Choosing a Cement for Choosing a Cement for Encapsulation of Encapsulation of Nuclear WastesNuclear Wastes

Neil MilestoneNeil MilestoneImmobilisation Science LaboratoryImmobilisation Science Laboratory

Engineering MaterialsEngineering MaterialsUniversity of University of SheffieldSheffield

OutlineOutlineIntroduction to nuclear wastesIntroduction to nuclear wastesWhy use a cement for encapsulationWhy use a cement for encapsulationCement formulations currently usedCement formulations currently used OPC basedOPC based

Reactions within cemented waste formsReactions within cemented waste formsPotential alternativesPotential alternatives GeopolymersGeopolymers Activated slagsActivated slags Calcium sulfoaluminatesCalcium sulfoaluminates

Need for a toolbox of cement systemsNeed for a toolbox of cement systems

Nuclear Wastes in UKNuclear Wastes in UKCemented wastesCemented wastes

Low level (LLW)Low level (LLW) Contain radioactive materials where activity does not Contain radioactive materials where activity does not

exceed 4 GBq/te of alpha or 12 GBq/te of beta/gammaexceed 4 GBq/te of alpha or 12 GBq/te of beta/gamma

Intermediate level (ILW)Intermediate level (ILW) Wastes where radioactivity levels exceed those for Wastes where radioactivity levels exceed those for

LLW, but where heating is not an issue for storage or LLW, but where heating is not an issue for storage or disposal facilitiesdisposal facilities

Vitrified wastesVitrified wastesHigh level (HLW) High level (HLW) Wastes where temperature rise as a result of their Wastes where temperature rise as a result of their

radioactivityradioactivity is an issue is an issue

Why use cements for Why use cements for encapsulation ?encapsulation ?

Systems are inexpensive and readily Systems are inexpensive and readily availableavailable

Can be readily prepared as a fluid grouts Can be readily prepared as a fluid grouts and used in remote operationsand used in remote operations

Hardened barrier which resists diffusion Hardened barrier which resists diffusion and not degraded by radiationand not degraded by radiation

Cemented waste formsCemented waste forms

External mixing Internal mixing

Advantages of cemented Advantages of cemented waste formswaste forms

High internal pH renders many High internal pH renders many radionucleides insolubleradionucleides insoluble

Hardened grout has low permeabilityHardened grout has low permeability

High surface area for adsorption of High surface area for adsorption of speciesspecies

Many properties are well known so Many properties are well known so predictions of durability can be madepredictions of durability can be made

Requirements of cement Requirements of cement system for encapsulationsystem for encapsulation

High fluidity and long working life before High fluidity and long working life before stiffeningstiffeningControlled heat of hydration to avoid Controlled heat of hydration to avoid excessive temperature risesexcessive temperature risesKnown thermal, mechanical and physical Known thermal, mechanical and physical propertiespropertiesLow free water content when hardenedLow free water content when hardenedAcceptable chemical/physical compatibility Acceptable chemical/physical compatibility with waste formwith waste form

Current formulationsCurrent formulationsOPC basedOPC based

Grouts used in large volumes – 500l or 3mGrouts used in large volumes – 500l or 3m 3 3

so central temperatures can reach ~80so central temperatures can reach ~80ooCC

To reduce heat output, high replacement To reduce heat output, high replacement levels of PFA or BFS used levels of PFA or BFS used Typically only ~60% of slag reactsTypically only ~60% of slag reacts

Relatively high w/s ratios used to maintain Relatively high w/s ratios used to maintain high fluidityhigh fluidity - leads to high free water - leads to high free water content, mostly in gel porescontent, mostly in gel pores

Reactions in OPC based systemsReactions in OPC based systemsCa silicates in cement react with water to Ca silicates in cement react with water to produce calcium silicate hydrateproduce calcium silicate hydrate

C-S-H plus Ca(OH)C-S-H plus Ca(OH)22

Ca(OH)Ca(OH)22 reacts with reactive silica to give reacts with reactive silica to give additional C-S-H – pozzolanic reactionadditional C-S-H – pozzolanic reactionTypical pH is >13 for OPC but reduced for Typical pH is >13 for OPC but reduced for PFA and BFS additions to as low as 12.PFA and BFS additions to as low as 12.Small amount of cement insufficient to Small amount of cement insufficient to activate hydration of all pozzolan – leaves activate hydration of all pozzolan – leaves unreacted material and free waterunreacted material and free water

Micrographs of hardened Micrographs of hardened pastespastes

9:1 BFS:OPC 1 yr at 20oC 5:4 PFA:OPC 90 days at 20oC

Chemical reactions within OPC Chemical reactions within OPC based cemented waste formsbased cemented waste forms

BaCOBaCO33 incorporation incorporation

Iron floc interactionsIron floc interactions

Al corrosionAl corrosion

Zeolite reactionsZeolite reactions

BaCOBaCO33 Interactions InteractionsBaCOBaCO33 slurry arises from gas scrubbing in EARP slurry arises from gas scrubbing in EARP and contains and contains 1414C. Cemented in a BFS/OPC C. Cemented in a BFS/OPC composite cement for encapsulationcomposite cement for encapsulation

Despite high insolubility of BaCODespite high insolubility of BaCO33 (K (Kspsp 2.6 x 10 2.6 x 10-9-9), it ), it reacts with any sulfate present to give more reacts with any sulfate present to give more insoluble BaSOinsoluble BaSO44 (K (Kspsp 1.1 x 10 1.1 x 10-10-10))

Reaction retards hydration of OPC but surprisingly Reaction retards hydration of OPC but surprisingly enhances hydration reaction of BFS.enhances hydration reaction of BFS.

BSE image of BaCOBSE image of BaCO33 in OPC in OPC

OPC with 30% BaCOOPC with 30% BaCO33 hydrated 90 days hydrated 90 days

EDS element mapsEDS element maps

Ba S

BaCOBaCO33 interactions interactions

BaCOBaCO33 + SO + SO44==

(aq)(aq) →→ BaSO BaSO44 + CO + CO33==

(aq)(aq)

BaSOBaSO44 precipitates around cement grains precipitates around cement grains

hindering access of waterhindering access of water

COCO33== interacts with interacts with hydratinghydrating aluminate phase to aluminate phase to

form monocarboaluminate which is most stable form monocarboaluminate which is most stable phase in alkaline solutionphase in alkaline solution

Reaction does not appear detrimental for durabilityReaction does not appear detrimental for durability

Iron Flocs in Composite Iron Flocs in Composite CementsCements

Iron flocs are generated in EARP at Sellafield Iron flocs are generated in EARP at Sellafield to scavenge last traces of actinides from to scavenge last traces of actinides from recycling liquorrecycling liquorTypically ~ 15% solids in a colloidal Typically ~ 15% solids in a colloidal suspension with pH ~8.5suspension with pH ~8.5Cemented only satisfactorily when pretreated Cemented only satisfactorily when pretreated with Ca(OH)with Ca(OH)22 before cementation in PFA/OPC before cementation in PFA/OPC

PretreatmentPretreatment

Lime interacts rapidly with floc suspension. At Lime interacts rapidly with floc suspension. At 10% Ca(OH)10% Ca(OH)22 addition, most reacts within a few addition, most reacts within a few hourshoursThe lime pretreatment allows the cement to The lime pretreatment allows the cement to hydrate normally, producing Ca(OH)hydrate normally, producing Ca(OH)22 for the for the pozzolanic reaction with PFApozzolanic reaction with PFAWhen this pretreated waste is cemented, the major When this pretreated waste is cemented, the major crystalline phase formed is an iron substituted crystalline phase formed is an iron substituted silicated katoite, 3CaO(Alsilicated katoite, 3CaO(Al22OO33,Fe,Fe22OO33)SiO)SiO22.4H.4H22O.O.

OPC/PFA system suitable for flocs with pretreatmentOPC/PFA system suitable for flocs with pretreatment

Reactions in cemented waste Reactions in cemented waste forms: Al corrosionforms: Al corrosion

Al present in historic wastes and associated Al present in historic wastes and associated with fuel claddings, along with Mgwith fuel claddings, along with MgAl is amphoteric and corrodes above pH 8.5Al is amphoteric and corrodes above pH 8.5pH of internal pore solution of Ca silicate pH of internal pore solution of Ca silicate cement typically above 12cement typically above 12

Al + 2OHAl + 2OH-- + 4H + 4H22O O →→ [Al(OH) [Al(OH)44.2H.2H22O]O]-- + H + H22

2Mg +2H2Mg +2H22O O →→ 2Mg(OH) 2Mg(OH)22 + H + H22

Al corrosionAl corrosion

Al in OPC w/s = 0.33 Al in 9:1 BFS:OPC w/s = 0.33 28 days 20oC 90 days 20oC

Al corrosion: our resultsAl corrosion: our resultsRate of corrosion dependent onRate of corrosion dependent on Al purityAl purity pH of pore solutionpH of pore solutionGeneration of porous zone surrounding Al from HGeneration of porous zone surrounding Al from H22

Formation of soluble hydroxy-Al species which Formation of soluble hydroxy-Al species which become incorporated in silicate phase to form become incorporated in silicate phase to form strsträtlingite, 2CaO.Alätlingite, 2CaO.Al22OO33.SiO.SiO22.8H.8H22OO

Cracking in laboratory specimens due to formation Cracking in laboratory specimens due to formation of Al(OH)of Al(OH)33

To reduce corrosion we need to limit the pH and To reduce corrosion we need to limit the pH and potentially the available waterpotentially the available water

Zeolites in cementZeolites in cement

Zeolites used to remove radio-Cs and Sr Zeolites used to remove radio-Cs and Sr from solution through ion exchangefrom solution through ion exchangeWill act as pozzolans in OPC systemsWill act as pozzolans in OPC systemsReaction can be reduced by addition of Reaction can be reduced by addition of PFAPFA

From pozzolanic reaction, adsorbed Cs is From pozzolanic reaction, adsorbed Cs is released and can be leached so an OPC released and can be leached so an OPC system is not suitablesystem is not suitable

What other systems meet the What other systems meet the criteria we set?criteria we set?

Alkali activated systemsAlkali activated systems Inorganic polymers ( Geopolymers)Inorganic polymers ( Geopolymers) SlagsSlags

Calcium sulphoaluminate cements (CSA)Calcium sulphoaluminate cements (CSA)

Inorganic polymersInorganic polymers

3-D polymerised network of alkali 3-D polymerised network of alkali aluminosilicate binderaluminosilicate binder

Components are alkaline alkali silicate Components are alkaline alkali silicate solution and solid aluminosilicate precursorsolution and solid aluminosilicate precursor

Precursors include metakaolinite, PFA, and Precursors include metakaolinite, PFA, and BFS and zeolitesBFS and zeolites

New system, so many properties not well New system, so many properties not well characterisedcharacterised

Geopolymer structureGeopolymer structure

Our work on geopolymersOur work on geopolymers

UK fly ash suitable as precursor UK fly ash suitable as precursor

Cations retained well but anions are Cations retained well but anions are not adsorbed.not adsorbed.

pH extremely high – Al corrosion pH extremely high – Al corrosion severesevere

Can use zeolites as precursorCan use zeolites as precursor

Activated slagsActivated slags

Traditionally BFS is activated with Traditionally BFS is activated with alkalisalkalis

Possible to use CaSOPossible to use CaSO44 or Na or Na22SOSO44

Low heat of hydration but slow to setLow heat of hydration but slow to set

Permeability lowPermeability low

Our work on activated slagsOur work on activated slags

Good products made with CaSOGood products made with CaSO44 and and

NaNa22SOSO44

pH reduced to ~11pH reduced to ~11

Al corrosion greatly reducedAl corrosion greatly reduced

Cs leaching slightly better than OPC Cs leaching slightly better than OPC based systemsbased systems

Calcium Sulphoaluminate (CSA) Calcium Sulphoaluminate (CSA) CementsCements

Used extensively in China for 30 yearsUsed extensively in China for 30 yearsClinker contains Clinker contains 4CaO.3Al4CaO.3Al22OO33.SO.SO33

Activated with CaSOActivated with CaSO44 to give ettringite, to give ettringite, 3CaO.Al3CaO.Al22OO33.3CaSO.3CaSO44.32H.32H22O, as main O, as main hydration product along with Alhydration product along with Al22OO33.nH.nH22OO

Very high fluidity grouts at high w/s ratios Very high fluidity grouts at high w/s ratios can be prepared can be prepared sets within 24 hours and develops sets within 24 hours and develops strength rapidly strength rapidly

CSA cementsCSA cements

Ettringite Ettringite retains large amounts of water in structureretains large amounts of water in structure Can substitute many species within Can substitute many species within

structurestructure

Questions over thermal stability and Questions over thermal stability and leachingleaching

CSA cements – our workCSA cements – our workHigh w/s ratio provides workable high fluidity High w/s ratio provides workable high fluidity formulation formulation

Little or no Al corrosionLittle or no Al corrosion pH ~11pH ~11 Internal desiccationInternal desiccation

Reduced leaching of Cs compared to OPC/BFSReduced leaching of Cs compared to OPC/BFS

Overall stability and heat output are key durability Overall stability and heat output are key durability issuesissues

Cementing toolboxCementing toolbox

Waste reactive ?Waste reactive ?

Conventional cementsConventional cementsCement ChemistryCement Chemistry

pHpHIon Ion

compatibilitycompatibility

NoNoYesYes

Corrosion

Pozzolans

Setting

Leaching

Is corrosion a concern?Is corrosion a concern?YesYes

AAS

NoNo

GeopolymerOPC

composite

Cementing Tool BoxCementing Tool Box

High pH SystemsLow pH systems

CSA cement

Low pH

GGBS/PFA

Zeolites

AcknowledgementsAcknowledgements

Nexia Solutions (formally NSTS – BNFL) Nexia Solutions (formally NSTS – BNFL) for financial supportfor financial support

Claire Utton, Anthony Setiadi, Nick Claire Utton, Anthony Setiadi, Nick Collier, Laura Gordon and Paulo Borges,Collier, Laura Gordon and Paulo Borges,

PhD studentsPhD students

Dr Yun Bai, Dr Jean-Phillipe Gorce and Dr Yun Bai, Dr Jean-Phillipe Gorce and Dr Qizhi Zhou, Post DocsDr Qizhi Zhou, Post Docs

Prof John Sharp and Dr Joanne HillProf John Sharp and Dr Joanne Hill