Effects of MgO on the Short- and Long-Term Stability of R7T7 and M7 Nuclear Waste Glass in Aqueous Media
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EFFECTS OF MgO ON THE SHORT- AND LONG-TERM STABILITY OF R7T7 AND M7 NUCLEAR WASTE GLASS IN AQUEOUS MEDIA T. ADVOCAT 1 , J.L. CROVISIER 2, J.L. DUSSOSSOY 1 and E. VERNAZ 1 ICEA CEN-VALRHO, Marcoule, BP 171, F-30207 Bagnols-sur-Chze,France 2 CGS-CNRS, 1 rue Blessig, F-67000 Strasbourg,France
ABSTRACT Experiments were conducted in initially pure water at 90'C and 100°C with glass specimens enriched in MgO and in alkali metals relative to the standard light water R7T7 reference glass. Three experimental protocols were implemented: Soxhlet testing with leachant renewal, static leaching according to a modified MCC-1 method with an SA/V ratio of 50 rn-1, and static leaching with glass powder for an SA/V ratio of 8000 rn-. The results clearly show that the short and, especially, the long term dissolution rates depend on the initial glass composition. Higher MgO, Na 20, Li 20 and B20 3 concentrations reduce the glass resistance to aqueous corrosion, as already indicated by the calculated hydration energy values. These experiments illustrate the importance of the glass composition in insuring long-term material integrity. INTRODUCTION The French "R7" vitrification unit at La Hague has been used since June 1989 to solidify fission product solutions produced by reprocessing spent LWR fuel[]. The chemical composition of the reference borosilicate glass designated SON 681817L1C2A2Z1 (or reference"R7T7") was defined for this industrial facility by specifying a range of oxide concentrationst 21 to allow for the inherent composition variations in fission product solutions and to ensure process flexibility, notably with regard to the molten glass temperature and viscosity. The permissible limits for R7T7 composition variations were defined by Pacaud et al. 121 following an experimental determination of the physical properties (density, viscosity, crystallization density, glass transition temperature) of a set of glass specimens, and short-term measurements of their aqueous corrosion resistance (28-day tests under Soxhlet conditions). The long term behavior in water of glass packages containing fission products intended for disposal in geologic repositories is a crucial problem, but has only been investigated experimentally in detail for the reference R7T7 glass. The research program described here was intended to extend this investigation, notably by quantifying the impact on the aqueous corrosion process of variations in the magnesium oxide content (from 0 to 5 wt% of oxide) in R7T7 glass and in a less viscous variant, M7 glass. The investigation concentrated on four major points: a phenomenological description of glass dissolution; the quantity of material dissolved; the dissolution rate laws*; and the nature and role of alteration products on the kinetics of glass corrosion. EXPERIMENTAL RESEARCH Materials and Methods Six glass compositions (Table I) were prepared by mixing oxides, carbonates and nitrates with the glass, heating to 12(X)°C in a platinum crucible and refining for 3 hours. The glass was cast into cylindrical graph
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