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E ffects of Zinc Addition on the Structure of Sodium Silicate Glass Thorsten Stechert

E ffects of Zinc Addition on the Structure of Sodium Silicate Glass Thorsten Stechert Imperial College London 11 April 2011 Huddersfield Supervised by: Prof. Robin Grimes and Dr. Luc Vandeperre. Outline. Problem definition Modelling glasses Zinc glasses Conclusions Further work.

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E ffects of Zinc Addition on the Structure of Sodium Silicate Glass Thorsten Stechert

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  1. Effects of Zinc Addition on the Structure of Sodium Silicate Glass Thorsten Stechert Imperial College London 11 April 2011 Huddersfield Supervised by: Prof. Robin Grimes and Dr. Luc Vandeperre

  2. Outline • Problem definition • Modelling glasses • Zinc glasses • Conclusions • Further work

  3. Problem Definition • Durability of HLW glass in humid environments is a key factor in long-term storage safety • Vitrified glass wastes are a key to the disposal of HLW from both reprocessing of current Magnox/MOX wastes and legacy wastes • Sodium borosilicate glasses are used, but are quite complex • The greatest advantage of glasses is the compositional variety in the waste that can be immobilised • BUT,that also means that these systems are highly complex (the base glass alone contains SiO2, B2O3, Na2O, Al2O3, CaO and ZrO2)

  4. Aim Modelling can help to isolate processes and trends, to help us understand: • The atomic structure of glasses • The effects of various elements on the glass structure • Stability of the glass phase, devitrification and segregation • Radiation damage due to recoil nuclei collisions • Effects of glass-crystal interfaces

  5. Why Zinc? • Several studies have shown improvements in oxide glass durability through small additions of ZnO • So far experimental studies have been unable to find conclusive information for the origin of the enhanced durability mechanism • Recent Diamond 2010 conference paper: “The role of Zn in model nuclear waste glasses studied by XAS” by N.J.Cassingham, M.C. Stennett, P.A. Bingham, G. Aquilanti and N.C. Hyatt • Zinc forms tetrahedral oxide structure, as proven by comparison of EXAFS results of a simulant glass with the mineral hemimorphite.

  6. What is Molecular Dynamics? • Molecular Dynamics is a technique whereby atoms are modelled by the interactions of ion pairs • The model relies on numerically solving Netwon’s second law of motion: • The potential energy of these pairs, which predicts the forces in the simulation consists of a short-range (van der Waal) and a long-range (electrostatic) part. The short-range potential used is by Pedone et al. • Molecular dynamics is originally intended for crystals, so how does it work for glass?

  7. Obtaining a Glass with Molecular Dynamics The potential energy of the Si-O pair interaction (Pedone potential)

  8. Obtaining a Glass with Molecular Dynamics • Glasses are non-crystalline, so an appropriate technique is needed to replicate the structure accurately • A melt-quench technique is used:

  9. Obtaining a Glass with Molecular Dynamics Modelled zinc sodium silicate glass with 20 mol% Na2O and 10 mol% ZnO content. Si shown in yellow, O in red, Zn in grey and Na in blue.

  10. Changes in the Pair Distribution Functions Si-O O-O Zn-O Simulated neutron pair distribution function, including a Zn-O peak at 1.86 Å. Modelled zinc sodium silicate glass with 20 mol% Na2O and 10 mol% ZnO content. Modelled sodium silicate glass with 20 mol% Na2O.

  11. Evaluating Mid-range Order • Network connectivity analysis measures the degree and the spread of crosslinking of the constituent polyhedra. E.g. Q4 is a tetrahedron with four bridging oxygens. • In addition to network connectivity analysis, we use ring size analysis to reveal structural information of the glass:

  12. Network Connectivity Change in network connectivity due to zinc addition

  13. Ringsize Distribution Change in ring size distribution due to zinc addition.

  14. Effect on Sodium Distribution 8 Å cross-section of sodium silicate glass (left) and zinc sodium silicate glass (right), showing the distribution of sodium atoms.

  15. Conclusions • Zinc is predicted to perform the role of a network former in sodium silicate glasses • Forms tetrahedra, which form joint polymeric chains with the SiOtetrahedra • ZnO addition seems to alter the distribution of sodium in sodium silicate glasses

  16. Further Work • Carry out further analysis to confirm a change in the alkali distribution for various alkali species (sodium, lithium, caesium?) • Investigate the effect on the radiation damage resistance of the glass • A more advanced model may also include Boron, as a secondary glass network former

  17. Questions? Thank you for listening! We would like to thank the DIAMOND consortium via the EPSRC and the Nuclear Decomissioning Authority (NDA) for funding this work thorsten.stechert@imperial.ac.uk

  18. Further Work

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