Thermo-hydro-chemical processes influence on buffer material degradation in high level
radioactive waste disposal
Wen-Sheng Lin1, Suu-Yan Liang2, Chen-Wuing Liu3
1Hydrotech Research Institute, National Taiwan University
Material and Chemical Research Laboratories, Industrial Technology Research Institute3
Department of Bioenvironmental Systems Engineering, National Taiwan University2

No. 1, Sec. 4, Roosevelt Road, Taipei, 10617 Taiwan

Key words: high-level radioactive waste, buffer material, bentonite degradation, performance assessment,
geological disposal

Abstract
The concept of geological disposal for high-level radioactive waste (HLW) is based on a multi-barrier systemthat comprises the natural geological barrier provided by the repository host rock and its surroundings, as well as
an engineered barrier system (EBS) that includes the waste form, waste canisters, buffer materials and backfill.
The multi-barrier system is expected to perform its desired functions and isolate the waste from the biosphere toachieve the performance of the HLW disposal. Due to the interaction processes of radionuclide decay heat,
hydraulic and water-mineral reaction in the radioactive waste repository, the bentonite buffer material
degradation that results from the external temperature and pressure warrants investigation. Smectite, a swellingclay mineral, is the main mineral in bentonite from 65 to 90 wt%. However, seldom addressed in Taiwan is theinfluence of thermo-hydro-chemical (T-H-C) processes on buffer material degradation in the EBS of HLW
disposal repository, as related to the chemical reaction of smectite clay dehydration. Therefore, we adopted thechemical kinetic model of smectite illitization to calculate the transformation mount of smectite-to-illite causedby the higher temperature of waste decay heat. The results show that smectite illitization is deemed insignificant
for the anticipated geological repository conditions. The results can form the advanced research results inevolution of bentonite degradation for the performance assessment and safety analysis on the final disposal of
HLW in Taiwan.
1. INTRODUCTION
The safety concept of a geological repository forthe disposal of radioactive waste is based on amulti-barrier system that includes the naturalgeological barrier and engineered barrier system(EBS). The natural geological barrier is provided by
the repository host rock and its surroundings, and theEBS comprised the waste form, waste canisters,
buffer materials and backfill. The multi-barriersystem should perform its desired functions to isolate
the waste from the biosphere. The EBS design usedin Taiwan is shown in Figure 1.
The buffer should have the functions, i.e. low
waterpermeability,
micro-porous
structure,
self-sealing, canister support, high swelling capacity,
colloid filtering, inhibition of microbial development,
resistance to rock shear movements, retardation ofradionuclide, and must able to effectively isolate
waste for at least 100,000 years. Bentonite has beenused as a buffer material in most HLW repository
concepts. Smectite clay is the main mineralcomponent of bentonite material and plays a key rolein the characteristic domination of buffer physicaland chemical behavior. The thermal constraint in
EBS for most HLW repository concept at thebentonite/canister interface should remain thebentonite buffer temperature at temperatures beyond
100 ℃. However, the long-term functions of buffer
clay could be lost through smectite illitization underprevailing temperature and temperature gradientconditions that arise from waste decay heat.
Therefore, the influence of higher waste decay
temperatures on bentonite performance requires
investigation.
The chemical evolution of bentonite buffer andbentonite degradation in the EBS of HLW disposal
repository caused by thermals affecting smectite
illitization and smectite dehydration are topics
seldom discussed in Taiwan. Therefore, in this study,
we used the chemical kinetic model of smectite
illitization to calculate the transformation mount ofsmectite-to-illite caused by higher temperatures ofwaste decay heat. The results indicated thatsmectite-to-illite conversion could cause volume
shrinkage due to loss of interlayer water of smectite
and lead to the bentonite buffer compression.

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japan

Therefore, investigating the evolution of bentonite
degradation on the final disposal of HLW can provide
further insights into performance assessments for thelong-term safety analysis of radioactive waste
disposal sites.

processed (COMSOL, 2016). The parameters for heattransport simulation in the buffer, backfill, and hostrock are shown in Table 2. The finite element meshof the whole system (i.e. the host rock and EBS) isshown in Figure 2.

RockBackfill

BufferCanister

Unit: cm
Figure 2 Finite element mesh for EBS and host rock.

Figure 1 Schematic of EBS design.
2. TEMPERATURE WITHIN BUFFER
Heat decay is expressed as power P(t), where t is
time in years, and ai is the coefficient Time andcoefficient are depicted in Table 1 (SKB, 2009).
7P(t )   ai exp(t / ti )(1)

i 1Table 1 Time and coefficient aii1234

ait[years]
2050
2005000.0601470.705024-0.054753
0.249767

i
567

t[years]
2000
5000
20000

ai
0.025407-0.009227
0.023877

Table 2 Parameters for heat transport simulation.
Parameter

Units

Description

Buffer

Backfill

Hostrock

1.95
830

1535
830

2750
712

25

25

29

1.1

1.1

2.91

0.435
1.56

0.46--ρ

g/cm3

Cp

Jkg-1K-1

Tint℃k

Wm-1K-1n-DensityHeatcapacity
Initialtemperature
Thermalconductivity
Porosity

ρD

g/m3

density

The initial canister power was 1315 W.
COMSOL Multiphysics software was used tosimulate the temperature evolution in the EBS.
COMSOL is a finite element software tool thatallows partial differential equations to be solved in
two-dimensional (2D) and three-dimensional (3D)
domains; the solutions can be visualized or further

3. SMECTITE HYDRATION PROPERTY
Montmorillonite is a swelling clay of the smectitegroup and the main mineral in bentonite (65 to 90wt%). Na-type smectite has superior swelling andlower permeability, and thus is favored as a buffer.
The isomorphic substitution of Al3+ and Mg2+ insmectite octahedral sites, or of Si4+ by Al3+ insmectite tetrahedral sites, generates an excess
negative charge on the smectite structure; this iscompensated for by the adsorption of cations on thesmectite layers. Figure 3 shows the smectite clay
mineral structure. If the interlayer of smectite adsorbs
water, that water is called interlayer water.
Colten-Bradley(1987)
andRansomandHelgeson(1994) stated that the smectite interlayer
includes three discontinuous basal spacings of ~10Å,~12.6Å(one-layer of water molecules), ~15.7Å
(two-layer of water molecules) in a subsurfacehydrogeological environment. The interlayer regionin the silicate structure can be considered to be asolvent; the water is the solute dissolved in thesolvent, and the solution is called a solid-solution.
Using O10(OH)2 as a base crystal-chemical unit,
Ransom and Helgeson (1989) expressed hydrated
Na-smectite (Na0.3Al1.9Si4O10(OH)2·nH2O, where n is
the number of moles of water in the fully hydrated
Na-smectite). The chemical and thermodynamic
properties of interlayer water differ from those ofpore water. Interlayer water can be considered aswater bonded to a mineral, forming a hydrated
mineral. When dehydration occurs, the interlayerwater will be released from the hydrated smectite toform H2O and a homologous anhydrous 2:1

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japanlayer-silicate counterpart. This behavior is analogous
to the reversible intracrystallization reaction of asolid-solution (Ransom and Helgeson, 1994, 1995;
Liu and Lin, 2005).

hs
as+nH2O(2)
where hs denotes hydrous smectite; as denotes
anhydrous smectite; n is the number of moles ofwater released from 1 mole of hydrous smectite.
When the smectite exposes to higher temperature ofwaste decay heat, the reaction (2) tends to move tothe right and the thermal condition could cause
smectite dehydration (i.e. interlayer water wasexpelled of the basal spacings from ~15.7 Å to ~10Å). In addition, smectite illitization may occur attemperatures as low as 70℃ (Jenning and Thompson,1986). Illite is a K-bearing, aluminous, non-swelling
2:1 layer clay (see Figure 3).
SiO4
9.6Å- AlO4(OH)2

SiO4(15.7 Å as shown )AlO4(OH)2

SiO4

H2OM+

10 ÅH2O

SiO4M+K+

M+H2O

M+M+H2O

K+K+K+K+

SiO4

SiO4

AlO4(OH)2

AlO4(OH)2SiO4

SiO4(a)(b)
Figure 3 Clay mineral structure: (a)Smectite; (b)Illite.
When smectite minerals exposed to higher
temperatures and pressures, the minerals are unstable
and transform to more stable silicate phases, such asillite/smectite mixed layers, illite. The transformation
process is strongly affected by kinetic and hydrologic
constraints, such as time, concentration ofexchangeable cations, saturation state in the bentonite
and permeability of the adjacent formation (Puschand (Karnland, 1988a; Madsen, 1998; Wersin et al.,2007). This process can be written by the following
chemical reaction (Karnland and Birgesson, 2006):
K+/Ca2+/Na+-smectite + K+ + (Al3+) → illite +silica + Ca2+/Na+(3)This leads to a devastation of smectite and to arelease of Si which may precipitate as SiO2 cement.
Thus, the illitisation process reduces swelling
capacity and plasticity of the bentonite material(Wersin et al., 2007). However, higher temperaturesat the onset of the smectite-to-illite transformation areproblematic in subsurface system. Furthermore, therate of smectite illitization is influenced by dissolved
potassium concentrations in groundwater. Wilson andBond(2016) indicated that smectite illitization would
occur over many thousands of years at thetemperature ≤ 100°C under near-neutral pHconditions. Their study shows that less than 0.003%
of the smectite is transformed to illite during the10,000 year period.
The interlayer molar volume (Vil) can bedetermined by (Ransom and Helgeson, 1994)
 abc  cos(90   ) Vil  N0(4)10 24 Z
where a = 5.17 Å, b = 8.99 Å, and β = 100°are
parameters associated with the smectite unit-cell; c is
the interlayer thickness; Z = 2 is the number of units
in the chemical formula of the smectite unit-cell, andN0 is Avogadro’s constant.
The number of moles of water (n) released by thedehydration of hydrous smectite is thusVn  il il(5)Mwwhere il is the density of the interlayer water;
Mw is themolecular weight of water and equals 18.05.
Hawkins and Egelstaff (1980) measured the averageinterlayer water density as 1.05 g/cm. The n is 4.51
for ~15.7Å(two layer of water molecules) computedfrom the equation (5).
3.2 Kinetic model for smectite to illite conversion
Huang et al. (1993) systematically investigated thekinetics for the conversion of a Na-saturated
montmorillonite to a mixed-layer smectite/illite as afunction of KCl concentration from 0.1 to 3mol/Lover a temperature range of 25℃ to 325℃ at 500
bar in cold seal pressure vessels using gold capsules.
The results of the study showed that the conversion
rate can be described by a simple empirical rateequation for a Na-rich fluid as follows:dS sm A  exp  Ea
 C1  S sm 2RTdt[K + ] [Na + ]C1 =[K + ]
22.3



(6)
(7)

where Ssm is the fraction of smectite layers in theI/Ssm, t is the time in seconds, A is the frequency
factor of 8.08×104 sec-1, Ea is the activation energyof 28 kcal/mole, R denotes the gas constant of 1.987
cal/deg-mole, T represents the temperature in Kelvin.
[K+] and [Na+] are the K+ ion of 9.22×10-5M andNa+ ion of 1.53×10-3M concentration in pore watercompositions of granite rock in Taiwan’s offshoreisland, respectively.
4. RESULT AND DISCISSION
This study adopted the chemical kinetic model ofsmectite dehydration and illitization to calculate theamount of water expelled from smectite clay minerals
caused by the higher temperature of waste decay heat.
The results show as follow:

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japan

The heat-generating spent fuel is contained inthe canister. The canister heat decay in less than140012001000P(w)Figure 5 Temperature profile at 1000 years forsimulation domain.

A800

600400BDDETemperature(℃)

Figure 6 Points for temperature calculation within thebuffer.
1009080706050403020100110100

1000

10000

Time(years)
Point(A)

Point(B)

Point(C)

Point(D)Point(E)

Figure 7 Temperature distribution at point A, B, C, Dand E.
10090Buffer temperature(℃)

20,000 years can be calculated by the equation (1) asshown in Figure 4. We use the COMSOL model tocalculate heat transport through the EBS to host rockduring the 10,000 year period in the calculation.
Parameters for heat transport simulation is tabulated
in Table 3. Temperature profile at 1000 years isshown in Figure 5. We select the five points, A B C
D E, as the represented points for temperature
calculation within buffer (see Figure 6). Temperature
distribution for the five represented points during the10,000 years period is shown in Figure 7. Figure 8
shows the average temperature evolution within thebuffer material. Notably, the temperature peak occurs
before 10 years. After approximately 10,000 years,
the thermal caused by the release of the canister had
dispersed and the temperature had reduced to nearlygeothermal background level. Figure 9 shows theaverage smectite fraction on illitization process. Less
than 0.009% of the smectite is transformed to illite
during the 10,000-year period because hightemperatures are not retained within the buffer for
extended periods. Crucially, the smectite-to-illite
conversion for the performance assessment isnegligible.
Ferrage et al. (2007) performed a kinetic study ofsmectite dehydration and reported that a smectitestructure exhibits various hydration states with theintercalation of zero, one, or two planes of watermolecules in the interlayer. Future study could adopt
the chemical kinetic model of smectite dehydration to
calculate the amount of water expelled from smectite
clay minerals because of higher temperatures ofwaste decay heat. Thus, it can quantify the hydrous
state porosity, smectite shrinkage, and swellingrecovery. A literature review indicated that thehorizontal confining stress from the surrounding hostrock is approximately 11.43–16.55 MPa (Taipower,2017) and the bentonite dry density is 1,533–1,692kg/m3 (density at water saturation 1,983–2,086kg/m3), which yields a swelling pressure 4.5–16 MPa(SKB, 2010). Therefore, further investigating theswelling pressure of smectite and the confining stressof the surrounding host rock could provide further
insights into the computation of bentonite
consolidation and compression as well as smectite
volume expansion.

807060504030
201020000110100

1000

10000

Time(years)

Figure 4 Canister power as a function of time.

1101001000

10000

Time (years)

Figure 8 Average temperature evolution within thebuffer material.

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japan

Average smectite fraction

1.00000
0.99999
0.99998
0.99997
0.999960.99995
0.99994
0.99993
0.99992
0.99991110100

1000

10000

Time (years)

Figure 9 Average smectite fraction as a function oftime on illitization process.
5. CONCLUSION AND SUGGESTIONThis study adopted the Kinetic dehydration ofinterlayer water, Kinetic model for smectite to illite
conversion and hydration state of the interlayer tocalculate the amount of water expelled from smectite
clay minerals caused by the higher temperature ofwaste decay heat. The results show as follows: Thetemperature peak occurred before 10 years. Afterapproximately 10,000 years, the thermal caused by
the release of canister has dispersed and thetemperature has reduced close to the geothermal
background. Less than 0.009% of the smectite istransformed to illite during the 10,000-year period.
Because high temperatures are not retained within thebuffer for extended periods of time. It is important tonote the smectite-to-illite conversion for theperformance assessment is negligible. The results canform the advanced research results in evolution of
bentonite degradation for the performance assessment
and safety analysis on the final disposal of HLW inTaiwan.
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6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japan