Thermodynamic Modeling of Cement/Groundwater Interaction as a Tool For Long-Tern Performance Assessment
- PDF / 825,999 Bytes
- 9 Pages / 420.48 x 639 pts Page_size
- 88 Downloads / 196 Views
THERMODYNAMIC MODELING OF CEMENT/GROUNDWATER INTERACTION AS A TOOL FOR LONG-TERN PERFORMANCE ASSESSMENT
kOUISE J. CRISCENTI* AND R. JEFF SERNE* Pacific Northwest Laboratory, P.O. Box 999, Richland, WA 99352 ABSTRACT The chemical behavior of cement in the natural environment is of interest because cement has been suggested as a means of containing low-level radioactive waste and as a barrier between radioactive-waste containers and the surrounding soil or rock. In the past, either experiments or computer modeling have been used independently to predict how cement will leach in the natural environment. In this study, a geochemical model for cement leaching was developed, and predictions by the model were compared to results from a static leach test. Calcium concentrations and pH were adequately predicted by the model; however, other major leachate constituents, such as Al, Si, and SO4 , were poorly predicted. Additional experimental data and refinement of the model are both required to better predict the observed leachate concentrations. Accurate prediction of experimental results by a geochemical model would increase confidence in the model for use in long-term performance assessment. INTRODUCTION The chemical behavior of cement in the natural environment is of interest because cement has been suggested as a means of containing low-level radioactive waste and as a barrier between radioactive-waste containers and the surrounding soil or rock. Depending on the purpose, two approaches have been taken to examine the interaction of cement and water. Researchers interested in the possibility of containing low-level radioactive waste in cement have conducted leaching experiments on cement waste forms and have measured empirical or effective diffusion coefficients from these experiments [1-4]. They have formulated several diffusion-based mass-transport models to describe how constituents leach from the waste forms. A wide range in effective diffusion coefficients for various constituents has been observed because the effects of diffusion, adsorption, dissolution, and precipitation are lumped into a single coefficient for each constituent. Although an effective diffusion model explains the laboratory data satisfactorily, the lack of information about the relative importance of each mechanism involved in the leaching process limits the usefulness of such a model for long-term performance assessment. Researchers who are interested in the use of cement as a barrier between radioactive waste containers and the environment have started to use equilibrium geochemical models to evaluate the long-term chemical stability of cement under repository conditions [5-7]. To completely describe cement alteration through time, it may also be necessary to consider such factors as the rates of dissolution and precipitation of the solid phases in the cement and the rate of diffusion of each constituent through the cement to the surrounding environment. However, because a steady-state or equilibrium condition between the cement and the environm
Data Loading...
