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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
2
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
8
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
10
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 …
11
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
12
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
13
Limits of predictability
14
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
16
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
17
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
18
Chemical evolution: Pore water chemistry of concrete
pH > 12.5 (> 80,000 years) pH is buffered by the dissolution of portlandite
19
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
20
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
21
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
22
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
23
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)
24
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
28
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)
32
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
33
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
35
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
36
Contents
1. General (disposal concepts, safety case)
2. The cement near field – geochemistry
3. The Boom clay far field – radionuclidespeciation and migration
4. Conclusions
37
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)
38
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
39
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
40
The financing through NIRAS/ONDRAF(the Belgian Agency for Radioactive Waste
and Enriched Fissile Materials)
and the European Commission is gratefullyacknowledged !
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