cement waste matrix evaluation and modelling of the long-term stability of cementitious waste...

Cement waste matrix evaluation and modelling of the long-term stability of cementitious waste

matrices

Thermodynamic modelling 2

Lisa Almkvist and Börje Torstenfelt

Swedish Nuclear Fuel and Waste Management Co (SKB)

Peter Cronstrand

Vattenfall Power Consultant (VPC)

Thermodynamic modelling


Cement waste matrix development(presented at the RCM in Moskow)

Solidification of operational intermediate-level waste– Ion exchange resins– Evaporator concentrates

Laboratory test programs studying:– Waste load– Water-to-cement ratio– Type of cement– Additives (liquid, solid)– Type of storage (dry, wet (deionised, salt or chalk

water))


A 1D cell-divided representation of a nuclear repository

The state of the engineered barriers is simulated through multi-component diffusive transport followed by thermo-dynamical calculations of relevant mineral species for each transport step.

Integrity of the repository concrete structure; short-term and long-term (presented at the RCM in Moskow)

Computational model

Cement encapsu-lated waste

Cl-

SO42-

CO32-

Na+

bentonite

Shotcrete Concrete wall

Ambient water with time-dependent ionic composition


Reactive-transport modelling - uncertainties

Transport:•Diffusion, (Ficks law or MCD)

•Advection

Thermodynamic database

Rates

Representation of CSH, (variable log k vs. solid solution)

Composition of the cement paste

Composition of infiltrating water

Reaction:• Thermodynamic equilibrium

• Kinetics

Transport

Reaction

Uncertainties

Flows

Diffusivities

Porosity evolution

Porosity-diffusivity-relation


Uncertainty assessment

• Uncertainties can initially be characterized in simplified leaching models before entering the full scale degradation scenario.

+ Easy to visualize the result

+ Easy to identify and isolate the influence of a specific input parameter

+ Easy to compare with experiments

• Degradation indicators:

• Dissolution of Ca(OH)2

• Decalcification of CSH


Leaching models

Reactive properties

• Thermodynamic database

• Log k vs. solid solution

• Water composition

Performed on crushed cement

Transport properties

• Porosity-diffusivity relations

• Flows

• Diffusivities

• Porosity evolution

Performed on solid samples


Database

Portlandite dissolution

0.00E+00

1.00E-01

2.00E-01

3.00E-01

4.00E-01

5.00E-01

6.00E-01

7.00E-01

8.00E-01

9.00E-01

0 20 40 60 80 100

Leaching step

M C

a(O

H)2

PCHatches-17

Minteq

Llnl

Wateq

Nagra/PSI

Thermoddem


Log k vs. Solid solution

Portlandite dissolution

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 20 40 60 80

Leaching step

M (

Ca(

OH

)2)

11.2

11.4

11.6

11.8

12

12.2

12.4

12.6

pH

Portlandite (ss, Walker2003)

Portlandite (stepwise log k)

pH (ss, Walker 2003)

pH (stepwise log k)


Water intrusionPortlandite dissolution

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 10 20 30 40 50 60 70

Leaching step

M (

Ca(

OH

)2)

Brine

Glacial

Litorina

Biogenic

Rain

Ramlösa

Evian

SFR-mean

Distilled water


Porosity-diffusion relation

Portlandite dissolution, porosity evolution

0

1

2

3

4

5

6

0 5 10 15

Year

M C

a(O

H)2

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

Po

rosi

ty

Portlandite, m=0

Portlandite, m=1

Portlandite, m=2

Portlandite, m=3

Porosity, m=0

Porosity, m=1

Porosity, m=2

Porosity, m=3

me dd 0Archies law:


Recipe for a conservative - yet realistic- estimate

• Databases yielding high dissolution rates (PCHatches or Thermoddem)

• Log k approach can reproduce the result obtained by a solid solution approach.

• Water: Low calcium, high NaCl. Carbonates have a non-trivial and twofold effect; enhance dissolution rates, but the precipitation of calcite reduces the porosity.

• Porosity-diffusion relation: Case-dependent (although a conservative estimate can always be achieved by choosing a sufficiently high diffusivity)


Full scale scenario - the Silo at SFR: initial state

Silo Year 0

0

100

200

300

400

500

600

700

800

900

1000

0 1 2 3 4

Distance (m)

Vo

lum

e (

cc

)

BiotiteK-feldsparAlbiteQuartzSaponite-hSaponite-mgSaponite-caSaponite-kSaponite-naMordenite-naMagnetiteKatoiteIlliteClinoptil-kPhillipsite-naPhillipsite-kMontmor-mgMontmor-caMontmor-kMontmor-naHydroxyapatiteHydrotalciteHydrogarnetBruciteMonocarboaluminateFriedelsaltEttringiteMonosulphateCalcitePortlanditeCSH_0.8CSH_1.1CSH_1.8

Bentonite Silowall ILW&LLW

Shotcrete Grout


Full scale scenario - the Silo at SFR: 100 000 years

E. Silo year 100 000

0

200

400

600

800

1000

1200

1400

0 1 2 3 4

Distance (m)

Vo

lum

e (

cc

)

BiotiteK-feldsparAlbiteQuartzSaponite-hSaponite-mgSaponite-caSaponite-kSaponite-naMordenite-naMagnetiteKatoiteIlliteClinoptil-kPhillipsite-naPhillipsite-kMontmor-mgMontmor-caMontmor-kMontmor-naHydroxyapatiteHydrotalciteHydrogarnetBruciteMonocarboaluminateFriedelsaltEttringiteMonosulphateCalcitePortlanditeCSH_0.8CSH_1.1CSH_1.8

Bentonite Silowall ILW&LLWShotcrete Grout


Full scale scenario - monitoring some parameters

Notation Diffusivity Water composition

Temperature

A Fixed Fixed Fixed

B Fixed Fixed Varying

C Fixed Varying Fixed

D Varying Fixed Fixed

E Varying Varying Varying


Full scale scenario - porosity distribution

Silo Year 100 000

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 1 2 3 4

Distance (m)

Po

ros

ity

A

B

C

D

E



Full scale scenario - pH distributionSilo Year 100 000

7

8

9

10

11

12

13

14

0 1 2 3 4

Distance (m)

pH

A

B

C

D

E



Some remaining uncertainties - clogging

• Introduces both a mesh-and time-step-dependency

• Is there a residual diffusivity (through the gel-pores) even in perfectly clogged material?

• Can remaining non-hydrated clinker materials lead to fractures?

• Are there types of waste that can accelerate the degradation process?


Some remaining uncertainties - fractures

The effect within the fracture

Pure advection Advection and diffusion to adjacent pores

Dual porosity approachOnly evaluates degradation in the fracture itself

The effect on the overall sample.

cement waste matrix evaluation and modelling of the long-term stability of cementitious waste...

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