Radiation Damage effects in alkali silicate glasses Thorsten Stechert Imperial College London 15/16 December 2010 Manche

1. Radiation Damage effects in alkali silicate glasses ThorstenStechert Imperial College London 15/16 December 2010 Manchester Supervised by: Prof. Robin Grimes and Dr. Luc Vandeperre

2. Outline • Problem definition • Modelling glasses • Radiation damage effects • Conclusions • Further Work

3. Problem Definition • 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 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 • Radiation damage due to recoil nuclei collisions • Stability of the glass phase, devitrification and segregation • Effects of glass-crystal interfaces

5. 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?

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

7. 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:

8. Obtaining a Glass with Molecular Dynamics Modelled silicate glass with 30 mol% Na2O content. Si shown in yellow, O in red and Na in blue.

9. Obtaining a Glass with Molecular Dynamics A comparison of the simulated total correlation function with neutron diffraction data by Wright et al. (Qmax = 22.88Å).

10. Modelling Radiation Damage • There are two principle types of radiation damage: • Collision damage • Ionisation damage • In molecular dynamics, only collision damage can be modelled directly • Alpha decay events generate recoil nuclei with energies of about 100 keV • We model the damage caused by recoil nuclei with the primary knock-on atom technique (PKA), where similarly high kinetic energy (e.g. 10 keV) is assigned to a single atom

11. The PKA model (a) (b) (d) (c) Evolution of a 10 keV silicon cascade: showing atoms displaced by 5 Å or more, with trajectories (a) and displaced atoms (b) shown after 50 timesteps. The evolution of damage is shown after 100 timesteps (c) and 250 timesteps (d).

12. Radiation Damage Effects • Even after 50 cascades, the observed volume change was below 0.2 % • Whilst there is hardly any change in the volume, structural changes may exist. • In addition to network connectivity analysis, we use ring size analysis to reveal structural information of the glass:

13. Radiation Damage Effects Change in ring size after 50 cascades

14. Radiation Damage Effects Change in network connectivity after 50 cascades

15. Conclusions • Molecular Dynamics can generate glass structures similar to those used for the vitrification of HLW • Good agreement with experiment has been achieved • Depolymerisation as a result of the damage cascades is observed • Collision radiation damage seems to be localised in the highly polymerised areas of the glass • Whilst the structure of the glass is affected, the volume of the glass remains largely unchanged

16. Further Work

17. Further Work • Due to processing limitations, the inclusion of glass-crystal interfaces within the glass cannot be avoided • Additionally, radioactive Isotopes of certain elements (ruthenium in particular) can segregate during the melting process and form a refractory layer • The study of these interfaces and the effect of self-irradiation on them may lead to new insights on the diffusion of radionuclides and the long-term durability of glasses • A more advanced model may also include Boron, as a secondary glass network former, and Cs, which occurs in nuclear waste and may act as a network modifier

18. Questions? Thank you for listening! We would like to thank the DIAMOND consortium, the EPSRC and the Nuclear Decomissioning Authority (NDA) for funding this work