite were found to be 15.2%, 2.8, 400% and 58%, respectively. The specific surface area for the bentonite was determined by the ethylene glycol mono-ethyl ether (EGME) method and was found to be 654 m2
/g. The ammonium acetate extraction method [25
] was used to determine the type and amount of cations present in the bentonite. The bentonite was found to contain Na+
(67.12 meq/100 g), K+
(1.95 meq/100 g), Ca2+
(44.67 meq/100 g) and Mg2+
(14.35 meq/100 g). The type and amount of soluble ions were determined by saturation extract method using water that enabled calculation of the fractional exchangeable cations as: Na+
(44.83 meq/100 g), K+
(1.20 meq/100 g), Ca2+
(42.11 meq/100 g), and Mg2+
(3.29 meq/100 g). The total cation exchange capacity of the bentonite was found to be 91.43 meq/100 g. The X-ray diffraction analysis was undertaken by using a Philips automated diffractometer (PANalytical, Cambridge, UK). The MX80 bentonite used in this study was found to contain 72% montmorillonite, 12% quartz and 16% other minerals (e.g., cristoballite, feldspars, etc.).
Two types of thermo-hydraulic column cells (TH column cells) were used in this study. The schematic diagrams of the devices are shown in Figure 1
. For the tests at 85 °C, a TH column cell (Device A in Figure 1
a) proposed by Tripathy et al. [18
] was used, whereas several new TH column cells were fabricated and used for the tests at 150 °C (Device B in Figure 1
b). The devices enabled applying both thermal and thermo-hydraulic gradients, and facilitated measuring the relative humidity and the temperature at predetermined locations along the depth of specimens and the axial stress at the top of the specimens.
The main features of the TH column cells were similar except that the dimensions and the accessories used were different. The main components of the TH column cells are a stainless steel base, a central section (100 mm diameter and 140 mm high for device A and 100 mm diameter and 360 mm high for device B), a connecting ring, and a top part that acts as a water reservoir for supplying water to the specimen during hydraulic tests. A polytetrafluoroethylene (PTFE) forms the liner of the central section for reducing the heat loss through the outer wall of the cells during tests. Metal heaters were used to apply thermal loading at the bottom end of the specimens. Dual thermocouples were attached at the centre of the heater. One of the thermocouple was connected to the heater controller and the other was connected to a data logger. During a test, the thermocouple that was connected to the heater controller maintained the predetermined temperature of the metal heater. The other thermocouple measured the actual temperature on the surface of the heater. The loading plunger in each cell was fitted with a metal porous disc which remained in contact with specimen during a test.
The diameter of commercially available relative humidity and temperature probes that are suitable for using at high temperature applications is usually larger than that of the probes which are used at lower temperatures. Therefore, in order to minimize the interference of electrical signal, it is preferable to maximize the distance between the probes, which in turn requires consideration of a thicker bentonite specimen. This was one of the main reasons behind development of the TH cell B. The relative humidity and temperature probes were procured from Vaisala (Helsinki, Finland). The accuracy of the probes (HMP110) used in case of device A was ±2.5% and ±3.5% for the range of relative humidity between 0 to 90% and 90 to 100%, respectively. The accuracy of temperature sensor in this case was ±0.4 °C for the range of temperature measurement between 40 and 80 °C. The accuracy of the probes (HMT315) used in device B was ± (1.5 + 0.015 × reading) % for the range of relative humidity between 0 to 100% and for an operating temperature range of −40 to 180 °C. The accuracy of the temperature sensor in this case was ±0.2 °C.
The central section of device A (Figure 1
a) contains three holes at distances of 20, 40, and 60 mm from the heater. The positions of the holes are staggered by an angle of 120°. The central section of device B (Figure 1
b) contains five holes at distances of 40, 60, 120, 180, and 240 mm from the heater. The positions of the holes are staggered by an angle of 90°. For the non-isothermal tests in device B, the hole at a distance of 40 mm from the heater was not used. Similarly, holes were drilled on compacted bentonite specimens, aligned with the holes in the central section of the cells for accommodating the relative humidity and temperature measurement probes.
In total, nine tests were carried. The duration of the tests was varied between about three to ten months. Four independent tests (two non-isothermal and two non-isothermal hydraulic tests) were carried out on compacted bentonite specimens using device A, whereas five tests (two non-isothermal and three non-isothermal hydraulic tests) were carried out using device B. Table 1
presents the details of the tests carried out in this investigation.
Cylindrical bentonite specimens were prepared by statically compacting bentonite powder (water content = 15.2%) to a targeted dry density of 1.65 ± 0.1 Mg/m3. A high capacity loading frame was used for the compaction process. The targeted dry density of the specimens was achieved following removal of the applied compaction stress which caused an axial expansion of the bentonite specimens. Compaction of bentonite powder was carried out inside the central section of the cells. Prior to preparing the specimens, the inner surfaces of the specimen cells were lubricated with silicon grease. For the tests in device A, compacted specimens (100 mm diameter, 80 mm high) were prepared in four layers, each 20 mm thick. To achieve the desired dry density, each layer was subjected to a static load of about 90 kN (applied static compaction pressure = 11.46 MPa). For the tests in device B, specimens (100 mm diameter, 300 mm high) were prepared in ten layers, each 30 mm thick. In this case, each layer was subjected to a static load of about 120 kN (applied static compaction pressure = 15.28 MPa).
Prior to the tests, the relative humidity and temperature measurement probes were inserted through the predrilled holes. Commercially available synthetic fluorinated oil based grease was applied at the junctions between different sections of the cell to prevent the vapour leakage. The TH column cells were covered with thermal insulations during the tests. For the non-isothermal tests using devices A and B (i.e., for Tests T1–T4) two types of thermal insulations were used, such as a 20 mm thick rock-wool covered with duct tape and a 40 mm thick rock-wool covered with reflective tape. All the non-isothermal hydraulic tests, except Test TH5 were conducted with the outer thermal insulation that was comprised of a 40 mm thick rock-wool covered with reflective tape. For Test TH5, a 20 mm thick insulation was used. During a test the load cell was positioned on top of the plunger and restrained against any movement by using a loading frame.
For the non-isothermal tests (for Tests T1 to T4), compacted bentonite specimens were either subjected to a temperature of 85 °C (device A) or 150 °C (device B) (see Table 1
) at the bottom end by raising the temperature of the heater, whereas at the top, a temperature of 25 °C was maintained throughout the tests. For the non-isothermal hydraulic tests (Tests TH5 to TH9), in addition to the applied temperatures (Table 1
), deionized water was supplied from the top end of the specimens. The applied water pressure was 600 kPa for Tests TH5, TH6, TH8, and TH9, whereas the applied water pressure was 5.0 kPa for Test TH7.
Temperature control at the top end of the specimens was achieved via circulation of water within a copper coil. A heating/refrigerating water circulator was used to control the temperature of the circulating water within the coil. The water circulator had a proportional integral-derivative (PID) controller and an adjustable over heating cut out. The empty space on top of the loading plunger was filled with deionized water for supplying water to the specimens during Tests TH5 to TH9. During Tests T1 to T4, the temperature of the water within the coil was maintained at 25 °C. The relative humidity, temperature, and axial stress were monitored during all tests. After termination of the tests, the specimens were extruded from the devices. The specimens were cut into 20 mm thick slices using a hacksaw. Each slice was then broken down into smaller parts for which the water content was determined by oven-drying method, whereas the volume was determined by molten wax method [26
]. The dry density and the degree of saturation for each slice were then calculated from the volume–mass relationships.
High density compacted bentonites have been proposed to form the buffer surrounding the waste canisters in the underground nuclear waste repositories. The buffer will be subjected to both thermal and hydraulic loadings in certain situations. Therefore, well-controlled laboratory tests involving simultaneous applications of thermal and hydraulic loadings or only thermal loading provide information that play crucial roles for understanding the behaviour of bentonites in terms of their efficiencies in satisfying the hydraulic, mechanical, thermal, and chemical criteria set out for the safety of the underground nuclear waste repositories [4
]. The laboratory test results have enabled validating numerical models for assessing the short and long-term behaviour of the engineered barrier systems [27
The heat flow through soil systems is complex due to the fact that the thermal conductivity of soil increases with an increase in the dry density, decreases with an increase in the porosity, and increases with an increase in the water content and degree of saturation [29
]. The host rock will provide the physical confinement to the bentonite buffer. Therefore, in addition to various factors the thermal and hydraulic properties of the host rock to some extent will govern the temperature and water content distributions within the buffer.
Both during the early-life and late-life phases of the repositories, the stress, thermal, and hydraulic boundary conditions surrounding compacted bentonites may change, which in turn may impact the thermal and hydraulic responses of compacted bentonites. In the past, research works have been focused in exploring the thermo-hydro-mechanical response of compacted bentonites for applied temperatures of less than 100 °C. In this study, compacted bentonites were subjected to both thermal and thermo-hydraulic gradients to explore the impact of high temperatures (85 °C and 150 °C) on the thermo-hydro-mechanical behaviour.
The test results clearly showed that the thermal response of compacted bentonites when subjected to temperature gradients depends upon the magnitude of applied temperature and the thermal conductivity of the surrounding material (i.e., thermal insulation). Similarly, the impact of thermo-hydraulic gradient at an applied temperature, and the impact of an elevated temperature on the thermo-hydraulic response of compacted bentonites could be derived from the test results. The main findings from this study are summarised in Table 4
and Table 5
presents the impact of thermal insulation and higher temperature (i.e., when the applied temperature was increased from 85 °C to 150 °C) on a variety of factors within compacted bentonites. Table 5
shows the changes that may occur for various factors due to a simultaneous application of thermal and hydraulic loadings as compared to that of the thermal loading, and the impact of higher temperature when thermo-hydraulic loading condition is considered as a reference.
shows that both the improved thermal insulation and high temperature application have the same influence on the various factors within compacted bentonites. The findings presented in Table 4
and Figure 15
suggest that thermal loading with improved insulation may cause an increase the shear strength of bentonite due to an increase in the dry density towards the warmer regions (i.e., towards the heat source), whereas a reverse trend can be expected at the opposite end. The development of axial stress occurs (Figure 14
), but of a small magnitude (about 125 kPa), primarily due to the wetting that occurs at the opposite end of the heat source (see Figure 15
). The dissipation of heat from compacted bentonites (in this case 20 mm thick insulation) may cause superior hydration, which in turn will cause a decrease in the hydraulic conductivity due to an increase in the water content (see Figure 15
). In both cases, an impermeable zone is expected to form at the interface between the host rock and buffer which will aid in retarding the transport of radionuclides.
shows that thermo-hydraulic loading at any applied thermal gradient have several advantages, for example, an increase in the water content causes a decrease in the hydraulic conductivity and an increase in the axial stress. The effect of high temperature (150 °C) although caused an overall decrease in the water content and degree of saturation; however, the axial stress remained similar to that occurred in case of a lower temperature (85 °C). This suggests that the stability of the underground repositories may not deteriorate upon an increase in the applied temperature from 85 °C to 150 °C.