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Radiation Damage effects in alkali silicate glasses Thorsten Stechert Imperial College London 15/16 December 2010 Manchester PowerPoint PPT Presentation


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Radiation Damage effects in alkali silicate glasses Thorsten Stechert Imperial College London 15/16 December 2010 Manchester Supervised by: Prof. Robin Grimes and Dr. Luc Vandeperre. Outline. Problem definition Modelling glasses Radiation damage effects Conclusions Further Work.

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

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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


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Outline

  • Problem definition

  • Modelling glasses

  • Radiation damage effects

  • Conclusions

  • Further Work


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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)


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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


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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?


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Obtaining a Glass with Molecular Dynamics

The potential energy of the Si-O pair interaction (Pedone potential)


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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:


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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.


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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).


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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


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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).


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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:


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Radiation Damage Effects

Change in ring size after 50 cascades


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Radiation Damage Effects

Change in network connectivity after 50 cascades


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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


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Further Work


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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


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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


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