This document summarizes research on modeling and experimental studies of bentonite clay behavior under high temperature conditions and long-term uranium sorption properties. It discusses: 1) Coupled thermo-hydro-mechanical-chemical modeling of bentonite alteration over long timescales, validated against data from the FEBEX in situ heating test. The model predicts changes in mineral fractions and stresses. 2) An experiment measuring uranium sorption capacity of bentonite samples from the FEBEX test exposed to 18 years of heating. Preliminary results show lower uranium sorption in the hottest sample, possibly due to differences in accessory minerals rather than clay structure. 3) Planned purification and characterization of the

1. Spent Fuel and Waste Science and Technology
Bentonite under high temperature conditions:
coupled THMC modeling and experimental
study
Liange Zheng, Hao Xu, Patricia Fox, Jonny Rutqvist, Jens T.
Birkholzer, Peter Nico, Christophe Tournassat, James Davis,
Ruth Tinnacher
Lawrence Berkeley National Laboratory (LBNL), Berkeley,
California 94720, USA
Las Vegas, NV
May 23-25, 2017

2. Spent Fuel and
Waste Science and
Technology Outline
Part 1: Studying the long term alteration of
bentonite using coupled THMC model
 Coupled THMC model using BExM for generic
gase
 Predictions based on FEBEX “in situ” test
Part 2: Measuring the Uranium sorption capacity
for bentonite samples undergone 18 years’
heating and hydration.

3. Spent Fuel and
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Motivation: to evaluate the thermal limit on EBS bentonite
Approach: coupled THMC modeling for generic scenarios
Coupled THMC model Recap (1):
Motivation and Approach
EBS Bentonite: Kunigel-
VI/FEBEX bentonite
Clay formation : Opalinus Clay
Two cases for comparison: a “high T”
case and a ‘”low T” case
20
40
60
80
100
120
140
160
180
200
220
0.001 0.1 10 1000 100000
T(oC)
Time (year)
point A
high T
low T
 Illitization can be modeled as smectite dissolution and neo-formation of illite
 MC coupling is formulated via an extended linear swelling model

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-0.2
-0.15
-0.1
-0.05
0
0.001 0.1 10 1000 100000
Volumefractionchange
Time (year)
smectite, B
high T, base
low T,base
no heat
Coupled THMC model Recap (2):
Key Findings
-0.2
-0.15
-0.1
-0.05
0
0.001 0.1 10 1000 100000
Volumefractionchange
Time (year)
smectite, B
high T, Kunigel
high T, FEBEX
low T, Kunigel
low T, FEBEX
Kunigel-VI bentonite FEBEX bentonite
Stress reduction by chemical changes Stress reduction by chemical changes
MPa % MPa %
Point A 0.16 16% 0.18 3.5%
Point B 0.53 53% 0.66 13%
Illitization occurs, T plays key role and bentonite/host rock interaction is
important
Type of bentonite matters and supply of K and Al is the key
Swelling stress decreases as a result of chemical changes and varies case
by case.

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Developing more rigorous approach to link chemistry to mechanics
through the micro-structure strain in Barcelona Expansive Clay model.
m
p
m
m
e
K

 ˆ

mspp 

om sss 
The effect of ionic strength is accounted through
osmotic suction
)ln(10 6
w
w
o a
V
RT
s 

The effect of exchangeable cations is accounted
through
i
i
i
mm x 
p
K
f
m
se
vm

dd 
The effect of the amount of smectite is accounted through the mass fraction of
smectite
m
sf
Coupled THMC model:
FY17 progress (1)
nH2O layers and
exchangeable cations

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Tests with combination of loading paths
Stress paths of tests (a) S1 (Lloret, et al., 2003)
Variation of void ratio over stress paths (S1 and S5)
Stress paths of tests (b) S5 (Lloret, et al., 2003)
Variation of void ratio over suction changes (S1 and S5)
Coupled THMC model:
FY17 progress (2)

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Model results from coupled THMC model with MC coupling via
BExM)
 The difference of stresses between “THM” and “THMC” cases:
• High T: 0.35 MPa (Point A), 1.1MPa (Point B)
• Low T: -0.65 MPa (Point A), 0.3 MPa (Point A),
 The chemical changes as a whole increase total stress in the bentonite
buffer at point A (except low T) and point B
Mean total stress at point A Mean total stress at point B
A
Buffer
B
Buffer
Coupled THMC model:
FY17 progress (3)

8. Spent Fuel and
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Future Work
Upgrading the simulator:
Improving the numerical efficiency and stability
Fine-tuning current model:
Impact of silicate cementation on the mechanical behavior
of bentonite
 Integration with PA model

9. Spent Fuel and
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Coupled THMC model:
Prediction based on “in situ” test (1)
The full-scale “in situ” test is located in Grimsel, Switzerland, heating at 100 °C started
in 1997, dismantled heater #1 in 2002 and heater #2 in 2015, extensive THMC data
were collected.
0
10
20
30
40
50
60
70
80
90
100
0 5 10 15 20
Relativehumidity(%)
Time (year)
R = 0.52 m
WCSE2-03
WCSE2-04
WCSE1-03
WCSE1-04
THMC-LSmodel
TH model
0.0E+00
2.0E-01
4.0E-01
6.0E-01
8.0E-01
1.0E+00
1.2E+00
0.4 0.6 0.8 1 1.2 1.4
Concentration(mol/L)
Radial distance (m)
Cl-
data S29,S28,S19, 5.3 yrs
Sq data,S47, 18.2 yrs
data S47,18.2 yrs
THMC,5.3 yrs
THMC,18.2 yrs
Coupled THMC model that was validated against the THMC data collected in FEBEX
“in situ” test will be extended to longer simulation time and higher temperature to
evaluate long term alteration of bentonite

10. Spent Fuel and
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Coupled THMC model:
Prediction based on “in situ” test (2)
-0.005
0.005
0.015
0.025
0.035
0.045
0.055
0.065
0.075
0.001 0.1 10 1000 100000
Volumefractionchange
Time (year)
illite, A
in situ prediction, 100 °C
FEBEX+Argillite, 100°C
Kunigel+Argillite, 100 °C
FEBEX+Argillite, 200 °C
-0.005
0.005
0.015
0.025
0.035
0.045
0.055
0.065
0.075
0.001 0.1 10 1000 100000
Volumefractionchange
Time (year)
illite, B
in situ prediction, 100 °C
FEBEX+Argillite, 100 °C
Kunigel+Argillite, 100 °C
FEBEX+Argillite, 200 °C
-0.15
-0.1
-0.05
0
0.05
0.001 0.1 10 1000 100000
Volumefractionchange
Time (year)
smectite, A
in situ prediction, 100 °C
FEBEX+Argillite, 100 °C
Kunigel+Argillite, 100 °C
FEBEX+Argillite, 200 °C
-0.15
-0.1
-0.05
0
0.05
0.001 0.1 10 1000 100000
Volumefractionchange
Time (year)
smectite, B
in situ prediction, 100 °C
FEBEX+Argillite, 100°C
Kunigel+Argillite, 100 °C
FEBEX+Argillite, 200 °C

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Coupled THMC model:
Prediction based on “in situ” test (3)
-0.03
-0.025
-0.02
-0.015
-0.01
-0.005
0
0.005
0.001 0.1 10 1000 100000
Volumefractionchange
Time (year)
feldspar, A
in situ prediction, 100 °C
FEBEX+Argillite, 100 °C
Kunigel+Argillite, 100 °C
FEBEX+Argillite, 200 °C
-0.002
-1E-17
0.002
0.004
0.006
0.008
0.01
0.012
0.014
0.001 0.1 10 1000 100000
Volumefractionchange
Time (year)
quartz, B
in situ prediction, 100 °C
FEBEX+Argillite, 100°C
Kunigel+Argillite, 100 °C
FEBEX+Argillite, 200 °C
-0.002
-1E-17
0.002
0.004
0.006
0.008
0.01
0.012
0.014
0.001 0.1 10 1000 100000
Volumefractionchange
Time (year)
quartz, A
in situ prediction, 100 °C
FEBEX+Argillite, 100 °C
Kunigel+Argillite, 100 °C
FEBEX+Argillite, 200 °C
-0.03
-0.025
-0.02
-0.015
-0.01
-0.005
0
0.005
0.001 0.1 10 1000 100000
Volumefractionchange
Time (year)
feldspar, B
in situ prediction, 100 °C
FEBEX+Argillite, 100°C
Kunigel+Argillite, 100 °C
FEBEX+Argillite, 200 °C
Predictions based on “ in situ” test for 200 °C is ongoing

12. Spent Fuel and
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Part 1: Studying the long term alteration of
bentonite using coupled THMC model
Part 2: Measuring the Uranium sorption capacity
for bentonite samples undergone 18 years’
heating and hydration.

13. Spent Fuel and
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U(VI) Sorption on FEBEX Bentonite
Samples
Goal: To evaluate the impacts of the 18-year exposure of bentonite to moderate heat and
various water saturation levels on U(VI) adsorption behavior.
20 cm
“Hot” Section B-D-48 Test samples near and far from heater; ‘cold’ section (no
heater, BD-59) and original bentonite will serve as
unheated controls.
Composite samples were created from 3 replicate blocks
from each location, air-dried and sieved to < 63 mm.
Sample
Distance to
axis (cm)
Moisture Content
(g water/g dry clay)
BD-48 50 0.14
BD-48 108 0.26
BD-59 50 0.22
BD-59 108 0.26
Original -- --
T, porewater Ca, Mg,
Na, K
water content
Experiment 1. U(VI) sorption kinetics on whole
bentonite under ‘native’ conditions.
This experiment will capture differences in porewater
chemistry and accessory minerals (e.g., Fe-oxides,
carbonates), and provide a basis for conditions to test
in Exp. 2.
Experiment 2. U(VI) sorption on purified
bentonite under field-relevant conditions.
This experiment will capture structural differences in
the clay fraction.

14. Spent Fuel and
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Exp 1. U Sorption Kinetics on bulk
FEBEX Bentonite
• Experiments performed in duplicate on <63
mm composite bentonite samples (un-
purified).
• BD-48, 50-cm sample (closest to heater)
shows ~10% less sorption than all other
samples.
• Very little difference in dissolved Ca between
samples; slight difference in dissolved Mg.
• Suggests that difference in U sorption is not
due to differences in aqueous U speciation.
• Testing a wider range of chemical conditions
(variable pH and Ca concentrations);
experiments are underway.

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Exp 2. Purification and
Characterization of FEBEX Bentonite
Clay Purification Procedure:
1. Water Soluble Salts.
Extract w/DI water at 100 g/L.
2. Carbonate Mineral Removal.
Extraction w/1M Na-acetate (pH 5),
followed by dialysis against 1M acetate
(pH 5).
3. Sodium Saturation and Rinsing.
Dialysis against 1 M NaCl, then dialysis
against DI water.
4. Removal of Quartz and Feldspars.
Centrifugation to remove particles >2 mm.
5. Dry and Grind Clay.
Are observed differences in U sorption due to (1) differences in accessory minerals and mass
fraction of clay or (2) structural differences in clay minerals?
Extraction Results:
• BD-48, 50 cm sample has higher concentrations
of soluble salts and acetate-extractable Ca and
Mg.
• BD-48, 50 cm sample has lower pH (7.96)
compared to BD-59, 50 cm sample (8.63).

16. Spent Fuel and
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Proposed Future Experimental
Work
Exp. 2. U(VI) sorption on purified FEBEX bentonite.
• 1 mM U(VI)
• pH 7-8
• 0-1 mM Ca
• 0.05 M NaCl
• 0.5 g/L clay
Diffusion Experiments with FEBEX bentonite
• Purified or un-purified?
• Details depend on results of batch U(VI) sorption experiments and
modeling of results.
Characterization of FEBEX bentonite samples
• XRD
• Carbonate content (i.e., through total inorganic carbon measurement).
• Surface area
• Cation exchange capacity
Modeling of U(VI) sorption on purified and un-purified FEBEX bentonite
samples

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