Bentonite FEBEX tests

In the FEBEX project blocks made of Ca-bentonite are tested for their suitability in radioactive waste isolation. This material has been tested in various laboratory experiment to identify the material parameters. The modelling of gas and water flow in engineered barriers demands beneath the intrinsic permeability and porosity constitutive equations for capillary pressure and relative permeabilities. 

Abstract The Canadian Nuclear Safety Commission (CNSC) used the finite element code FRACON to perform blind predictions of the FEBEX heater experiment. The FRACON code numerically solves the extended equations of Biot s poro-elasticity. The rock was assumed to be linearly elastic, however, the poro-elastic coefficients of variably saturated bentonite were expressed as functions of net stress and void ratio using the state surface equation obtained from suction-controlled oedometer tests. In this paper, we will summarize our approach and predictive results for the Thermo-Hydro-Mechanical response of the bentonite. It is shown that the model correctly predicts drying of the bentonite near the heaters and re-saturation near the rock interface. The evolution of temperature and the heater thermal output were reasonably well predicted by the model. The trends in the total stresses developed in the bentonite were also correctly predicted, however the absolute values were underestimated probably due to the neglect of pore pressure build-up in the rock mass. 

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. 

UPC, 2000, Task definition DECOVALEX III, Task 1 Modelling of FEBEX in-situ test. Part B Thermo-hydro-mechanical analysis of the bentonite behaviour. Polytechnical University of Catalonia, Barcelona, Spain. 

Abstract Coupled THM simulation of the FEBEX, which is the full-scale in-situ Engineered Barrier System Experiment performed in Grimsel Test Site in Switzerland, is one Task in the international cooperation project DECOVALEX III. In the Task, the simulation of the thermal, hydraulic and mechanical behaviour in the buffer during heating phase is required, e.g. the evolutions and the distributions of stress, relative humidity and temperature at the specified points in bentonite buffer material. 

First, we identified the input parameter for THAMES on properties of FEBEX bentonite, because the fundamental properties of FEBEX bentonite had been obtained by various laboratory tests to identify the input data for the numerical code CODE BRIGHT (enresa (1998)). After calibrations of the all required parameters for THAMES, such as thermal vapour flow diffusivity and intrinsic permeability, the coupled THM simulations were carried out. 

The model is fitted to a suction experiment for Febex bentonite and applied to the TH simulation of the bentonite buffer of the Febex in situ test, which is considered in the international Decovalex 3 project. The present approach is to describe the essential features of the TH behaviour of the buffer in a simple ID geometry. The results calculated with FEM are compared to the measurements. 

The FEBEX in situ test has been conducted as a multi-national project coordinated by the Spanish organization ENRESA. In essence, the experiment involves the installation of two cylindrical heaters (4.54 m long and 0.97 m in diameter) centered in a tunnel (diameter 2.27 m) and surrounded by a barrier (thickness 0.64 m) made of highly compacted unsaturated blocks of bentonite (Figure 1). The tunnel is located in a mountainous area of massive saturated granite about 420 m below ground surface. 

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... 

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 tests have been performed with the FEBEX bentonite, which is the clay used for the FEBEX Project in the in situ (Grimsel, Switzerland) and the mock-up (Madrid, Spain) tests (ENRESA 2000). 

In laboratory experiments specimens from the FEBEX bentonite with a height of 0.13 m and a diameter of 0.15 m have been tested. In a series of tests the specimens have been subjected to different combinations of heating and water injection. In one of these tests the specimen had an initial dry density of about 1.65 g/cm and an initial water content of 13.1 %. At the bottom end of the specimen water has been injected with a pressure of 1 MPa and after 148 d the distribution of water content along the height of the specimen has been measured, DECOVALEX III (2001). 

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