International Symposium on Energy Geotechnics

Saturation of barrier materials under thermal gradient

M.V. Villar, P.L. Martín, F.J. Romero, R.J. Iglesias, V. Gutiérrez-Rodrigo

Barcelona, June 3rd, 2015

�FRAMEWORK

�THE HE-E EXPERIMENT

�AIMS

�MATERIALS

�TH TESTS IN CELLS

�ONLINE RESULTS

�POSTMORTEM RESULTS

OUTLINE

Saturation of barrier materials under thermal gradi ent

Saturation of barrier materials under thermal gradi ent

FRAMEWORK: GEOLOGICAL DISPOSAL OF NUCLEAR WASTE

Waste canisters Engineered barrier

Concrete plug • Swelling capacity

• Low permeability

• Retention capacity

Bentonite-based materials are

proposed as sealing/backfilling

material in underground

repositories (engineered barrier)

THE HE-E EXPERIMENT

• Located at the Mont Terri URL (Switzerland) in a 50-m long non-lined horizontal

microtunnel of 1.3 m diameter excavated in 1999 in the shaly facies of the

Opalinus Clay. Characterised during the Ventilation Experiment

• 1:2 scale heating experiment considering natural resaturation of the EBS

• Heaters supported by MX80 bentonite blocks (ρd=1.81 g/cm3, w=10.3%)

• Granular material filling the rest of the gallery

THE HE-E EXPERIMENT

Two symmetrical sections, with different sealing materials

•MX-80 bentonite pellets (B) (ρd=1.46 g/cm3, w=5.9%)

•65/35 sand/bentonite mixture (S/B) (ρd=1.5 g/cm3, w=4%)

Maximum heater surface temperature of

140°C, increased almost linearly to its

maximum value in a period of 1 year

Natural hydration

AIMS

• Reproduce in the laboratory the conditions of the granular

material of the HE-E in situ test to provide support for its

modelling

• Check the thermo-hydro-mechanical behaviour of the HE-E

materials at T>100°C

• Compare the THM behaviour of two different sealing

materials

MATERIALS

0

10

20

30

40

50

60

70

80

90

1 00

0 .010 .111 0

D iam eter (m m )

Pe

rce

nta

ge

pa

ssin

g

B

S/B

MX-80 BENTONITE PELLETS

Specific heat capacity: 0.64 J/g·K (at 22°C) - 0.97 J/g·K (at 115°C)

Thermal conductivity: 0.12 W/m·K

Grain density: 2.75 g/cm3

BET as= 33 m2/g

SAND/BENTONITE

MIXTURE

Grain density: 2.71 g/cm3

BET as= 5 m2/g

Thermal conductivity: 0.33 W/m·K

Specific heat capacity: 0.74 J/g·K (at 22°C) - 0.90 J/g·K (at 115°C) Dry sieving

MATERIALS

0

10

20

30

40

50

60

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80

90

100

1101001000100001000001000000

Incr

emen

tal

po

re v

olu

me

per

gra

m (

%)

Pore diameter (nm)

S/B

B

MACROPOROSITY MESO

PORE SIZE DISTRIBUTION BY MIP

Cl- SO42- HCO3

- Mg2+ Ca2+ Na+ K+ Sr+ pH

10636 1354 26 413 1034 5550 63 47 7.6

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

1 1.2 1.4 1.6

Sw

elli

ng

pre

ssu

re (

MP

a)

Final dry density (g/cm3)

MX-80, deionised water

S/B mixture, deionised water

S/B mixture, Pearson water

ln Ps = 5.44 ρd – 6.94

SWELLING PRESSURE TEST IN STANDARD OEDOMETERS

(SMALL SAMPLES)

kw MX-80 (1.5 g/cm3) = 4·10-13 m/s

kw S/B (1.4 g/cm3) = 2·10-10 m/s

Pearson water composition (mg/L)

THM TESTS IN CELLS

THM TESTS IN CELLS

TESTS CHARACTERISTICS

Material: HE-E sand/bentonite (65/35)

mixture, MX-80 pellets

Height of column: 50 cm

Initial dry density and water content: 1.46

g/cm3 and 3.6%, 1.52 g/cm3 and 6.4%

Heater T bottom: 100°C – 140°C

Upper T: room

Hydration: Pearson water, 0.06 bar

Data provided: online measurements of

RH, T, water intake, heater power, axial

pressure

MX-80 pellets

1 5

2 0

2 5

3 0

3 5

4 0

4 5

5 0

5 5

6 0

0 1 000 2 000 30 00 400 0 5000

T im e (h)

Te

mp

era

ture

(°

C)

T 1 T 2

T 3 lab

1 00° C 1 40° C

ONLINE RESULTS: HEATING PHASE

Quick thermal steady state

Low temperatures in thematerial due to its low thermalconductivity and heat losses

16

21

26

31

36

41

46

51

56

0 1000 20 00 3000 40 00

T im e (h)

Te

mp

era

ture

(°

C)

T 1 T 2

T 3 lab

100°C 140°C

S/B mixture

S/B mixture

ONLINE RESULTS: HEATING PHASE

20

30

40

50

60

70

80

90

0 1 000 2000 3000 4000

T im e (h)

Re

lati

ve

hu

mid

ity

(%

)

RH 1

RH 2

RH 3

100°C

140°C

2 0

3 0

4 0

5 0

6 0

7 0

0 100 0 2 000 3000 400 0 50 00

T im e (h)

Re

lati

ve

hu

mid

ity

(%

)

RH 1

RH 2

RH 3

100° C 140° C MX-80 pellets

Quick movement of thevapour phase towards coolerareas

Much longer time forhydraulic steady state

Heating phase: steady values

Higher T and heater power in B pellets cell:

• Steel reinforcement

• Better thermal contact with heater

ONLINE RESULTS

15

30

45

60

75

90

105

120

135

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30

40

50

60

70

80

90

0 20 40Te

mp

era

ture

(°

C)

Re

lati

ve

hu

mid

ity

(%

)

Distance from heater (cm)

HEATER T 140°C

RH B

RH S/B

T B

T S/B

For ρd ≈ 1.5 g/cm3

Faster vapour movement in cell S/B:

• Higher gas permeability

• Lower retention capacity

0

50

100

150

200

250

300

350

400

10

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30

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50

60

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100

0 10000 20000 30000

Wa

ter in

tak

e (g

)

Re

lati

ve

hu

mid

ity

(%

)

Time hydration (h)

RH1 RH2

RH3 water

MX-80 pellets: heating + hydration phase

ONLINE RESULTS

w=18.8%, Sr=65%

15

20

25

30

35

40

45

50

55

60

0 5000 10000 15000 20000 25000 30000

Tem

pe

ratu

re

(°C

)

Time hydration (h)

T1 T2

T3 lab

0

1

2

3

4

5

6

7

8

0

50

100

150

200

250

0 2000 4000 6000 8000

Axi

al

pre

ssu

re (

MP

a)

Wa

ter

inta

ke

(cm

3)

Time (h)

MGR18

water intake

pressure

0

50

100

150

200

250

300

350

400

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

0 10000 20000 30000

Wa

ter in

tak

e (g

)

Ax

ial

pre

ssu

re (

MP

a)

Time hydration (h)

pressure

water

Expected equilibrium Ps 5 MPa (deionised water)

ONLINE RESULTS

MX-80 pellets: heating + hydration phaseSr=65%

Isothermal, deionised water, 10 cm high

15

25

35

45

55

65

0.01 1 100 10000

Tem

pe

ratu

re

(°C

)

Time hydration (h)

T1 T2

T3 lab

20

30

40

50

60

70

80

90

100

0.01 1 100 10000

Time hydration (h)

Re

lati

ve

hu

mid

ity

(%

)

30

130

230

330

430

530

630

730

830

930

1030

Wa

ter

inta

ke

(g

)

RH1

RH2

HR3

water

Sand/bentonite mixture: heating + hydration phase

ONLINE RESULTS

Sand/bentonite mixture: heating + hydration phase

ONLINE RESULTS

20

30

40

50

60

70

0 2 4 6 8

Tem

pe

ratu

re

(°C

)

Distance to axis (cm)

40 cm

22 cm

10 cm

Distance from heater

S/B mixture Teflon Rockwool

Average T, but they tend to decrease

9

10

11

12

13

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15

10

20

30

40

50

60

70

0 10000 20000 30000

He

ate

r p

ow

er

(W)

Tem

pe

ratu

re

(°C

)

Time hydration (h)

T3 Lab T power

DISMANTLING CELL SAND/BENTONITE

DISMANTLING CELL SAND/BENTONITE

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5

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25

30

35

40

0 10 20 30 40 50

Distance to hydration surface (cm)

Wa

ter

con

ten

t (%

)

1.30

1.35

1.40

1.45

1.50

1.55

1.60

1.65

1.70

Dry

de

nsi

ty (

g/c

m3)

w.c.

d.d.

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50

Distance to hydration surface (cm)D

eg

ree

of

satu

rati

on

(%

)

Actual water intake: 676 g

Online measurement: 887 g

Final w = 28.2%

Dry density = 1.46 g/cm3

Average w = 28.3%, ρd = 1.44 g/cm3, Sr = 93%

POSTMORTEM RESULTS CELL SAND/BENTONITE

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1101001000100001000001000000

Incr

em

en

tal

po

re v

olu

me

pe

r g

ram

(%

)

Pore diameter (nm)

1 9

17 25

33 41

47 49

original

MACROPOROSITY MESO

Distance from hydration surface (cm)

POSTMORTEM RESULTS CELL SAND/BENTONITE

0

5

10

15

20

25

1.0E+04

1.0E+05

1.0E+06

0 10 20 30 40 50

Me

sop

ore

mo

de

(nm

)Ma

cro

po

re m

od

e (

nm

)

Distance from hydration surface (cm)

macro

meso

macro

meso

Sample Macropores (>50 nm), %

Mode macropores, µm

Mesopores(50-7 nm), %

Mode mesopores, nm

Micropores (<7 nm), %

Original 92 204 8 19

Sections 1-23 a 83±6 58±21 5±1 16±4 12±7

Section 24 b 53 110 8 18 39

POSTMORTEM RESULTS CELL SAND/BENTONITE

Ca-Na sulphates

NaCl

Close to heater

SEM OBSERVATIONS

CONCLUSIONS

� Low thermal conductivity of the dry materials and heat losses: steep

thermal gradient and lower temperatures than in in situ test

� The different gas and water permeabilities of both materials implied:

• Different pace and extent of water redistribution in the vapour

phase: different relative humidity gradient at the end of the heating

phase

• Faster hydration and quicker dissipation of the water content

gradient for test with S/B mixture

CONCLUSIONS

CELL B

• During heating phase, axial pressure was related to temperature,

afterwards to hydration state

• Probably non-monotonic development of swelling pressure in pellets

CELL S/B

• The arrival of the water front implied a sudden increase in temperature

for S/B mixture, and an overall soft decrease of T after saturation

• The S/B 50-cm long column was completely saturated after 2.8 years of

hydration

• Water content, dry density, degree of saturation and pore size

distribution were homogeneous along the S/B column, except in the 2

cm closest to the heater

The research leading to these results received funding from the European Atomic Energy Community's Seventh

Framework Programme (FP7/2007-2011) under grant agreement n°249681 and was additionally financed by

ENRESA through a CIEMAT-ENRESA General Agreement.

It is being currently financed by the Mont Terri Consortium