Relationship Between Glass Leaching Mechanism and Geochemical Transport of Radionuclides
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RELATIONSHIP BETWEEN GLASS LEACHING MECHANISM AND GEOCHEMICAL TRANSPORT OF RADIONUCLIDES
A. AVOGADRO and F. LANZA Commission of the European Communities Joint Research Centre - Ispra Establishment 1-21020 Ispra (Va), Italy
When the leaching of glasses is taken into consideration in risk analysis models,
it is necessary to make a distinction between accidental and nor-
mal conditions in the repository. In accidental conditions, it is assumed that the glass will be leached by ground-water which can also be renewed. The composition of this water will be related to the aquifer existing around the repository. Under normal repository conditions the leaching process has to be analyzed separately for each geological formation considered. In a salt repository the brine migrates, due to the thermal gradient, towards the conditioned waste. The leaching of the glass will correspond then to a leaching in a closed system evolving towards saturation. In a hard rock system water flows slowly through the fracture network. The water around the glass will then be slowly renewed and only near saturation conditions will be reached. In a porous system, like clay or marine sediments, water is practically stagnant and transport could only be due to diffusion of the soluble species. It is possible that, due to Brownian motion, even microcolloids could diffuse. It is known that in a closed system the leaching of glass tends to decrease as the solution approaches saturation reaching eventually a zero I or near zero value . In the two other formations considered, a reduced transport exists due to diffusion in the interstitial water or to water flow in fractures which does not allow to attain complete saturation. The leaching rate will then depend upon the types and rate of transport of the dissolved species. Moreover, the transport will depend upon the physico-
104 chemical characteristics of the different species as well as on their interaction with the geological material; it is obvious therefore, that each element has to be considered separately. In this paper an attempt has been made to subdivide the different elements present in the vitrified waste into classes in order to simplify the description of the release of the radioelements in the various repository conditions. In the release of the different radionuclides from borosilicate glasses, a prominent role is played by the leaching rate of silicon. Silicon dioxide represents the skeleton of the glass matrix and its leaching rate establishes the rate of degradation of the glass. The leaching of silicon will be limited in a closed system by the saturation concentration of dissolved silica. Amorphous silica has a relatively high solubility in pure water, reaching 0 2,3 at 80 C a value around 300 ppm . Silica dissolves in pure water in a non-ionic form. However, when alkalies are present in the solution, even silicate ions will increase the solubility of silica; such an effect is reported to be effective already at pH 8-9 3 . It is usually assumed that an increase of the concentra
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