Vitrification of high zirconia nuclear waste streams:

1. Vitrification of high zirconia nuclear waste streams: An EXAFS study

3. Introduction Two of the major issues for UK high level waste vitrification New and legacy waste streams Long term durability of vitrified product Man-made glass existed since Egyptians Glass science only really existed in last hundred years Prof. W. E. S. Turner

4. Structural model First recognisable model proposed by Zachariasen (1932): Similarity in structure of crystals and glass e.g. bonding, extended 3D “network” Glass network non-periodic Silicate glass was made up of silica (network forming) tetrahedra (SiO4) Network modifiers (e.g. Na+) breaks silicate network. i.e. acts as a flux

5. Zachariasen-Warren, 1941 The continuous random network theory Still widely used and still contentious Shows short and some medium range order Boron acts as a network former

6. Cations and the structure of glass Many elements added to change properties of glass Only relatively recently people have begun to understood why the properties change E.g. colour of glass Co2+ in silicate glass (CN=4) give a blue colour In certain borosilicate glasses (CN=6) the colour is pink Any colour so long as it black!!Any colour so long as it black!!

7. Glass and nuclear waste Glass developed into important nuclear waste technology Vitrification plays a vital role in the disposal of nuclear waste (i.e. reprocessed calcine) Glass held at ~1050oC for 7 hours Tm <1150-1200 ºC to minimize volatilisation of fission products (e.g. 137Cs) Long term durability and high waste loadings critical

8. Zirconia and nuclear waste Appears in fuel rods as a fission product and as uranium oxide fuel cladding (Zircalloy) Zr causes problems such as: Low solubility in borosilicate glass Refractory nature of oxides (Tm= 2700oC) Crystallisation of Zr oxides Increases viscosity Advantage Increases durability Increases strength

9. Alternative dissolution techniques New reprocessing process Complete chemical dissolution of Zircalloy fuel rods Involves significant amounts of fuel assembly components being taken into solution HELP!!! Waste information and shall I say ISL STANDARD WASTE COMPOTIONSHELP!!! Waste information and shall I say ISL STANDARD WASTE COMPOTIONS

10. Glass melting Started with basic ‘MW’ sodium lithium borosilicate base glass Added various amounts of ZrO2 Noticeable increase in viscosity with Zr additions Great difficulty in getting ZrO2 to dissolve in glass Made worse if used alumina crucible as compared to platinum crucibles. Also maded full (simulant) waste stream glasses Blend and Magnox - 25 wt% waste loading (ISL reference compositions) Chemical dissolution (High Zr) – 15 wt% waste loading (Matlack, 1999)

11. Glass compositions studied

12. Check where Zr is SEM and XRD confirm Zr is in glass matrix not present as crystals RuO2 crystals in blend, high Zr and Magnox glass High Zr glass shows very small number of ZrO2 crystals

13. X-ray Absorption Spectroscopy Technique for examining short range order in materials Uses characteristic X-rays to probe local environment of a specific ion Very useful for amorphous materials where there is no long range structure X-rays from a synchrotron radiation source are transmitted through sample

14. EXAFS details Absorption of X-ray photon, emmission of photoelectron In a monatomic gas get smooth decrease in absorption In all other materials get ‘wiggles’ Caused by interaction of emitted photoelectron with neighboring atoms Interference effects changes the probability of X-ray absorption EXAFS oscillations 30-2000 eV past edge

15. EXAFS - what it can show us These EXAFS oscillations determined by: Number, Nj, and type of scatters in successive co-ordination shells Absorber – scatterer distance, Rj Static and dynamic disorder – Debye-Waller factor, 2s2

16. EXAFS data CO I PUT IN THE EXAFS EQUATIONCO I PUT IN THE EXAFS EQUATION

17. Data analysis

18. Elements not resolved Elements with low electron density E.g. Li, B, etc.

19. First model

20. Second model

21. Third model

22. Fourth model

23. Fifth model

24. EXAFS results

25. All results for Second model

26. Change in properties with composition Zirconium increases strength of silicate network by direct bonding Sodium needed for charge balance This strengthening of silicate network believed to cause variation in chemical and mechanical properties

27. Durability – Soda-silica glass Work done in Sheffield in 1925 was first systematic survey of glass durability Powdered and washed samples boiled for 1 hr variously in: Water, NaOH, Na2CO3, and HCl(aq) ZrO2 bearing glasses found to most durable under all conditions

28. Durability – borosilicate glass Powdered and washed samples under room temperature static leach conditions Shows overall increase in durability for all elements Abnormality at low n for Na and B Possibly caused by small scale phase separation or change in alternation layer morphology

29. Practicalities High zirconia waste can be vitrified with a waste loading of at least 15 wt% Zirconia: interacts and strengthens silicate glass network significantly improvement on durability of borosilicate glass increases viscosity significantly however, additions allow low temperature melting May cause phase separation Further work to be carried out on Zr and durability

30. Conclusions Chemical dissolution sourced waste can be vitrified with a waste loading of at least 15 wt% Zr becomes part of the silicate network Zr increases durability of glass Work still needed into role of boron and lithium in high zirconia borosilicate glasses.

31. Any Questions? Thank you to: Neil Hyatt, Karl Travis, Russell Hand and Ewan Madrell EPSRC and Nexia Solutions for financial support