Experimental study on the physico-mechanical properties of Tamusu mudstone-A potential host rock for high-level radioactive waste in Inner Mongolia of China

Chen Lu,Hongdn Yu,Honghui Li,Weizhong Chen,**

a State Key Laboratory of Geomechanics and Geotechnical Engineering,Institute of Rock and Soil Mechanics,Chinese Academy of Sciences,Wuhan,430071,China

b University of Chinese Academy of Sciences,Beijing,100049,China

c China Institute for Radiation Protection,Taiyuan,030006,China

Keywords:Tamusu mudstone Physico-mechanical properties High-level radioactive waste (HLW)repository Disintegration test Permeability test

ABSTRACT Tamusu mudstone,located in Bayin Gobi Basin in Inner Mongolia of China,has been selected as a potential host rock for high-level radioactive waste (HLW) disposal in China.A series of tests has been carried out,including X-ray diffraction (XRD) tests,scanning electron microscopy (SEM) tests,disintegration tests,permeability tests and triaxial compression tests,to estimate the physico-mechanical properties of Tamusu mudstone in this work.The mineral composition of Tamusu mudstone was analyzed and it was considered as a stable rock due to its low disintegration rate,i.e.approximately 0.11% after several wet/dry cycles.Based on the results of permeability test,it was found that Tamusu mudstone has a low permeability,with the magnitude of about 10-20 m2.The low permeability makes the mudstone well prevent nuclide migration and diffusion,and might be influenced by temperature.The triaxial tests show that Tamusu mudstone is a stiff mudstone with high compressive strength,which means that the excavation disturbed zone would be smaller compared to other types of mudstone due to construction and operation of HLW repositories.Finally,the properties of Tamusu mudstone were compared with those of Opalinus clay,Callovo-Oxfordian(COx)argillite,and Boom clay to further discuss the possibility of using Tamusu mudstone as a potential nuclear waste disposal medium.

1.Introduction

In recent years,many scientific issues attributed to the widespread use of nuclear energy have received increasing attention.Among them,the disposal of radioactive nuclear waste has received the most attention.Scholars have proposed many ideas for disposing high-level radioactive waste (HLW),such as ice cover,ocean disposal,seabed disposal,outer space disposal,and deep geological disposal.At present,the most acceptable way is the deep geological disposal.Clayey rocks due to their favorable advantages,such as low permeability,strong ion exchange,strong adsorption,and fracture self-sealing,have been selected as a potential surrounding rock in HLW repositories in many countries.

Switzerland built an underground rock laboratory called Mont Terri in the Canton of Jura.At this laboratory,many field and laboratory tests have been performed to study the physical properties of Opalinus clay,including physico-mechanical characteristics such as the mineral composition,permeability characteristics,excavation disturbed zone (EDZ),and thermo-hydro-mechanical (THM)characteristics(Thury and Bossart,1999;Martin and Lanyon,2003;Bossart et al.,2004,2017; Croise et al.,2004; Wersin et al.,2004;Kull et al.,2007; Amann et al.,2011,2012).

The French National Radioactive Waste Management Agency(Andra)has built an underground HLW laboratory in Bure,Meuse/Haute Marne Province,to study the feasibility of Callovo-Oxfordian(COx) argillite to be the surrounding rock of the repository.Many tests have been conducted at this laboratory.The field tests at this site have mainly investigated the evolution of the EDZ permeability characteristics,hydro-mechanical (HM) and THM properties,and long-term creep properties of COx argillite (Andra,2005; de la Vaissière et al.,2015; Armand et al.,2017a).Some scholars have studied the HM properties of COx argillite through laboratory tests.For example,Homand et al.(2004) used the pulse method to measure the permeability of saturated COx argillite samples and improved the equipment and methods to measure the permeability of unsaturated rock samples.Zhang(2016)studied the relationship between permeability and volumetric strain of COx argillite,and his results showed that the permeability first decreased and subsequently increased with increasing volumetric strain.Armand et al.(2017b) systematically studied the HM characteristics of COx argillite.Zhang (2018) summarized the THM properties of COx argillite in detail and improved the understanding of COx argillite THM properties.

Belgian Nuclear Research Centre (SCK.CEN) established the HADES underground laboratory at a preselected site in the Boom clay layer within the Mol-Dessel region and invited experts from many countries to study the physico-mechanical properties of Boom clay under multifield coupling.De Bruyn and Thimus(1996)performed triaxial tests at high temperatures and found that the strength and elastic modulus of Boom clay were dependent on the temperature.Delage et al.(2000) studied the permeability coefficient of Boom clay at different temperatures.Their results showed that the hydraulic conductivity of Boom clay increased at higher temperature.Based on field tests at the HADES underground research laboratory,the evolution of the permeability within the EDZ of the surrounding rock of Boom clay was studied (Bastiaens et al.,2007; Van Marcke and Bastiaens,2010).These studies showed that from 2004 to 2005,the hydraulic conductivity of the surrounding rock in the connecting gallery in the horizontal and vertical directions tended to decrease,and the difference of the hydraulic conductivity in these two directions gradually decreased.

During the past decade,our research team focused on studying the THM properties of Boom clay(Yu et al.,2012,2014,2015,2018;Ma et al.,2016).For example,Ma et al.(2016)studied the evolution law of the permeability of Boom clay in both horizontal and vertical directions under complicated conditions,i.e.different temperatures and loading paths.Yu et al.(2018) performed triaxial tests under THM coupling,and their results showed that the shear strength,elastic modulus and cohesion of Boom clay decreased when temperature increased.However,the effect of temperature on the friction angle is not substantial.

In China,much more effort has been made to granite in Beishan,Gansu Province,for HLW underground disposal during the past decades.Indeed,the study on HLW disposal in mudstone has just begun.A few researches have been performed related to the HLW disposal in mudstones,such as regional preselection,section preselection and site evaluation.According to the natural,social and economic conditions,Tamusu mudstone in the Bayin Gobi Basin,which is located in Inner Mongolia of China,has been selected as a candidate host rock of the HLW repository(Du et al.,2017;He et al.,2019;Xiang et al.,2020).On the basis of previous studies,this paper presents basic information on physico-mechanical properties of Tamusu mudstone and gives a comparison with other clayey rocks mentioned above.The core samples used in this paper were drilled from the Yingejing Sub-basin,with the core hole number of TZK-2.The storage method and properties of the cores were shown in Xiang et al.(2020).

2.Analyses of mineral compositions of Tamusu mudstone

2.1.Methodology

Representative rock blocks were selected for analyzing the mineral compositions of Tamusu mudstone using scanning electron microscopy (SEM) and X-ray diffraction (XRD).The rock samples were ground into powder and passed through a 40 μm square hole sieve.Then the screened powder was dried at 40°C for 24 h.Finally,the powder was sealed and stored.In order to obtain the clay mineral content,the clay fractions (＜2 μm) of the samples were investigated using oriented mounts in the air-dried state after ethylene glycol solvation and after heating at 495°C.

A Bruker D8 ADVANCE X-ray diffractometer was adopted to analyze the mineral compositions of Tamusu mudstone at room temperature.The diffractometer is operated by CuKα radiation with the tube current of 40 mA and the tube voltage of 40 kV.The scanning range of the diffractometer is 5°-50°with a step size of 0.02°,and the counting time is 0.3 s.

A FEI Quanta 250 scanning electron microscope with INCA XMAX50 EDX detector produced by the FEI Company(USA)was used to analyze rock samples.Since the rock sample was not conductive,gold spraying treatment was carried out.The SEM tests were conducted under an accelerating voltage of 20 kV and a working distance of 15.4 mm.

2.2.Analysis of XRD and SEM results

Different mineral compositions of Tamusu mudstone can be distinguished from the XRD traces according to the value and angle of the characteristic peak.Results of the mineral composition analysis are shown in Table 1 and Fig.1,and SEM images of the samples are shown in Fig.2.

Table 1 Mineral composition (%) of Tamusu mudstone.

The mineral composition analysis shows that the percentages of dolomite,plagioclase,and quartz,which have high strength(Wang et al.,2017),are relatively high,occupying 77% of the sample.Combined with XRD results,we can find quartz,dolomite,plagioclase and clay minerals in Fig.2.

The clay minerals occupy around 13%in the Tamusu mudstone,which mainly consists of illite and illite/smectite interstratified minerals.These clay minerals have strong ion exchange characteristics and adsorption capacity,which can effectively prevent the migration of radionuclides (He et al.,2019; Zhang et al.,2020).Furthermore,the Tamusu mudstone contains 1%of analcime which may enhance the radionuclide adsorption properties (Xiang et al.,2020).It will benefit for absorption of nuclide for high-level radioactive waste underground disposal.Moreover,Fe2+in pyrite,which accounts for 5% of the total sample,can reduce the high valence radionuclides and reduce their migration and diffusion capacity.S-in pyrite can consume H2O2produced by the radiation to groundwater,which is beneficial to maintain the reduction atmosphere of the repository(Chen et al.,2010).

The massive particles of Tamusu mudstone are closely arranged,and the anisotropy is observed in the microstructure (see Fig.2).There are many massive grain crystal minerals in the sample,with different sizes and irregular shapes.The sample micrographs show that the largest grain crystal is approximately 7 μm,whereas the smallest one is approximately 0.1 μm.Pores are observed in the micrographs among the clay mineral particles or between clay and non-clay mineral particles,some of which are filled with solid particles.Microcracks can be observed in the micrographs,primarily in the intergranular and intragranular regions.Most of these microcracks are irregular in shape,with good extensibility and different lengths.

SEM images reveal that the microstructure of Tamusu mudstone is anisotropic.Former researches (Dehandschutter et al.,2004,2005) on Boom clay samples taken at a depth of several metres in the sidewall of a tunnel also revealed obvious differences in the microscopic junction between the perpendicular and parallel directions to the bedding plane.The anisotropy is an important factor that affects the mechanical properties of clayey rock and plays an important role in THM coupling.The anisotropy of the mechanical properties of Tamusu mudstone must be in-depth investigated in future studies.

Fig.1.XRD traces of representative rock blocks.

3.Disintegration tests on Tamusu mudstone

3.1.Test preparation

Six cylindrical samples,each of which has a diameter of 25 mm and a length of 50 mm,were prepared and numbered as BJ-1 to BJ-6.The basic details of the disintegration test samples are shown in Table 2.A photograph of the sample BJ-6 is shown in Fig.3.

Table 2 Basic characteristics of the disintegration test samples.

3.2.Test procedure

The disintegration tests were performed according to the following procedure:

(1) The samples were placed in an oven at 105°C for 24 h,during which the samples were continuously dried.

(2) The samples were moved out of the oven and cooled to room temperature,after which they were ground,sieved and weighed.

(3) The samples were placed into distilled water and soaked for 24 h.

(4) Steps (1)-(3) were repeated 5 times in total.

The electronic scale in this test had a capacity of 1000 g and an accuracy of 0.01 g.The temperature of the test environment was controlled at 25°C.Rubber gloves were worn when contacting the sample during weighing,and the measurements were conducted immediately after the room temperature was reached to avoid the dried samples absorbing moisture from the air and minimize the temperature changing effects.

3.3.Test results

The results obtained from the disintegration tests are shown in Table 3.After 5 wet/dry cycles,few disintegrated particles and blocks were found in the samples,and the integrities were maintained.According to a visual inspection,Tamusu mudstones did not collapse or sustain any visible cracks or defects.The samples at the end of the test seemed approximately identical to their state before testing.The disintegration rate of Tamusu mudstone can be defined as follows (Wu et al.,2010):

Table 3 Measurements from the disintegration tests.

where Dmis the disintegration rate,m1is the mass of the rock sample during testing,and m2is the mass of the first weighting.The results of the sample disintegration rates are shown in Fig.4.

As the sample is small,the test results inevitably have a certain degree of discreteness.The average of the test results is shown by the dotted line in Fig.4.The disintegration rate of the sample increases when the number of wet/dry cycles increases.The average disintegration rate is approximately linear,especially after the second wet/dry cycle.

Fig.2.SEM images of Tamusu mudstone: (a) Perpendicular and (b) parallel to the bedding plane.

Fig.3.Photograph of the sample BJ-6.

Fig.4.Disintegration test results.

The disintegration characteristics of Tamusu mudstone are very weak.The disintegration rates of samples BJ-1 to BJ-6 were 0.15%,0.16%,0.05%,0.11%,0.04% and 0.17%,respectively,with an average value of 0.11% after 5 wet/dry cycles.The results of the mineral composition analysis in Section 2 revealed that the clay mineral content was 13%,including 8% illite/smectite.This composition contributes to Tamusu mudstone expansion(Helmy,1998;Hensen and Smit,2002) and disintegration (Morrow et al.,2000; Kong et al.,2018; Yang et al.,2019) when the mudstone comes in contact with water.

The physical properties of Tamusu mudstone remain stable after water exposure,which makes it difficult to disintegrate.Therefore,when a saturated rock sample is required before testing,it can be saturated in a vacuum saturation cylinder,which can improve the saturation efficiency.

4.TH coupled properties of Tamusu mudstone

4.1.Test preparation

For the permeability measurement,two cylindrical Tamusu mudstone samples,each with a diameter of 38 mm and a height of 20 mm,were prepared and labelled as TMS01 and TMS02 (Fig.5),respectively.The samples were collected in the direction parallel to the bedding plane and thus the measured permeability of the mudstone was normal to the bedding plane.Following the suggestions in the literature (Boulin et al.,2012),the hydraulic conductivity and intrinsic permeability of Tamusu mudstone were measured by the steady-state method.According to the in situ field test results provided by Du et al.(2017) and Huang (2018),if the geological disposal repository of nuclear waste is built at a depth of 500 m underground,both the horizontal and vertical stresses will be approximately 10 MPa,and the pore water pressure will be 4-5 MPa.Therefore,for the permeability test,the hydrostatic pressure is 10 MPa (σ1=σ2=σ3=10 MPa) and the backpressure is 4 MPa (Δp=4 MPa).The radioactive nuclide in HLW will release heat due to decay during geological disposal,which results in a substantial increase in the temperature of the surrounding rock of the disposal repository (Weetjens and Sillen,2005).Therefore,in thepermeabilitytest,thetemperaturechangeis 20°C→40°C→60°C→80°C→60°C→40°C→20°C,and the heating/cooling rate is 1°C/h.The thermo-mechanical loading path is shown in Fig.6.

4.2.Test results

The permeability test results are shown in Fig.7.The hydraulic conductivity of Tamusu mudstone varies from 1 × 10-13m/s to 1 × 10-12m/s with respect to changes in temperature,and the intrinsic permeability is verylow,in the rangeof 1×10-20-4×10-20m2.According to the SEM analysis in Section 2,Tamusu mudstone particles are closely arranged,and there are pores among the clay mineral particles or between clay and non-clay mineral particles.Some pores are filled with crystallized mineral particles,which may slow or even stop the flow of pore water,and thus lead to the low permeability of Tamusu mudstone.

The variations in hydraulic conductivity and intrinsic permeability of Tamusu mudstone have the same trend as the temperature varies.When the temperature increases from 20°C to 80°C,the hydraulic conductivity of Tamusu mudstone increases from 1.05 ×10-13m/s to 1.01 ×10-12m/s (a 10-fold increase),and the intrinsic permeability increases from 1.07×10-20m2to 3.78×10-20m2(a 3.5-fold increase).These results indicate that the increase of the hydraulic conductivity during heating is mainly due to the decreases of the pore water density and dynamic viscosity coefficient.On a macroscopic level,the Tamusu mudstone sample may exhibit volumetric expansion.Whereas on a microscopic level,the sample may exhibit pore expansion and microfracture system propagation or coalescence.

The changes in hydraulic conductivity and intrinsic permeability of Tamusu mudstone during cooling are smaller than those during heating.Thus,at the same temperature,the hydraulic conductivity and intrinsic permeability during cooling are larger than the values during heating.When the temperature returns to 20°C,the hydraulic conductivity and intrinsic permeability increase by a factor of 2.3,which shows that the heating stage may cause Tamusu mudstone to exhibit thermal expansion.This thermal expansion results in irreversible changes in the microstructure of Tamusu mudstone,resulting in the permeability that cannot be restored to its original state in the cooling stage.

5.Mechanical properties of Tamusu mudstone

Fig.5.Photograph of Tamusu mudstone samples: (a) TMS01 and (b) TMS02.

Fig.6.Thermo-mechanical loading path.

5.1.Test preparation

Cylindrical samples with a diameter of 50 mm and a length of 100 mm were used for the conventional triaxial compression test(the confining pressure was 5 MPa,10 MPa and 15 MPa; and the water pressure was 5 MPa).A photograph of the test sample is shown in Fig.8.The environmental temperature during testing was controlled at 22°C.During the test,the sample was subjected to a constant strain rate of 10-6s-1and could be regarded as quasistatic (Bieniawski and Bernede,1979).The loading axis was perpendicular to the bedding plane.

5.2.Test results

The results of the conventional triaxial compression test are shown in Fig.9.It can be observed that the whole process can be divided into three stages: compaction stage,elastic stage,and plastic strengthening stage.In the compaction stage,the stress slowly increases,and the strain rapidly increases.Meanwhile,the volumetric strain increases in this stage,and the sample is compacted.Then,the sample enters the elastic stage,where the stressstrain relationship is linear,the volumetric strain continues to increase,and the sample continues to be compressed.When the strain increases and the sample comes into the plastic strengthening stage,wherein the stress nonlinearly increases with respect to the strain.The volumetric strain first increases and subsequently decreases,and the sample begins to expand.Finally,the stress reaches the peak value,after which the sample is destroyed.

SEM images of the intact Tamusu mudstone (Fig.2) show that there are microcracks and pores in the intergranular regions of the sample.Thus,in the early stage of loading,the fractures and pores in the sample are closed,which are the characteristic of the compaction stage.Then,due to the friction between closed fractures,no relative sliding occurs between the microfractures,which is the characteristic of the elastic stage.In the plastic strengthening stage,plastic deformation and volumetric expansion occur in response to the initiation and development of the microfracture system and slip between fracture surfaces.Finally,the microcracks expand and connect to form macroscopic cracks,and then the sample fails.

The confining pressure also affects the mechanical properties of Tamusu mudstone.A higher confining pressure corresponds to a greater ultimate strain.The ductility increases from 6.3 × 10-3at 5 MPa to 7.7 × 10-3at 15 MPa.With the increase in confining pressure from 5 MPa to 15 MPa,the peak strength of Tamusu mudstone tends to increase from 24.1 MPa to 68.4 MPa.Moreover,at the confining pressure of 15 MPa,strain softening occurs after the peak strength is reached,which is different from the brittle failure of other samples.

Fig.7.Permeability test results.

Fig.8.Photograph of sample for the conventional triaxial compression test.

Fig.9.Stress-strain response from the conventional triaxial compression test.

5.3.Transversely isotropic elastoplastic constitutive model

The Tamusu region is composed of a continuous layer of thick lacustrine mudstone and samples from this region contain typical laminar structures (Xiang et al.,2020).According to the layered characteristics of Tamusu mudstone,a transversely isotropic elastoplastic constitutive model is proposed for describing its stressstrain behavior using the approach and framework proposed by Yu et al.(2014) and Ma et al.(2018).

The effective stress tensor is σ=σ′-δu,where σ′is the total stress tensor,δ is the Kronecker delta,and u is the pore water pressure.The effective stress tensor σ is linked with the strain tensor ε through the general Hooke law (Hong et al.,2014):

where [C]epis the elastoplastic tangential stiffness matrix,and it can be expressed as follows:

where Fsis the plastic yield surface; Gsis the plastic potential function; and [C] is the elastic stiffness matrix,which is given as follows (Yu et al.,2014):

where Evand Ehare the elastic moduli in the directions perpendicular and parallel to the bedding plane,respectively;νhhand νvhand are the Poisson’s ratios; and Gvhis the shear modulus.

The plastic hardening modulus A in Eq.(3) is calculated as follows:

where εpis the plastic strain tensor; and hpis the plastic damage parameter,which is introduced in a hyperbolic form (Ma et al.,2018):

where hp0and bpare the model parameters; and ξpis the equivalent plastic deviatoric strain,which is given as

where epis the plastic deviatoric strain.

The relationship between the cohesion cpand the plastic damage parameter hpis assumed as follows:

where c0is the initial cohesion.

The relationship between the friction angle φ and the plastic damage parameter hpis assumed as follows:

where φ0is the initial friction angle.

The plastic yield surface Fsis consistent with the Drucker-Prager model:

where α and k are the slope and intercept on the q-p plane of the shear failure surface,respectively,which are represented as

where s is the deviatoric stress tensor.

The expressions of α and k are related to the cohesion and friction angle:

The plastic potential surface function Gsis taken as follows:

The presence of confining pressure will affect the elastic modulus in the directions parallel and perpendicular to the bedding plane,i.e.Ehand Evare related to the confining pressure and calculated from the test results,as shown in Table 4.The remaining parameters are listed in Table 5.

The simulation results match well with the experimental results by calling the UMAT subroutine in ABAQUS (Fig.10).Given the limited experimental data,the transversely isotropic elastoplastic constitutive model of Tamusu mudstone requires more in-depth study.

6.Discussion

To further discuss the possibility of using Tamusu mudstone as a host rock for the geological disposal of HLW,the basic physicomechanical properties of Tamusu mudstone are compared with those of Opalinus clay in Switzerland,COx argillite in France and Boom clay in Belgium,as shown in Table 6.

Table 4 Values of Eh and Ev at different confining pressures.

Table 5 Parameter values of the numerical model.

Table 6 Basic physico-mechanical parameters of candidate host clayey rocks for HLW repositories.

Compared to the other three types of clayey rocks,Tamusu mudstone has higher density,elastic modulus in the direction perpendicular to the bedding plane and compressive strength,which are 2.55 g/cm3,4 GPa,and 24.1 MPa,respectively.The higher strength of Tamusu mudstone may ensure its stability during excavation.Thus,if using this material as the surrounding rock for a HLW repository,the cost of the project may be reduced,and the construction efficiency may be improved.

The water content of Boom clay is relatively high.Bastiaens et al.(2006) and Yu et al.(2015) showed that Boom clay has strong HM characteristics,and similar properties have been found in the tests of Opalinus clay and COx argillite under HM coupling (Armand et al.,2017b).However,Tamusu mudstone has the lowest water content (approximately 2.2%),which indicates that the HM characteristics of Tamusu mudstone may not be obvious.Moreover,although the intact Tamusu mudstone sample displays lowest permeability of 1 ×10-20m2compared with other candidate host clayey rocks,the SEM image shows that it contains many pores and microfractures,which indicates that the connectivity of the pores ispoor.These characteristics may weaken the seepage effect during the radioactive waste geological disposal,which would be beneficial to the long-term stability of the repository.

Fig.10.Conventional triaxial test (exp.) results and simulations (num.).

In addition,the permeability of clayey rocks varies with the temperature.In the range of 20°C-80°C,the intrinsic permeability of Tamusu mudstone increases with increasing temperature,whereas those of Opalinus clay and COx argillite decrease with increasing temperature (Monfared et al.,2014; Zhang et al.,2017).The intrinsic permeability of Boom clay first slightly decreases with increasing temperature and subsequently remains constant (Ma et al.,2016).During cooling,the intrinsic permeability of Tamusu mudstone,Opalinus clay,and COx argillite decreases with decreasing temperature(Monfared et al.,2014;Zhang et al.,2017),whereas that of Boom clay first remains constant and subsequently slightly increases with decreasing temperature (Ma et al.,2016).Different trends of the intrinsic permeability of clayey rocks may be related to the sensitivity of structure to the temperature.Although the permeability of Tamusu mudstone increases in the heating stage,its highest value is about 4×10-20m2(at 80°C),which is still at a very low level.In general,the permeability of Tamusu mudstone is beneficial to prevent the spread of nuclides during the long-term operation of the HLW repository.

7.Conclusions

In this paper,the physico-mechanical properties of Tamusu mudstone are obtained by a series of experimental studies,including XRD,SEM,disintegration tests,permeability tests and conventional triaxial compression tests.The results show that Tamusu mudstone could be an ideal candidate host rock for HLW repositories.The main conclusions can be drawn as follows:

(1) Tamusu mudstone displays a high granular mineral content and closely arranged microstructure,which makes it maintain its physical properties when exposed to water and make it difficult to disintegrate.The clay minerals and analcite in Tamusu mudstone have strong ion exchange characteristics and adsorption capacity,which can effectively prevent radionuclide migration.Pyrite in Tamusu mudstone can reduce the high-valence radionuclides,reduce their migration and diffusion capacity,and consume the H2O2produced by radiation.

(2) Tamusu mudstone displays a high density of 2.55 g/cm3,an elastic modulus of 4 GPa in the direction perpendicular to the bedding plane,and a compressive strength of about 24 MPa,which is a kind of hard clayey rock.The SEM analysis shows that the microstructure of Tamusu mudstone is closely arranged,contains some microcracks and pores,and has obvious anisotropy.The mineral composition of Tamusu mudstone is primarily quartz,plagioclase,and dolomite,which are responsible for the high compressive strength,brittle failure,and other mechanical characteristics of the material.

(3) The permeability test shows that the hydraulic conductivity and intrinsic permeability of Tamusu mudstone are relatively low.Even when the temperature increases to 80°C,the hydraulic conductivity of Tamusu mudstone is only 1×10-12m/s.Moreover,its microstructural characteristics can well explain its low permeability.The low permeability can also effectively prevent radionuclide migration and diffusion and ensure the safety of the ecosystem outside the surrounding rock.The trends of the hydraulic conductivity and intrinsic permeability of Tamusu mudstone are in accordance with the temperature changes.Under the same temperature,the hydraulic conductivity and intrinsic permeability during cooling are larger than those during heating.This phenomenon indicates that the microstructure of Tamusu mudstone will irreversibly change when it is heated.

(4) Compared with Opalinus clay in Switzerland,COx argillite in France and Boom clay in Belgium,Tamusu mudstone shows itself advantages in terms of the geological disposal of HLW,which will be beneficial to the long-term safety and stability of the repository.Furthermore,analcime in Tamusu mudstone will enhance the radionuclide adsorption properties more than other clayey rocks.Future studies should focus on the mechanical properties,seepage flow,selfsealing characteristics and long-term rheological properties of Tamusu mudstone under THM coupling.

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

The authors gratefully thank the general programs of the National Natural Science Foundation of China (Grant Nos.51979266 and 51879258)and the Youth Innovation Promotion Association of the Chinese Academy of Sciences for their support of this study.The authors are also grateful to the editors and reviewers for their valuable comments,which have substantially improved this paper.