Bentonite FEBEX experiments

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. 

The equations of poro-elasticity were solved with the finite element code FRACON. The FRACON code was used to predict the in-situ T-H-M experiment at the FEBEX gallery in Grimsel, Switzerland. The FRACON correctly predicted that the bentonite would resaturate from the rock interface. Near the heater, it also correctly predicted that initial drying will take place, followed by a slow resaturation. The model correctly predicted that at the end of 1000 days of heating, resaturation of the bentonite was still... 

ENRESA. 1998. FEBEX full-scale engineered barriers experiment in crystalline host rock-Bentonite origin, properties and fabrication of blocks. Publicacion Tecnica 05/98. ENRESA. Emilio Vargas, 7-28043, Madrid... 

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. 

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. 

This paper will focus on the hydraulic properties of the swelling materials, especially on the bentonite that is used in the Full-scale Engineered Barrieres Experiment in Crystalline Rock (FEBEX). The influence of swelling will be shown in a simulated laboratory experiment and in the application on the FEBEX in situ experiment. 

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. 

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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