Apparent Solubility Limit of Nuclear Glass
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Apparent Solubility Limit of Nuclear Glass Étienne Vernaz and Stéphane Gin Commissariat à l’Énergie Atomique (CEA/Valrhô), DCC/SCD/LEAM BP 171, 30207 Bagnols-sur-Cèze Cedex, France ABSTRACT Most nuclear glass alteration models are based on a first-order kinetic law of the general type r = r0 (1−Csi/C*) or r = r0 (1−Q/K). It is generally assumed that the establishment of quasi steady-state concentrations in solution corresponds to an intrinsic glass solubility limit that may be expressed either in terms of the dissolved silicon concentration C* or as an ion activity product at saturation K. Experimental research over the last five years in France has shown that the quasi steady-state concentrations observed in solution do not correspond to a thermodynamic solubility limit with respect to the glass. This assertion is supported by a large body of experimental evidence that is discussed in this paper. The signification of the C* parameter is therefore reconsidered, and its implications on long-term behavior modeling are discussed. EVOLUTION OF THE APPARENT SOLUBILITY C* OVER TIME In most static experiments with borosilicate nuclear waste glass samples, the silicon concentration in solution is observed to rise to a quasi steady-state value that is reached after a time that depends on the glass-surface-area-to-solution-volume (S/V) ratio and is generally considered to be the glass solubility limit. The glass alteration rate as measured by a tracer element such as boron (which is not retained in the gel layer) is then observed to drop considerably [1,2]. This major drop in the alteration rate was long associated with the onset of a thermodynamic solubility limit and modeled by a first-order law [3]. During experiments at moderate S/V ratios (typically not exceeding 2000 m-1), the final evolution of the boron concentrations over time became nearly unmeasurable. Under these conditions, the evolution of the dissolved silicon concentration - much slower than that of boron because of silicon retention in the gel as saturation conditions were reached - was no longer detectable, suggesting that a solubility limit had been reached. During an 800-day experiment with SON68 (R7T7-type) glass at 90°C at a very high S/V ratio (200 000 m-1), the Si and B concentrations in solution were observed (Figure 1) to increase in a virtually linear manner between 28 and 800 days, resulting in constant alteration rates under these conditions of 2.0 x 10-4 g·m-2d-1 for boron and 4.3 × 10-6 g·m-2d-1 for silicon. The difference in the Si and B release rates corresponds to a silicon retention factor of some 98% in the gel, a value consistent with high silicon concentrations in solution [4]. The Si concentration in solution rose from 136 ppm after 28 days to 268 ppm after 800 days. Identical results were measured in solution before and after ultrafiltration, confirming that this was indeed dissolved silicon and not colloidal forms. No significant pH variation was observed: the pH remained constant at 9.44 ± 0.05 between 28 and 800 days.
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