R7T7 Nuclear Waste Glass Behavior in Moist Clay: Role of the Clay Mass/Glass Surface Area Ratio
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R7T7 NUCLEAR WASTE GLASS BEHAVIOR IN MOIST CLAY: ROLE OF THE CLAY MASS/GLASS SURFACE AREA RATIO
N. GODON and E. VERNAZ CEN-Valrh6, SDHA, BP 171, 30205 Bagnols-sur-C~ze Cedex, France
ABSTRACT R7T7 glass alteration was investigated in the presence of various moist clays. In contact with smectite 4a, selected in France as a potential engineered barrier material, the glass was significantly corroded: after 6 months the glass corrosion rate was practically the same as the initial alteration rate in double-distilled water. Substantially lower alteration was observed in contact with bentonite 6 activated by sodium carbonate. Smectite 4a consumes silicon released by glass corrosion, and thus retards the rise to high silicon concentrations in solution at which the glass corrosion rate diminishes. Glass can therefore in some cases be altered as much in moist clay as in water with high renewal rates. Other experiments with smaller quantities of smectite 4a showed that the phenomenon becomes less important in time: its duration is proportional to the ratio between the clay mass (C) and the glass surface area (SA). Comparing the results of studies at different C/SA ratios indicates that low glass corrosion rates are obtained more slowly at higher C/SA ratios. Tests with 2-"9Pu-doped R7T7 glass also showed that the radionuclide retention factor in the alteration film at the glass surface is only 6, compared with a factor of nearly 50 in double-distilled water. INTRODUCTION Previous experiments1 1' 21 revealed the decisive role of the type of clay in determining the R7TF7 glass alteration rate. All of these experiments were performed with a large water volume in
contact with a small volume of clay in order to ensure satisfactory permeability and homogeneity. This situation, however, does not correspond to the granite repository scenario currently under consideration in France, in which a large clay mass would be packed between the glass and bedrock to ensure impermeability. In the event of water ingress, the glass would thus be in contact with a large mass of moist clay rather than a liquid leachant. Under these conditions, will the locally low SA/V ratio be favorable to the glass or, on the contrary, will the large clay mass impose high dissolution rates? In order to answer this question, the authors investigated the alteration of R7T7 glass specimens in contact with three types of moist clay over periods of up to one year at 90°C. EXPERIMENTAL PROTOCOL The experiments were conducted in 50 cm 3 stainless steel vessels maintained at a temperature of 90°'C by a circumferential heater. The temperature inside the vessel was monitored by a thermocouple in a thimble penetrating through the cover (Figure 1). Dry clay was placed in the leaching vessel with a 2.5 x 2.5 ×0.2 cm glass test coupon at the center. The specimen surface finish was saw-cut and the total surface area was about 14.5 cm 2. The vessel was then hermetically sealed, supplied with Volvic mineral water (composition: Table I) and pressurized to 10 MPa with a hydropneu
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