FRACON Code

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. 

In order to predict the T-H-M response of the bentonite, a coupled T-H-M transient analysis was performed with the Finite Element Code FRACON. The governing equations incorporated in the FRACON code were derived from an extension of Biot s (1941) theory of poro-elasticity to include the T-H-M behaviour of the unsaturated FEBEX bentonite. The model formulation(Nguyen, Selvadurai and Armand, 2003) resulted in three governing equations where the primary unknowns are temperature, the displacement vector and the pore fluid pressure, as follows ... 

Sequence of transient analysis using the FRACON code... 

In modelling the response of the bentonite, we mathematically simulate as closely as possible the actual test sequence. Each of the following phases were modelled with the FRACON code, with the results of the last time step of each phase being used as starting conditions for the subsequent phase ... 

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... 

In the FRACON code developed and used by the CNSC, the bentonite was assumed to be a poroelastic continuum of a generalized Biot s type. The physical processes considered are the heat conduction, pore water flow in saturated /unsaturated porous media, vapor flow driven by temperature gradient, and mechanical deformation of the skeleton. These processes are described by the following governing equations. 

Four research teams—AECB, CLAY, KIPH and LBNL—studied the task with different computational models. The computer codes applied to the task were ROCMAS, FRACON, THAMES and ABAQUS-CLAY. All of them were based on the finite-element method (FEM). Figure 6 presents an overview of the geometry and the boundary conditions of respective models, including the nearfield rock, bentonite buffer, concrete lid, and heater. The LBNL model is the largest and explicitly includes nearby drifts as well as three main fractures... 

Nguyen, T.S. 1996. Description of the computer code FRACON. In Stephansson, O., Jing, L., and Tsang, C.-F. editors. Coupled Thermohydro-mechanical Processes of Fractured Media. Developments in Geotechnical Engineering, Elsevier, 79 pp. 539-544. 

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