Research Projects: Glasses, Cements and Waste Immobilisation

This list is not intended to be fully comprehensive; other topics can be negotiated with individual staff members. Additional projects will be notified in supplements issued periodically during the session. Under each major heading the projects are grouped according to the prime supervisor (with the supervisors in alphabetical order of surname).

Dr R J Hand
32	THE MECHANICAL PROPERTIES OF GLASSES AS A FUNCTION OF COMPOSITION Supervisor: Dr R J Hand
The effects of composition on the mechanical properties of silicate glasses are not well understood. Recent work both at Sheffield and elsewhere suggests that composition does have an effect on the mechanical properties of silicate glasses. Similar compositional dependence is expected with other glass types. Projects are available to look at the mechanical properties of silicate, borosilicate or iron phosphate glasses as a function of composition. All of the projects will utilise the glass-making facilities available at Sheffield to produce a range of glass compositions of the chosen glass type. A variety of mechanical properties including strength and toughness will be measured. Nanoindentation will be used to assess the near surface mechanical properties of the glasses after exposure to different environments. The flaws that cause failure in these glasses will also be investigated by optical and electron microscopy. The overall aim of the project will be to elucidate links between the mechanical properties and composition with a specific aim of trying to understand what compositional parameters control the ease of Griffith flaw formation in glasses. If a project on the mechanical properties of glasses interests you please contact Dr RJ Hand to discuss details of the projects currently available.
33	VITRIFICATION AND RE-USE OF WASTE MATERIALS Supervisors: Dr R J Hand and Dr P A Bingham
There are a number of industrial wastes that can potentially be vitrified so that can be safely immobilized, such as municipal waste incinerator ash, sewage sludge ash and asbestos. Although vitrification is attractive in principle it is an energy intensive process and thus secondary re-use of the product is required to make vitrification an attractive proposition. Secondary re-use requires us to have a detailed knowledge both of the materials produced by this process and of the process variables that can affect their final properties. We have ongoing research in this area and projects on a variety of wastes are available. All the projects will involve the study of the network structure and redox state of the vitrified wastes, with other aspects including chemical durability, viscosity, crystallisation, melting behaviour and refractory corrosion. All the projects will involve a substantial glass melting programme. Actual wastes will be studied where it is safe to do so, and surrogates where it is not, for example with asbestos. Analysis techniques to be used will include thermal analysis, FT-IR and Raman spectroscopy, viscometry and electron microscopy. Some projects may involve collaboration with Dr S Forder of Sheffield Hallam University for Mössbauer studies. Other properties of the wasteforms, such as the mechanical properties, will be investigated as required. If a project on toxic waste immobilization interests you please contact Dr RJ Hand to discuss details of the projects currently available.
34	FINAL RATE OF GLASS DISSOLUTION: CASE STUDY OF ALKALI-BOROSILICATE GLASSES Supervisors: Dr R J Hand and Dr M I Ojovan
Glass has emerged as an optimal material for the immobilisation of high level nuclear waste (HLW) as it provides excellent retention of radionuclides at acceptable costs. An imperative issue is to assess the long-term behaviour of vitrified nuclear waste in real disposal conditions. Various studies designed to understand the main features of the behaviour of nuclear waste glasses in disposal environments are currently underway internationally and many important results have already been obtained. However, despite significant achievements a complete consensus on the governing mechanisms in the long-term aqueous corrosion of glass has not yet been achieved. A key role in the long-term behaviour of alkali-borosilicate glasses is played by so-called final rate of glass dissolution determining the bulk potential release of radionuclides. This information is very important in predicting the long term performance of vitrified wasteforms in disposal environments. This project will involve studying the most important mechanisms of alkali-borosilicate glasses corrosion and in particular determining of final rate of glass dissolution of various HLW glasses, although there will be a particular focus on assessing the performance of UK HLW glass compositions. Data generated from laboratory tests will be mathematically processed and the derived calculations used in model calculations. Derived parameters will be used to predict the behaviour of glasses in a disposal environment. The project will involve collaboration with SCK-CEN, Mol, Belgium and it is envisaged that student will spend time at the SCK-CEN laboratories in Mol.
Professor J Harding
35	THE EFFECT OF WATER ON CERAMICS (AND VICE VERSA) Supervisor: Professor J Harding
Water is everywhere. Many minerals are precipitated from water and most dissolve in it eventually. Aqueous corrosion is a major industrial problem. How all these processes happens depends on the details of what happens where water meets the solid. Both sides are affected by this interface. The structure of water close to the surface of a ceramic is completely different to the structure of bulk water. Ceramics surfaces hydrolyse in the presence of tiny concentrations of water vapour. This project will use dynamical simulation methods to investigate what the structure of the water/ceramic interface is, how it changes with different ceramics or different surfaces of the same ceramic and how this interface controls the growth and dissolution of ceramics. The project will use existing molecular dynamics and ab initio programs, but opportunities for students to write their own programs and scripts will also be available.
Dr N C Hyatt
36	CERAMIC WASTEFORMS FOR IMMOBILISATION OF FISSION PRODUCTS, MINOR ACTINIDES AND PLUTONIUM Supervisors: Dr N C Hyatt and Dr E R Maddrell (Visiting Lecturer – Nexia Solutions/BNFL)
Background. Nuclear fuel reprocessing produces separated plutonium and a radio-toxic waste comprising fission products (e.g. cesium and iodine) and minor actinides (e.g. neptunium and americium). Excess plutonium requires immobilisation to safeguard against misuse and proliferation, whereas the waste components require immobilisation to prevent dispersal in the environment. Immobilisation of plutonium, fission products and minor actinides in crystalline ceramics may be achieved by targeting the substitution of these species on specific cation / anion sites with appropriate charge compensation. PhD top-up awards are available to support projects in this area – please ask for details. Projects. The common aims of projects in this area are: firstly, to understand the substitution and charge compensation mechanisms required to immobilise fission products, minor actinides or plutonium in crystalline ceramics; and secondly, to identify the reactions leading to release of these species in accelerated corrosion experiments. Projects in this area will focus on one of the following topics: 1. Cesium immobilisation in hollandite ceramics, based on: Ba_xTi_8-2xM_2xO₁₆ 2. Minor actinide immobilisation in zirconolite ceramics, based on CaZrTi₂O₇ 3. Plutonium immobilisation in pyrochlore and apatite ceramics, based on Gd₂Ti₂O₇ and Ca₂Y₈Si₆O₂₆ 4. Iodine immobilisation in iodo-apatite ceramics, based on Pb₅(VO₄)₃I Work involving plutonium and minor actinides will initially use non-radioactive simulants (e.g. lanthanide elements) or low activity a- isotopes (U, Th), with the aim of extending this work using Pu and Np through collaboration with the Institute of Trans-Uranics at Forschungszentrum Karlsrhue and Centre for Radiochemistry Research at The University of Manchester. Training. Projects in this area will involve solid and liquid based ceramic processing methods and the use of a range of characterisation techniques, including: X-ray, neutron and electron diffraction, electron microscopy, and Raman, X-ray absorption and solid state nuclear magnetic resonance spectroscopies. Full training for working with radioisotopes will be provided. German language tuition would be provided for placement at ITU Karlsrhue.
37	STRUCTURE, COMPOSITION AND DURABILITY OF NUCLEAR WASTE GLASSES Supervisors: Dr N C Hyatt
Background. Alkali borosilicate glasses are currently employed for the immobilisation of UK high level (heat generating) nuclear waste, comprising the fission products separated by nuclear fuel reprocessing. The radionuclides contained within high level waste glasses may be chemically bound within the polymeric glass network or immobilised in extra-network cavities together with network modifier cations. To predict the behaviour of waste loaded glasses under repository conditions, where they are likely to be in contact with water over geologic time scales, it is necessary to understand the fundamental mechanisms of glass corrosion in aqueous solution. PhD top-up awards are available to support projects in this area – please ask for details. Project. The aim of this project is to determine the effect of glass structure and composition on the key mechanisms controlling corrosion of simulated (non-active) waste glass in aqueous solution, using a range of bulk and surface analytical techniques, combined with chemical analysis of corrosion solutions containing species leached from the waste glass. These data will be compared with the results of geochemical modelling studies, in order to assess the performance of geochemical models in predicting the short term corrosion behaviour of nuclear waste glasses in aqueous solution. Particular emphasis will be placed on optimisation of the glass composition to improve glass durability. Training. This project will involve high temperature glass melting, the use of accelerated leach testing methods and the application of a range of characterisation techniques, including: X-ray and electron diffraction, electron microscopy, and Raman, X-ray absorption and solid state nuclear magnetic resonance spectroscopies. Fabrication of uranium bearing glasses will be undertaken using a dedicated glove box facility.
Dr H Kinoshita
38	ELECTROCHEMICAL AGING OF RADIOACTIVE WASTE FORMS Supervisors: Dr H Kinoshita and Dr N B Milestone
This project aims to establish a new technique to predict the integrity of radioactive waste forms in the future. Because a long-term safety and integrity is the key for the storage of radioactive materials, such technique would have a large impact world widely in this field. Electrochemical aging is a process that accelerates diffusion of ionic species in porous matrices. This technique developed at Tokyo Institute of Technology is capable of simulating the aging process over 100 years. We are aiming to apply this technique to predict the integrity of the radioactive waste forms in the future. In this project, we focus on (i) initial investigation and establishment of the electrochemical aging process, (ii) application of the technique to investigate integrity of radioactive waste forms in the future. There is also a possibility of collaboration with Tokyo Institute of Technology.
39	CO₂ STORAGE IN RECYCLED CEMENTITIOUS MATERIALS Supervisors: Dr H Kinoshita and Dr N B Milestone
This project aims to immobilise CO₂ using recycled cementitious materials, to reduce the concentrations of this greenhouse gas in the atmosphere. Most of cementitious materials have very high concentration of Ca, which would readily form stable carbonates reacting with CO₂ without any energy input and thus, they have a great potential for CO₂ immobilisation and storage. Effective utilisation of recycled cementitious materials would benefit our society not only in reduction of wastes but also in reduction of CO₂ in the environment. In this project, we focus on (i) experimental investigation on various conditions of carbonation processes for different cementitious materials, and (ii) study on the physico-chemical properties of the carbonated materials. This experimental study complements with a theoretical study by thermodynamic modelling to investigate the optimal condition for CO₂ capturing and storage.
40	BEHAVIOUR OF URANIUM IN CEMENTITIOUS MATRICES Supervisors: Dr H Kinoshita and Dr N B Milestone
Understanding of the behaviour of uranium metal in cementitious system is crucially important in substantiating the performance of the radioactive waste products. It is known that uranium metal reacts with cementitious materials. However, the chemistry behind the reaction is still unclear, which makes it difficult to identify whether the reaction is advantageous or disadvantageous for the encapsulation of uranium and how to modify it in case the reaction is a disadvantage. In this project, we focus on (i) thermodynamic stability/reaction of uranium metal in cementitious systems via thermodynamic modelling and supporting experimental works and (ii) identify the possible problems and scientific background to them and the corresponding solutions for the immobilisation of uranium.
Dr N B Milestone
41	THE ROLE OF PORE WATER IN CEMENTED WASTES Supervisor: Dr N B Milestone
Portland cement based composites rely on the presence of water to hydrate the cementitious material. However, the water/cement ratio is usually sufficiently high that large amounts of unreacted water remain within a series of pores. This highly alkaline pore water provides protection to steel in construction concrete but in waste encapsulation may cause problems with expansive metal corrosion, waste/matrix interactions and migration of ions. Waste immobilisation composite cements currently contain large amount of supplementary cementing materials, primarily to reduce heat output but this can mean a low degree of hydration of the supplementary material and a high porosity giving rise to a matrix which is permeable. The chemistry of the pore solution can be changed by using different cement systems and different additives as well as This PhD project will examine how the pore solution changes in pH, composition, redox potential as well as how the water is held for a series of different types of cement systems. How the water is held will also be explored using techniques such as NMR, and selective extraction.
42	SULPHATE BASED CEMENTS Supervisor: Dr N B Milestone
PC based composite cements containing up to 90% replacement of OPC with BFS are routinely used for encapsulation of low and intermediate levels of nuclear waste (LLW and ILW). Hydration of the BFS is usually activated by an alkaline solution such as that generated by hydrating OPC or by using NaOH or Na silicate but these give rise to an internal pore solution pH around 12-13 which has been shown to corrode Al and attack zeolites used as selective ion exchange materials. Supersulphated cements, which largely rely on gypsum activation of slag hydration, are commercially available but have not been used extensively for construction because of their low rate of strength development. This system, along with calcium and sodium sulphate activated slags give rise to the formation of a mixture of ettringite, 3CaO.Al₂O₃.3CaSO₄.32H₂O, and C-S-H as the binders. Preliminary work has shown there is reduced corrosion of Al in these types of systems. Calcium sulfoaluminate cements, which rely on the hydration of the phase 4CaO.3Al₂O₃.SO₄ with additional CaSO₄ also give ettringite the binder phase, , have been widely used as construction cements in China for over 30 years. Their manufacture is more environmentally friendly that of OPC as a lower temperature of clinkering is used and they emit less CO₂. Preliminary experiments have shown that the internal pH is lower than that of OPC so that reactive metals such as Al or Mg found in nuclear wastes show less corrosion if any. This project will investigate the potential of one of these sulphate based systems as an alternative cementitious encapsulant. Suitable formulations will be developed which optimise workability, setting time and strength. The hydration products will be characterised and their potential for encapsulation of reactive metals such as Al and Mg investigated. Internal pH and water availability will be determined along with characterisation of corrosion product if any. Long term durability will be assessed through corrosion rate studies. The work will be undertaken in conjunction with Nexia Solutions as part of a wider programme investigating the viability of alternative cements for encapsulation of difficult or reactive wastes.
43	ENCAPSULATION OF HOST SPECIES WITHIN ZEOLITIC FRAMEWORKS Supervisor: Dr N B Milestone
Zeolites are porous aluminosilicates which are made up of a series of cages and pore networks. They have wide applications as ion exchangers as well as adsorbents as well as their extensive use as catalysts in the petrochemical industry. It is possible to synthesize some zeolites which incorporate anions or cations within the cages, which can give additional properties. Some of the best examples of this type of material are sodalites and ultramarines which are widely used as pigments as the occluded anion is stable. One of the best known is lapis lazuli or blue ultramarine used as the only blue pigment in historical times. This relies on the presence of the S3- species held within the cage. Work already conducted at Sheffield has shown that other colours can be generated using Se and Te species prepared by a novel process. This project will look at different synthesis methods to prepare several zeolite systems which contain closed cages such as sodalite and cancrinite and examine their potential for trapping species such coloured anions, toxic species and metal nanoparticles. The properties of such species and will determined together with their stabilities and possible uses.
Dr G Möbus
44	CO-ORDINATION AND VALENCE CHEMISTRY OF CATIONS IN OXIDE GLASSES AND NANOSTRUCTRED MATERIALS Supervisors: Dr G Möbus, Dr M I Ojovan and Dr R J Hand
The knowledge of the structural units, coordination numbers, and bond types constitutes an essential part of the structural solution of complex oxide glasses and ceramics. An important tool to assess such parameters is the fine structure in spectra of electron energy losses in the field-emission transmission electron microscope. The very high spatial resolution enables the local examination of nanoscale inhomogeneous materials such as glass-nanoparticle composites. The project will focus on, while not being restricted to, a collection of borosilicate based nanoparticle composites with CeO2 and/or Ag precipitates. Examinations include the formation mechanism and process control of precipitate-generated nanoparticles, their influence on mechanical properties and irradiation-induced flow. Application fields of these materials range from immobilisation of cations to functional optical materials.
Dr M I Ojovan
45	SINTERING OF GLASS COMPOSITE MATERIALS TO IMMOBILISE PROBLEMATIC WASTES Supervisors Dr M I Ojovan and Professor W E Lee (Imperial College, London)
Glass composite materials are the focus of attention as possible matrices for immobilisation of difficult waste streams for example toxic and nuclear wastes containing large amounts of glass-immiscible components such as sulphates, chlorides and molybdates or refractory materials requiring unacceptably high melting temperatures for vitrification. Moreover glass composites may be used to immobilise long-lived radionuclides (such as actinide species) by incorporating them into the more durable crystalline phases, whereas the short-lived radionuclides may be accommodated in the less durable vitreous phase. Acceptable durability will result if the active species are locked into the crystal phases that are encapsulated in a durable glass matrix. Sintering of glass composites offers the advantage of lower processing temperatures and higher waste loading resulting in significantly higher efficiency of immobilisation process. The purpose of this work is to analyse the feasibility of sintering route in immobilisation of toxic and nuclear wastes containing refractory oxides. Microstructure and leach durability of glass composites will be analysed as a function of waste loading and sintering process parameters. The partitioning of simulant waste elements between the vitreous and crystalline components of glass composite materials will be investigated using a range of analytical techniques, including SEM / EDS, TEM, XRD and optical microscopy with image analysis and standard leaching procedures together with ICP MS analysis of leachates.
46	HIGH-TEMPERATURE VISCOSITY OF OXIDE SYSTEMS SiO₂-cAo-Na₂O and B₂O₃: SEARCH FOR A NON-ACTIVATED TEMPERA\TURE BEHAVIOUR Supervisors: Dr M I Ojovan, Dr R J Hand and Dr K P Travis
Defect-mediated viscosity model relates the viscosity of amorphous materials η(T) to thermodynamic parameters of broken bonds e.g. configurons. The viscosity equation derived from this model η(T) = A₁T[1 + A₂ exp(B/RT)][1 + C exp(D/RT)] is in excellent agreement with the experimental data for most amorphous materials in both glassy and liquid states. It shows that the viscosity has an Arrhenius-type temperature dependence with high activation energies for glassy materials whereas for melts the viscosity has an Arrhenius dependence with low activation energies. In addition to this the defect-mediated model predicts that at very high temperatures the viscosity transits to a non-activated regime η(T) α T. The experimental data available for melted soda lime silica glasses and B₂O₃ show a systematic diminution in activation energies however there are no solid experimental data to consistently demonstrate the non-activated regime. This project aims to study the high temperature viscosity temperature relationships for oxide systems SiO₂-CaO-Na₂O and B₂O₃ in a search for non-activated temperature behaviour of viscosities. The experimental part of project will include melting of suitable oxide systems and measurements of their viscosities over a range of temperature. The analysis of viscosities will be carried out using genetic algorithms and other curve fitting approaches.
47	CORROSION OF REFRACTORY METALS IN SILICATE MELTS: MATERIALS FOR SELF-DESCENDING CAPSULES TO REACH MOHO DISCONTINUITY Supervisors: Dr M I Ojovan and Professor F G F Gibb
Exploration the uppermost hundreds km of the Earth and particularly of Moho discontinuity which separates the crust from the mantle is a challenge for the current technique. Recently is was suggested to use for this purpose self-descending metallic capsules heated by decaying radionuclides [M.I. Ojovan, F.G.F. Gibb, P.P. Poluektov, E.P. Emets. Sounding internal Earth’s layers by probe self-descending capsules. Atomic Energy, v.99, No. 2, 120-127 (2005)]. The probe can melts its way down through the crust and mantle while its position and progress could be tracked by acoustic signals generated in the rocks yielding new insights into the physical properties of the rocks through which they pass. Combined with other geophysical methods self-descending probes should provide unequivocal information on the nature and composition of the Earth’s interior. As descending times last for many years an important issue is the interaction between the probe material and melted rocks. This project aims to investigate the corrosion of refractory metals (tungsten, molybdenum) as possible probe materials in silicate melts (basalts and granites) and will attempt to evaluate probes survivability. Probe corrosion and damaging reactions will be investigated under conditions envisaged in the deep Earth’s interior both experimentally and developing appropriate survivability models.
Dr J M Parker
48	STRUCTURAL AND CHEMICAL FACTORS CONTROLLING GLASS COLOUR Supervisor: Dr J M Parker
A programme is being developed to allow the prediction of the absorption spectrum and hence the colour of solution coloured glasses. While this is relatively straightforward for singly doped glasses the problem becomes more complex where multiple doping occurs because of redox interactions, and the colour then can also be sensitive to the thermal history. This programme has so far allowed the deconvolution of some complex spectra and the identification of a number of unexpected optical effects. Interest in this work also lies in: the colour control of glass containers particularly where high levels of cullet are being used in its manufacture; in the development of optimised solar control glasses; in reducing UV transmission for containers and hence extending product life; and in computing techniques that allow prediction of recipes that give a glass with a defined colour at minimum cost.
49	ELECTRON PROCESSES IN COLOURED GLASSES Supervisor: Dr J M Parker
Redox ion pairs in glasses can exchange electrons down to the glass transition temperature approximately. The aim of this project is to examine both the thermodynamic and kinetic aspects of these processes. The project will also study how the separation of the redox ions influences electron exchange. Such processes are important in understanding UV, visible and IR transmission of glasses; they are therefore important in the development of decolorisers such as cerium as alternatives to selenium for making clear glass articles. Similar processes also occur in making glasses coloured by nanoparticles e.g. gold ruby glasses, in photochromic glasses, and in photodarkening processes in certain glasses when subject to high intensity laser beams.
50	NANO CRYSTALLISATION IN GLASS CERAMICS Supervisors: Dr J M Parker
Glass ceramics based on Cr-doped zinc oxide-alumina-silica system can precipitate gahnite crystals into which Cr3+ ions segregate. The early stages of crystallisation produce a metastable phase of different structure to the final precipitate. The phases produced offer an octahedral site for chromium with an exceptionally high crystal field energy. This project will involve the optimisation of the steps in this process in order to produce glasses of novel optical properties.
Dr K P Travis
51	NUMERICAL MODELLING APPLIED TO DEEP GEOLOGICAL DISPOSAL OF NUCLEAR WASTE Supervisors: Dr K P Travis and Professor F G F Gibb
Disposal in deep boreholes is emerging as a potentially better alternative to mined repositories for the geological disposal of heat-generating high-level nuclear wastes. In order to predict the behaviour of the waste forms and materials involved, and make performance assessments of the disposal, it is necessary to combine sophisticated numerical modelling studies with a programme of experimental work. This project will build on our existing work which has concentrated on modelling the conductive flow of heat in realistic waste disposal scenarios using finite difference methods, extending it to cover heat transfer by convection and modelling container failure. The project requires a high competency in mathematics and would suit students whose first degree is in Physics/Applied Mathematics/materials Science/Chemistry or an appropriate engineering discipline.
52	MODELLING MATERIALS FAILURE USING SMOOTH PARTICLE APPLIED MECHANICS (SPAM) Supervisor: Dr K P Travis
Smooth Particle Applied Mechanics (SPAM) is a quite general simulation method which uses particles to solve problems in continuum mechanics. The basic idea is to express all of the continuum field variables (density, stress, heat flux etc) on a grid composed of moving particles. SPAM is best suited to solving problems which present extreme difficulty for the more usual continuum methods such as Finite Elements (FE), Finite Differences (FD); The FE approach has problems when large scale irregular deformations are involved while the FD approach runs into difficulty when there are moving boundaries resulting from materials flow. SPAM has been used in a wide variety of applications such as cutting and machining, understanding the stability of naval vessels to waves, the dynamics of ice in the Arctic Ocean and even problems in Astrophysics. It also shows great potential for solving problems in Materials Engineering, particularly material failure. This project will involve developing SPAM code and applying SPAM to study material failure problems of current interest. It requires a high competency in mathematics and would suit students whose first degree is in Physics/Applied Mathematics/Materials Science/Chemistry or an appropriate engineering discipline.
53	IMPROVED ALGORITHMS FOR REVERSE STRUCTURE DETERMINATION Supervisors: Dr K P Travis, Mr Mark Bankhead and Mr Scott Owens (Nexia Solutions)
Reverse Monte Carlo (RMC) is a useful tool for determining the atomic structure of a material given sets of experimental measurements such as X-ray diffraction and neutron scattering data. The basic method involves evolving an initial trial arrangement of atoms, one by one, accepting the new configuration with a Boltzmann weighted probability criterion based on a measure of the square difference between a calculated structure factor and the experimental structure factor. RMC is especially useful for helping to resolve the structures of amorphous materials. However, it has a number of drawbacks. For example, unless the starting configuration is close to the final (unknown) configuration, the algorithm will either be extremely slow to converge or not converge at all. This project will involve the use of Genetic and evolutionary algorithms to improve the RMC method. Key to the success of this approach is having a clever means of performing the crossover operator. Part of this project will therefore explore new ways of performing crossovers, borrowing ideas from Dynamical Systems Theory and Nonequilibrium statistical mechanics. The performance of the new algorithm will be assessed using data generated using molecular dynamics and/or dissipative particle dynamics. Later, real experimental data will be used to try to resolve the structure of an amorphous material of industrial importance. This project would suit students whose first degree is in Computer Science/Physics/Applied Mathematics/Materials Science/Chemistry or other appropriate engineering discipline.
Immobilisation Science Laboratory
Dr N C Hyatt
The following two projects are available immediately, under the supervision Dr Neil Hyatt, Dr Russell Hand and Dr Michael Ojovan. The projects will attract the normal EPSRC stipend with a CASE top up of £2000 per annum. Both projects offer the opportunity of industrial placement and placement at an overseas laboratory, as an integral part of the project.
54	UNDERSTANDING SPINEL FORMATION IN NUCLEAR WASTE GLASSESSSupervisors: Dr N C Hyatt, Dr R Hand and Dr M I Ojovan Project sponsor: National Nuclear Laboratory
This project aims to achieve a basic understanding of the reaction mechanisms of refractory spinel formation and dissolution vitrification of high activity liqours (HALs) in the Sellafield Waste Vitrification Plant. A statistical experimental design approach will be applied to determine the behaviour of key chemical and process variables governing spinel formation / dissolution during glass processing. The data acquired will be used to develop empirical models to predict the effects of these variables on the spinel volume fraction and crystallite size, glass viscosity and durability, by validation against full scale inactive glass samples prepared on the Vitrification Test Rig. This study will underpin optimisation of calcination and vitrification processes on WVP to minimise spinel formation, delivering dual benefits of reduced final waste volume and increased HAL throughput leading to hazard reduction associated with the storage of liquid wastes. This project will be based in ISL laboratories at Sheffield, but with the opportunity for extended placements at the Sellafield site and secondment to the full scale inactive vitrification plant. The project also offers the opportunity for a short placement at a US or French research group.
55	VITRIFICATION OF ORGANIC ION-EXCHANGE RESINS IN SUPPORT OF NUCLEAR DECOMMISSIONINGSupervisors: Dr N C Hyatt, Dr R Hand and Dr M I Ojovan Project sponsor: Magnox South Sites
Several hundred cubic metres of sulphonate organic ion exchange (IEX) resins are present on existing civil and MoD UK nuclear sites. Civil and defence nuclear power systems employ these resins in the decontamination of radioactive process fluids. However, such resins constitute a considerable storage and disposal challenge due to the radioactive inventory and polymer backbone, which is prone to radiolysis and degradation (leading to hydrogen generation and a colloidal transport pathway for adsorbed radionuclides). This project, in collaboration with Magnox South / Hinkley Point A Site aims to develop an understanding of chemical and physical processes involved in organic IEX resin vitrification to support definition of the full scale process envelope, selection of vitrification technology, design of melter off-gas system, and definition of product performance criteria. The project will focus on addressing three key knowledge gaps: 1. Understanding the interplay of glass composition and processing conditions on the volatilisation of Cs and Ru radioisotopes; 2. Determination of oxygen transfer rate in alkali borosilicate and alumino silicate glass, to control melt redox; 3. Empirical modelling and verification of volatilisation and redox behaviour to define a process envelope. The project will provide the opportunity for a research placement at Hinkley Point A radiochemical laboratories with specialist training in radiochemistry and radiation protection. The project also offers the opportunity for a secondment to a US research group.
Dr M I Ojovan
56	SOLID-STATE VITRIFICATION OF LEGACY NUCLEAR WASTES Supervisors: Dr M I Ojovan and Dr R J Hand
Vitrification of toxic wastes provides the best route for their immobilization but is expensive and requires high (>1100 ^oC) temperatures. These are very undesirable when dealing with nuclear wastes, the consequences of which are secondary radioactive waste, equipment corrosion and increased costs. This project will study the feasibility of standard temperature and pressure (STP) solid-state synthesis via mechanical milling (MM) to vitrify legacy nuclear waste streams. MM vitrification of crystalline solids is achieved by severe cyclic deformation in a ball-milling processes, which induces extensive plastic deformation. MM vitrification (amorphisation) of materials is not well understood. Solid-state amorphisation is controlled by the relative energetics of the defected crystalline and amorphous phases. It is assumed that amorphisation occurs when the free energy increase due to the mechanical defects G_d is higher that the free energy difference between the crystalline and amorphous states (G_c–G_a). Variables of the MM process critical in controlling amorphisation include mill energy and milling temperature. A more energetic milling process provides more lattice strain, development of finer grain sizes and finally more effective amorphisation. Typically a lower milling temperature accelerates the amorphisation process. MM has been used to produce amorphous structures in many metallic, oxide and non-oxide systems. This Immobilisation Science Laboratory project aims to study the feasibility of ball-milling (mechanochemical) synthesis to vitrify legacy nuclear waste streams.
Dr N B Milestone
57	INTERACTIONS OF HARDENED WASTEFORMS Supervisors: Dr N B Milestone and Dr Yun Bai
Currently, the effects of leaching on cemented wasteforms are not criteria for Nirex’s current Letter of Compliance. However, with the CoRWM decision to move to geological disposal, it is important that this be considered. If alternative cements are used as encapsulants it is important that any interactions, either with ground waters or between different cement types be known. Most leach tests currently used as standards in the industry have been designed around glass and are not applicable for cement based matrices. This project would develop a suitable leach testing procedure which could be accepted as a standard throughout the industry both in UK and hopefully EU who are further ahead in this area. This test would be applicable for testing both wasteforms and concrete with which the repository is likely to be lined. It is important to know which ions can be mobilised from the waste form and the effects they may have on surrounding concrete which is likely to be based on OPC. It will require an understanding of the likely ground waters as well as chemistry of cement.
58	USE OF SELECTIVE ADSORBENTS FOR REMOVAL OF SPECIFIC IONS FROM WASTE STREAMS Supervisors Dr N B Milestone and Dr N C Hyatt
In many waste streams, the concentration of toxic species may be low so that large quantities of solution must be treated to immobilise the species. Concentration through selective extraction offers potential to reduce the volume of waste that must be conditioned. This is already carried out through the use of natural zeolites such as clinoptilolite to remove Cs and Sr from waste processing liquors in the nuclear industry. Most industrial adsorbents are able to remove cations but the selective extraction of anions such as I^-, TcO₄^-. New work developed at PNNL based on grafting selective groups to supports such as zeolites or mesoporous silicas has shown that it is possible to prepare adsorbents which can selectively remove certain ions such as Hg²⁺ and anions such as ReO₄^-. This project will build on that work in collaboration with PNNL to prepare a series of selective adsorbents and examine ways in which these adsorbents can be prepared in a monolith form suitable for disposal in a suitable repository.