Research Projects: Nuclear Materials

113 VITRIFICATION AND RE-USE OF WASTE MATERIALS
Supervisors: Dr R J Hand and Dr H Kinoshita

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.

114 SIMULATING DIFFUSION IN CERAMICS AND MINERALS
Supervisor: Professor J Harding

How fast atoms move and where they end up is a major issue in understanding (and so controlling the properties of ceramics). The first is the problem of diffusion; the second the problem of segregation. This project will investigate the mechanisms of diffusion for a range of ceramics from perovskites to pyrochlores and garnets. It will also consider the segregation of atoms and ions to surfaces and grain boundaries and what difference this makes to the properties of these structures. A range of static and dynamic simulation methods will be used including ab initio methods. Depending on the materials chosen, the project could link to a range of experimental work in the Department: from batteries and ferroelectric materials to nuclear waste disposal. The codes to do the calculations are already exist, but there will be opportunities for writing scripts and codes if people are interested.

115 CERAMIC WASTEFORMS FOR IMMOBILISATION OF FISSION PRODUCTS, MINOR ACTINIDES AND PLUTONIUM
Supervisors: Prof N C Hyatt and Dr E R Maddrell (National Nuclear Laboratory)

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: BaxTi8-2xM2xO16
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 - 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. There is also the opportunity to take relevant MSc level modules in nuclear science and engineering, and a programme of professional skills development activities, offered by the Nuclear First Doctoral Training Centre.

PhD top-up awards are available to support projects in this area – please ask for details.

116 RADIATION DAMAGE EFFECTS IN NUCLEAR WASTE GLASSES STUDIED BY MULTI-ELEMENT X-RAY ABSORPTION SPECTROSCOPY
Supervisors: Prof 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. Understanding of the effect of radiation damage (from β,γ and α decay) on the long term stability of nuclear waste glasses is currently lacking. However, such an understanding is critical in order to accurately predict the long term behaviour of waste loaded glasses under disposal conditions. Preliminary studies have demonstrated that, radiation enhanced diffusion of alkali cations, may lead to depolymerisation of the glass network. Ultimately this could lead to radiation induced phase separation, with a detrimental effect on glass durability.

Project. The aim of this project is to develop an holistic understanding of structure-composition-radiation damage effects in alkali borosilicate glasses. This study will take a systematic approach using electron and swift ion implantation of non-radioactive specimens, to simulate the effects β and α decay in a range of different glass compositions. A combination of nuclear magnetic resonance, Mossbauer and X-ray absorption spectroscopies will be applied to quantify the changes in the local structure (i.e. oxidation state, co-ordination number, and nearest neighbours) of key glass elements (B, Si, alkali, fission product and actinide surrogate elements). The results of this study will be compared with the results of modelling simulations of radiation damage effects undertaken by collaborating research groups.

Training. This project will involve melting and laboratory characterisation of nuclear waste glass compositions, combined with the use ion implantation facilities and synchrotron radiation sources in the UK, USA and Europe. Training will be provided in the theoretical and basis of spectroscopic techniques and analysis of data. There is also the opportunity to take relevant MSc level modules in nuclear science and engineering, and a programme of professional skills development activities, offered by the Nuclear First Doctoral Training Centre.

PhD top-up awards are available to support projects in this area – please ask for details.

117 THE NANOSCALE CHEMISTRY AND MICROSTRUCTURE OF HYDROTHEMALLY ALTERED MATERIALS FOR NUCLEAR WASTE IMMOBILISATION
Supervisors: Prof N C Hyatt and Prof. W M Rainforth

Background. Geological waste disposal in a mined repository is now accepted as the “best available approach” for long term management of the UK nuclear waste legacy, following the report of CoRWM. Wasteform dissolution is a key contributor to the source term in the performance assessment model for a Geological Disposal Facility (GDF). Hence, an understanding of the mechanism of wasteform dissolution underpins both prediction of disposal site evolution and public confidence in the safety of the chosen disposal option.

Project. Dissolution involves reaction between the exposed surface area of the wasteform material and the alteration fluid, resulting in an reaction interface between altered surface and pristine bulk. Characterisation of such alteration microstructures is essential in differentiating, for example, between diffusion controlled vs. dissolution – re-precipitation mechanisms of alteration, characterised by chemical gradient or discontinuity on the nano-scale at the alteration interface. This project will apply a combination of TEM techniques to investigate the microstructure and chemistry of hydrothermally altered ceramics, glasses and natural mineral analogues of relevance to nuclear waste immobilisation and disposal. In particular, we will develop FIB preparation methods to extract TEM specimens in the location of etch pits, phase separated regions, and grain boundaries to develop an understanding of the role of local microstructure and composition on alteration behaviour.

Training. This project will involve solid and liquid based ceramic processing methods and the use of a range of characterisation techniques, including: electron microscopy, focussed ion beam milling, and Raman and X-ray absorption spectroscopies. There is also the opportunity to take relevant MSc level modules in nuclear science and engineering, and a programme of professional skills development activities, offered by the Nuclear First Doctoral Training Centre.

PhD top-up awards are available to support projects in this area – please ask for details.

118 ELECTROCHEMICAL LEACHING OF RADIOACTIVE WASTE FORMS
Supervisor: Dr H Kinoshita

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.

119 CO2 STORAGE IN RECYCLED CEMENTITIOUS MATERIALS
Supervisor: Dr H Kinoshita

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.

120 BEHAVIOUR OF URANIUM IN CEMENTITIOUS MATRICES
Supervisor: Dr H Kinoshita

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.

121 SPECTROSCOPIC AND 3D-IMAGING CHARACTERISATION OF GLASS-NANOCOMPOSITES FOR VITRIFICATION OF RADIONUCLIDES
Supervisors: Dr G Möbus and Prof R J Hand

Physical and chemical properties of glasses and ceramics often depend on the coordination and valence states of spurious elements or other functional cations. A particular example is the multi-element glass matrix used to immobilise radioactive elements. In this project, the emphasis is on glass-compositions with high load of extra cations (non-radioactive simulants, e.g. lanthanides), such that solubility limits are exceeded, and precipitation of small crystals occurs. The technique of electron energy loss spectroscopy in the electron microscope is used to spatially map the distribution of coordination and valence parameters in a selection of such glass nanocomposites. Solubility limits, crystallisation sequences, and annealing processes of multi-component borosilicate glasses will be explored. Metrological analysis of the three-dimensional particle distribution in the glass matrix will form another essential part of the project.

122 STEEL CORROSION IN PORE SOLUTIONS OF ALKALI-ACTIVATED CONCRETES
Supervisor: Professor J Provis

The most fundamental factor required for the durability of any steel-reinforced concrete is the ability to develop and maintain an environment with appropriate chemical (and in particular electrochemical) conditions to enable the embedded steel to remain in a passive state. Steel corrosion is, on a worldwide basis, the predominant cause of premature failure of reinforced concrete elements; when steel rusts, it expands, and the resulting tensile stresses cause cracking and loss of performance in a structural sense. The chemistry of steel corrosion in the pore solution environment of concrete is complex, and far from fully understood, even for traditional Portland cement-based concretes. With the development and introduction of less environmentally-damaging binders, in particular alkali-activated/”geopolymer” concretes, the situation becomes even more complex. These materials can have pore solution environments which are very different (in both chemical and electrochemical senses) from the pore solution environments within Portland cement concretes, but are similarly required to hold the steel in a passive state if they are to act successfully as reinforced concretes. Decades of in-service observations of steel-reinforced alkali-activated concretes in various parts of the world have shown that they are able to provide a high degree of protection for embedded steel components, and on this basis, commercialization of these materials is proceeding. However, given that the chemical environment within a geopolymer binder is so different from that which is observed within Portland cement, detailed scientific investigation in this area is necessary to answer durability-related questions from a fundamental standpoint. This project will address this question in detail. Suitable chemical environments to simulate alkali-activated binder pore solutions will be developed and validated, and in-situ and ex-situ determination of the factors controlling corrosion will be conducted through the use of standard and advanced analytical tools.

123 THE ROLE OF MINOR ELEMENTS IN GEOPOLYMER FORMATION
Professor J.L. Provis

While a good deal of attention has been paid in recent years to the roles played by the silicon, aluminium and calcium supplied to the geopolymer formation reaction by different (solid and solution) mix components, there has been much less detailed investigation of the (often very important) roles of “minor” elements in geopolymer gel formation. This project will be focused on important “minor” elements (present at levels of 1-10% in fly ash and slag), in particular iron (ferric and ferrous), sulfur (in various oxidation states), boron and magnesium. Speciation and reactivity studies on each of these elements under conditions simulating geopolymer-forming reaction mixes will be conducted.

Iron in particular is generally supplied by fly ash particles, and its role in geopolymer formation is as yet unclear; some researchers have claimed that it has no influence on geopolymer formation, others have identified it as playing a key role in immobilisation of specific toxic species when geopolymer binders are used to immobilise hazardous wastes, and yet others have claimed that it shows a strongly deleterious effect on geopolymer gel formation. The effect of the form in which iron is supplied, including its availability from the different glassy phases which are present in fly ash, will be analysed in detail through the development and study of model systems, with a view towards developing a more detailed understanding of how these phases interact with aluminosilicates during geopolymer formation.

124 ENVIRONMENTAL CYCLING RESISTANCE OF GEOPOLYMER BINDERS
Supervisor: Professor J L Provis

Geopolymers have been proposed, and are currently being commercialised, as an environmentally-friendly alternative to Portland cement as a binder material in concretes. Concrete production is currently responsible for around 5-8% of worldwide human-derived CO2 emissions, and geopolymer technology provides the potential to reduce this by around 80-90% per ton of concrete, for comparable or better performance.

Determining and analysing the durability of geopolymer binders and concretes exposed in a cyclic manner to aggressive environments (in particular wet/dry cycling and ‘normal’ environmental exposure, as well as chloride and freeze/thaw conditions) has been identified as being important in the wider commercial acceptance of geopolymer technology. This project will form part of a major international research effort directed at determining the durability, degradation mechanisms and expected lifetimes of geopolymer concretes, and will be conducted with key input and collaboration from industrial geopolymer producers.

The project will involve analysis of geopolymers by a variety of experimental techniques, ranging from straightforward gravimetric analysis and strength testing to the use of advanced synchrotron beamline instruments to study areas such as tomography and phase evolution over time. The outcomes will provide for the first time detailed descriptions of the nanostructural and microstructural evolution of geopolymer binders and concretes during cyclic exposure to aggressive environments.

125 TETRAGONAL TUNGSTEN BRONZE STRUCTURED CERAMICS AS POTENTIAL NEW IMMOBILISATION MATRICIES FOR ACTINIDE RICH WASTE STREAMS
Supervisor: Dr M C Stennett

The tetragonal tungsten bronze (TTB) structure is a derivative of the perovskite structure (ABO3) but unlike simple perovskites, which are composed of a simple cubic array of corner sharing octahedral, sharing occurs in such a way as to create an arrangement of pentagonal, square and triangular sites in projection down the c-axis. As a result of the presence of these different sites the TTB structure is potentially suitable for the immobilisation of a wide variety of different size radionuclides. The TTB structure can be described by the formula [A12A2]C2B5O15 where the A sites will accomodate large monovalent (K+, Rb+), divalent (Ba2+, Sr2+) and trivalent (La3+, Ce3+) cations and the B sites will accommodate smaller cations such as Nb5+, Ti4+, Zr4+, Al3+, and Fe3+. The aim of the project is to synthesis TTB structured compounds and investigate the solubility limits of actinide analogues such as Ce3+/Ce4+ (and time permitting U4+/U6+) by a solid state ceramic route. The project will involve the application of solid state ceramic synthesis techniques to investigate the solid solution limits of actinide analogues in the TTB structure. A range of techniques, including X-ray diffraction, electron diffraction and electron microscopy will be employed to characterise the materials produced.

126 MOLTEN SALT SYNTHESIS OF CERAMICS FOR THE IMMOBILISATION OF RADIONUCLIDES
Supervisors: Dr M C Stennett

Crystalline ceramic phases are promising hosts for radionuclides arising from the nuclear fuel cycle. Single phase ceramics are usually produced by solid-state synthesis (SSS) which consists of solid-state reaction between oxide and/or carbonate precursors via repeated cycles of milling and reaction at high temperature. Repeated cycling at high temperature can lead to unwanted volatilisation / decomposition of reactants / product phases and is expensive. Molten salt synthesis (MSS) is an alternative to SSS and used low melting point water soluble salts as fluxes to promote diffusion of constituent components and improve homogeneity. MSS promotes phase formation at lower temperatures and can yield high surface area reactive powders which when consolidated will densify at relatively low temperatures compared to powders derived by SSS. The aim of this project is to explore the use of MSS to prepared single phase ceramics phases and demonstrate the suitability of the route for the incorporation of radionuclides. This project will involve the synthesis of ceramic phases by solid state and molten salt synthesis routes. A range of techniques, including: X-ray diffraction, electron microscopy, particle size and surface area analysis will be employed to characterise the materials produced.

127 RADIATION DAMAGE EFFECTS
Supervisors: Dr K Whittle and Prof W M Rainforth

For future nuclear technologies to be viable, the effects of high levels of radiation damage need to be quantified, understood, and mitigated. Materials currently being proposed, e.g. ODS materials, oxide and carbide based ceramics are poorly understood to high levels of damage, e.g. 400 dpa. This project will address these issues by using both in-situ and ex-situ irradiations. These irradiations will be undertaken with characterisation before and after irradiation, e.g. TEM, XRD, Raman spectroscopy, XAFS etc, on a range of specific model materials developing models to predict long term radiation damage effects. This work will involve collaborations with Argonne National Laboratory, USA, the Australian National University (ANU), and ANSTO (Australian Nuclear Science and Technology Organisation).

128 RADIATION DAMAGE AND RECOVERY PROCESSES
Supervisors: Dr K Whittle and Prof N C Hyatt

Understanding the processes involved, and the factors that influence, radiation damage and recovery are vital in understanding the long-term effects of damage and how it can be mitigated. This is difficult to do experimentally but can be modelled using a range of techniques. The project will use a range of simulation techniques examining specific systems for which there are experimental damage results. The results can then be used to potentially develop new materials with higher immunity to damage.

129 NEW WASTE FORMS FOR A NEW GENERATION
Supervisors: Dr K Whittle and Prof N C Hyatt

Development of new materials, primarily for use as Inert Matrix Fuels, and problematic waste streams is an important component in the future development of GenIV fission reactors. However, there are still important questions that need answering in the basic materials science, e.g. what is the structure adopted by the waste form, and where would the radioactive species go? This question directly impacts the future acceptance of a material, both in regulatory and predictive modelling for a repository. This project, using a range of experimental techniques, on both active and inactive materials will study both new and proposed waste forms, to more fully understand the structure and how it can be improved in the next generation of materials.

130 LONG RANGE DISORDER – SHORT RANGE ORDER
Supervisors: Dr K Whittle and Prof W M Rainforth

In many systems there is the potential for long-range disorder, but which retains short-range order. The short-range order is visible in electron diffraction, but poorly visible in X-ray and neutron diffraction. This observation has two potential uses, firstly it can be used to develop new materials with nuclear applications, e.g. fluorites as both a fuel and waste form. While at the same time the diffuse scattering can be used to study the effects of damage. This project will use ion beam irradiation, coupled with actinide doping to investigate this observation further and use it to develop further the models that exist in predicting radiation damage effects. Predominantly this work will examine ceramics, but ceramic-metallic hybrids are possible. This work will involve collaborations with ANSTO (Australian Nuclear Science and Technology Organisation), and the Australian National University.