Temperature FEBEX tests

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. 

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. 

Figure 5 shows the schematic view of the FEBEX. FEBEX has two heaters. Vertical sections, as D, El, E2 and G, in the test tunnel are instrumented sections that are selected for comparison between the prediction and the monitored data for part B. Sections of El and E2 are for relative humidity simulation. Sections of D and G are for temperature simulation. Section of E2 is for total pressure simulation. 

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. 

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