Barriers bentonite

Finally, the montmorillonite weathering reactions also contribute in a kinetic fashion to the pH buffering capacity of the bentonite barrier according to the following... 

Khandelwal, A., Rabideau, A. J., and Shen, P. (1998). Analysis of diffusion and sorption of volatile organic contaminants in soil/bentonite barrier materials, Environmental Science Technology, 32(9), 1333-1339. 

Khandelwal, A., and Rabideau, A. J. (1998). Enhancement of soil/bentonite barrier performance by the addition of natural humus, Proceedings of the 4th International Symposium on Environmental Geotechnology, Boston, MA, 9-12 August, 1998. 

Before discussing the practical applications of bentonite, its different properties have to be studied in order to determine how the various bentonite rocks can be used. In the scope of this book, some agricultural and environmental applications are mentioned. In these applications, natural or chemically modified bentonite rocks are in direct contact with the soil or the geological environment. As an example, we can mention the bentonite barriers used for waste disposals. 

Based on the characterization of the bentonite and on the details of the process of test installation, a thermo-hydro-mechanical model for the bentonite barrier and the heaters was to be prepared. Using this model, the thermo-hydro-mechanical response of the bentonite barrier as a result of the heat released by the heaters and the hydration from the host rock was required. Local field variables such as temperature, relative humidity, pore water pressure, stresses and displacements, as well as global... 

As in Part B, only a reduced number of modelling teams provided blind predictions for the rock behavior, once the expansive bentonite barrier was in place. Coupled THM models are also required for this part of the Benchmark although the temperature increase plays a dominant effect on the rock behavior. As it is frequently the case, temperature changes are well reproduced in general terms. Rock water pressures development integrates two separate phenomena the modification of the... 

The FEBEX T-H-M experiment is a valuable and important project which should lead to an improvement in the understanding of the behaviour of the bentonite barrier around heat-emitting Nuclear Fuel Waste(NFW) containers. Such large field experiments should always be undertaken with the simultaneous development of constitutive and computational models to interpret the experiments. The FEBEX bentonite possesses strong nonlinear behaviour in the unsaturated state. In order to simulate that behaviour, we have adopted a nonlinear poro-elastic approach. In this approach, the coefficients of the poroelastic equations are assumed to be functions of suction and the void ratio. These functions are derived from the state-surface equation which has been experimentally obtained from suction-controlled oedometric tests performed by the Spanish research organizations UPC and CIEMAT. 

These blind predictions of the FEBEX data do not make a strong case that, for this particular geomechanical situation, a coupled analysis is entirely necessary. The granite in this case is sparsely fractured, and most of the inflow occurs at the lamprophyre and other more fractured areas. Also, the rock mass is sufficiently nonporous and saturated that inelastic deformation of the rock matrix is not a significant issue for repository performance. However, the exercise was very valuable for developing rationale for modeling the more complex coupled problems associated with the introduction of the bentonite barrier and the heat of the simulated waste. 

A FULLY COUPLED THREE-DIMENSIONAL THM ANALYSIS OF THE FEDEX IN SITU TEST WITH THE ROCMAS CODE PREDICTION OF THM BEHAVIOR IN A BENTONITE BARRIER... 

The FEBEX in situ test started in 1995 with rock-mass characterization and tunnel excavation. After the installation of heaters, the bentonite barrier, and monitoring equipment, the heating... 

In the following subsections, simulated and measured system responses are compared at selected monitoring points within the bentonite barrier. 

Figure 5 presents simulated temperature in the bentonite barrier after 1,000 days. The figure shows the location of the hottest point on the heater surface, which is kept at a constant temperature of 100°C. At the drift wall the temperature reaches a maximum of about 40-50°C, giving rise to a thermal gradient of about 80 to 90°C/m. 

Figure 10 presents simulated and measured evolution of stress normal to the drift wall at two locations (E2G2 and B2G in Figure 9). The simulated stress began to develop as soon as the wetting commenced and increased to about 2 to 2.5 MPa at 1,000 days. The measured stress indicates that the swelling stress might not have begun to develop until several months after heater tum-on. This delay in the development of swelling stress was a common observation at many monitoring points in the bentonite barrier at FEBEX.

The numerical model ROCMAS was applied to predict coupled THM processes in a bentonite barrier at the FEBEX in situ test. The results indicate that numerical modeling can provide highly reliable predictions for temperature distribution, and reasonably reliable predictions for moisture flow and stress in a bentonite barrier. Moreover, field observations and modeling shows that resaturation of the buffer was controlled by the properties of the bentonite barrier whereas the permeability of the rock was sufficiently high to act as an unrestriced water source. Therefore, the wetting of the bentonite took place uniformly from the rock and was not impacted by the permeability difference between the Lamprophyres dykes and surrounding rock. 

The evolution of stress in the bentonite barrier at FEBEX was affected by the existence of gaps between the pre-fabricated bentonite blocks. The swelling pressure did not develop until moisture swelling of the bentoninte blocks had closed the gaps completely. 

The test schedule involves switching on the heaters an applying increasing power until the temperature reaches a value of 100°C at some point in the bentonite. From that moment on, the power of the heaters is constantly adjusted in order to keep the maximum temperature in the bentonite barrier at the lOOn mark. The test was run in this way for five years until one of the heaters was switched off and dismantled. The experiment continues with the other heater without a planned finishing date. 

Because of space limitations only a brief overview of the analysis characteristics can be given. More information is reported in Febex (2000) and Gens et al. (2002). The numerical computations have been carried out using the fully coupled THMC version of CODE BRIOHT. Radial symmetry has been assumed resulting in a 1-D axisymmetric analysis that is quite appropriate for examining the behaviour of the bentonite barrier and immediate adjacent rock. 

Figure 3. Variation of temperature with time (5 years) at three points of the bentonite barrier...

Figure 6. Distribution of ionic strength in the bentonite barrier and rock at various times...

DRYING AND RESATURATION OF THE BENTONITE BARRIER IN A NUCLEAR WASTE REPOSITORY. ANALYSES BASED ON AN ANALYTICAL SOLUTION... 

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