Dissolution of R7T7 Glass in Static and Flowing Conditions: Influence of Si Diffusion Mechanism in the Leached Layer
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DISSOLUTION OF R7T7 GLASS IN STATIC AND FLOWING CONDITIONS: INFLUENCE OF Si DIFFUSION MECHANISM IN THE LEACHED LAYER F. DELAGE1, F. LARCHE 2 and E. VERNAZ' I2CEA CEN-VALRHO, SCD/SDMC, BP 171, F-30207Bagnols-sur-Cze, France Universitjde MontpellierIA GDPC, F-34090 Montpellier, France ABSTRACT Leach tests with R7T7 nuclear waste glass in distilled water were conducted at 50 and 90'C 1 under static and slow flowing conditions, with an SAN ratio of 50 m- . A computer model for glass dissolution (LIXIVER) Was used to interpret the experimental data. This mechanistic model is based on a combination of the first-order law governing surface reactions, the silicon mass transport equation for the interstitial solution in the alteration film, and an empirical law for partial silicon retention in the alteration layer. The LIXIVER model satisfactorily accounts for most of the experimental results. The importance of the silicon diffusion mechanism in the diffusion layer is stressed. Values are indicated for the apparent silicon diffusion coefficient, 2 2 Dsi, which ranged from 10-16 to 10-14 m .s-1 at 50'C, and from 10-15 to 10-13 m .s-1 at 90'C. INTRODUCTION Glass dissolution involves two potentially limiting stages: the surface reaction at the interface between the glass and the alteration layer, and the transport of dissolved reaction products through the interstitial solution in the alteration layer. Depending on the surface reaction kinetics, the diffusion coefficients of the species and the hydrodynamic conditions at the reaction interface, the dissolution rate may be limited by thet transport, by the surface reaction, or by a combination of both (mixed kinetics). Grambowl l suggested that the glass silica hydrolysis reaction was the limiting step in the dissolution process for borosilicate glasses, although he later[2] noted that silicon diffusion was a non-negligible factor in interpreting the results of flowing leach tests. This paper deals with the influence of the surface reaction and diffusion mechanisms on the overall dissolution process for R7T7 nuclear waste glass at 50 and 90'C. Two approaches are discussed: experimentation and modeling. Dynamic and static leaching experiments were 3 conducted, and the LIXIVER computer calculation codel ] was used to interpret the experimental results. The dynamic tests were intended to investigate the influence of the silicon diffusion on the global dissolution process, and the static tests to provide data for the model. The LIXIVER code is based on mixed kinetics; the formation of the alteration layer is simulated by precipitation of hydroxides and carbonates of insoluble elements, and by partial silicon 4 retention according to an empirical law as a function of the silicon concentration in solution[ l. EXPERIMENTAL PROTOCOLS The experiments were conducted with monolithic R7T7 specimens polished with silicon carbide to grade 4000. The Savillex leaching cells, teflon containers with a unit volume of 3 1000 cm , were placed in a controlled-temperature chamber at 50 and 90'C. The
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