The incorporation and solubility of sulphate, chloride and molybdate anions in borosilicate and aluminosilicate glasses

This thesis investigates the incorporation and solubility behaviour of three anionic species (sulphate, chloride and molybdate) in two different types of glasses (borosilicate and aluminosilicate glasses). These anions can be often found in nuclear waste and their poor solubilities in nuclear waste glasses are a main factor that controls the loading capacity of nuclear waste vitrification. The investigations in this thesis are therefore focused on the compositional dependence of their solubilities in glass, together with the effects of their incorporations on glass structure and properties. A variety of glass properties have been assessed. Glass densities steadily increased with increasing incorporation of sulphate and molybdate but showed maxima with chloride incorporation. Glass transition temperatures Tg all decreased with initial anionic loadings, whereas further loadings results in either decreased or unchanged Tg depending on anionic species and glass composition. Intense Raman peaks are created due to sulphate and molybdate additions; these characteristic peaks are assigned to the vibrations of SO42– and MoO42–, respectively. The shift of these peaks with variation of alkaline earth species in glass suggests the association of SO42– and MoO42– with alkaline earth cations in glass network. The incorporation of chloride does not cause significant changes in the Raman spectra, however. Based on X-ray diffraction results the visibly homogeneous glasses were completely amorphous while the phase separated glasses contained a number of crystals. There are two mechanisms of phase separation occurring in the glasses with excess sulphate and molybdate: liquid-liquid separation and thereafter crystallisation, which occurs during cooling within glass melts with critical amounts of sulphate or molybdate; or a segregated layer, which occurs if the addition of sulphate or molybdate is too excessive to be completely dissolved in the melt. The crystals formed through the former mechanism are mostly spherical, submicron in size and randomly dispersed. These crystals are more likely to be alkaline earth salts while the segregated layers are essentially sodium salts. The phase separation caused by excess chloride in melt is different. The separated phases in aluminosilicate glasses are all non-chlorine containing and are formed through nucleation and growth during cooling. Sulphate solubility is observed to steadily increase with the replacement of larger for smaller alkaline earths in borosilicate glasses. Sulphate solubility in aluminosilicate glasses is not achieved as no sulphate can be retained in these compositions. Chloride solubility also increases from MgO-containing to BaO-containing borosilicate glasses like sulphate solubility. However, the retention of chloride in aluminosilicate glasses is selective and sensitive to compositions; barium aluminosilicate glass possesses the highest chloride solubility with the highest chloride retention. In contrast, molybdate solubility increases from BaO-containing to MgO-containing aluminosilicate glasses and from BaO-containing to CaO-containing borosilicate glasses. Molybdate is poorly soluble in magnesium borosilicate glass. Comparison of the behaviour of these three anionic species in glass suggests that the controlling factors for molybdate solubility may be very different from the other two. Finally three compositional parameters normalised cation field strength (NCFS), electronegativity index (XR) and cationic size (SR), which are related to cationic charge and size, but which differ from each other with respect to the contributions of each aspect, are used to express the solubility dependence of each species. Within narrow compositional variations in this study (equimolar substitution among alkaline earths) the above parameters seems to be quite applicable. But the compositional variations in literature glasses are much more complicated and the fittings may not apply. When combined with literature data, the best fitting for sulphate solubility is found with SR, the index of cationic size, with an increasing exponential relationship between solubility and SR. For chloride solubility with best fit is obtained with NCFS, the index of cation field strength, with a decreasing exponential relationship between solubility and NCFS. Nevertheless, no convincing correlation for molybdate has been achieved, although XR, the index of electronegativity of network modifiers, does show a general trend of increasing solubility with linearly decreasing XR

Incorporation and phase separation of Cl in alkaline earth aluminosilicate glasses

Pyrochemical reprocessing of spent nuclear fuels may lead to the generation of chloride containing wastes. 36Cl wastes may also arise from the treatment of irradiated graphite. Such wastes will have limited solubility in the borosilicates currently used for waste vitrification. Despite requiring higher processing temperatures aluminosilicate glasses show promise for this application. In a series of alkaline earth aluminosilicate glasses we demonstrate that chloride solubility is related to the alkaline earth species as follows Sr > Sr+Ba > Ba > Ca > Mg, with the strontium aluminosilicate glass accommodating up to 5.92 at% Cl. Typical chloride retention rates are ~80% of the batched chloride content at 1400ºC. It has also been observed that, when Cl is present in the glass in excess, phase separation firstly occurs as formation of non-Cl crystals (mainly alkaline earth aluminosilicates, with a minority of aluminates); a segregated chloride layer is only formed at higher chlorine loadings. This indicates that chlorine solubility in glass is not only controlled by the capacity of glass network to accommodate Cl– but also by the stability of glass network after Cl– incorporation. In addition, increased incorporation of Cl– in glass results in steadily decreased glass densities and glass transition temperatures

Influence of Radioactive Sludge Content on Vitrification of High-Level Liquid Waste

The radioactive sludges formed at the bottom of high-level liquid waste (HLW) storage tanks pose challenges when the HLWs are vitrified. This study aims to determine the influence of the sludge content (enriched in Na2O, Al2O3, NiO, Fe2O3, and BaSO4) on the structure and properties of waste glasses in order to find the optimal ratio of sludges to HLW during vitrification. In the experiments, the simulated sludge and simulated HLW were mixed at different ratios from 0:8 to 4:4, with an overall waste content of 16 wt %, in a borosilicate glass wasteform. It is found that the glass density, molar volume, sulfur retention, and glass transition temperature changed little when increasing the sludge content of the glasses, while the viscosity, chemical durability, and crystallization features of the glasses varied notably. The crystals formed in the glasses during the thermal treatment were exclusively Fe-substituted diopside (Ca, Mg, Fe)2Si2O6. An increase in the Al2O3 and NiO content of the glasses may have been responsible for the increased crystallinity at high temperatures. The leaching rate of Si, B, Na, and Cs from the glasses declined with the increasing addition of sludge to the glasses. Although all the glasses fulfilled the requirements for vitrification processing and glass-product performance, it is recommended that the sludge content of the whole waste should not exceed 25 wt %. This study guides further research on the immobilization of high-level sludges