Modeling of Glass Dissolution and Transport With the Code SUGAR
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ABSTRACT SUGAR (Simplified Understading of Glass And environment Reactions) is a one dimensional (iD) reactive transport code which sequentially couples dissolved chemical species transport and sorption with, chemical speciation, mineral precipitation and kinetic controlled glass dissolution. SUGAR also has the capability to model radioactive decay and steel corrosion. This paper describes the computer model and presents results of model testing against experimental data. The capabilities of the model are demonstrated by a preliminary investigation of the geochemical evolution of glass in crystalline rock formations.
INTRODUCTION The dissolution behaviour of borosilicate glass has been studied extensively by various batch and flow leaching experiments and surface characteris~ition techniques to investigate the mechanism of glass dissolution under varying fluid composition conditions. This has led to a generalised kinetic model of glass dissolution [1] based broadly on transition state theory [2] where dissolution rate is a function of the solubility of a silica controlling phase representing the hydrated gel layer formed on surface of the leached glass. The main parameters in this model are: dissolved silica activity, pH, and diffusion in the surface hydrated gel layer. The modelling of long-term dissolution of borosilicate glass is more problematic; experimental evidence shows that dissolution rate never approaches zero owing to the thermodynamically unstable nature of glass. Long term dissolution rate is a complex function of the crystallisation of secondary phases formed on the surface of the glass and diffusive limited transport of dissolved species through the surface alteration layer. In addition to these chemical controls on glass dissolution both short-term and longterm dissolution will be controlled by the effects of fluid flow [3], since in an open system fluid composition will be a net result of the rate of species addition by chemical reaction and the removal of a species by advection and dispersion/diffusion. Modelling the combined effects of chemical reaction and transport can be achieved through various strategies; ideally all the kinetic dissolution, chemical speciation, mineral precipitation, transport/sorption and radioactive decay equations should be solved simultaneously [4]. In practice however limitations of cpu time necessitate that transport and chemical reactions are considered successively at each time step. In this paper we describe the reaction-transport model SUGAR which simulates borosilicate glass dissolution sequentially coupled with 1D transport. Ini order to justify the use of such partial coupling SUGAR has been tested against both analytical solutions to simplified processes and against glass leaching experiments. 253 Mat. Res. Soc. Symp. Proc. Vol. 506 ©01998 Materials Research Society
MODEL DESCRIPTION The equation for the transport and chemical reactions modelled by SUGAR which describes the change in concentration of a species is:
act-
=
D(
Ox
cqD- D
d c) .x-
- Pc +
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