Buffer Mass Test - thermal calculations for the high temperature test

the calculations have been made by using a Finite Element Method program for instationary thermal flow. The model used has been an axi-symmetric one, with the centre-line passing through the centre of the heater. The outer radius of the model was 6.25 m and the boundary conditions were set to fixed temperatures of +10degreeC i.e. the average rock temperature. The effects of the boundary conditions on the calculated temperature have been analysed and the effects are within 1.5degreeC during the time period studied (10 weeks). The largest influence is noticed at the boundaries. In the buffer mass and at the heaters surface no influence on the calculated temperatures could be detected due to the boundary conditions during the time period studied. The temperature at the rock/bentonite interface is shown to be almost independent of the thermal conductivity of the bentonite. 10 weeks after the start of the heater this temperature has reached +55degreeC. In contrast to this, the temperature at the surface of the heater varies considerably for different values of the thermal conductivity of the bentonite (195degreeC at Λ=0.5 W/m,K and 141degreeC at Λ=0.8 W/m,K). The amount of drying has been estimated on the basis of sorption curves for Na-montmorillonite found in literature and sorption curves for organic material (pine-wood) with almost the same specific surface as the bentonite. Here, the water content will be close to 0percent within a distance of 7.5 cm from the heaters surface. In the middle part of the buffer mass (7.5 cm to 15 cm distance) the water content will be in the range of 0-2percent and in the outermost part it will be in the range of 2-3.8percent. The resulting thermal conductivity of the buffer mass, is thus predicted to be in the range 0.65-0.71 W/m,K, which gives a surface temperature of the heater of 155-164degreeC. (author)