A Justification for the USE of Data from Accelerated Leach Tests of Glass
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A JUSTIFICATION FOR THE USE OF DATA FROM ACCELERATED LEACH TESTS OF GLASS TAE M. AHN, CHARLES G. INTERRANTE, AND RICHARD A. WELLER U. S. Nuclear Regulatory Commission, Washington DC 20555
ABSTRACT A case is made for the use of short-term laboratory data in making predictions on the likelihood of significant colloid formation in supersaturated leachates of glass, under long-term repository conditions, using "accelerated tests" with a large ratio of the surface area of the glass to the leachate volume. In the repository conditions in which colloids can form, long-term leaching may be a kinetically-controlled process that involves the continuous formation of colloids. If this kinetic process dominates, it could lead to a significant increase in the predicted rates of radionuclide release. The question is whether or not colloids may form after prolonged times; the delayed formation would make it difficult to use short-term laboratory test results to represent (or predict) the long-term and cumulative effects of radionuclides. In this work, the pertinent long-term kinetic processes are identified in part. Classical nucleation theory for particle formation, as a potential condensation mechanism for colloid formation, is applied to explain pertinent experimental data on colloid formation. The classical theory, which is justified for this discussion, indicates that as supersaturation of a leachate is decreased, the nucleation rate decreases most significantly, while the incubation time increases at a small rate. As a result of this decreased nucleation rate, the significance of colloids tends to vanish, and usefulness of data from "accelerated" laboratory tests may be applicable to long-term behavior. INTRODUCTION When compared with dissolved ions, colloids, polynuclei or species " are important vehicles for transport of released radionuclides because they can be charged negatively. A negative charge can facilitate their migration through negatively charged host rocks. Colloids are known to form in many different ways, either by a condensation process or by a dispersion process [1]. Laboratory tests of High-Level Waste (HLW) glasses indicate colloids form by both processes [2-5]. In earlier work in static tests [2,3], colloids formed in the leachate could be interpreted to have formed adjacent to or far from the glass surface by a nucleation process, which is a condensation process. Alternatively, colloids could have formed by a mechanical spallation (or erosion) of altered layer on the glass surface and that would be termed a dispersion process. The indications from those earlier data [2,3] do not exclude the likelihood of one of these processes or both. Data without agitation in static tests [2,4] appear to come from nucleation in the condensation process, whereas dripping water tests [5,6] likely involve both the dispersion and the condensation processes. In the present work, the nucleation in the condensation process is considered as the mechanism for colloid formation. When colloids form from supersaturated condition
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