The consotldatfcki and loss of permeability of salt crystal aggregates, importa.it in assessing the effects of water in salt repositories, has been studied as a function of several variables. The kinetic behavior was shatter to that often observed in sinter ing and suggested the following expression for the time dependence of the void fraction: • ft) • •«>)-W/fi)ln(1 • «/z(0) s) , where A and B are rate constants and z(0) is initial average particle size. With brine present, A and 4(0) varied linearly with stress. The initial void fraction was also dependent to some extent on the particle size distribution. The rate of consolidation was most rapid In brine and least rapid In the presence yf only air as the fluid. A brine containing 6 m MgC1 2 showed an Intermediate rate, presumably because of the greatly reduced ^ofubHity of NaCt. A substantial «all effect was Indicated by an observed increase in the void fraction of consolidated columns with distance from the top where the stress was applied and by a dependence of consolidation rate on the column height and radius. The distance through which the stress fell by a factor of e was estimeted to change Inversely as the fourth power of the column diameter. With increasing temperature (to 86*C), consolidation proceeded somewhat more rapkRy and the wail effect was reduced. The permeability of the columns dropped rapidly with consolidation, decreasing with about the sixth power of the void fraction. In general, extrapolation of the results to repository conditions confirms the eeif-eeaOng properties of bedded sett as a storage medium for radioactive waste.
To investigate the movement of brine along grain boundaries in pobrrystalline salt, measure ments have been made of the radial flow of brine through the interface between cylindrical salt crystals under axial stresses to 140 bar and temperature to 80°C. For constant conditions, the total flow of brine showed a linen dependence on the logarithm of time, end the reciprocal permeability increased linearly with time. Loss of salt from the interface by pressure solution effects was more than enough to account for the decrease in the apparent thkknes» of the interface (i.e., that which r. ay be estimted for an interface of the same permeability formed by plane parallel surfaces). This apparent thickness, initially as large as 10 /un, decreased to as little as 0.2 pm with exposure to stress and flowing bn.ie. It decreased quickly with sudden increases in axial stress and usually increased, though not reversr'bly, with decreases in stress. The rate of increase in the reciprocal per meability with time was roughly proportional to the stress and to the square of the hydraulic pres sure drop. Assuming similar apparent thicknesses for the grain boundaries in polycrystalline salt, permeabilities are predicted that are quite consistent with the low values reported for stressed core specimens.
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