Temperature effect on the thermal and hydraulic conductivity of Korean bentonite buffer material

High-level radioactive waste (HLW) disposal is considered to constitute a disposal system within a deep rock mass using deep geological repository. A deep geological disposal system has an engineered barrier system (EBS) consisting of canisters, buffer material, and backfill material.

Available online 4 May 2021

0149-1970/© 2021 The Authors Published by Elsevier Ltd This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)

Temperature effect on the thermal and hydraulic conductivity of Korean

bentonite buffer material

Seunghun Parka,1, Seok Yoonb,*,1, Sangki Kwona, Min-Su Leeb, Geon-Young Kimb

aDept of Energy Resources Engineering, Inha University, 100 Inha-ro, Michuhol-gu, Incheon, South Korea

bRadioactive Waste Disposal Division, Korea Atomic Energy Research Institute, 111, Daedeok-daero 989, Beon-gil, Yuseong-gu, Daejeon, South Korea

A R T I C L E I N F O

Keywords:

Gyeongju bentonite

Thermal conductivity

Hydraulic conductivity

Temperature effect

A B S T R A C T High-level radioactive waste (HLW) disposal is considered to constitute a disposal system within a deep rock mass using deep geological repository A deep geological disposal system has an engineered barrier system (EBS) consisting of canisters, buffer material, and backfill material Among these items, the buffer material protects a canister from groundwater inflow and prevents radionuclide outflow It is an also very important factor for evaluating the stability of a disposal system in which heat is propagated from the canisters The aim of this study was to evaluate the thermal and hydraulic properties of Gyeongju bentonite, a buffer material from Korea The thermal conductivity and hydraulic conductivity of Gyeongju bentonite were measured according to different degrees of saturation and to dry density The whole process is based on the temperature change induced in the disposal environment The thermal conductivity increased as temperature increased, and as did the temperature effect with high initial degree of saturation Additionally, the hydraulic conductivity also increased as temper-ature did, and decreased with high dry density After the process of heating and cooling, the thermal and hy-draulic conductivity of the bentonite presented irreversible changes

1 Introduction

A geological repository for high-level radioactive waste (HLW) is an

underground facility for which stable management is essential This

management includes consideration of risk factors such as leakage of

radioactive nuclides and high heat emission from the spent fuel The

deep geological disposal with an engineered barrier system (EBS) is

recommended to dispose HLW (Kim et al., 2011; Kwon et al., 2013) The

components of an EBS include canisters, backfill, buffers, and the

near-field rock (Fig 1) The buffer is an important component and can be

utilized for the reduction of groundwater inflow from the near-field

rock It is both for blockage of leaks of radioactive nuclides dissolved

in groundwater from the spent fuel and for protection of the canister

from physical shocks such as rock shear behavior The materials used in

designing the buffer were evaluated and bentonite was found to be most

suitable It has been chosen in many studies related to the characteristics

of candidate buffer materials (Lee et al., 2011, 2017) Bentonite has the

characteristics of low permeability and high radionuclide retention

ca-pacity However, these properties might be deteriorated with increasing

temperature due to the decay heat from the spent fuel Therefore, it is

important to understand the thermal and hydraulic characteristics of the bentonite They are considered to be affected by decay heat generated by the spent fuel

The EBS in the disposal environment is necessary to dissipate heat generated by spent fuel in the canisters and to prevent the release of nuclides due to inflow of groundwater from the surrounding rock Above all, the buffer plays an important role in ensuring that the heat in the disposal environment leads to damage to canisters and change in the rock behavior, on account of excessive thermal stress and thermal expansion respectively Many countries have limited the maximum value of allowable temperature of the buffer to less than 100 ◦C (Choi

et al., 2008; Lee et al., 2007; Lee et al., 2014; JNC, 1999) For this reason, much research has been done on change in the characteristics of bentonite due to exposure to high temperature in a repository where long-term performance of the system is critical The researches on bentonite indicate that temperature change has an luence on its hy-draulic and thermal characteristics (Daniels et al., 2017; Ye et al., 2013, Yoon et al., 2018; Zihms and Harrington, 2015) The hydraulic con-ductivity of bentonite is known to increase as temperature increases (Cho et al., 1999).The reasons for change in hydraulic conductivity due

* Corresponding author

E-mail address: syoon@kaeri.re.kr (S Yoon)

1 These authors contributed equally to this work

Contents lists available at ScienceDirect Progress in Nuclear Energy journal homepage: www.elsevier.com/locate/pnucene

https://doi.org/10.1016/j.pnucene.2021.103759

Received 3 November 2019; Received in revised form 21 April 2021; Accepted 23 April 2021

to increasing temperature can be explained as contributions from the

change of both water viscosity and density (Lide, 1995; Villar et al.,

2010) When the degree of saturation might decline, the thermal

con-ductivity of bentonite is known to decrease with increasing temperature

(Yoon et al., 2018) It is thought that the temperature contributes to the

evaporation of water in the pores of bentonite (Cho, 2019) On the other

hand, the thermal conductivity of bentonite increases in some cases with

increasing temperature (Xu et al., 2019), which can vary depending on

the degree of saturation of bentonite and the sealing environment of the

experiment In a disposal repository subject to environmental change in

temperature and groundwater inflow, research on the thermal and

hy-draulic properties of bentonite is definitely critical in terms of long-term

integrity and sealing performance of the buffer

The temperature of EBS is expected to change for a long period of

time with the heat emissions from the canisters In addition,

tempera-ture change is considered as a necessary factor for the repository system

management Most of the researches studying the temperature effects on

bentonite have been carried out to evaluate the bentonite properties In

the process, the condition only up to the maximum temperature created

by decay heat has been considered (Cho et al., 2017) In a geological

repository where change in the long-term behaviors occur, a

tempera-ture decreases occurs after the maximum temperatempera-ture is reached, as

shown in Fig 2 Such phenomenon on the temperature change of the

buffer in a geological repository was predicted from computer modeling

This explains why the rise and fall of the buffer temperature in the re-pository environment will inevitably occur This is also the reason the buffer properties need to be considered from the viewpoint of long-term integrity given the temperature change phenomena in a geological re-pository Some evaluations of the buffer properties have been already performed, considering the rise and the fall in temperature (Chen et al.,

2017; Daniels et al., 2017; Zihms and Harrington, 2015) In most of the previous research, although Na-type bentonite was used to understand the thermal and hydraulic behaviors of bentonite on the temperature change, there has been little research on Ca-type bentonite in terms of diversity of bentonite researches Furthermore, there has been little evaluation for the thermal and hydraulic properties of a buffer consid-ering various dry density and water evaporation under conditions in which the temperature rises and falls

In this study, an investigation of the thermal and hydraulic proper-ties of a buffer was conducted Not only did it consider the rise and fall of temperature, but it involved other variables such as the degree of saturation and various dry density, which were not considered in pre-vious researches In Korea, Gyeongju bentonite, which is Ca-type pro-duced from Gyeongju province, is regarded as a candidate for the buffer material for Korean reference disposal system Therefore, this research aimed to help understand the thermal and hydraulic properties of this Ca-type bentonite

2 Materials and test methods

2.1 Characterization of Gyeongju bentonite

Bentonite composes of the minerals such as montmorillonite, quartz, feldspar, halloysite, clinoptilorite, cristobalite, and a small amount of organic matter The bentonite mined in the Gyeongju area of South Korea is called Gyeongju bentonite Gyeongju bentonite is classified as a Ca-type bentonite in which Ca cations occur between layers of mont-morillonite The representative bentonite of the Na-type is MX-80, which is mined in Wyoming (U.S.A) The main mineral content of MX-

80 includes montmorillonite (90%) and others (10%) such as cristoba-lite, feldspar, quartz, etc (Zihms and Harrington, 2015) In contrast, the main minerals in Gyeongju bentonite are montmorillonite (62%), feld-spar (21%), and small amounts of other minerals (~17% of such as quartz (5%), cristobalite (4%), calcite (5%),heulandite (3%)) In Korea, researches on physical, hydraulic, thermal, and chemical characteriza-tions were carried out to investigate the possibility that Gyeongju bentonite can be used as a candidate for buffer material (Lee et al.,

2011) The typical characterizations of Gyeongju bentonite are pre-sented in Table 1

2.2 Thermal conductivity test

Measurement of the thermal conductivity of bentonite was con-ducted using QTM-500 When it comes to measurement, QTM-500 has the advantage that it can easily and rapidly obtain thermal conductivity using the transient hot-wire method The transient hot-wire method is a calculation method using the relationship between the temperature of a hot wire and the time needed for the flow of heat (thermal flow) to pass through the specimen to reach the probe The probe used for the thermal

Fig 1 Concept of the engineered barrier system in a deep disposal repository

(Yoon et al., 2019)

Fig 2 Temperature change of buffer material over time as predicted from

modeling (Lee et al., 2019)

Table 1

Various properties of Gyeongju bentonite (Yoo et al., 2015; Yoon et al., 2019)

Specific surface area (m 2 /g) 61.5 Cation Exchange Capacity (meq/100g) 64.7

conductivity measurement was PD-11, which was designed to make it

only possible for the heat to flow into specimen from the hot wire, as

shown in Fig 3(a) The bentonite powder was compressed in accordance

with the dry density of 1.61 g/cm3, and the compacted specimen had the

dimensions 100 × 50 × 20 mm The thermal conductivity was

deter-mined using Eq (1)

λ = K t×R × I2×ln t2/ t1

T2− T1

where λ is the thermal conductivity (W/m∙K) K t and H t, the correction

factor on the measurement device were calculated from the data on the

reference samples which were quartz: 1.42 W/m∙K, silicon rubber: 0.24

W/m∙K, and plastic: 0.036 W/m∙K R is the electric resistance of the

probe (Ω/m) per unit length, I is the heating current (A), t2 and t1 are the

time from the yield of electronic current (t1 =30 s, t2 =60 s), and T2 and

T1 are the temperatures (K) at t2 and t1.The specimen and probe were

sealed with thermo tape to prevent water from escaping the specimen

They were placed in a furnace during the experiment in order to control

the temperature variation, as shown in Fig 3(a) The thermal

conduc-tivity of the compacted bentonite at a certain temperature was measured

until the temperature became constant

2.3 Hydraulic conductivity test

The hydraulic conductivity test for a soil material can be measured

by the constant head or failing head method Both methods are suitable

for applications with highly permeable soil If the constant head or

failing head test were used to measure the hydraulic conductivity of

bentonite, it would be possible to get erroneous results due to the lead

time of the experiment and the evaporation of water (Cho, 2019) Many

researchers have measured the hydraulic conductivity of bentonite

using the modified constant head test (Chen et al., 2017; Cho et al.,

1999; Daniels et al., 2017) The modified constant head test has the same methodology as the other experiments do; the difference is whether the constant pressure is supplied or not According to Darcy’s law, the hy-draulic conductivity of fluid saturated bentonite can be calculated as follows:

where K h is the hydraulic conductivity (m/s), Q is the volumetric flow rate (m3/s), A is the surface area of the specimen (m2), and H h is the hydraulic gradient (m/m)

Darcy’s law assumes that linearity may exist between the hydraulic gradient and the flow rate This linearity comes from the proportion in the hydraulic gradient generated by the amount of fluid running through

a porous medium For this reason, devices were required that could provide a constant hydraulic gradient in order to measure the hydraulic conductivity In this study, a GDS Advanced Pressure/Volume Controller (ADVDPC) was used to measure the hydraulic conductivity of the bentonite This device consisted of a gauge which can measure and control the fluid pressure and change in volume The fluid from this device can be injected to water or oil, and an additional device is needed

to inject air In this experiment, distilled water was used as the fluid The gauge was used in the pressure range 0.1–8 MPa, depending on the solid volume The specimen in the cell was maintained at a differential pressure of 1.2–1.3 MPa of injection pressure and 0.2 MPa of back pressure The experimental setup measured the amount of water that was penetrated and accumulated in the bentonite to provide a flow of water from the top of the cell

In order for the experiment to be conducted, compacted circular blocks with diameter of 50 mm and height of 10 mm, were produced using Gyeongju bentonite with about 11% of water content The test condition of hydraulic conductivity was maintained as measurement

Fig 3 Setup for the measurement of thermal conductivity and hydraulic conductivity

concept of Fig 3(b) Before the hydraulic conductivity test, the

bentonite was fully saturated by slowly supplying 0.1–1 MPa of water

pressure inside the cell from Day 1 to Day 10

2.4 Conditions for tests of thermal and hydraulic conductivity

All experiments were conducted with respect to the expected

perature hysteresis of the buffer in the disposal repository The

tem-perature hysteresis means the variation of the temtem-perature change in the

repository condition It was reflected in the test procedure with

consideration of the research results of Cho et al (2017) They have

predicted the long-term temperature change of the buffer In this study,

the temperature hysteresis was related to the heating and cooling

pro-cess, which means the temperature of the buffer material changes

Ac-cording to the previous researches, in a disposal repository, it is

inevitable that the temperature of the buffer increases and decreases

For this reason, changes were observed in the thermal and hydraulic

properties of bentonite during heating and cooling process It was to

investigate the reversibility of the bentonite properties after heating and

cooling

The unsaturated bentonite specimens for the thermal conductivity

test were used with various degrees of water saturation The heating and

cooling process (Test conditions: 2–6) was reflected, as listed in Table 2

According to the test conditions regarding the degree of saturation, the

bentonite for the thermal conductivity was used in the heating and

cooling process for certain periods of time (4–10 h)

The compacted bentonite specimens with various dry densities from

1.3 to 1.74 g/cm3 were used for the hydraulic conductivity test and it

reflected the heating and cooling processes (Test conditions: 2–6) as

listed in Table 3 Under each test condition, the bentonite was allowed to

be heated and cooled for certain periods of time (0.5–1 day) to achieve

temperature equilibrium During all of the cooling processes in the

thermal and hydraulic conductivity tests, the temperature of the

bentonite was decreased by natural cooling at a constant temperature

3 Results

3.1 Results for thermal conductivity

Because the suggested dry density of compacted bentonite is more

than 1.6 g/cm3, compacted bentonite with a dry density of 1.61 g/cm3

was used to measure the thermal conductivity The thermal conductivity

increases as the temperature increases, as shown in Fig 4 The bentonite

is composed of three phases (i.e., soil particles, water, and air) The

thermal conductivities of soil particles, water, and air are proportional

to the temperature increase in the range 20–90 ◦C (Cengel and Chajar,

2011) Thus, it is thought that thermal conductivity of compacted

bentonite would also increase with temperature increase The higher the

compacted bentonite’s initial degree of saturation is, the higher the

thermal conductivity should increase with temperature It was assumed

that degree of saturation was almost constant because it was impossible

to measure volume change during the experiment Based on the previous

research conducted by Xu et al (2019), it was reported that the transfer

of latent heat had an impact on the thermal conductivity of soils The transfer of latent heat requires enough water and air movement for water vapor to pass through during temperature increase Furthermore, the thermal conductivity was 1–6%, when cooling which was less than heating The soil’s particle size increased during heating and decreased during cooling (Dong et al., 2019) From this, it is inferred that the thermal conductivity of soils is related to the particle size during heating and cooling Moreover, the density of water decreases as temperature increases (Cengel and Chajar, 2011), which, consequently, is likely that

it could not be recovered fully during the cooling process

On account of heating and cooling, the variations show an irrevers-ible phenomenon: the decrease in the thermal conductivity with each degree of saturation (Fig 4) In addition, such phenomenon can be explained by a change in the thermal behavior of particles and water, from when soil is unsaturated Ferrari and Laloui (2017) explained that unsaturated soil subjected to cooling would generate an irreversible volume change due to the rearrangement of the particles during their thermal contraction They also noted that heating and cooling of un-saturated soil would induce rearrangement of the particles and reduce the volume of mass of water which is affected Typically, when the temperature drops, it is known that the thermal contraction coefficient

of water is 17 times greater than solids The thermal contraction coef-ficient means the relative contraction rate of thermal strain between water and solid after cooling (Sultan et al., 2002) It is believed that this phenomenon contributes to the decrease of thermal conductivity due to water The water has higher thermal contractibility during cooling after

a temperature rise During heating and cooling, the results of bentonite properties change appeared to present a similar result in a previous study related to irreversible phenomena (Sultan et al., 2002) The study explained that irreversible phenomena led to the change in bentonite properties due to the thermal behavior of bentonite (i.e the change in its properties) during heating and cooling

Table 2

Details of the experimental thermal conductivity test conditions

Density

(g/cm 3 ) Test Condition Temperature ( ◦C) Test Duration

(hours)

Initial Degree of Saturation

Initial Water content (%) 1.61 1 – No Heating 25 4

7

10

4

4

0.21, 0.47, 0.61, 0.83 5.3, 11.9, 15.3 20.7

2 – Heating 43–45

3 – Heating 61–63

4 – Heating 83–86

5 - Cooling 55–60

6 - Cooling 25

Table 3

Details of experimental process of the hydraulic conductivity test

Density (g/cm 3 ) Test Condition Temperature ( ◦C) Test Duration

(days)

Injection Pressure (MPa)

Back Pressure (MPa) 1.3, 1.4,

1.5, 1.6, 1.74

1 - Saturation 25 1–10 0.1–1.0 –

2 - Heating 30 1.5 1.2–1.3 0.2

3 - Heating 60 1.5

4 - Heating 90 1.5

5 - Cooling 60 2

6 - Cooling 30 2

Fig 4 Change in thermal conductivity of Gyeongju bentonite with saturation

and temperature

3.2 Results for hydraulic conductivity

Hydraulic conductivity tests were conducted on compacted

bentonite samples of various densities Tests considering the

tempera-ture change at 30, 60, and 90 ◦C measured the volume change over time

with water being injected Fig 5 shows change in the slope of the

vol-ume dependent on time at a fixed injection pressure and back pressure

The slope change was found to increase with temperature rise In

addition, the correlation coefficient was close to 1.0 with the volume

change over time The consequences of the inflow and outflow showed

that the slope change was similar to each other and that linearity was

shown in both of them

As shown in Fig 6, the hydraulic conductivity was found to change

with temperature increase (from 30 to 90 ◦C) and decrease (from 90 to

30 ◦C) An effect of temperature on bentonite which has various dry

densities tended to increase the hydraulic conductivity with heating In

the heating process, temperature increase from 30 to 90 ◦C resulted in

increase in hydraulic conductivity of about 0.2–2 times at each dry

density The cause of this increase during heating might be due to the

change in water viscosity Based on the previous study, water viscosity is

reported to be more sensitive to temperature increase Moreover, water

viscosity is known to decrease when temperature rises in the range

below 100 ◦C (Cho, 2019)

Hydraulic conductivity during cooling was observed to be lower than

during heating, as shown in Fig 6 Table 4 represent the rate of change

in hydraulic conductivity during heating and cooling process It is

believed that an irreversible phenomenon occurred due to change in the

bentonite microstructures during cooling after heating occurred This

irreversible phenomenon is explained by the change of fully saturated bentonite characteristics during the heating and cooling processes which Punch (2015) introduced Firstly, reduction of the swelling pressure of bentonite was generated during the heating process This would take place when the particles were rearranged due to change in the bentonite microstructure with the temperature increase Then, the reduction of swelling pressure would be expected to induce fluid penetration because the binding force weakened between the particles For this reason, it is expected to influence the increase of hydraulic conductivity Secondly, the swelling pressure of the bentonite increased in the cooling process This is expected to be when the temperature drops, so that the filling of the pores would occur due to contraction of the microchannels the contraction was caused by the increase in swelling-pressure between the rearranged particles Therefore, it is thought hydraulic conductivity during cooling reflects a slight decrease from the initial hydraulic con-ductivity during heating As a result, irreversible change in the hydraulic conductivity generated during heating and cooling seems to be due to microstructural alteration by changes in the swelling pressure of the bentonite

The hydraulic conductivity of the bentonite with a given dry density presents a non-linear change It decreases hydraulic conductivity when the dry density increases, as shown in Fig 7 The tendency of the hy-draulic conductivity to decrease is reflected, at each dry density, by the different effects of the swelling, which leads to decrease in the size of pores between the particles of the compacted bentonite with increasing dry density In other words, the bentonite particles surface when they come into contact with water In addition, the combination of the behavior of bentonite particles with water molecules presents pore

Fig 5 Volume change of inflow and outflow at a fixed injection and back pressure

reduction This is the reason water molecules in the hydrated layer of the

bentonite surface are strongly bound to the oxygen of the silicate surface

of the bentonite particles, so that the water cannot flow An influence of

the hydraulic conductivity on the temperature with increasing dry

density is observed to have a difference of about 1–2 orders according to

each experimental temperature Thus, the hydraulic behavior of the buffer bentonite in a disposal environment is thought to generate a change in the hydraulic conductivity property with temperature in-crease, and to follow a decrease in hydraulic conductivity if the buffer material were used at high density

Among the bentonite candidate materials, many studies have been carried out using Ca- and Na-type bentonites Zihams and Harrington (2015) have drawn a conclusion that the permeability changes during the heating and cooling process using Mx-80 (Na-type bentonite) Similar to the results in this study (Ca-type bentonite) compared with a previous study (Na-type bentonite), it was indicated that irreversible phenomena of hydraulic properties generated with heating and cooling The intrinsic permeability was calculated using Eq (3) to compare these results with previous studies

k = K h×ρ w× g

Fig 6 Change in the hydraulic conductivity of Gyeongju bentonite with temperature

Table 4

Rate of change in hydraulic conductivity with the heating and cooling process

Temperature change Change (%) in the hydraulic conductivity with different density

1.3 (g/cm 3 ) 1.4 (g/cm 3 ) 1.5 (g/cm 3 ) 1.6 (g/cm 3 ) 1.74 (g/cm 3 )

30 ◦C -> 60 ◦ C 34% 86% 207% 103% 95%

60 ◦C -> 90 ◦ C 48% 68% 458% 51% 78%

90 ◦C -> 60 ◦ C − 34% − 42% − 86% − 39% − 50%

60 ◦C -> 30 ◦ C − 36% − 48% − 62% − 50% − 45%

where k is the intrinsic permeability (m2), K h is the hydraulic

conduc-tivity (m/s), ρ w is the water density (kg/m3), g is the acceleration of

gravity (m/s2), and μ w is the water viscosity (Pa∙s, kg/m∙s)

The result of calculating the intrinsic permeability is shown in Fig 8

The MX-80 results were for dry density of 1.56 g/cm3 and Gyeongju

bentonite was for dry density of 1.6 g/cm3 There was a significant

difference in permeability change with the heating and cooling process,

and the permeability of Gyeongju bentonite was greater than of MX-80

Moreover, Ca-type and Na-type bentonites present a difference of about

1 order of permeability This is because the pore size is increased due to

the lower swelling effect of Ca-type bentonite, which is relatively less

than that of Na-type bentonite Consequently, Ca-type bentonite is

ex-pected to have greater permeability than Na-type is, and it is believed

that irreversible phenomena similarly occur with respect to the heating

and cooling process for both types of bentonite Therefore, it is thought

that the swelling of bentonite has an influence on its permeability when

subject to heating and cooling processes, and that this is an intrinsic

feature of Ca- and Na-type bentonites In the disposal environment, it is

necessary to take into account the effect of the change in permeability

generated by swelling as an intrinsic feature of bentonite which subject

to temperature changes

4 Conclusion

In this study, the thermal and hydraulic properties of Gyeongju

bentonite as buffer material were evaluated with regard to the

tem-perature change due to the heat expected in a high-level radioactive

waste disposal site Since the bentonite buffer materials are subjected to

rising and falling temperatures in a canister, it is important to identify

the thermal and hydraulic properties of bentonite with a consideration

of heat changes There have been many previous studies on the

evalu-ation for properties considering the effects of changes on Na-type

bentonite in the heat environment, while the study of Gyeongju

bentonite (Ca-type) was insufficient Therefore, in this paper, thermal

conductivity and hydraulic conductivity, which are normally considered

as the most important on the thermal and hydraulic performance of

bentonite buffer materials, were measured for Gyeongju bentonite with

a consideration of the temperature changes From this study, the

following conclusions were drawn:

(1) In the thermal conductivity experiment considering the

unsatu-rated state, the thermal conductivity increased linearly with

temperature rise In the heating process, the thermal conductivity

increased up to about 0.1–0.2 W/m∙K with different degrees of

saturation According to each degree of saturation, the thermal

conductivity during cooling was about 1–6% different in thermal

conductivity after heating process

(2) In the saturation hydraulic conductivity test, change of the hy-draulic gradient was clearly measured with increasing tempera-ture The hydraulic conductivity increased linearly with increasing temperature The hydraulic conductivity increased at

an approximately up to 1 order in the heating process with each change in dry density In addition, all of the tests indicated an irreversible change of hydraulic conductivity during cooling The hydraulic conductivity with increasing dry density presented a tendency of non-linear decrease

(3) Comparison of the intrinsic permeability of Gyeongju (Ca-type) with MX-80 (Na-type) bentonites was performed during heating and cooling processes Both types of bentonite showed an irre-versible change in intrinsic permeability that decreased further during cooling processes In addition, the intrinsic permeability

of Ca-type and Na-type bentonite was about 1 order different, and that of Ca-type was greater Although the intrinsic permeability

of bentonite was confirmed, but these properties could be affected by the intrinsic features of bentonite

For future studies, it is recommended that bentonite particle behavior, bentonite mineral properties, and bentonite water behavior be considered It is for better understanding of the thermal and hydraulic properties of the bentonite buffer material In addition, the thermal and hydraulic properties performed in this study may support analysis of the buffer behavior using computer modeling of the disposal repository Furthermore, under the similar conditions to this study, future studies are necessary to reflect the environmental factors in a disposal re-pository, including certain factors such as long-term behaviors, chemical properties of the fluids, and constraints of the bentonite

Credit author statement Seunghun Park: Investigation, Writing-review & editing Seok Yoon: Writing & editing, Supervision Sangki Kwon: Editing Min-Su Lee: Investigation Geon-Young Kim: Methodology

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper

Acknowledgments

This research was supported by the Nuclear Research and Develop-ment Program of the National Research Foundation of Korea (NRF- 2021M2C9A1018633), and Basic Research Project (NRF- 2020R1FA1072379) funded by the Minister of Science and ICT

Fig 7 Change in the hydraulic conductivity of Gyeongju bentonite at various

Appendix A Supplementary data

Supplementary data to this article can be found online at https://doi

org/10.1016/j.pnucene.2021.103759

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