Thermal conductivity bentonite

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Thermal Conductivity. Thermal conductivity measurements were made on clay-waste slurries and on the dried cancrinite product. These measurements represent the possible extremes. The values for the actual product, depending on the process, are somewhere between these extremes. The commercial clays, KCS (kaolin 2) and MC-101 (bentonite 2) were used with standard synthetic waste to make the clay-waste slurries and the dried cancrinite product. The results of these measurements are given in Table VIII. These values are near those of dried salt cake and are high enough so that large temperature gradients should not occur in the cancrinite product made from stored Hanford wastes. 

The measured power decreases steadily with tintie for the first 400 days after the beginning of the automatic operation. This is consistent with the progressive drying of the inner annulus of the barrier and the associated decrease of thermal conductivity of the bentonite. The slight increase in power in the second part of the period represented is attributed to the progressive hydration of the barrier due to the incoming water. Some predictions reproduce accurately the observed behavior. 

The considered radial process in the bentonite annulus is a complicated one with coupled, highly nonlinear flows that involve many things. There are liquid flow and vapor flow as well as conductive and convective heat flow depending on gradients in pressure, water vapor density and temperature. The flow coefficients depend on water properties such as saturation water vapor pressure and dynamic viscosity of water. They also depend on the properties of bentonite water retention curve, hydraulic conductivity and water vapor diffusion coefficient, and thermal conductivity, all of which are functions of degree of water saturation. 

The moisture flux g (kg/(m, s)) in the bentonite has a liquid and a vapor component. The liquid flux g, is proportional to the gradient of the pore water pressure P with a hydraulic conductivity k(S) that is a function of the degree of water saturation S. The flux is inversely proportional to the viscosity rj(T). The water vapor flux g, is proportional to the gradient of the water vapor density in the gas phases in the pores with a vapor conductivity factor D,(5) that is a decreasing function of S. The heat flux q (W/m ) has a conductive part with a thermal conductivity. /i(S). There is also a negligible convective part. We have... 

On the other hand, for the thermal problem a dependence of the thermal conductivity coefficient on the degree of saturation was taken into account. The specific heat, however, was assumed constant. Most of the parameters involved were obtained from previous works related to bentonite and rock characterization (i.e. Borgesson, 2001), although information from other similar projects was also used (FEBEX, 2000). 

Since the model involves the Komine s theoretical model for evaluating swelling pressure in saturation, it has the potential to include the effects of chemical change in bentonite and pore water. Using this model, fully coupled thermal, hydraulic, mechanical and chemical analysis can be conducted in the future. 

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