The Incorporation Of P S Cr F Cl I Mn Ti U And Bi Into Simulated Nuclear Waste Glasses

The understanding of dissolution of the British nuclear waste glass, as compared to other nuclear waste glasses, is limited. The few studies that have been performed were generally done in static leach tests with the exception of the single pass flow through test performed by Abratis during his PhD, producing a number of papers (Abraitis, P.K., Univ. Manchester, 1999; Abraitis, P.K. et. al., Applied Geochemistry 15 (2000) 1399; Abraitis, P.K. et. al., Mat. Res. Soc. Sym. Pro. 556 (1999) 401). In order to gain further insight to the British nuclear waste glasses X-ray absorption spectroscopy was used along with dissolution studies of British simulated nuclear waste glasses to determine the role of Zn and to ascertain the mechanisms of corrosion. Both baseline glasses along with a Blend simulated nuclear waste glass were studied to give better comparisons with previous results by other authors. The Blend waste glass is composed of a mixture of Magnox reactor waste and thorium oxide reprocessing (ThORP) waste at a wt% of 25wt% Magnox waste and 75 wt% ThORP. Zn K edge X-ray absorption spectroscopy was applied to determine the Zn coordination environment in model inactive UK high level waste (HLW) glasses. Quantitative analysis of the X-ray absorption fine structure (EXAFS) data provided conclusive evidence for the presence of ZnO4 species participating in network formation, linking, on average, to 2 ± 1 SiO4 units via bridging oxygen atoms. Excellent quantitative agreement was observed between the Zn-O contact distance and coordination number determined from EXAFS and previous Molecular Dynamics simulations of glasses with the same nominal composition. Analysis also provided evidence in support of the network forming role of Zn as predicted from Molecular Dynamics simulation, but it was not possible to confirm the predicted clustering of Zn species at high Zn concentration in simple soda lime silica glasses. The single pass flow through (SPFT) work in this study has shown that the steady state dissolution of a base glass with 25 wt% waste loading (MW+25wt%) and a base glass with 30 wt% waste loading (MW+30wt%) of Blend simulated nuclear waste glasses is similar to that of previous studies. The SPFT flow per surface area (Q/S) results are consistent with previous studies. The dissolution rate dependency on temperature and pH of was also consistent with similar glasses. The NRi with respect to B, Si, Na and Al, of the glasses in this study increased with an increase in solution pH. The average activation energy with respect to Si was consistent for a surface controlled reaction with these glasses as was shown in previous studies on MW+Magnox, a similar glass. One of the current designs for a geological repository of HLW is a co-located repository with intermediate level waste (ILW). At long times, there is the possibility of hyper alkaline solution from the ILW repository interacting with the vitrified HLW glass wasteforms. Dissolution studies were performed to understand the mechanisms by which the UK HLW glasses will corrode under hyper alkaline solutions. The addition of an alkali buffer to high purity H2O resulted in a decrease of the dissolution rate for the MW Blend glasses during the product consistency type tests. The concentration of Si in solution for all of the samples suggests that Ca and Si are co-precipitating as was evident from the decrease in Ca concentrations at short times. It is suggested that the presence of Ca in solution forms a passive reactive interface (PRI) which decreases the rate of formation of a hydrated surface layer decreasing the overall dissolution rate. The results from the geochemical modelling of the product consistency type tests showed that Ca and Si are co-precipitating at short times decreasing the normalised mass loss. This is evident from the Ca and Si phases which are most probable to precipitate out of solution as suggested by the geochemical modelling results from PhREEQC In order to understand the mechanisms of alteration layer formation of the UK HLW glasses, vapour hydration tests (VHT) were performed. Results from the time dependant VHT experiments have shown changes in the mechanisms of alteration layer formation with the addition of the simulated Blend waste to the base glasses. It was also shown that iron may be a suppressor to zinc dissolution. Both a Ca and Zn doped Blend waste glass and a base glass doped with Fe Zn and Zr did not show significant layers of zinc in the alteration layer as shown in the Ca and Zn doped base glass in both the reproducibility or time-dependant VHT experiments. Giesler et al. have similar results to the VHT results in this study, which correlate to an interface-coupled dissolution-reprecipitation mechanism when simulated Blend waste is added to the base glasses (Giesler, T. et. al., J. Non-Cryst. Solids 356 (2010) 1458). However, further.

For isolation of nuclear wastes through the vitrification process, waste slurry is mixed with borosilicate based glass and remelted at high temperature. During these processes, water can enter into the final waste glass. It is known that water in silica and silicate glasses changes various glass properties, such as chemical durability, viscosity and electrical conductivity. These properties are very important for processing and assuring the quality and safety controls of the waste glasses. The objective of this project was to investigate the effect of water in the simulated nuclear waste glasses on various glass properties, including chemical durability, glass transition temperature, liquidus temperature, viscosity and electrical conductivity. This report summarizes the results of this investigation conducted at Rensselaer during the past one year.

The chemical durability of simulated nuclear waste glasses having different water contents was studied. Results from the product consistency test (PCT) showed that glass dissolution increased with water content in the glass. This trend was not observed during MCC-1 testing. This difference was attributed to the differences in reactions between glass and water. In the PCT, the glass network dissolution controlled the elemental releases, and water in the glass accelerated the reaction rate. On the other hand, alkali ion exchange with hydronium played an important role in the MCC-1. For the latter, the amount of water introduced into a leached layer from ion-exchange was found to be much greater than that of initially incorporated water in the glass. Hence, the initial water content has no effect on glass dissolution as measured by the MCC-1 test.

Immobilisation of high level toxic wastes by vitrification is a well established process that has been studied extensively over last 40 years. A suitable glass host is used to dissolve the high level nuclear waste to form a glassy (vitreous) homogeneous product that can be cast into suitable forms, including large glass blocks. The main advantages of the vitrification route include the fact glass is a good solvent for waste, glasses can be processed at reasonably low temperatures, glass is very tolerant of variations in waste composition, glass exhibits reasonable chemical durability, glass is radiation resistant and can accommodate changes occurring during decay of high level nuclear waste constituents. This book analyses the immobilisation of high level toxic wastes through the use of an appropriate glass host.

The purpose of this report is to review previous work on the weathering of natural glasses; and to make recommendations for further work with respect to studying the alteration of natural glasses as it relates quantifying rates of dissolution. the first task was greatly simplified by the published papers of Jercinovic and Ewing (1987) and Byers, Jercinovic, and Ewing (1987). The second task is obviously the more difficult of the two and the author makes no claim of completeness in this regard. Glasses weather in the natural environment by reacting with aqueous solutions producing a rind of secondary solid phases. It had been proposed by some workers that the thickness of this rind is a function of the age of the glass and thus could be used to estimate glass dissolution rates. However, Jercinovic and Ewing (1987) point out that in general the rind thickness does not correlate with the age of the glass owing to the differences in time of contact with the solution compared to the actual age of the sample. It should be noted that the rate of glass dissolution is also a function of the composition of both the glass and the solution, and the temperature. Quantification of the effects of these parameters (as well as time of contact with the aqueous phase and flow rates) would thus permit a prediction of the consequences of glass-fluid interactions under varying environmental conditions. Defense high- level nuclear waste (DHLW), consisting primarily of liquid and sludge, will be encapsulated by and dispersed in a borosilicate glass before permanent storage in a HLW repository. This glass containing the DHLW serves to dilute the radionuclides and to retard their dispersion into the environment. 318 refs.