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 2011Huddersfield

Supervised by: Prof. Robin Grimes and Dr. Luc Vandeperre

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

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 SiO2, B2O3, Na2O, Al2O3, CaO and ZrO2)

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

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.

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?

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)

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:

DIAMOND Decommissioning, Immobilisation and Management of Nuclear Wastes for Disposal

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.

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% Na2O and 10 mol% ZnO content. Modelled sodium silicate glass with 20 mol% Na2O.

Si-O

Zn-O

O-O

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:

DIAMOND Decommissioning, Immobilisation and Management of Nuclear Wastes for Disposal

Network Connectivity

Change in network connectivity due to zinc addition

DIAMOND Decommissioning, Immobilisation and Management of Nuclear Wastes for Disposal

Ringsize Distribution

Change in ring size distribution due to zinc addition.

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.

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

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

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

DIAMOND Decommissioning, Immobilisation and Management of Nuclear Wastes for Disposal

Further Work