Kinetics of alkali ion exchange of irradiated glasses
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Kinetics of alkali ion exchange of irradiated glasses Michael I. Ojovan, William E. Lee Immobilisation Science Laboratory, Department of Engineering Materials, University of Sheffield, Mappin Street, S1 3JD, UK. E-mail: [email protected] Abstract The kinetics of alkali ion exchange of irradiated glasses were investigated using the structural energy barrier model for ion exchange of glasses. Derived rates of alkali ion exchange depend both on irradiation dose D(Gy) and dose rate q(Gy/s) illustrating that some effects cannot be simulated by external irradiation and require in-situ measurements. Higher D and q lead to increased ion exchange rates. Significant changes occur in the activation energies demonstrating a 4 - 6 times decrease depending on glass composition. Radiation-induced changes are higher at relatively low temperatures and are diminished by increased glass temperature. Numerical estimations show that changes in alkali ion exchange kinetics occur at D far below damaging doses. Introduction The kinetics of alkali leaching from silicate glasses are controlled by the extent and rate of reactions between aqueous solutions and the glass network. It is broadly accepted that the early stage of alkali leaching is controlled by alkali ion exchange whereas the later stages are controlled by network hydrolysis and dissolution [1]. However, the duration of the initial stage may be very long, many hundreds of years depending on specific conditions where network hydrolysis and dissolution is suppressed, such as for silica saturated solutions or at low temperatures. Hence the ion exchange may control the behaviour of alkali silicate glasses, this being the case for a number of glasses, including low radioactivity sodium silicate glasses [2-5]. The role and effect of irradiation (including self-irradiation inherent to nuclear waste glasses) on alkali ion exchange has been the subject of few studies. A particular goal is a model description of ion exchange accounting for radiation-induced effects at relatively small doses and dose rates characteristic of radioactive glasses. McGrail et. al. [4] developed a theoretical model describing quantitatively the kinetics of ion exchange from alkali silicate glasses, e.g. the structural energy barrier model. This model gives for the alkali ion exchange rate (mol/m2s): rx = ω exp(− E x / RT ) ,
(1)
where ω is the exchange attempt frequency, Ex is the energy barrier to ion exchange, R is the universal gas constant and T is temperature. The structural energy barrier model gives an excellent description of the kinetics of alkali ion exchange accounting for changes in composition of alkali silicate glasses and isotope effects [4]. The purpose of this paper is to demonstrate that the structural energy barrier model can be readily used to account for the effects of irradiation on the kinetics of alkali ion exchange. It will be shown, that the rate of alkali ion exchange of irradiated alkali silicate glasses can be determined by the equation: rx = ω exp(− E x / RT ) + ωf (
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