Hydrothermal Leaching of R7-T7 Borosilicate Glass
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HYDROTHERMAL LEACHING OF R7-T7 BOROSILICATE GLASS
J. Caurel*, E. Vemaz* and D. Beaufort** * SDHA/SEMC, CEN-ValrhO, BP171, 30205 Bagnols-sur-C~ze Cedex, France, ** Laboratoire de P~trologie des Alterations Hydrothermales, Universit6 de Poitiers,
40 avenue du recteur Pineau, 86022 Poitiers Cedex, France. ABSTRACT The results of hydrothermal leach tests are intended to be used to predict long-term low-temperature glass dissolution. It is often assumed that data can be extrapolated to other conditions using an Arrhenius-type equation. Hydrothermal leaching mechanisms and their temperature dependence in R7T7 glass were investigated in static experiments lasting from 7 days to 1 year at 150'C and 250'C. Leachates, surface layers and crystalline products were analyzed by ICP, TEM, SEM, EMP, XRD and cathodoluminescence. Unexpectedly, no actual saturation conditions in solutions were reached after one year leaching at 150'C nor at 250°C. The effect of precipitation of alteration products (a silica-enriched amorphous layer and aluminosilicates [smectite at 150'C, smectite and zeolites at 250°C]) is discussed. However, the formation of large cracks in the bulk glass results in a higher glass reacting surface and a higher dissolution rate at 250'C. Arrhenius calculations cannot be used to extrapolate our hydrothermal data to lower-temperatures. INTRODUCTION The long-term behavior of nuclear waste glass is studied through short-term experiments and predictive models of glass dissolution result. Hydrothermal leaching experiments have been investigated as a possible way to increase dissolution rates. Hydrothermal experiment results could be used for predictive modelling, if the dissolution mechanisms do not change with temperature. PRELIMINARY STUDIES Preliminary hydrothermal leaching experiments with R7T7 (French nuclear waste glass composition [1]) were carried out by Fillet [1] and by Vemaz and Dussossoy [2]. Fillet [11 evaluated the R7T7 corrosion kinetics at 100, 150, 200, 250 and 300'C, in double-distilled water, between I and 28 days (static leaching apparatus, SA/V= 5 m-1, P=10 MPa): the dissolution rates were 0.06 g.m- 2 .d- 1 at 100'C, 0.64 g.m- 2 .d- 1 at 150°C, 1.7 g.m' 2 .d" 1 at 200'C, 3.6 g.m- 2 .d" 1 at 250'C and drastically increased at 300'C. The calculations are based on normalized mass loss for boron, which has the highest release rate at all temperatures. Vernaz and Dussossoy [2] performed R7T7 leaching experiments in Volvic water (chemical composition of Volvic is given in L2]) at 150 and 250'C, between I and 84 days (static leaching apparatus, SA/V=350 mi , P=10 MPa). The main results for both experiments are: - high dissolution rates even after 28 days in double-distilled water or 84 days in Volvic water: Fillet considers that rates are constant between 3 and 28 days, - the alteration products are: the gel layer covering the glass; from 100 to 200*C, a lamellar smectite-like structure (the mineral could not be identified by XRD because the number of stacked sheets in its structure is too low); at 250*C,
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