A series of tests have been conducted to evaluate the leaching of glass kept in contact with sea sediments. Some tests were also conducted using sediment loaded with iron corrosion products. The mass loss rate, even after 240 days, was constant No specific effect of the iron corrosion products was observed. When the glass is degraded, the elements comprised in the matrix enter solution and migrate through the sediments. An analysis of the sediment column has been performed and the distribution of some elements has been obtained in order to evaluate the diffusion coefficient and the Kd value The presence of iron corrosion products decreases the diffusion coefficient of uranium.
The technical feasibility and environmental acceptability of disposing of high-level nuclear waste in geological formations under selected zones of the oceans has been investigated by the Joint Research Centre in the framework of the International Seabed Working Group of the NEA.
In particular, studies have been conducted on the leaching of borosilicate glasses containing simulated high-level radioactive wastes in contact with seabed sediments The tests have been performed simulating where possible the conditions existing in the sediments around the canister (near field).
A very simple case for the near field has been considered, that is that the glass is contained directly in the corrosion resistant canister which is in contact with the seabed sediments.
It should be noted that no data exist in the literature on the speafic effects of the seabed sediments and that very few data exist even on the leaching of glasses by seawater. As a consequence it was decided firstly to investigate systematically the leaching of glass in static seawater (Lanza et al, 1987)
These tests have shown that the leaching of the borosilicate glass evolves in time. In an initial period, direct dissolution is prevalent and the degradation of the glass increases linearly with time. The duration of this period decreases with increasing temperature. In a second period, the Increase of silica concentration in solution, which facilitates the reprecipitation of some compounds, begins to play a major role and, as a consequence, the degradation rate decreases with time An analysis of the surface layer of the samples corroded at 90 OC has detected the presence of magnesium of about 5 at% and an equal amount of aluminium, suggesting that, after the initial period, the degradation is controlled by the formulation of complex aluminium-magnesium silicate J H. Thomassin (1984) in his study of the surface degradation of basaltic glasses, was able to identify complex Mg-A1 silicates in the surface layer Unfortunately, the layer obtained in our test was so thin that no X-ray analysis was possible. For the definition of the near field conditions we will assume that the glass comes into contact with sediments after the complete corrosion of the canister Usually, two possibilities are considered for the canister material a material, such as for instance Ticode 12, having a high resistance to corrosion due to the formation of a very stable oxide, or a material, such as mild steel, which corrodes homogeneously.