Michael I. Ojovan

Immobilisation of radioactive waste in glasses, glass composite materials and ceramics

Abstract The basic principles of incorporating high level radioactive waste into glasses, ceramics (Synroc type) and glass composites including glass ceramics are described. Current UK technology uses glass wasteforms for the products of reprocessing, although many countries are temporarily storing the ceramic spent fuel for eventual disposal. Some waste streams may be incorporated into ceramics, but difficult or legacy wastes will require the development of other wasteforms comprising composite systems of crystals and glass. The importance of processing–property–structure (especially durability) relations in such systems over size scales from the atomic to the geological and on timescales to hundreds of thousands of years is highlighted.

Thermodynamic parameters of bonds in glassy materials from viscosity–temperature relationships

Doremuss model of viscosity assumes that viscous flow in amorphous materials is mediated by broken bonds (configurons). The resulting equation contains four coefficients, which are directly related to the entropies and enthalpies of formation and motion of the configurons. Thus by fitting this viscosity equation to experimental viscosity data these enthalpy and entropy terms can be obtained. The non-linear nature of the equation obtained means that the fitting process is non-trivial. A genetic algorithm based approach has been developed to fit the equation to experimental viscosity data for a number of glassy materials, including SiO2, GeO2, B2O3, anorthite, diopside, xNa2O-(1-x)SiO2, xPbO-(1-x)SiO2, soda-lime-silica glasses, salol, and α-phenyl-o-cresol. Excellent fits of the equation to the viscosity data were obtained over the entire temperature range. The fitting parameters were used to quantitatively determine the enthalpies and entropies of formation and motion of configurons in the analysed systems and the activation energies for flow at high and low temperatures as well as fragility ratios using the Doremus criterion for fragility. A direct anti-correlation between fragility ratio and configuron percolation threshold, which determines the glass transition temperature in the analysed materials, was found.

Viscosity and Glass Transition in Amorphous Oxides

An overview is given of amorphous oxide materials viscosity and glass-liquid transition phenomena. The viscosity is a continuous function of temperature, whereas the glass-liquid transition is accompanied by explicit discontinuities in the derivative parameters such as the specific heat or thermal expansion coefficient. A compendium of viscosity models is given including recent data on viscous flow model based on network defects in which thermodynamic parameters of configurons—elementary excitations resulting from broken bonds—are found from viscosity-temperature relationships. Glass-liquid transition phenomena are described including the configuron model of glass transition which shows a reduction of Hausdorff dimension of bonds at glass-liquid transition.

Long-term field and laboratory leaching tests of cemented radioactive wastes

Experiments with real and simulated radioactive cementitious wasteforms were set up to compare the leaching behaviour of cementitious wasteforms containing nuclear power plant operational waste in field and laboratory test conditions. Experiments revealed that the average annual (137)Cs leach rate in deionised water was about thirty-five times greater compared with the measured average value for the 1st year of the field test. Cumulative leached fraction of (137)Cs for 1st year (3.74%) was close to values reported in literature for similar laboratory experiments in deionised water, however more than two orders of magnitude higher than the 1st year leached fraction of (137)Cs in the repository test (0.01%). Therefore, to compare field and laboratory test results, a scaling factor is required in order to account for surface to volume factor difference, multiplied by a temperature factor and a leach rate decrease coefficient related to the ground water composition.

Configurons: Thermodynamic Parameters and Symmetry Changes at Glass Transition

Thermodynamic parameters of configurons - elementary excitations resulting from broken bonds in amorphous materials - are found from viscosity-temperature relationships. Glass-liquid transition phenomena and most popular models are described along with the configuron model of glass transition. The symmetry breaking, which occurs as a change of Hausdorff dimension of bonds, is examined at glass-liquid transition. Thermal history effects in the glass-liquid transition are interpreted in terms of configuron relaxation.

Glass formation in amorphous SiO2 as a percolation phase transition in a system of network defects

Thermodynamic parameters of defects (presumably, defective SiO molecules) in the network of amorphous SiO2 are obtained by analyzing the viscosity of the melt with the use of the Doremus model. The best agreement between the experimental data on viscosity and the calculations is achieved when the enthalpy and entropy of the defect formation in the amorphous SiO2 network are Hd=220 kJ/mol and Sd=16.13R, respectively. The analysis of the network defect concentration shows that, above the glass-transition temperature (Tg), the defects form dynamic percolation clusters. This result agrees well with the results of molecular dynamics modeling, which means that the glass transition in amorphous SiO2 can be considered as a percolation phase transition. Below Tg, the geometry of the distribution of network defects is Euclidean and has a dimension d=3. Above the glass-transition temperature, the geometry of the network defect distribution is non-Euclidean and has a fractal dimension of df=2.5. The temperature Tg can be calculated from the condition that percolation arises in the defect system. This approach leads to a simple analytic formula for the glass-transition temperature: Tg=Hd/((Sd+1.735R). The calculated value of the glass-transition temperature (1482 K) agrees well with that obtained from the recent measurements of Tg for amorphous SiO2 (1475 K).

Topologically disordered systems at the glass transition

The thermodynamic approach to the viscosity and fragility of amorphous oxides was used to determine the topological characteristics of the disordered network-forming systems. Instead of the disordered system of atoms we considered the congruent disordered system of interconnecting bonds. The Gibbs free energy of network-breaking defects (configurons) was found based on available viscosity data. Amorphous silica and germania were used as reference disordered systems for which we found an excellent agreement of calculated and measured glass transition temperatures. We reveal that the Hausdorff dimension of the system of bonds changes from Euclidian three-dimensional below to fractal 2.55 ± 0.05-dimensional geometry above the glass transition temperature.

Viscosity of network liquids within Doremus approach

Defect-mediated diffusion and viscous flow of network forming liquids have been investigated. An analytical formula of viscosity has been derived with two-exponential forms demonstrating high activation energy at low temperatures and low activation energy at high temperatures with Arrhenius-type behavior for both high and low temperature limits. Calculated data for the viscosity of silica glass are in excellent agreement with the experimental data.

Crystalline materials for actinide immobilisation

Physical and Chemical Properties of Actinides Areas of Actinide Use Nuclear Waste Immobilisation of Actinides Synthesis of Chemically Durable Actinides Analytical Methods for Actinide Future Potential of Actinide Containing Materials.

Handbook of advanced radioactive waste conditioning technologies

Introduction. Part 1 Radioactive waste treatment processes and conditioning technologies: Compaction processes Incineration and plasma processes Application of inorganic cements Calcination and vitrification processes Historical development of vitrification Decommissioning of nuclear facilities and environmental remediation. Part 2 Advanced materials and technologies for the immobilisation of radioactive wastes: Geopolymers Glass matrices Ceramic matrices Waste packages Metal containers. Part 3 Long-term performance assessment and knowledge management techniques: Failure mechanisms of nuclear waste forms Long-term performance models Knowledge management techniques.