Vitrification of high zirconia nuclear waste streams:

1
Vitrification of high zirconia nuclear waste
streams

• An EXAFS study

Andrew Connelly,
Dr. N. C. Hyatt and Dr. K. P. Travis
2
Content

• Glass and nuclear waste
• Zirconia nuclear waste and glass
• EXAFS techniques and data analysis
• Results and discussion
• Conclusions
• Questions

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

For better or worse I am part and parcel of
the glass industry
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

O
Si
Zachariasen, W. H., Jou. Am. Chem. Soc, 54,
3841-3851, 1932
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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

Warren, B. E. Jou. Am. Chem. Soc.,24, 256-261,
1941
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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 (CN4) give a blue colour
• In certain borosilicate glasses (CN6) the colour
  is pink

http//antoine.frostburg.edu/chem/senese/101/index
.shtml
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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 lt1150-1200 ºC to minimize volatilisation of
  fission products (e.g. 137Cs)
• Long term durability and high waste loadings
  critical

After Marples, J. Glass Tech., 1988, 29(6)
p.230-247
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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

http//www.areva.com
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Alternative dissolution techniques

• New reprocessing process
• Complete chemical dissolution of Zircalloy fuel
  rods
• Involves significant amounts of fuel assembly
  components being taken into solution

http//www.areva.com
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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)

Matlack, K. mat. Sci. Res. Soc. Sym. Pro, 1999,
556, 247-254
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Glass compositions studied
Can be reduced to 1050 ºC with small additions as
replacement for SiO2
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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

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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

http//www.p-ng.si/arcon/xas/xas/xas.htm
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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

http//www.p-ng.si/arcon/xas/xas/xas.htm
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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

Phase shift
Absorption
Amplitude factor
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EXAFS data
1. Data was collected on station 16.5 at the SRS
in Daresbury. Energy scanned to 12 Å-1 using
(220) Si monochromator.
XANES
2. Two traces per sample, data summed and then
background subtracted using Exspline.
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Data analysis
3. Map a theoretical fit to the experimental data
using Excurv98 and the EXAFS equation.
4. Fourier Transform to give radial distribution
function (RDF) with Zr at 0 Å
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Elements not resolved

• Elements with low electron density
• E.g. Li, B, etc.

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First model
Zr
O
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Second model
Zr
O
Si
Previous best
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Third model
Na
Zr
O
Si
Previous best
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Fourth model
Zr
O
Zr
Si
Comparison
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Fifth model
Na
Zr
O
Si
Comparison
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EXAFS results
Zr
O
Si
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All results for Second model
Zr
O
Si
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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

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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

6SiO2(2-x) Na2O xRO or R2O3 or RO2
Dimbleby, V. Turner, W. E. S. Jou. Soc. Gla.
Tech., 1925, 10, p304-358
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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

Lobanova, M. Mat. Res. Soc. Proc., 713, 571-579,
2002
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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

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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.

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Any Questions?
Andrew Connelly ISL University of
Sheffield a.connelly_at_shef.ac.uk

• Thank you to
• Neil Hyatt, Karl Travis, Russell Hand and Ewan
  Madrell
• EPSRC and Nexia Solutions for financial support