French SON 68 nuclear glass alteration mechanisms on contact with clay media

The leaching behavior of the nonradioactive French SON 68 (R7T7-type) nuclear glass was investigated in the presence of FoCa7 clay. Static experiments were carried out at 90 degreesC under conditions favorable to convective transfer. Three test environments were compared: a clay medium, the same clay medium doped with 2.5 wt.% silica gel, and pure water. These experiments showed that in raw clay the glass alteration rate remained near the initial value for several hundred days, whereas in pure water at the same temperature and with the same glass-surface-area-to-solution-volume (S/V) ratio the alteration rate quickly dropped by several orders of magnitude after the formation of a protective gel layer. This major difference between the two media can be attributed to transfer of the principal gel-forming elements (Si, Al, Ca) into the clay medium as a result of sorption phenomena and probably the precipitation of silicate phases. The addition of silica gel to the clay not only neutralizes the clay sorption capacity, but also protects the glass from significant alteration (the glass was altered appreciably less than in pure water). This article discusses the mechanisms limiting the glass alteration kinetics in the test media. The gel that forms by reconstitution of hydrolyzed glass species at the glass/clay interface is shown to constitute a diffusion barrier whose effectiveness depends to a large degree on the conditions under which species are removed by the external medium. Interpreting the experimental data from this standpoint leads to a reexamination of the classic model in which the glass alteration kinetics are inhibited by the dissolved silicon content. (C) 2001 Elsevier Science Ltd. All rights reserved.