Diffusion of Cesium in Sodium Borosilicate Glasses for Nuclear Waste Immobilisation, Studied by Low-Energy Ion Scattering

Low-energy ion scattering (LEIS) is shown to be a convenient technique for measuring the diffusion coefficient of Cs in sodium borosilicate glass. A 3 keV exp 4 He exp + ion beam if first used to create an alkali depletion layer in the outermost 60 nm of the glass. After annealing, to remove the vacancies from the glass matrix, the return of Cs to the surface is monitored. Diffusion coefficients at 815 and 842K are determined. Both are found to agree, within experimental error, with the values from the concentration couple method. The latter analyzes the Cs profiles by SEM/EDS. The present technique, however, can be applied at lower temperatures, where the concentration couple method would require too long times. This gives the possibility to predict the diffusion coefficient of Cs at actual temperatures in nuclear waste glass cylinders.