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Radionuclide behaviour and geochemistryupon geological disposal of high-level waste in

Boom Clay: overview and critical assessment

Pierre Van Iseghem

SCK•CEN, 2400 Mol, Belgium

Co-authors: L. Wang, D. Jacques, N. Maes, C. Bruggeman,

S. Salah, J. Govaerts

University of Manchester, U.K. December 2, 2009

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Contents

1. General (disposal concepts, safety case)

2. The cement near field – geochemistry

3. The Boom Clay far field – radionuclide speciationand migration

4. Conclusions

3

Geological disposal – current lay-out forthe ~10,000 m3 of the Belgian programme(40 y operation of the NPP’s)

4

Boom Clay acts as the main barrier

• High plasticity

• Low hydraulic conductivity

• Transport is mainly diffusion

controlled

• High sorption properties

• Slightly alkaline (pH = 8.2) – cfr

carbonates

• Strongly reducing (Eh< -300 mV) –

cfr pyrite, organic matter

5

SAFIR-2 disposal design forhigh-level waste till 2004

Clay

Backfill

Disposal Tube

Waste Canister with Overpack

Boom

FoCa7-clay

Concrete lining

Reducing conditionsin the far-field

Oxidizing, reducing in the long-term !?

6

The supercontainer disposal concept for high-level waste is the actual

reference

7

Safety assessment of a disposal concept. Pathway for natural spreading of

radionuclides.

Geosphere (Bq/m)3

Receptor [mSv/y]

Activity flux (Bq/y)

Conditioned waste (Bq)

Aquifer model

Biosphere model

Aquifer

Biosphere

Boom Clay

Aquiferm

River

HLW disposal gallery

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Main processes occurring

Vitrified HLW orSpent fuel :* Dissolution processes

* radiolysis* actinide solubility

Disturbed zone : * thermo-hydro-mechanical-chemical coupling

Concrete liner : * alcaline plume effects

Stainless steel overpack :* corrosion mechanisms

Backfill : * thermo-hydro-geochemical- coupling

* geochemical perturbations

Vitrified HLW or Spent Fuel•Dissolution processes•Radiolysis•Actinide solubility

Carbon steel overpack•Corrosion mechanisms

Concrete buffer, backfill, lining•Alkaline plume

Disturbed zone•Thermo-hydro-mechanical-chemical coupling

Underlying aquifer :* regional scale hydrogeology

Overlying aquifer :* palaeohydrogeology

* computer code development

Biosphere :* water-soil-plant transfer

Boom Clay :

* radionuclide sorptionmechanisms

* organic complexation* natural trace elements

(U, Th, REE) geochemistry

*undisturbed geochemistry

9

Recent assessments: V-HLW(normal evolution scenario)

1x102 1x103 1x104 1x105 1x106

time after repository closure (a)

1x10-12

1x10-11

1x10-10

1x10-9

1x10-8

1x10-7

1x10-6

1x10-5

1x10-4

1x10-3

1x10-2

Dos

e (w

ell p

athw

ay) (

Sv/a

)

14C36Cl79Se99Tc107Pd126Sn129ITotal

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Post-closure safety case

• Report (or a series of reports) prepared to obtain a license for construction, operation or closure of a repository

• It has to show that the repository is safe

• The safety case is an integration of evidence, arguments and analyses that describe, quantify and substantiate the claim that a repository will be safe after closure …

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Safety functions

• Isolation of wastefrom changes at the surfacefrom man (intrusion, safeguards, terrorism, ...)

• Containmentwatertight container

• Delaying and spreading of releasesSlow release♣waste matrix♣low solubility

No water flowSlow transport through buffer and host formation

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BC

tem

p. in

ac

cept

. Ran

ge

And

Loss

of in

tegr

ityof

EC

B

Poss

ible

rele

ase

of n

on re

tard

edco

ntam

inan

ts to

be

iosp

here

0 106100 00010 0001 000

years

Isolation (I)

Eng. Cont. (C)

Delay and attenuate (R)

Clo

sure

of

disp

osal

Engineered Containment phase

System containment phase (non retarded)

Stable geological barrier phase

System containment phase (retarded)

Thermal phase

Boom Clay

Boom Clay

EBS

Waste?

Example of application of the safetyfunctions: spent fuel disposal

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Limits of predictability

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Contents

1. General (disposal concepts, safety case)

2. The cement near field – geochemistry

3. The Boom clay far field – radionuclidespeciation and migration

4. Conclusions

15

The use of cement materials in deep disposal of radwaste

Cementitious supercontainer as the backfill, thus

55,000 tons concrete will be inserted in Boom Clay

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Experimental and modeling studies to assess the influence of cement

• Near-field chemistryCorrosion studiesSource term of RN retention

• Far-field chemistryNegative effect: high pH plume tends to dissolve clays and organic mattersPositive effect: precipitation of zeolite, C-H-S, and calcite enhances RN retention

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Laboratory drill-core experiments

Outlet(Percolate)

(131I, HTO, H14CO3–)

Impulse injection

Inlet(Cement Water)

Inle

t filt

er

Out

let f

ilter

Clay core

Outlet(Percolate)

(131I, HTO, H14CO3–)

Impulse injection

Inlet(Cement Water)

Inle

t filt

er

Out

let f

ilter

Clay core• Chemical analysis of the percolate

• Mineralogical and radio analysis of the core

∅38×32 mm

0

100

200

300

-3 -2 -1 0 1 2 3 4Distance (cm)

[134 C

s], k

Bq/

cm³

Water Flow

Darcy velocity = 2.2 10-9 m.s-1

D = 1.0 10-13 m².s-1

ηR = 1 086η estimated to be 0.30, soR estimated to be 3 600corresponding Kd = 639 ml.g-1

Boom Clay Gallery

(c)

YCW and ECW

cement water

N2

filter paper with tracer

out flowfilter plates

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Chemical evolution: Pore water chemistry of concrete

pH > 12.5 (> 80,000 years) pH is buffered by the dissolution of portlandite

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Experimental results: effects on pH

pH front advances faster in YCW than ECW

YCW: Na/K, pH 13.2 ECW: Ca(OH)2 solution, pH 12.5

8

9

10

11

12

13

14

0 500 1000 1500 2000time (days)

pH

Young Cement Water (YCW)

pH0 = 13.2

8

9

10

11

12

0 500 1000 1500 2000time (days)

pH

Evolved Cement Water (ECW)

pH0 = 12.5

breakthrough breakthrough

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Experimental results: retarded diffusion of bicarbonate

0.0

0.2

0.4

0.6

0.8

1.0

0 50 100 150 200 250 300

Nor

mal

ized

Act

ivit

y

reference

Evolved cement water

Young cement waterH 14CO 3-

0

50

100

150

200

250

0 100 200 300 400 500 600Time (days)

Con

cent

ratio

n (B

q/cm

3 )Experimental data

Fit result

Migration of H14CO3- in evolved cement water (ECW)

H14CO3- is moderately retarded in (ECW)

R = 1.82if = 0.11, then, R = 16.6Dapp = 8.3 × 10-12 m2 s-1

Deff = 1.5 × 10-11 m2 s-1

Volume percolated [ml]

• H14CO3- is retarded in ECW

• precipitation of calcite

• isotopic exchange unlikely

• the retardation can be explained by

a decreased apparent diffusion

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Geochemical coupled modeling

Radial diffusion

No feed-back from chemistry on porosity and D

r 1

r 2

r 3

Boom Clay

OPC buffer

lining concrete wedge blocks and backfill

overpackstainless steel envelope

r 1 = 0.21 mr 2 = 0.96 mr 3 = 1.62 m

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Modeling result: after 105 years

-1 0 1 2 3Distance from concrete - Boom Clay boundary (m)

8

9

10

11

12

13pH

25000 y50000 y75000 y100000 y

Concrete Boom Clay

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Modeling result: mineralogy in concrete

0

0.2

0.4

0.6P

ortla

ndite

(mol

/0.0

25l)

-1 -0.5 0Distance from concrete - Boom Clay boundary (m)

6

7

8

9

Cal

cite

(mol

/0.0

25l)

25000 y50000y75000 y100000 y

-1 -0.5 0Distance from waste - Boom Clay boundary (m)

0

0.1

0.2

0.3

0.4

Afw

illite

(mol

/0.0

25l)

0

0.04

0.08

0.12

Hyd

roga

rnet

(mol

/0.0

25l)

(a)

(b)

(c)

(d)

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Contents

1. General (disposal concepts, safety case)

2. The cement near field – geochemistry

3. The Boom clay far field – radionuclidespeciation and migration

4. Conclusions

Safety Case (2013)

« Hard » data =>quantitative performance/safety analysis

« Supporting » data => demonstrating confidence in

the system

e.g. Kd,Cs, D, η

e.g. Conceptual model confirmation, SCM-IEx model,

Colloidal transport model, Spectroscopic evidence, …

Uncertainty treatment

RN migration in geological disposal: What is needed? Understanding and Quantification

Underlying processes in Radionuclidemigration with respect to geological disposal

BiosphereNear Field Host RockGeochemical prop Physical prop

RN Speciation

RN Transport

Water/solid composition, pH, Eh, pCO2,…

Density, porosity, poresize,…

RN in solution

RN Uptake

• Precipitation/dissolution

• Complexation

• Sorption/desorption RN in solution

• Diffusion (∆C)

• Advection (∆P)

• Colloidal transport (size)

R3 R2Safety functions:

Study of the influence of DOC on the Solubility of trivalent radionuclides: ex. Eu(III) in Boom Clay water

Solubility increase was succesfully described using Tipping’s Humic-Ionbinding model VI

-10 0 10 20 30 40 50 60 70 80 90 100 1101E-7

1E-6

1E-5

1E-4

RBCW < 30000 kDa RBCW < 0,45 µm

Eu s

olut

ion

conc

. (m

ol·l-1

)

DOC (mg·l-1)

filtration effect

•“generic” OM model is used

• 1 fitting parameter: LogKMA(median Metal-OM binding constant)

Log KMA (NOM<30kDa)~2.2-2.5

Log KMA (NOM>30kDa)~4-8

=> Succesful Incorporation in PhreeqC

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OM complexation parameters used (Tipping model)

Log KMA30000 Da = 2.20, PEC = 4.00 meq/g

Log KMA0.45 µm = 2.20, PEC = 4.00 meq/g

Cation exchange reaction Kc 3 Na-illite + Eu3+ ⇔ Eu-illite + 3 Na+ 76 Surface complexation reactions on strong sites Log KSC ≡SSOH + Eu3+ ⇔ ≡SSOEu2+ + H+ ≡SSOH + Eu3+ + H2O ⇔ ≡SSOEuOH+ + 2 H+ ≡SSOH + Eu3+ + H2O ⇔ ≡SSOEu(OH)2

0 + 3 H+

3.1 -4.4 -12.7

Surface complexation reactions on weak sites Log KSC ≡SW1OH + Eu3+ ⇔ ≡SW1OEu2+ + H+ ≡SW1OH + Eu3+ + H2O ⇔ ≡SW1OEuOH+ + 2 H+

0.3 -6.2

Surface complexation parameters used

-10 -9 -8 -7 -6-9

-8

-7

-6

illite (no NOM) illite + RBCW centr illite + RBCW filtr illite + RBCW+BC centr illite + RBCW+BC filtr illite + SBCW+BC centr illite + SBCW+BC filtr

log[

Eu eq

, sol

id] (

mol

·g-1)

log[Eueq, solution] (mol·l-1)

Study of the influence of DOC on the sorption of trivalent RN onto pure minerals as a first step to

the natural system: Eu/Illite

• Sorption to illite decreases due to NOM complexation but… in a compact system, large NOM molecules become filtered out…

• Model description (solid and dashed lines – “blind” predictions) gives quiteaccurate fit – Additivity approach is suitable

Methodology to obtain migration data: direct determination on intact clay cores

clay water

Tracer solution

out flow/Breakthrough curve

clay slice ~ 1 mm

pressure

filter plates

N2/0.4%CO2

0

50

100

150

200

250

300

350

-3 -2 -1 0 1 2 3 4

Distance to the source (cm)

134 C

s Con

cent

ratio

n (k

Bq

· cm

-3)

Water Flow

Migration parameters for Cs+

in Boom Clay

Dapp = 1.0 E-13 m²/snR = 1 086nRD = 1.1 E-10 m²/sn estimated to 0.3, soR estimated to 3 600corresponding Kd = 639 cm³/g

FromFormationside

ToGallery

side

Distribution profile after 7 yearsof percolation (linear y scale)

134Cs

(Source)SCK•CEN/PDC/00/08

Close to real situation - Direct determination of K, Dapp,ηR and apparentSolubility is possible

NOM diffusion parameters

0

50

100

150

200

250

300

350

0 20 40 60 80 100 120 140

Time since injection [Days]

C-1

4 ac

tivity

con

cent

ratio

n [B

q/m

l]

Experimental values

Fit result

Current interpretation is based on “simple” fitting of the D-A equation (no colloids)

(Put et al. (1998) Rad. Acta 82, 375.)

Dapp~3 10-11 m²/s

nR~0.51 with

n=0.16 (anionic species)

R~3.2

Different colloidal transport models are currently under test as the “simpleapproach” is limited when applied to large-scale simulations

NOM diffusion at large-scale: filtrationeffects

0 500 1000 1500 2000 2500 3000 3500time (days)

0.0x100

2.0x107

4.0x107

6.0x107

8.0x107

1.0x108

1.2x108

14C

act

ivity

(Bq/

m³)

Dp= 1.5×10-10 m²/s

Dp= 1.0×10-10 m²/s

Dp= 8.5×10-11 m²/s

experimental

0 500 1000 1500 2000 2500 3000 3500time (days)

0.0x100

3.0x106

6.0x106

9.0x106

1.2x107

1.5x107

1.8x107

14C

act

ivity

(Bq/

m³)

Dp= 8.5×10-11 m²/s

experimental

(a) (b)

0 500 1000 1500 2000 2500 3000 3500time (days)

0.0x100

5.0x106

1.0x107

1.5x107

2.0x107

2.5x107

14C

act

ivity

(Bq/

m³)

diffusion onlydiffusion + advectionexperimental

(c)

0 500 1000 1500 2000 2500 3000 3500time (days)

0.0x100

1.0x107

2.0x107

3.0x107

14C

act

ivity

(Bq/

m³)

diffusion + advectiondiffusion, advection + colloid transportexperimental

(d)

• Validation of lab derived migration parameters• Colloid filtration effects described by simple

Attachement/detachment kinetics (ka=8 10-9 s-1 and kd=8 10-10 s-1)

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TrivalentTrivalent RNRN--NOM coupled transport NOM coupled transport --kineticskinetics are important: are important:

conceptualconceptual transport modeltransport model

[ RN ][ RN ]liquidliquid RNOMRNOM

mobile RN-OM complex

RNRN

Linear Sorption

Boom Clay solid phase

RNOMRNOM

RRN RRNOM

kkdecompdecomp

kkcompcomp

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TestingTesting of the of the conceptualconceptual model model forfortrivalenttrivalent RNRN

Cm244

Transport of the RN-NOM complex is based on the migration parameters of NOM including dissociation kinetics and sorption to the mineral phase(conceptual model also works for tetravalent RN – not shown)

Parameters used:

• RRN~11000

•RRN-NOM~38

• kdecompl~1.7 10-6 s-1

(LogKRN-NOM~5.6)

• Dpore~1.4 10-10 m²/s

Conclusions from part III

• « Bottom-up » approach starting from studies on individual phases and gradually building up towardsmore complex systems and experiments, providesinsight in the migration behaviour of trivalent RN in the Boom Clay.

• A defendable conceptual model describing the migration behaviour of trivalent RN in Boom Clay can betransfered for PA purposes

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Migration parameters used in the safety assessment study

PAGIS (1985) New data (2006) Da Da C_sol (m2/s) (m2/s) (mol/l)

C-14 - 7.0E-11 - Cl-36 - 1.4E-10 -

Se-79 (VI) 1.0E-12 3.0E-11 - Se-79 (0, -II) 1.0E-12 1.4E-10 5.0E-08

Tc-99 3.0E-12 2.0E-10 3.0E-08 I-129 - 1.4E-10 -

Pd-107 1.0E-12 1.0E-11 1.0E-07 Sn-126 1.0E-12 1.0E-11 5.5E-07 Cs-135 8.0E-13 1.0E-13 - Np-237 3.0E-13 2.0E-13 1.0E-06

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Contents

1. General (disposal concepts, safety case)

2. The cement near field – geochemistry

3. The Boom clay far field – radionuclidespeciation and migration

4. Conclusions

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The R&D ’09-’14 on the disposal of radioactive waste includes following

topics

• High-level waste (glass, spent fuel)

• “B-waste” (bitumen)

• Safety assessments (incl. alternative host formations)

• Radionuclide migration and retention processes in Boom Clay(speciation, sorption, migration)

• Perturbations (oxidation, alkaline plume, temperature, gas, thermo-hydro-mechanical-chemical coupled processes)

• Geosynthesis

• EBS evolution (corrosion, sorption)

• CAT A waste (surface disposal)

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Conclusions - general

• The long-term safety of geological disposal in Boom Clayrelies mainly on the Boom Clay far field

• The 3 main R&D approaches in radwaste disposal(phenomenology, safety assessment, engineering) must interact

• Both the basic understanding of processes and the data generation for safety assessment are crucial

• Crucial milestones in the Belgian programme are approaching

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Interactions between the different actions within the radioactive waste

management

Technology

Safety strategy & Performance Assessment

Phenomenology

Characterizationof waste & waste

packages

Conception, construction, operation, closure

Deep understanding of physical, chemical,

geological, … processes

Radiological evaluation & environmental impact assessment

Immobilization of waste

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The financing through NIRAS/ONDRAF(the Belgian Agency for Radioactive Waste

and Enriched Fissile Materials)

and the European Commission is gratefullyacknowledged !