The Role of Water Diffusion in the Corrosion of the French Nuclear Waste Glass SON 68 under Solution Saturation Conditio
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Karine Ferrand1,Abdesselam Abdelouas1, Bernd Grambow1 and Jean-Louis Crovisier2 1 SUBATECH (UMR 6457), 4, rue Alfred Kastler, B.P. 20722 44307 Nantes cedex 3, France. 2 Ecole et Observatoire des Sciences de la Terre, Centre de Géochimie de la Surface - CNRS (UMR 7517), 1, rue Blessig, 67084 Strasbourg Cedex, France. $%675$&7 The alteration kinetics of the French borosilicate glass SON 68 have been investigated in a dynamic system at 50°C and 90°C under solution saturation conditions. The pH was adjusted to 4.8, 7.2 and 9.8 with addition of chemical buffers or/and by bubbling CO2 in solution. In all experiments, Li and Cs leaching seems to be controlled by a diffusion process. The Li- and Csconcentrations were used to calculate the sum of ionic exchange and matrix dissolution rates of the glass while Mo-concentrations indicate matrix dissolution. The final leaching rates in saturation condition of Mo were in the order of 10-4-10-5 g.m-2.d-1 in good agreement with those given in literature for static tests. The glass surface was studied by scanning and transmission electron microscopy (SEM, STEM) for analysis of the corrosion products and by infrared spectroscopy (FTIR) for water speciation and concentration. A good inverse correlation between the water content and the alkali concentrations released from the glass has been obtained. About three hydrogen ions replaced one alkali ion. Modeling of the experimental data using GM 2001 model gives water diffusion coefficients between 10-20 and 10-22 m2.s-1. ,1752'8&7,21 The alteration of nuclear waste glasses by groundwater is the principal source for radionuclide release from this waste matrix. A lot of work was realized since 35 years to characterize this alteration process. Principal results are that in contact with glass, water diffuses and ionic exchange occurs between protons and network-modifying alkali metal ions. Then, the hydrated silicate network dissolves congruently and a partially gel-like surface layer of alteration products is formed. Glass constituents which are not incorporated into this surface layer are released congruently into the aqueous phase. Accumulation of dissolved glass constituents, mainly of silica, leads to a decrease of corrosion rates with time, indicating either affinity controlled glass corrosion or solution stabilized protective layer formation. The first order rate law is used since 1984 to predict long-term corrosion of nuclear glasses but recent work indicated that there might be problems with its applicability. Jegou [1] has shown that corrosion rate decreased only by a factor 5 and not by 5000 as predicted by the first order law when pristine glass or prealtered glass had been corroded in static tests at 90°C in an over-saturated solution. Decrease in corrosion rate would then be due to a protective alteration layer formation. Today, the r(t) model currently developed by CEA [2] is based on this conc
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