Postgraduate Study in the Immobilisation Science Laboratory

The ISL is seeking suitably qualified candidates for the following postgraduate research projects available from October 1st 2008. Some of the projects will involve collaboration with Nexia Solutions who will supply expertise and research support. Please note that for UK students many projects will attract the EPSRC award recommended studentship rate (£12,900 p.a. as from 1st October 2008).

For details on how to apply please contact the departmental postgraduate admissions secretary Mrs Karen Burton

E-mail: k.a.burton@sheffield.ac.uk

28 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.

29 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.

31 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 tests. 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 (cerium and lanthanide elements, respectively), with the aim of extending this work using radionuclides 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. German language tuition would be provided for placement at ITU Karlsrhue.

32 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.

33 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.

35 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.

36 THE ROLE OF PORE WATER IN CEMENTED WASTES
Supervisor: Dr NB 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.

37 SULPHATE BASED CEMENTS
Supervisors: 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.

39 ION-IMPLANTATION AND IRRADIATION STUDIES IN OXIDE GLASSES
Supervisors: G Möbus

High-resolution microstructure and chemical co-ordination of ion and electron irradiated oxide glasses are to be analysed using state-of-the-art electron microscopy and spectroscopy equipment. Special emphasis is on the accumulation of defects into clusters, induced phase separation and formation of crystalline microphases in the glass matrix. The background of this study comes from its relevance to two fields of applications: i) simulation of irradiation damage in radioactive glasses for actinide immobilisation, ii) optical and electronic properties of modified glass near-surface regions (graded index, graded absorption, microphase quantum confinement etc). Characterisation techniques employed comprises: field-emission transmission electron microscopy and focused ion beam sectioning.

40 CO-ORDINATION AND VALENCE CHEMISTRY OF CATIONS IN OXIDE GLASSES, CERAMICS AND NANOSTRUCTURED 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 electron microscope. Unlike with most other spectroscopies, the TEM provides measurements with high spatial resolution and can also observe dynamical changes whether due to irradiation or temperature changes. Furthermore, fine details in such spectra can be correlated to the oxidation state of structural cations or doping atoms in such phases. A choice of individual groups of materials is available to be selected and arranged upon the start of the project:
(i) Oxide glasses with various doping levels and nanoscale precipitates for optical or biomedical applications.
(ii) Coatings and multilayered structures, such as diamond like carbon or metallic multilayers.
(iii) Materials for radionuclide immobilisation including multi-component glasses and titanate ceramics.

41 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.

47 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

49 DEVELOPMENT OF DISSIPATIVE PARTICLE DYNAMICS METHODOLOGY
Supervisors: Dr K P Travis, Mr Mark Bankhead and Mr Scott Owens (Nexia Solutions)

Dissipative Particle Dynamics (DPD) is one of the most promising methods developed in the last 20 years for modelling complex multi-phase materials from above the molecular level to the continuum. Until very recently the lack of a coherent method of parameterising DPD from experimental or molecular simulation data has limited it to studying the generic phase behaviour of polymer solutions. Research at Sheffield has partly solved this problem by showing a deep correspondence between DPD and Regular Solution Theory, paving the way to model solid/solid, solid/liquid and liquid/liquid phase equilibria, as well as modelling the rheology of complex multiphase systems. This project comprises of two parts: The first part is concerned with creating a DPD modelling code incorporating the latest advances made in the methodology. The second part of the work will be concerned with improving the methodology and applying it to problems of interest to the nuclear industry. This project would suit candidates with an interest in thermodynamics, mathematics, programming, and nuclear waste remediation.

50 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 ?C) 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 Gd is higher that the free energy difference between the crystalline and amorphous states (Gc–Ga). 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.

51 THE INFLUENCE OF CATIONIC AND ANIONIC SUBSTITUTIONS ON THE STRUCTURE OF CALCIUM PHOSPHATE MINERAL PHASES
Supervisors: Dr R J Hand and Dr N C Hyatt
(This project has a CASE award with AWE and is only open to UK Nationals)

Calcium phosphate is currently a candidate for the immobilization of special categories of halide-containing radioactive wastes. In addition to Cl- and F- these wastes also contain a variety of cationic species, A, including, for example, tri- and tetra-valent actinides, together with K+, Ni2+, Mg2+, Zn2+, Al3+ and Ta5+. These substitutions are incorporated chemically into apatite and spodiosite mineral phases, ie. A5(PO4)3(Cl, F, O) and A2(PO4)(Cl, F, O). These phases are notoriously non-stoichiometric; hence their ability to substitute a very wide variety of different valent species. Little is known about the detailed structures of these substituted phases, and the precise composition ranges over which they will form are also unclear. Knowledge of the way in which the resulting structures are likely to influence the stability of the mineral phases formed is also limited. The investigation will therefore be aimed at gaining a clearer understanding of the structure of the apatite and spodiosite mineral phases and their solid solutions in which a variety of different cations and anions are substituted, and how these substitutions affect the overall stability of the resulting products. Studies will be based on a variety of analytical techniques including XRD, NMR, ESR, neutron diffraction and EXAFS. Both scanning and transmission electron microscopy coupled with EDS will also be utilised. The generation of appropriate thermodynamic data and phase diagrams may also form an integral part of the investigation. All of the materials studied will be non-radioactive and thus actinide surrogates, eg. Hf, Ce and Sm will be used where appropriate.

52 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.

53 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.