Analysis of surface leaching processes in vitrified high-level nuclear wastes using in-situ raman imaging and atomistic modeling. 1997 annual progress report
'The objectives of this report are: (1) To investigate the development of Raman spectroscopy for remote, in-situ, real-time measurement of the processes underlying chemical corrosion of glasses, (2) To conduct Raman spectroscopy measurements and quantum mechanical modelling studies of the transition states, corrosion products, and transition state energies for the hydrate species of higher valence and multivalent ions formed in the reconstructed glass surface. (3) To use these results to model long-term corrosion behavior of complex borosilicate wasteform glasses. (4) To apply the Raman spectroscopy and modelling methods developed here for the remote analysis of leaching processes in waste glasses containing radioactive components, and for imaging of variations in leaching behavior due to composition inhomogeneities in large scale waste glass products. Results of First Year Research During the first year, the authors primarily addressed Objective (1) which is to develop a methodology for the remote monitoring of leaching processes in glasses by Raman spectroscopy. The authors assembled a micro and macro Raman system for examining surface structure in glass samples, in-situ within the leaching vessel. The Raman spectrometer was prepared for imaging by installing a CCD detector which gives 2-dimensional information. The latter can be used to obtain spectrographic data in one dimension and to scan variations in materials behavior across the other dimension. By scanning the sample in a perpendicular direction, it is possible to conduct 2-dimensional spectral analysis of the sample surface. The plan is to select one or several identifying Raman lines that follow the leaching process After that, it will be possible to introduce a slit configuration and use both dimensions of the CCD detector for scanning the sample surface while examining only the selected spectral feature. Glass samples consisting of alkali silicates were first examined. The samples were melted in a conventional electric furnace in 100 gr batches and poured into brass molds. The samples were cut and polished under water-free conditions and then immersed under water in petri dishes. By leaving the upper surface uncovered, the authors focused the objective lens of a microscope on the submerged glass surface. Light from an Argon laser operating at 514.5 nm with approximately 100 mW of power was directed to the glass surface at a large angle of incidence (55 ). Raman spectra were collected in a 0.6 m spectrometer with a 1,200 line/mm grating using a supernotch filter to block the laser line. Typical data are shown in the attached figure. Samples of the following compositions were tested: 20% Na,O--80% SiO, and 30% L and O--70% SiO, . The attached figure reports the results from the Li,O-SiO, glass. The results show a significant difference between the unleached and water-exposed samples. This is evident in the relative amplitudes of the Raman peaks at 436 cm{sup -2} and 582 cm{sup -2}. These 2 peaks are associated with vibrations of the alkali coordinated non- bridging oxygen. In fact, the ratio of the 582 to the 436 peaks gives a measure of the alkali content of the material. By examining the sample surface only, and conducting the tests while the sample is submerged, the authors observed a decrease in the 582 cm{sup -1} peak and an increase in the 436 cm{sup -1} peak with increased exposure of the sample to the aqueous environment. This is shown in the attached figure and is clear indication of the progressive dealkalization of the glass surface by water. The peaks can be fitted to Gaussian shapes and the integrated intensity will yield a direct measure of the alkali content of the glass surface. In addition, the angle of incidence of the laser beam can be adjusted to provide more penetration into the sample and yield composition profiles.'