1. DIAMOND Decommissioning, Immobilisation and Management of Nuclear Wastes for Disposal 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. DIAMOND Decommissioning, Immobilisation and Management of Nuclear Wastes for Disposal Outline I.Problem definition II.Modelling glasses III.Zinc glasses IV.Conclusions V.Further work

3. DIAMOND Decommissioning, Immobilisation and Management of Nuclear Wastes for Disposal 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 SiO 2, B 2 O 3, Na 2 O, Al 2 O 3, CaO and ZrO 2 )

4. DIAMOND Decommissioning, Immobilisation and Management of Nuclear Wastes for Disposal 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. DIAMOND Decommissioning, Immobilisation and Management of Nuclear Wastes for Disposal 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. DIAMOND Decommissioning, Immobilisation and Management of Nuclear Wastes for Disposal 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? What is Molecular Dynamics?

7. DIAMOND Decommissioning, Immobilisation and Management of Nuclear Wastes for Disposal Obtaining a Glass with Molecular Dynamics The potential energy of the Si-O pair interaction (Pedone potential)

8. DIAMOND Decommissioning, Immobilisation and Management of Nuclear Wastes for Disposal 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. DIAMOND Decommissioning, Immobilisation and Management of Nuclear Wastes for Disposal Obtaining a Glass with Molecular Dynamics Modelled zinc sodium silicate glass with 20 mol% Na 2 O and 10 mol% ZnO content. Si shown in yellow, O in red, Zn in grey and Na in blue.

10. DIAMOND Decommissioning, Immobilisation and Management of Nuclear Wastes for Disposal Changes in the Pair Distribution Functions Simulated neutron pair distribution function, including a Zn-O peak at 1.86 Å. Modelled zinc sodium silicate glass with 20 mol% Na 2 O and 10 mol% ZnO content. Modelled sodium silicate glass with 20 mol% Na 2 O. Si-O Zn-O O-O

11. DIAMOND Decommissioning, Immobilisation and Management of Nuclear Wastes for Disposal 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. DIAMOND Decommissioning, Immobilisation and Management of Nuclear Wastes for Disposal Network Connectivity Change in network connectivity due to zinc addition

13. DIAMOND Decommissioning, Immobilisation and Management of Nuclear Wastes for Disposal Ringsize Distribution Change in ring size distribution due to zinc addition.

14. DIAMOND Decommissioning, Immobilisation and Management of Nuclear Wastes for Disposal 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. DIAMOND Decommissioning, Immobilisation and Management of Nuclear Wastes for Disposal 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 SiO tetrahedra ZnO addition seems to alter the distribution of sodium in sodium silicate glasses

16. DIAMOND Decommissioning, Immobilisation and Management of Nuclear Wastes for Disposal 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. DIAMOND Decommissioning, Immobilisation and Management of Nuclear Wastes for Disposal 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. DIAMOND Decommissioning, Immobilisation and Management of Nuclear Wastes for Disposal Further Work