Structural characterization of sulphate and chloride doped glasses for radioactive waste immobilisation

Abstract

Silicate and borosilicate glasses are of great importance for both the commercial and nuclear waste glass industries. Anionic species such as chloride and, more famously, sulphate are known to play a role in melting, fining and homogenization of commercial silicate glasses. At the same time, these anions can be present in abundance in several kinds of nuclear wastes destined for confinement in a multi-oxide borosilicate glass matrix (the most widely employed glass for radioactive waste immobilisation). Beyond a certain extent, it is difficult to incorporate the anionic species in commercially used radioactive waste borosilicate glasses due to their poor solubility in the molten glass and this can limit waste loading in the final waste forms, thus increasing the waste volumes and having substantial logistical and economic impacts. Besides the technological and economic repercussions faced by the waste vitrification facilities, the undissolved sulphates that precipitate as a water-soluble salt layer, are often enriched in 137Cs, 99Tc and 90Sr and can dissolve in the groundwater in geological waste repositories which raises environmental concerns. Similarly, the pyrochemical reprocessed chloride containing waste contains significant quantities of actinides such as 239Pu and 241Am which have low solubilities in the borosilicate waste glasses. Therefore, in order to formulate glass compositions with enhanced anionic capacities, a careful investigation is required to determine the anion solubility factors in such glasses. Since the structural characterisation of the industrially used borosilicate glass compositions is challenging due to the presence of multiple elements in the glass, we have designed a simplerapproach by developing binary and ternary silicate and borosilicate glasses with and without the anions added. The key focus of the thesis is to look at largely sulphate capacities of two glass systems: silicate and borosilicate glasses. Thorough spectroscopic investigation of chloride doped glasses has not been carried out due to insufficient levels of chloride retained in the two glass systems and hence makes just a side-study for the research. The undoped and sulphate doped glasses are subjected to comparative compositional and structural investigation using a range of techniques including density measurements, XRF, ICP, XRD, MAS - NMR, Raman spectroscopy, ND, and DTA, SEM - EDS, and Mossbauer spectroscopy. This thesis mostly elucidates the influence of different essential modifier oxides that are either already present or can be added / removed in the industrial waste glass compositions to enhance sulphate solubility. Density measurements show that incorporation of sulphate slightly decreases the densities of the doped glasses. XRF, ICP, SEM-EDS analyses identify the dependence of sulphate capacity on glass composition. Raman spectroscopy and 29Si MAS-NMR techniques provide measures to link sulphate capacity with structural changes in the Si/O and B/O/Si network. 23Na, 7Li MAS-NMR, though less informative, suggest changes in modifier environment with sulphate addition. Raman spectroscopy of sulphate doped glasses shows distinct bands corresponding to S-O stretching modes in silicate and borosilicate systems. The centre position of these modes is compared with S-O modes for corresponding crystalline sulphates to establish the cation environment around the sulphate ions. The increase in the degree of polymerisation of the glass network upon sulphate addition is consistent between silicate and borosilicate systems, with only a few exceptions. Neutron Diffraction is employed to study the change in coordination environment of the cations and anions in the glass upon sulphate addition. The results were collectively put intodifferent empirical models proposed to study sulphate solubility and checked for the models’ versatility. Simulated sulphate doped, multi-oxide borosilicate glasses which are broadly representative of industrial U.S. HLW / LAW waste glasses, are prepared and modified as per our investigations on simpler glasses, to study changes in its sulphur solubility. Additionally, we have also considered the effect of V2O5 addition on sulphate incorporation in the simulated radioactive waste glasses.