High Level Wasteform Microstructures from Crystalcontaining Glasses to Glasscontaining Ceramics

1
High Level Wasteform Microstructures from
Crystal-containing Glasses to Glass-containing
Ceramics

• WE Lee, RJ Hand, PB Rose, MC Stennett and MI
  Ojovan,
• Immobilisation Science Laboratory,
• University of Sheffield, UK.

RWIN Conf. on Waste Immobilisation Challenges,
Sheffield, UK July 6-7th 2004
2
HLW Immobilisation.

• Currently, either left as ceramic spent fuel or
  U/Pu removed and reprocessing waste vitrified in
  glass.
• Some legacy wastes (including Pu) will require
  alternative immobilising matrices based on
  ceramics and glasses.
• Wastes from future (e.g. Gen IV) reactors need to
  be considered now.

3
Ceramic and Glass Densification Processes.

• VGS Viscous Glass SinteringAll ceramic powder
  becomes liquid. Viscous flow.
• VCS Viscous Composite Sinteringgt 15vol but lt
  60vol of ceramic becomes liquid. Common in
  clay-derived ceramics.
• LPS Liquid Phase SinteringLess than 15vol of
  ceramic becomes liquid.
• SSS Solid State SinteringOnly solid involved in
  mass transport.

4
Relation Between Ceramic Microstructure and
Densification Process.

• Vitreous or Viscous Composite Sintered.Multiphase
  grain and bond system.
• Liquid Phase Sintered.Second phase at grain
  boundaries, often glassy.
• Solid State Sintered.Typically single phase,
  clean grain boundaries.

5
Glass Ceramic Processing and Crystallisation.

• Glass melting, crystallise on cooling (via hold)
  or in separate (2-step) operation.
• Nucleating agent to encourage heterogeneous
  nucleation and fine microstructure.
• Frequently form metastable phases which transform
  to thermodynamically stable phases on heat
  treatment.

6
Categories of Wasteform.

• Glasses.
• Glass Composite Materials (GCMs) 1. Glass
  ceramics Zirconolite-based for
  separated long-lived actinides.
  2. Melt dispersed wasteforms
  U/Mo-containing wastes via Cold Crucible
  Melter (CCM). 3. Self-sustaining reacted
  wasteforms.
• Ceramics 1. Single phase ZrSiO4,
  ZrO2 2. Dual phase ZrSiO4/ZrO2,
  Garnet/perovskite 3. Multiphase
  Titanate and zirconate systems e.g. Synroc.

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Glasses.

• Made by melting, ideally all batch becomes
  liquid, viscous flow.
• Accommodate large part of Periodic Table using
  low melting temperatures (1100-1150oC). Proven
  technology.
• Some elements difficult (I, Cl, Ru, Mo, Pu) or
  need high melting temperatures (e.g. ZrO2,
  Al2O3-rich wastes).
• Processing problems e.g. heel at bottom of melter
  in French AVH process, products of refractory
  erosion/corrosion in Joule melters. Need to
  immobilise these.

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Crystals in Glasses.
Historically, regarded as unwanted due to
decreasing wasteform durability.

• Remnant refractory crystals e.g. Pd-Te-Rh and
  (Ru,Rh)O2 in melted glass.
• Yellow phase containing alkali sulphates,
  chromates, and molybdates.
• Crystallisation over time due to radiogenic
  heating e.g. Fe,Mn,Cr spinels.

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Crystals in Glasses

• Presence of crystals not always detrimental. E.g.
  can increase waste loading so reducing waste
  volume or enhance durability by partitioning
  long-lived radionuclides into stable crystals.
• Glass is a good host for many wastes but for
  difficult legacy (often small volume) wastes need
  to examine potential of other systems
  (ceramics/glass ceramics).

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Glass Composite Materials (GCMs)

• Comprise both vitreous and crystalline
  components.
• The major component may be a crystalline
  phase with residual vitreous phase, or the
  vitreous phase may be the major component,
  with particles of a crystalline phase
  dispersed in the glass matrix.
• In this sense can regard all wasteforms as
  GCM.

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Production of GCMs.
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GCMs Glass Ceramics.

• Desirable to separate long lived actinides from
  wasteform (HLW glass) and incorporate into more
  durable and small volume form.
• E.g. zirconolite-based (CaZrxTi3-xO7, 0.8?x?1.37)
  glass ceramics in CAS glass.

P. Loiseau, D Caurant et al. Phys. Chem. Glasses
43C 201 (2002)
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Actinide Incorporating Zirconolite Glass Ceramics.
P. Loiseau, D Caurant et al. Phys. Chem. Glasses
43C 195 (2002)

• Tm 1550-1650oC.
• Tc 950oC bulk nucleation metastable fluorite
  structure zirconolite dendrites.
• Tc 1050 1200oC elongated zirconolite in bulk,
  complex dendritic titanate acicular anorthite
  at surface.

Z
T
B
sample surface
R
A
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Simulants for Radionuclides.

• 6wt Nd3, Gd3, Yb3 and Th4 as simulant
  waste.
• Acted as nucleating agents.
• Suggests waste will beneficially act as nucleant.

 


Nd
Gd
Yb
Th
P. Loiseau, D Caurant et al. Phys. Chem. Glasses
43C 201 (2002)
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Other Observations.

• Zirconolite thermodynamically unstable and
  transforms to titanate after long times (20h) at
  high (1200oC) temperatures. Not expected to occur
  in disposal environment.
• Can make viable wasteforms by controlled cooling
  from melt rather than using separate heat
  treatment.

P. Loiseau, D Caurant et al. Phys. Chem. Glasses
43C 201 (2002)
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Other Types of Glass Composite Material.

• Product of CCM of Mo-rich wastes.
• Product of self-sustaining reaction of e.g.
  contaminated soil powder metal fuel.

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GCM by dispersing crystalline particles
orliquids in a glass melt using a cold crucible
melter (CCM).
Experience in Russia for incineration ashes and
sulphate-chloride containing radioactive wastes
(ILW) in France for U-Mo radioactive wastes (HLW)
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CCM GCMs U/Mo-rich Waste.

• CEA have U-Mo-Sn-Al-fuel from gas cooled
  reactors.
• High Mo and P melt is corrosive and requires high
  temperature glass formulation to incorporate
  enough Mo (12wt).
• Developed CCM in which waste and CaO-ZrO2
  enriched alumino-borosilicate glass additives
  melted by direct high frequency induction.
  Cooling of melter walls produces protective solid
  glass layer (in situ refractory).

Quang et al. WM03, Tucson, AZ, USA.
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Product Morphology and Composition.
Product in simple Mo/P silicate glass simulant
system melted at 1400oC is cooling rate
dependent. In all cases liquid-liquid phase
separation was followed by crystallisation within
separated phases.

• Rapid cool 100nm-1?m heterogeneous Mo-rich
  microsphere crystals in a silicate glass
  matrix.
• Very slow cool range of large crystalline
  phases e.g. 1) NaCaPO4, 2) CaMoO4 and 3)
  Na2MoO4.2H2O in glass matrix which also contains
  the microspheres formed on rapid cool.

Schuller and Bart, Glass Odysee 2002, Montpelier,
France.
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U/Mo GCM Wasteform Microstructure
µ-spheres enriched in Mo, P, Ca.

• Ideally, water soluble molybdate microspheres
  isolated in R7T7 type glass matrix.

Courtesy T. Advocat, CEA Marcoule, France.
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GCMs by Self-Sustaining Immobilisation (SSI)
utilising energy released on interaction of
powder metal fuels (PMF) and waste constituents.
Ceramic or GCM
Waste PMF
SSI enables production of durable GCM which can
be tailored to host both short- and long-lived
radionuclides.
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Microstructure of GCM by SSI.

• GCM obtained by a self-sustaining thermochemical
  process containing corundum, zirconia, leucite
  and glass

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Ceramic Wasteforms.

• A wide variety of ceramic wasteforms have been
  considered for immobilisation of actinides.
• Desire durable, high-density, solid solution
  ceramics made by sintering or melting while
  avoiding formation of unexpected separate
  actinide-containing phases (e.g. grain boundary
  glass).
• Complex, multicomponent wastestreams need more
  than one crystal host. Need for multiphase
  systems and for intimate mixing of waste and
  hosts for full incorporation of active species in
  crystals.

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Candidate Ceramics

• Pyrochlores (A2B2X6Y)
• Apatites (A5B3O12Y)
• Zirconolite (Ca2ZrTi2O7)
• Orthosilicates (ZrSiO4)
• Perovskites (ABO3)
• Orthophosphates (ABO4)
• Titanate (CaTiSiO5)

Highlighted by Lumpkin G. R., BNFL Report, March
2004.
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Lumpkin G. R., BNFL Report, March 2004
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Microstructures of Single- and Dual-Phase
Ceramic Wasteforms.

• Single-phase zircon (Zr,Pu)SiO4 zirconia
  (Zr,Gd,An)O2.
• Dual-phase ceramics better for immobilising
  multi-valent actinides. E.g. sintered
  zircon/zirconia, melted garnet/perovskite and
  garnet/zirconia.

An (U,Pu,Np,Am,Cm)
Anderson and Burakov, Burakov et al. Proc. Int.
Conf. Global 01, Paris, France (2001). Burakov
and Anderson, ICEM 01, Bruges, Belgium (2001).
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Single-Phase Sintered Ceramic Microstructures.
(Zr,Pu)SiO4, 6.1wt Pu.lt80 dense.
(Zr,Gd,Pu)O2, 10.3wt Pu. Cubic, lt90 dense.
Not fully dense or single phase?
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Dual-Phase Ceramic Microstructures.
Light (Zr,Pu)O2 in zircon matrix. Sintered.
Light (Zr,Gd,Pu)O2 in garnet matrix. Melted.
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Complex Multiphase Ceramics.

• E.g. Synroc and other titanate systems made via
  complex processing routes and calcination of
  resulting powders under reducing conditions hot
  pressing.
• Multiphase microstructures with each crystal
  phase designed to accommodate particular
  radioactive species.
• Waste incorporation often accompanied by
  structural modification e.g. via planar defects
  twins or crystallographic shear planes.
• Concerns over glass location and composition?

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Multiphase Ceramics.

• Typically consist of fine grains of up to 6 phase
  types fluorite derivatives (zirconolite),
  perovskites, rutile, hollandites,
  magnetoplumbite/?-alumina types and alloys.
• Various formulations contain different
  proportions of these phases.
• E.g. Synroc C with 20 waste is 30 zirconolite,
  30 hollandite, 20 perovskite, 10 rutile, lt5
  magnetoplumbite and lt5 alloy.

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Synroc Complex Waste Stream/Processing
Contaminants.

• Incorporation of common waste stream impurities
  individually stabilises new phases e.g. monazite
  CePO4 (P2O5), pseudobrookite MgTi2O5 (MgO) and
  pollucite CsAlSi2O6 (SiO2).
• Adding impurities simultaneously leads to
  formation of soluble glassy phase containing
  active species.

Buykx et al. J. Am. Ceram. Soc. 73 217 (1990)
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Glass/Ceramic Wasteforms
Glass Composite Materials
Ceramics
Glasses
SSS or LPS
Vitrified or VCS

• Pressureless Sintered or Hot Pressed.
• Predominantly single phase e.g. Zircon or
  multiphase e.g. Synroc.

Vitrified

• Glass Ceramics.
• Melted Wasteforms (cold or hot crucible).
• Self-sustaining Reacted Wasteforms.

• R7T7.
• Magnox.
• RBMK.

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Conclusions and Challenges.

• Gamut of candidate hosts for HLW from established
  borosilicate glasses (with some crystals present)
  to untried ceramics (with grain boundary glass
  present).
• Need to match difficult wastes to suitable host
  systems.
• Need to understand microstructural evolution in
  wasteforms on processing in particular the firing
  (heating/cooling) scheme.
• Need to characterise location and chemistry of
  crystal and glassy phases in all wasteforms due
  to effect on durability.
• Need to correlate microstructures with durability.