Numerical Modeling and Column Experiments to Simulate the Alkaline Disturbed Zone Around a Cementitious Radioactive Wast
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consisted of two reservoirs which contained the reactant fluids kept under a nitrogen atmosphere held at room temperature. These were connected to a series of heated conditioning vessels maintained at the experimental temperature of 70 °C, which were connected via peristaltic pumps to the columns (0.75 cm internal diameter, length 30 cm). The columns were immersed in two constant temperature water baths and held at 70 'C. Reacted fluids were collected and samples taken for chemical analysis. The reacted minerals were subjected to detailed mineralogical analysis on the termination of the experiments. All experiments were of approximately 6 months duration.
N2 Gas
Peristaltic pump
Reactant Fluid
S~Packed
• C
Vessel Conditioning containing Reactant Fluid at 70'C
Water Bath
Collection vessel
Figure 1
Schematic of experimental setup
PREDICTIVE MODELLING The computer code PRECIP [2] has been used to provide 'blind' predictive calculations based upon known experimental parameters and kinetic and thermodynamic data data available from the literature. The rates of mineral reactions are explicitly included by using a kinetic formulation. The conceptual model is of a one-dimensional flow path along which precipitation and dissolution reactions may occur. As the chemical reactions proceed in response to the advection reactions in the flow field, the masses of the precipitates change at each point on the profile with consequent changes in the porosity and permeability. The transport and reaction equations are fully coupled and solved simultaneously. By restricting the calculation to just the major aqueous components the computing times are kept relatively short, allowing the use of refined spatial grids and the simulation of long time periods. The dissolution rates for quartz and albite were taken from Knauss and Wolery [3]. The rate for calcite dissolution was been based on the review work of Rochelle [4] and that for muscovite from Knauss and Wolery [5]. Precipitation rates for all products were set to be at least an order of magnitude greater than that of the mineral dissolution rate, for all conditions likely to be found in the experiments. Thermodynamic data for reacting minerals and potential secondary phases were taken from the literature [6].
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The predictions for the reaction of calcite with both 'young' and 'evolved' fluids showed dissolution to occur only immediately adjacent to the inlet end of the column, with the accompanying formation of small amounts of portlandite. The predictions for reactions of quartz with the 'young' fluid, show quartz being dissolved along the length of the column, though with much more dissolution near the inlet than towards the outlet. Product minerals, however, were restricted to close to the inlet end, hillebrandite forming first and a smaller quantity of tobermorite forming next. Beyond this point, all available calcium was removed from the incoming fluid and products were not predicted to form. There was a large increase in the porosity over most of the column except
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