Repository For Radioactive Waste Disposal Research Articles

Constant-Head Step-Injection Tests Using a Conventional Straddle-Sliding-Packer System for Investigating the Shear Capabilities of Minor Faults

AbstractLow-permeability rock is suitable as the host rock of an underground repository for radioactive waste disposal; however, minor faults might develop there. Investigating the shear capability (= shear compliance) of those faults is crucial because they could be elastically sheared by the thermal effect of the waste to damage the waste’s engineered barriers. This study performed constant-head step-injection tests along with a recently developed packer-pressure-based extensometer method for assessing the applicability of this method to investigate the shear capability of minor faults. Herein, two neighboring minor faults (A and B) in siliceous mudstone were evaluated. The results showed that fault A, with centimeter-thick fault breccia, exhibited high shear capability, whereas fault B, with millimeters or less-thick fault breccia, displayed low shear capability despite containing an incohesive fault rock. An elastic shear displacement occurred for fault A during injection and reached 15–66 mm when the test-section pressure increased from 4.1 to 4.3 MPa. Here, the shear capability was 101 mm/MPa or more. Conversely, fault B had cohesion, and no shear displacement was detected even when the test-section pressure increased from 4.0 to 6.0 MPa. In this case, the shear capability was 10−1 mm/MPa or less. The estimated shear capabilities were consistent with the results from previous shear experiments, and therefore, the applied method helps investigate the shear capabilities of minor faults.

Performance and mechanism of siderite/zero-valent nickel composite material for removing Se(IV) from solution

79Se is a fission product in spent fuel with a half-life of up to 6.5 × 104 a, which is not controlled by solubility in aqueous solution and has strong mobility. These characteristics make it highly susceptible to migration in groundwater. Meanwhile, due to the negative charge of selenium ions, they are difficult to be fixed by buffer/backfill materials, engineering materials, geological materials and commonly used adsorption methods in near-field geochemical environments. Therefore, the fixation of 79Se has become a hot and difficult research topic both domestically and internationally. Therefore, this article aimed to simulate the removal of radioactive nuclide Se(IV) from high-level radioactive waste disposal repositories. On the basis of siderite (FeCO3), by adding zero-valent nickel (Ni0) and continuously optimizing sample preparation conditions, an economical, efficient and stable siderite/zero-valent nickel (FeCO3/Ni0) composite material was successfully prepared. Experimental studies on the removal of Se(IV) from solution were conducted under different conditions and the material was characterized using various modern methods from macro and micro perspectives, the performance and mechanism of FeCO3/Ni0 composite material for removing Se(IV) was explored deeply. The research results indicated that FeCO3/Ni0 had high potential as a candidate adsorbent for the removal of Se(IV) due to its easy synthesis, low-cost, high adsorption capacity and strong reduction.

Effects of temperature and confining pressure on the permeability of Beishan granite from high-level radioactive waste disposal repository

Understanding the variation of rock permeability with temperature and confining pressure is the foundation for projects such as high-level radioactive waste disposal, deep geothermal exploitation, and carbon dioxide storage. This paper presents an experimental study of the permeability evolution of thermally damaged Beishan granite under effective stress. The results show that temperature has a significant impact on the permeability of granite, and there is an obvious threshold temperature (400–500 °C), initial permeability and permeability change rate increased gradually with temperature. The permeability of Beishan granite shows non-linear decreasing trend with the increase of effective stress, ultimately tending towards stability. The microstructure of granite after thermal damage can be observed by scanning electron microscopy and used to assist in explaining the microscopic mechanism of permeability changes. A new granite permeability prediction model is proposed, which integrates permeability with temperature and effective stress, providing an important reference for the safety of high-level radioactive waste disposal.

Compaction and clay content control mudrock porosity

Mudrocks, ubiquitous yet poorly understood sedimentary rocks with significant variations in composition and physical properties, form seals for geological carbon dioxide and energy (e.g., hydrogen and methane) storage, repositories for radioactive waste disposal, and reservoirs for natural gas. Understanding the controls on mudrock pore structure is essential for evaluating their porosity. The identification and quantification of controls depend on the nano-to micron scale pore network, which are the subject of this study. Small-angle (SANS) and very small-angle neutron scattering (VSANS) experiments were conducted on 13 diverse mudrock sets, characterised by differences in mineralogy, stratigraphy, maturity, and depositional environment. We performed multivariate statistics to systematically characterise the pore structure in 71 samples cross a 5 μm–2 nm pore size range. Our results indicate a multivariate approach more effectively captures the complex controls on porosity rather than single parameters. Compaction and clay content emerge as key primary and secondary controls on mudrock porosity, respectively, upon which we introduce a new porosity classification. Our complementary experimental-statistical assessment involving SANS-derived multiscale porosity sheds new light on the influence of structural controls on storage or production capacity in mudrocks.

Using multiple isotopes to determine groundwater source, age, and renewal rate in the Beishan preselected area for geological disposal of high-level radioactive waste in China

Understanding hydrogeological conditions is crucial for selecting and assessing the long-term safety performance of a high-level radioactive waste (HLW) disposal repository. Utilizing environmental isotopes as effective markers for analysing groundwater movement, this study investigates groundwater recharge sources, age, and renewal rates using multiple isotopes in China's potential HLW repository site, the Beishan area. The results indicated deep bedrock groundwater primarily derives from ancient precipitation infiltration under cold climatic conditions. A noteworthy distinction is that loose sedimentary groundwater exhibits higher tritium content (>10 TU) compared to bedrock groundwater (<3.2 TU). Groundwater within the recharge area, especially within gullies and piedmont slope deposits, is relatively youthful, with an age of less than 30 years and an annual renewal rate exceeding 5 %. In contrast, the shallow groundwater age in the intermountain basins and depressions of the discharge area generally exceeds 50 years, with an annual renewal rate often falling below 0.5 %. At the Beishan underground research laboratory site, deep groundwater at the disposal repository depth displays a corrected 14C age exceeding 8,000 years, indicating an extremely slow movement and alteration rate. As a result, the hydrogeological conditions in the Beishan area are expected to be relatively beneficial for ensuring the safety of HLW repository.

Analytical solution on temperature evolution in three-layer barrier high-level radioactive waste repository and required pre-cooling time determination

An analytical model of the temperature field in a high-level radioactive waste (HLW) disposal repository containing three-layer barrier of bentonite blocks, bentonite pellets and surrounding rock was established, and a series of mathematical methods were used to obtain a fully analytical solution for the model, which can visually represent the relationships between temperature and various parameters and provide a reference for designing repository. Through comparisons to other existing analytical and numerical simulation results, the validity of our solution was verified. Subsequently, temperature evolution analyses of one-canister and one-panel conditions were performed using the derived solution. Additionally, the temperature evolution at the point where peak temperatures are expected (i.e., mid-height of central canister's side surface in one disposal panel) was investigated with different geometric design dimensions, and it was observed that temperature remains virtually unchanged when the tunnel spacing exceeds 40 m. The required pre-cooling times for four types of HLW under different geometric and thermal parameters were analyzed using the obtained solution, with all canisters considered in one disposal panel.

Insight on the Swelling Pressure-Suction Relationship of Compacted Bentonite during Hydration.

Investigation of the swelling pressure of buffer/backfill materials is a critical aspect in the design of high-level radioactive waste (HLW) disposal repositories. In this study, to clarify the swelling pressure-suction relation for compacted bentonite upon the hydration path, constant-volume swelling pressure tests with suction control were conducted. The swelling pressure-suction curves indicated that the swelling pressure of the specimens increased significantly with increasing dry density, while the shape of the curves during hydration depended on the dry density. Moreover, the swelling pressure-suction curves exhibited a distinction between unsaturated and saturated segments divided by the critical saturated state (CSS) curve, which proves the unique existence of a CSS curve in the stress space independent of the stress path. With the introduction of the CSS curve into the s-p space, the conventional stress space of unsaturated soil could expand to that of unsaturated expansive soil. The results obtained in this study could provide the mechanical parameters for the construction of disposal repositories. In addition, the stress space with CSS curve proposed in this study provides a new approach to building constitutive models of bentonite materials.

Importance of Stress and Temperature-Dependent Permeability of Rocks and its Application in Underground Nuclear Waste Disposal

Water flow is an essential factor in the sealability of any underground cavern, including those for nuclear waste disposal, and is significantly affected by the permeability of the rock. The permeability of rocks is affected by various factors, including stress and temperature. The rock stress changes by excavating a cavern, and rock temperature changes by decay heat from nuclear waste, and the temperature change induces thermal stress. Therefore, water flow around such caverns must be evaluated considering the effects of stress and temperature. Numerical analyses of water migration around underground nuclear waste disposal caverns have been carried out. However, studies considering the stress and temperature-dependent permeability may not be published yet. To demonstrate the necessity to consider the stress and temperature-dependency in permeability, equations that represent the post-failure permeability as a function of average effective stress and temperature were proposed. The water inflow was numerically calculated for a simple underground nuclear waste disposal cavern with or without stress and temperature dependency which showed the significance of the dependency. Also, the importance of rock types was demonstrated by considering the three rocks of granite, sandstone, and tuff for a full-scale underground radioactive disposal site for the stress and temperature-dependent permeability. A high sealability could be expected for granite and sandstone but not for the tuff. Introducing the stress and temperature-dependent permeability could contribute to the thoughtful design of an underground repository for radioactive waste disposal considering rock types.

Validation of the fully-analytical solution with temperature superposition for the nuclear waste repository

Temperature distributions and evolutions in high-level radioactive waste (HLW) disposal repository are usually obtained by numerical simulation or analytical solution. For the use of analytical solution to calculate the temperature distributions of one disposal panel in the repository, the temperatures from every nuclear waste canister were superposed for calculating the whole effects of all the canisters, and the disposal tunnels, which are filled with backfill materials, were ignored to simplify the deducing process. Yet the accuracy of the temperature superposition has not been validated and the influence of tunnels on temperature has not been evaluated. In this paper, a disposal panel with 1189 waste canisters was modelled by the finite element method for analyzing the temperature distributions and evolutions. The size of numerical domain was determined by studying the temperature distributions. When the distance between the outer edges and outermost canisters surface was set as 700 m or above, the influence of boundaries on calculated results can be ignored. A disposal panel with tunnels was modelled to study the influence of tunnels on temperature evolution, and the results show that the tunnels have practically no effect on the temperature evolution. Finally, the comparison of temperatures calculated by numerical simulation and analytical solution was done to validate the analytical solution with the superposition method.

Scaling effect on cesium diffusion in compacted MX-80 bentonite for buffer materials in HLW repository

Abstract In this study, radionuclide behavior in high-level radioactive waste (HLW) disposal repositories is complicated because of the spatial heterogeneity of porous media, coupled flow-transport mechanisms, and multiple chemical reaction processes. Discrepancies in the diffusion behavior of a non-sorbing tracer (HTO) and a reactive tracer (137Cs) in porous media have long been recognized but are not yet fully understood, which hinders effective assessment of the capabilities of buffer materials. This paper was dedicated to exploring and explaining the discrepancies in the transport behavior of non-sorbing and reactive tracers through laboratory experiments and an investigation of contributing mechanisms. Our results showed that for a bentonite sample of the same thickness, 137Cs has smaller apparent and less effective diffusion coefficients than those for HTO. These discrepancies can be attributed to the negative surface electric effects, atomic properties, and chemical reactions. In the case of bentonite samples with different thicknesses (0.5, 0.75, 2.0, 2.5 cm), the apparent and effective diffusion coefficients show an increasing trend with bentonite thickness. According to the experimental data and fitting results, the apparent and effective diffusion coefficients are highly related to bentonite thickness. Thus, scaling effects on transport parameters were proposed to explain the results, which were attributed to the nonuniform distribution of the pore space in the bentonite sample. The scale effect behavior of radionuclide was quantified through a regression analysis. The results can be used to improve buffer designs for radionuclides diffusion.

Synthetic metaschoepite mineral film on silicon: An analog surface for weathered spent nuclear fuel

Incorporation of fission products in spent nuclear fuel is of interest in the frame of safe storage of high-level nuclear waste in a deep geological repository. Uranyl oxyhydroxide films were grown on silicon wafer to represent the corrosion product formed on the surfaces of the fuel. Films were obtained by hydrolysis of uranyl ions (9 mM) in mild conditions (≤60 °C) using either Mg(OH)2, raw brucite, La(OH)3, or NH4OH to control the solution pH to ≈ 5.0. To identify the mineral phases we used the grazing-incidence X-ray technique. Atomic force microscopy data indicated that the surfaces of metaschoepite films prepared using either Mg(OH)2 or brucite at 50 °C for 3 h were self-affine fractal (Df = 2.5). The concentration (wt.%) of reactant ions detected in a metaschoepite film after reaction at 70 °C for three days as determined by X-ray fluorescence was: La (8.90) > Nd (5.6) > Ce (3.2) > Eu (2.5) > Pb (0.7) > Sr (0.14). Metaschoepite and sodium uranate hydrate were identified in a film formed in Cl-SO4-Na type groundwater from the Beishan area (China), the preselected area of a radioactive waste disposal repository.

Coupled thermal-hydro model tests of double-layered buffer for nuclear waste repository

In the high-level radioactive waste disposal repository, bentonite is a preferred buffer material. As the buffer material, the hydraulic conductivity must be less than 10−13 m/s and the thermal conductivity should be higher than 0.8 W/(m·K) recommended by International Atomic Energy Agency (IAEA). Due to the decay heat of radionuclide and low thermal conductivity of bentonite, the released heat cannot be quickly transferred to the host rock through bentonite buffer layer and the repository may be in a high temperature state (>100 °C), which compromises both the hydro-mechanical performance of the bentonite and the anti-corrosion behavior of nuclear waste canister, therefore the barrier function of the buffer layer is severely damaged. Thus, a double-layered buffer concept which is based on the dual requirements of thermal conductivity and permeability was proposed for the nuclear waste disposal repository. In this paper, coupled thermal-hydro model tests of double-layered buffer with graphite-bentonite mixture layer and bentonite layer (the layer thickness ratio of graphite-bentonite to bentonite was 2:3) were carried out to investigate the temperature and relative humidity distribution laws and determine whether the physicochemical properties of double-layered buffer change after experiencing the coupled thermal-hydro process. In the model test, a heater simulating the decay heat generated by HLW was placed below the graphite-bentonite layer, and a hydration system simulating groundwater infiltration was installed above the bentonite layer, thus, the heat was transferred from the graphite-bentonite layer to the bentonite layer while water was permeated from the bentonite layer to the graphite-bentonite layer. The temperature and relative humidity distribution were synchronously measured by temperature-humidity sensors. After the model tests, the heater was switched off, and the tested bentonite/graphite-bentonite mixture samples were taken out for subsequent water content, scanning electron microscopy (SEM) and X-ray diffraction (XRD) tests. The main conclusions from the test results are as follows. The distributions of temperature and relative humidity in the buffers were mainly governed by the competition between the thermal effect and the water permeation effect, the results of water content at different distances also indirectly verify this conclusion. And SEM results showed that macropores of the bentonite close to the water inlet side were significantly reduced and the pore distribution tended to be uniform along the water injection direction during wetting. XRD results indicated that the mineral composition of the graphite-bentonite mixtures close to the heater changed while that of the bentonite away from the heater remained unchanged. Obviously, even if the high thermal conductivity graphite was added to the buffer layer near the heat source, the mineral composition of the bentonite at the heat source side cannot be avoided completely unaffected. Therefore, it is necessary to conduct heat from the inner layer to outer layer to reduce the overall temperature in the repository and reduce adverse effects of temperature and water synchronization on outer bentonite layer. The research results in this paper can serve as a guideline for the buffer layer design in the high-level radioactive waste repository.

Fully analytical solution in time and space domains on temperature in multi-barrier nuclear waste repository

Temperature evolution is one of the crucial factors in safe operation and geometric design of high-level radioactive waste disposal repositories. A three-dimensional two-layer axisymmetric heat transfer model was established to study the temperature evolution of the repository in the case of single canister. Applying a series of mathematical tools such as the finite Fourier sine transform, separation of variables and Duhamel's theorem, a fully analytical solution was obtained, which can visually and quickly represent the temperature evolution in both time and space. The validity of the model and solution was confirmed by comparing it with the existing line heat source solution, semi-analytical solution and finite element simulation results. The canister surface temperature was obtained by superimposing the temperature difference between the internal and external surfaces of buffer layer on the rock wall temperature. A single-panel calculation model was established, and dimensional and parameter analysis were carried out using the obtained fully analytical solution. According to the quantitative analysis using the obtained solution, increasing the canister spacing has a better effect on reducing the peak temperature than increasing the tunnel spacing. The effect of the two ways in reducing the peak temperature weakens with increasing the spacings.

Constant-head step injection tests to quantify the stress dependence of fracture transmissivity in an excavation damaged zone: A case study from the Horonobe Underground Research Laboratory

In an underground repository for high-level radioactive waste disposal , fracture transmissivity in an excavation damaged zone (EDZ) along tunnels or deposition holes can decrease during the post-closure period via processes such as self-sealing by clay-swelling at the EDZ's fracture surface or an increase in effective normal stress acting on the fractures owing to swelling of backfilling or buffer materials. Hydromechanical coupling models for the stress-dependence of fracture transmissivity are helpful to estimate the change in an EDZ's fracture transmissivity after closure. The applicability of the applied models should be confirmed by in situ tests at the given site; this appears to be facilitated by using constant-head step injection tests. However, injection testing is rarely applied to EDZ fractures . To investigate the applicability of injection tests, the present study performed them on single, tensile EDZ fractures in the Horonobe Underground Research Laboratory hosted by poorly swelling mudstone . Furthermore, the Barton–Bandis normal stress-dependent fracture-closure model quantified the stress-dependence of EDZ facture transmissivity. The fracture's hydraulic aperture increased gradually during injection, and its variation was well reproduced by fitting the model. Although the model requires the normal stress, this parameter was reasonably estimated by the fitting analyses. Constant-head step injection tests coupled with the Barton–Bandis model is believed to be a convenient method for preliminarily quantifying or verifying the stress dependence of EDZ fracture transmissivity, at least for poorly self-sealed, tensile EDZ-fractures. • Assessment combines step injection testing and Barton–Bandis fracture-closure model. • It quantifies fracture transmissivity stress dependence in excavation damaged zone. • The method can also potentially estimate total normal stress acting on a fracture.

Plutonium reactive transport in fractured granite: Multi-species experiments and simulations

Plutonium (Pu) in the subsurface environment can transport in different oxidation states as an aqueous solute or as colloidal particles. The transport behavior of Pu is affected by the relative abundances of these species and can be difficult to predict when they simultaneously exist. This study investigates the concurrent transport of Pu intrinsic colloids, Pu(IV)(aq) and Pu(V-VI)(aq) through a combination of controlled experiments and semi-analytical dual-porosity transport modeling. Pu transport experiments were conducted in a fractured granite at high and low flow rates to elucidate sorption processes and their scaling behavior. In the experiments, Pu(IV)(aq) was the least mobile of the Pu species, Pu(V-VI)(aq) had intermediate mobility, and the colloidal Pu, which consisted mainly of precipitated and/or hydrolyzed Pu(IV), was the most mobile. The semi-analytical modeling revealed that the sorption of each Pu species was rate-limited, as the sorption could not be described by assuming local equilibrium in the experiments. The model was able to describe the sorption of the different Pu species that occurring either on fracture surfaces, in the pores of the rock matrix, or simultaneously in both locations. While equally good fits to the data could be achieved using any of these assumptions, a fracture-dominated process was considered to be the most plausible because it provided the most reasonable estimates of sorption rate constants. Importantly, a key result of this work is that the sorption rate constant of all Pu species tends to decrease with increasing time scales, which implies that Pu will tend to be more mobile at longer time scales than observations at shorter time scales suggest. This result has important implications for predicting the environmental impacts of Pu in the safety assessments of geologic repositories for radioactive waste disposal, and we explore potential mechanistic bases for upscaling the sorption rate constants to time and distance scales that cannot be practically evaluated in experiments.

Insight on the Void Ratio-Suction Relationship of Compacted Bentonite during Hydration.

Investigation on swelling characteristics of buffer/backfill materials during hydration is an important issue in the design of artificial barriers in high-level radioactive waste (HLW) disposal repositories. In this work, for clarifying the characteristic of void ratio–suction relationship for compacted bentonite on hydration path, suction-controlled swelling deformation tests under constant vertical stresses 0.001~40 MPa were carried out on compacted bentonite specimens. Four different types of void ratio-suction curves indicated that swelling-collapse behavior under hydration depends on suction and over-consolidation ratio (OCR), based on which the swelling index was defined. Then, equations were proposed for describing the swelling-collapse characteristic of void ratio–suction curves. Simulation results of suction-controlled swelling deformation tests show that the different types of the hydration deformation curves could be well described by the proposed equations. Obviously, the proposed equations could be used for description and prediction of swelling characteristics of compacted bentonite during hydration, which is also of great importance for the safety assessment of the HLW repositories.

Experimental Determination of Polycrystalline Salt Rock Thermal Conductivity, Diffusivity and Specific Heat From 20 to 240°C

Salt rock is recognized as one of the most suitable parent rocks for geological disposal of high level radioactive waste due to its low permeability, good ductility, good thermal conductivity and damage self-healing properties. The thermal conductivity of salt rock directly affects the temperature of disposal storage and surrounding rock, high temperature will lead to a series of problems such as nuclear waste storage tank rupture, mechanical and permeability reduction of surrounding rock. In this paper, the thermal conductivity, specific heat and thermal diffusion coefficient of NaCl single crystal made by crystallization method in the laboratory and polycrystalline salt rock from Khewra salt mine were measured in the range of 22–240°C by the transient plane source method. The results showed that the thermal conductivity and thermal diffusivity of salt rock decrease gradually with the increase of temperature, while the specific heat capacity increases with the increase of temperature. The thermal conductivity of salt rock is slightly lower than that of single-crystal NaCl. The reason for this phenomenon may be that there are a few pores in salt rock. Based on the experimental data, the models of thermal conductivity, thermal diffusivity and specific heat of salt rock with temperature were established. The results can provide a reference for the construction of underground salt rock high-level radioactive waste disposal repository. Based on the thermal conductivity model of polycrystalline salt rock established in this paper, the temperature field evolution during the operation of the underground salt rock high-level nuclear waste repository in 1,000 years was studied. It was found that the temperature of the glass solidified of high-level radioactive waste reached the highest (177.6°C) and then dropped rapidly. The decay heat radiation influence radius of the nuclear waste reaches its maximum in about 50 years of operation of the repository and then gradually decreases.

Co-transport and co-release of Eu(III) with bentonite colloids in saturated porous sand columns: Controlling factors and governing mechanisms

Accurate prediction of the colloid-driven transport of radionuclides in porous media is critical for the long-term safety assessment of radioactive waste disposal repository. However, the co-transport and corelease process of radionuclides with colloids have not been well documented, the intrinsic mechanisms for colloids-driven retention/transport of radionuclides are still pending for further discussion. Thus the controlling factors and governing mechanisms of co-transport and co-release behavior of Eu(III) with bentonite colloids (BC) were discussed and quantified by combining laboratory-scale column experiments, colloid filtration theory and advection dispersion equation model. The results showed that the role of colloids in facilitating or retarding the Eu(III) transport in porous media varied with cations concentration, pH, and humic acid (HA). The transport of Eu(III) was facilitated by the dispersed colloids under the low ionic strength and high pH conditions, while was impeded by the aggregated colloids cluster. The enhancement of Eu(III) transport was not monotonically risen with the increase of colloids concentration, the most optimized colloids concentration in facilitating Eu(III) transport was approximately 150 mg L−1. HA showed significant promotion on both Eu(III) and colloid transport because of not only its strong Eu(III) complexion ability but also the increased dispersion of HA-coated colloid particles. The HA and BC displayed a synergistic effect on Eu(III) transport, the co-transport occurred by forming the ternary BC-HA-Eu(III) hybrid. The transport patterns could be simulated well with a two-site model that used the advection dispersion equation by reflecting the blocking effect. The retarded Eu(III) on the stationary phase was released and remobilized by the introduction of colloids, or by a transient reduction in cation concentration. The findings are essential for predicting the geological fate and the migration risk of radionuclides in the repository environment.

Experimental Study on Ultrasonic Velocity, Suction Curve, and Gas Permeability of Damaged Granite

The excavation damaged zone (EDZ) of a high-level radioactive waste (HLW) disposal repository is a key factor related to long-term safety. The essence of the damage evolution of rocks is the development of microcracks, which change the ultrasonic velocity, suction curve, and gas permeability of rocks. In this work, some granite samples with different damage degrees were generated by uniaxial compression, which is similar to the stress condition of the surrounding rocks of HLW disposal tunnels. The ultrasonic velocity was simultaneously tested during the compression process. The suction curve was then obtained by drying the damaged samples under different levels of relative humidity. The gas permeability was finally measured at different levels of water saturation. Test results indicated that the unstable development of microcracks led to a decrease in Young’s modulus and enlarged the horizontal deformability of granite. The development of microcracks did not change the longitudinal wave velocity, which tended to remain constant when the ratio of the axial stress to the crack damage stress was greater than one. However, the shear wave velocity of damaged granite was significantly decreased. Such effect indicated that the microcracks generated at the stable and unstable development stages mainly extended along the axial direction rather than the radial direction. The suction curve of damaged granite was obtained and was found to comply with van Genuchten’s model and the dual-porosity model. The results of the permeability tests revealed that almost no new gas pathways were generated at the stable development stage of the microcracks. Meanwhile, the unstable development of microcracks led to a great increase in gas permeability. Moreover, the gas permeability of the intact and damaged granite tended to be constant when the water saturation level was less than 0.5. These results will benefit the performance assessment of the EDZs in the surrounding rocks of HLW disposal repositories.

Investigations of the mechanical behavior and failure mechanism of fractured rock samples

As excavation is often very deep with large unloading forces in the construction of a high-level radioactive waste (HLW) disposal repository, microcracks in the surrounding rock are unavoidable. In this work, a series of uniaxial compression tests were conducted on intact samples and samples with fractures at different inclination angles (30°, 45°, and 60°), to investigate the mechanical behavior and failure mechanism of fractured rock samples. According to the experimental results, a more inclined angle of the fractures would dramatically reduce the compressive strength of the sample. After the compression tests, the granite samples with different fracture inclinations have different failure modes and distribution characteristics of cracks on sample outside surfaces and the premade fracture planes. Higher dip angles of the premade fractures have less surface density of cracks on the sample outside surface after compression. The instability failure of granite samples with 60° fracture inclination is entirely due to shear deformation between the premade fractures. Based on this single failure mode, the compressive strength of a granite sample with 60° fracture inclination has a positive correlation with the difference in fractal dimensions induced by compression. The greater the difference in fractal dimensions, the higher the compressive strength.