Introduction and overview of problems related to the gel

During alteration of most nuclear glasses in aqueous media, an alteration film or gel forms on the glass surface. This gel is characterized by 2 significant properties: -1) Its role as a diffusion barrier, the apparent silicon diffusion coefficients in the interstitial water of the gel, derived by fitting the curves for R7T7 glass leaching experiments, are very low: in the order of 10-16 m2.s-1 at 90 Celsius degrees. These values assume very small gel pore dimension (a few nanometers) and strong silicon adsorption on the gel. -2) Its significant retention capacity for some radionuclides such as rare earth elements and actinides, which significantly diminishes the flow of radioelements released from the glass package. A lot of experimental and more theoretical studies are needed to understand nuclear gels as material in order to model them and estimate their evolution over long periods of time. (A.C.)