Choosing a Cement for Encapsulation of Nuclear Wastes

1
Choosing a Cement for Encapsulation of Nuclear
Wastes

• Neil Milestone
• Immobilisation Science Laboratory
• Engineering Materials
• University of Sheffield

2
Outline

• Introduction to nuclear wastes
• Why use a cement for encapsulation
• Cement formulations currently used
• OPC based
• Reactions within cemented waste forms
• Potential alternatives
• Geopolymers
• Activated slags
• Calcium sulfoaluminates
• Need for a toolbox of cement systems

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Nuclear Wastes in UK

• Cemented wastes
• Low level (LLW)
• Contain radioactive materials where activity does
  not exceed 4 GBq/te of alpha or 12 GBq/te of
  beta/gamma
• Intermediate level (ILW)
• Wastes where radioactivity levels exceed those
  for LLW, but where heating is not an issue for
  storage or disposal facilities
• Vitrified wastes
• High level (HLW)
• Wastes where temperature rise as a result of
  their radioactivity is an issue

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Why use cements for encapsulation ?

• Systems are inexpensive and readily available
• Can be readily prepared as a fluid grouts and
  used in remote operations
• Hardened barrier which resists diffusion and not
  degraded by radiation

5
Cemented waste forms
External mixing
Internal mixing
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Advantages of cemented waste forms

• High internal pH renders many radionucleides
  insoluble
• Hardened grout has low permeability
• High surface area for adsorption of species
• Many properties are well known so predictions of
  durability can be made

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Requirements of cement system for encapsulation

• High fluidity and long working life before
  stiffening
• Controlled heat of hydration to avoid excessive
  temperature rises
• Known thermal, mechanical and physical properties
• Low free water content when hardened
• Acceptable chemical/physical compatibility with
  waste form

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Current formulations

• OPC based
• Grouts used in large volumes 500l or 3m3 so
  central temperatures can reach 80oC
• To reduce heat output, high replacement levels of
  PFA or BFS used
• Typically only 60 of slag reacts
• Relatively high w/s ratios used to maintain high
  fluidity - leads to high free water content,
  mostly in gel pores

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Reactions in OPC based systems

• Ca silicates in cement react with water to
  produce calcium silicate hydrate
• C-S-H plus Ca(OH)2
• Ca(OH)2 reacts with reactive silica to give
  additional C-S-H pozzolanic reaction
• Typical pH is gt13 for OPC but reduced for PFA and
  BFS additions to as low as 12.
• Small amount of cement insufficient to activate
  hydration of all pozzolan leaves unreacted
  material and free water

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Micrographs of hardened pastes
91 BFSOPC 1 yr at 20oC
54 PFAOPC 90 days at 20oC
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Chemical reactions within OPC based cemented
waste forms

• BaCO3 incorporation
• Iron floc interactions
• Al corrosion
• Zeolite reactions

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BaCO3 Interactions

• BaCO3 slurry arises from gas scrubbing in EARP
  and contains 14C. Cemented in a BFS/OPC
  composite cement for encapsulation
• Despite high insolubility of BaCO3 (Ksp 2.6 x
  10-9), it reacts with any sulfate present to give
  more insoluble BaSO4 (Ksp 1.1 x 10-10)
• Reaction retards hydration of OPC but
  surprisingly enhances hydration reaction of BFS.

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BSE image of BaCO3 in OPC
OPC with 30 BaCO3 hydrated 90 days
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EDS element maps
Ba
S
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BaCO3 interactions

• BaCO3 SO4(aq) ? BaSO4 CO3(aq)
• BaSO4 precipitates around cement grains hindering
  access of water
• CO3 interacts with hydrating aluminate phase to
  form monocarboaluminate which is most stable
  phase in alkaline solution
• Reaction does not appear detrimental for
  durability

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Iron Flocs in Composite Cements

• Iron flocs are generated in EARP at Sellafield to
  scavenge last traces of actinides from recycling
  liquor
• Typically 15 solids in a colloidal suspension
  with pH 8.5
• Cemented only satisfactorily when pretreated with
  Ca(OH)2 before cementation in PFA/OPC

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Pretreatment

• Lime interacts rapidly with floc suspension. At
  10 Ca(OH)2 addition, most reacts within a few
  hours
• The lime pretreatment allows the cement to
  hydrate normally, producing Ca(OH)2 for the
  pozzolanic reaction with PFA
• When this pretreated waste is cemented, the major
  crystalline phase formed is an iron substituted
  silicated katoite, 3CaO(Al2O3,Fe2O3)SiO2.4H2O.
• OPC/PFA system suitable for flocs with
  pretreatment

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Reactions in cemented waste forms Al corrosion

• Al present in historic wastes and associated with
  fuel claddings, along with Mg
• Al is amphoteric and corrodes above pH 8.5
• pH of internal pore solution of Ca silicate
  cement typically above 12
• Al 2OH- 4H2O ? Al(OH)4.2H2O- H2
• 2Mg 2H2O ? 2Mg(OH)2 H2

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Al corrosion
Al in OPC w/s 0.33
Al in 91 BFSOPC w/s
0.33 28 days 20oC 90 days 20oC
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Al corrosion our results

• Rate of corrosion dependent on
• Al purity
• pH of pore solution
• Generation of porous zone surrounding Al from H2
• Formation of soluble hydroxy-Al species which
  become incorporated in silicate phase to form
  strätlingite, 2CaO.Al2O3.SiO2.8H2O
• Cracking in laboratory specimens due to formation
  of Al(OH)3
• To reduce corrosion we need to limit the pH and
  potentially the available water

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Zeolites in cement

• Zeolites used to remove radio-Cs and Sr from
  solution through ion exchange
• Will act as pozzolans in OPC systems
• Reaction can be reduced by addition of PFA
• From pozzolanic reaction, adsorbed Cs is released
  and can be leached so an OPC system is not
  suitable

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What other systems meet the criteria we set?

• Alkali activated systems
• Inorganic polymers ( Geopolymers)
• Slags
• Calcium sulphoaluminate cements (CSA)

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Inorganic polymers

• 3-D polymerised network of alkali aluminosilicate
  binder
• Components are alkaline alkali silicate solution
  and solid aluminosilicate precursor
• Precursors include metakaolinite, PFA, and BFS
  and zeolites
• New system, so many properties not well
  characterised

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Geopolymer structure
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Our work on geopolymers

• UK fly ash suitable as precursor
• Cations retained well but anions are not
  adsorbed.
• pH extremely high Al corrosion severe
• Can use zeolites as precursor

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Activated slags

• Traditionally BFS is activated with alkalis
• Possible to use CaSO4 or Na2SO4
• Low heat of hydration but slow to set
• Permeability low

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Our work on activated slags

• Good products made with CaSO4 and Na2SO4
• pH reduced to 11
• Al corrosion greatly reduced
• Cs leaching slightly better than OPC based systems

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Calcium Sulphoaluminate (CSA) Cements

• Used extensively in China for 30 years
• Clinker contains 4CaO.3Al2O3.SO3
• Activated with CaSO4 to give ettringite,
  3CaO.Al2O3.3CaSO4.32H2O, as main hydration
  product along with Al2O3.nH2O
• Very high fluidity grouts at high w/s ratios can
  be prepared
• sets within 24 hours and develops strength
  rapidly

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CSA cements

• Ettringite
• retains large amounts of water in structure
• Can substitute many species within structure
• Questions over thermal stability and leaching

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CSA cements our work

• High w/s ratio provides workable high fluidity
  formulation
• Little or no Al corrosion
• pH 11
• Internal desiccation
• Reduced leaching of Cs compared to OPC/BFS
• Overall stability and heat output are key
  durability issues

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Cementing toolbox
Yes
No
Waste reactive ?
Conventional cements
Cement Chemistry
Ion compatibility
pH
Setting Leaching
Corrosion Pozzolans
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Cementing Tool Box
Is corrosion a concern?
Yes
No
High pH Systems
Low pH systems
CSA cement
AAS
Geopolymer
OPC composite
Low pH GGBS/PFA
Zeolites
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Acknowledgements

• Nexia Solutions (formally NSTS BNFL) for
  financial support
• Claire Utton, Anthony Setiadi, Nick Collier,
  Laura Gordon and Paulo Borges,
• PhD students
• Dr Yun Bai, Dr Jean-Phillipe Gorce and Dr Qizhi
  Zhou, Post Docs
• Prof John Sharp and Dr Joanne Hill