Abstract:
An improved filtration set-up has been designed for the experimental simulation of solution-rock interaction in a flow regime within a wide range of temperature (20-300°C), pressure (1-100 MPa), and solution pH (2-14). The independent control of pressure on the solution and on the rock allows one to model the conditions of isotropic confining pressure (σx, = σy, = σz) and transverse isotropic stress (σx = σy ≠ σz). Two experiments were conducted on the filtration of Na-bearing alkalic solutions through a sandstone sample from a monitoring borehole at the area of underground disposal of liquid radioactive wastes from the Mining Chemical Plant. The experimental were done at a temperature of 70°C, a pressure on the sample of 6.5 MPa, a solution pressure of 3.5 MPa, and a filtration rate of about 1.5 cm3 per day. The initial solution of the first experiment (2-BG) contained 1.18 g/l Al, and that of the second experiment (3-BG) was Al-free. Changes in solution composition were observed in the course of filtration in both experiments. The maximum changes occurred during the first 15-20 days of the experiments. After this time a steady-state solution composition was established in run 2-BG, whereas such a state was not evident in run 3-BG. Changes in the mineral composition of sandstone were much more extensive in run 3-BG, where a massive zone cemented by abundant newly formed zeolite was formed in the frontal part during the experiment. Mineral changes in run 2-BG were negligible and included mainly mica chloritization and hydration. The variations in mineral composition were directly correlated with changes in sample permeability: it slightly increased in run 2-BG and significantly decreased in run 3-BG with increasing experimental duration. Both the experiments showed an increase in Sr content in the solution at the filtration front, which was related to the undersaturation of the initial solutions in Sr. Under the experimental conditions, the partition coefficient of Sr between rock and solution was 2-3. Although the total concentration of Sr in solution increased, the amount of 90Sr must decrease at the expense of isotopic exchange with the rock. Isotopic exchange is probably the most efficient mechanism of the purification of liquid radioactive waste.