Simulating Radio Element Release and Water-Rock Interactions During Dissolution of Borosilicate Glass
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and associated release of radioelements. As borosilicate glass dissolves, an altered region forms at the surface, comprising a mixture of leached glass and secondary reaction products. Precipitation or sorption of radioelements within the altered region at the glass surface may significantly retard transport away from the wasteform. A demonstrated capability to model these processes over long timescales may be necessary to support a performance assessment of HLW repositories. In this paper, a combined glass dissolution and chemistry model is presented which can be used to assist in the interpretation of experimental studies of borosilicate glass, and to support the development of a conceptual model for the behaviour of glass in a HLW repository. DESCRIPTION OF THE MODEL Glass dissolution The AEA Technology code, GLAD, uses the approach based on the Grambow rate equation [1] to model glass dissolution. In general terms the mathematical model employed in GLAD is of the form, =
aiR + Req
where, c, is the aqueous concentration of element i, cxiis the mole fraction of the element i, R is the rate of dissolution of the solid in question and Req represents the other chemical processes of the system. If the release of element i is controlled by the rate of dissolution of the solid, 391
Mat. Res. Soc. Symp. Proc. Vol. 506 01998 Materials Research Society
where k0 is the initial leach rate of the solid, Q(t) is the activity product for the dissolution of the bulk solid material and K is related to the solubility product of the material and the long-term rate of leaching. This approach links the rate of glass dissolution to the aqueous concentration of the H 4 SiO 4 species. At short times, dissolution is relatively rapid and the aqueous concentration of H4 SiO 4 in the leachate increases sharply. As dissolution proceeds, the H 4 SiO 4 concentration becomes constant (glass 'saturation' is reached). Following 'saturation', the glass continues to dissolve but at a much reduced rate. Thus, to parameterise the model, both a short-term and a long-term dissolution rate are required. It is assumed that all components present within the glass dissolve
at the same rate. The H 4 SiO 4 concentration at glass 'saturation' is estimated with reference to the measured concentration of silica in the leachate solution after long times (making an appropriate correction for the aqueous speciation of silica at the measured pH and temperature). The software can also consider cases where the dissolution process is controlled by diffusion of material through the altered region that forms at the glass surface over time. However, in the calculations presented here, it is assumed that this diffusion is relatively fast and that the kinetics of the dissolution reaction limit the dissolution rate. Water-rock interactions GLAD is linked to the geochemical modelling code, HARPHRQ, so that mineral reactions can be modelled in detail during the calculation. HARPHRQ is a thermodynamic equilibrium code [2] which is based on PHREEQE [3]. Using HARPHRQ, it
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