Nuclear High-Level Radioactive Waste Research Articles

Beishan exploration tunnel surrounding rock discontinuity identification based on structure from motion photogrammetry technology

AbstractThe Beishan Exploration Tunnel (BET) is a facility built to develop technologies associated with the safety of China's first high‐level radioactive nuclear waste(HLW) disposal. The surrounding rock discontinuity identification is a key research topic in BET, which could provide essential geological data for future HLW disposal stability and integrity research. This article presents the rock discontinuity identification research progress in BET based on Structure from Motion (SfM) photogrammetry technology. The discontinuity identification algorithm is improved by introducing the region‐growing algorithm to optimize the candidate subplane. This algorithm automatically picks the seed, avoids human intervention, and thus increases the work efficiency of the discontinuity identification. The FCM method is improved by embedding with the CFSFDP algorithm in the discontinuity sets grouping. The CFSFDP algorithm coincides well with the Fisher distribution of discontinuity orientations, which is suitable for the Beishan situation. A parallel scheme is used when implementing the method, which accelerates the discontinuity calculation. This improved rock discontinuity identification method was tested on a slope above the BET and applied in the BET. The discontinuity identification results were compared with the results from the manual field measurement and the open‐source software DSE. The results show that the improved discontinuity identification method obtains reliable discontinuity data and costs less time and human workload than the other two methods. The surrounding rock discontinuity identification research provides a powerful tool for the Beishan HLW disposal geological investigation.

Laboratory Study of Effective Stress Coefficient for Saturated Claystone

Claystone is potentially the main rock formation for the deep geological disposal of high-level radioactive nuclear waste. A major factor that affects the deformation of the host medium is effective stress. Therefore, studying the effective stress principle of claystone is essential for a stability analysis of waste disposal facilities. Consolidated drained (CD) tests were carried out on claystone samples to study their effective stress principle in this paper. Firstly, two samples were saturated under a specified confining pressure and pore pressure for about one month. Secondly, the confining pressure and pore pressure were increased to a specified value simultaneously and then reverted to the previous stress state (the deformations of the samples were recorded during the whole process). Different incremental combinations of the confining pressure and pore pressure were tried at this step. Finally, the effective stress coefficients of the samples were obtained through a back analysis. Furthermore, some potential influencing factors (the neutral stress and loading rate) of the effective stress coefficient were also studied through additional tests. Some interesting results are worth mentioning: (1) the effective stress coefficient of claystone is close to one; (2) the neutral stress and loading rate may have little effect on the effective stress coefficient of claystone.

Disposal of high-level radioactive waste in crystalline rock: On coupled processes and site development

Safe disposal of high-level radioactive nuclear waste (HLW) is crucial for human health and the environment, as well as for sustainable development. Deep geological disposal in sparsely fractured crystalline rock is considered one of the most favorable methods for final disposal of HLW. Extensive research has been conducted worldwide and many countries have initiated their own national development programs for deep geological disposal. Significant advancements of national programs for deep geological disposal of HLW in crystalline rock have been achieved in Sweden and Finland, which are currently under site development stage, focusing on detailed site characterization, repository construction, and post-closure safety analysis. Continued research and development remain important in the site development stage to ensure long-term safety of the HLW disposal repository. This work presents an overview and discussion of the progress as well as remaining open scientific issues and possibilities related to site development for safe disposal of HLW in crystalline rock. We emphasize that developing a comprehensive and convergent understanding of the coupled thermal, hydraulic, mechanical, chemical and biological (THMCB) processes in fractured crystalline rock remains the most important yet challenging topic for future studies towards safe disposal of HLW in crystalline rock. Advancements in laboratory facilities/techniques and computational models, as well as available comprehensive field data from site developments, provide new opportunities to enhance our understanding of the coupled processes and thereby repository design for safe geological disposal of HLW in crystalline rock.

Effect of thermal ageing on physical properties of MX80 bentonite under high-temperature conditions

In a deep geological repository (DGR) system, the buffer layer is indispensable to ensuring the safe disposal of high-level radioactive nuclear waste (HLW). Because the heat generated by the decay of the spent nuclear fuel in a canister is released to the surrounding buffer layers, the bentonite buffer material experiences long-term high-temperature conditions. Therefore, the variations in physical properties of bentonite buffer material under high-temperature conditions are one of the important parameters in the DGRs for HLW. A series of tests on the specific gravity, specific surface area (SSA), Atterberg's limits, and free swelling ratio of MX80 bentonite after heating for different times at a high temperature of 200 °C were conducted, to investigate the influence of thermal ageing time on its physical properties. Then, the microscopic investigations, including X-ray diffraction (XRD), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM), were conducted to explain the aforementioned variations from the microscopic point of view. Based on the crystal layer structure characteristics of montmorillonite, the quantitative relationships between the physical properties of bentonite and its mineral composition or bound water content were established to further explain the mechanism by which thermal ageing affects the physical properties of MX80 bentonite. The results indicate that with increasing heating time, the specific gravity, SSA, liquid limit, plastic limit, plastic index and free swelling ratio decrease sharply by 2.5%, 4.5%, 3.1%, 5.5%, 2.8% and 30.2%, respectively, within 15–30 days of heating. After 30 days of heating, their variations are negligible. Under high-temperature conditions, the transformation of mineral composition, desorption of bound water, and changes in the micro-morphology are the fundamental reasons for the variations in the physical properties of bentonite, and they influence and interact with each other.

Evaluation of hydro-mechano-chemical behaviour of bentonite-sand mixtures

Bentonite-sand mixtures are widely used in engineering barrier of deep geological disposal of high-level radioactive nuclear waste and anti-seepage barrier of civil geotechnical engineering. Under the action of groundwater solution infiltration and external stress, the hydro-mechanical (HM) behaviour of bentonite-sand mixtures, i.e. the swelling characteristics and permeability, will change. Once the anti-seepage and filtration effect is weakened or lost, the pollutants will spread to the biosphere. Therefore, it is necessary to study the swelling characteristics and permeability of bentonite-sand mixtures under coupled mechano-chemical (MC) effect and to establish corresponding prediction model. For this reason, swelling tests under salt solution with different concentrations are conducted on pure bentonite and its mixtures with 30%, 70% and 90% sand contents, the compression tests are carried out on saturated samples, and the saturated permeability coefficient k of the sample under each load is calculated by Terzaghi's one-dimensional consolidation theory. The concepts of true effective stress p e , montmorillonite void ratio e m and critical sand content α s are introduced to determine the e m - p e relationship and finally the k - e m relationship of bentonite-sand mixtures. It is found that when the sand content α ≤ α s , the e m - p e relationship of the mixture is linear and independent of the salt solution concentration, and when α > α s , the e m - p e relationship of bentonite-sand mixture is bi-linear with the true effective deviatoric stress p esα as the intersection. In addition, the e m - k relationship also shows the linear trend when α ≤ α s , and the slope of the line increases with the increase of the salt solution concentration. When α > α s , the k - e m relationship will deviate from the linear relationship. Moreover, the larger the sand content is, the farther the deviation is. On the basis of summing the regularity, a model for predicting the HM behaviour of bentonite-sand mixture under the coupled MC effect is proposed. By comparing the swelling and permeability test results with model prediction results of different types of bentonite and its sand mixtures, the predictive model is verified. The study on the HM behaviour of bentonite-sand mixtures under salt solution infiltration and the model establishment can provide experimental and theoretical basis for the design and construction of anti-seepage engineering by bentonite-sand mixtures.

Transformation of inherent microorganisms in Wyoming-type bentonite and their effects on structural iron

Bentonite is one of the materials used to construct engineered barriers in high-level radioactive nuclear waste geological disposal with its many advantageous features such as low hydraulic conductivity, self-sealing ability, durability, adsorption and immobilization of metals and radionuclides and reduction of microbial activity. Many of these properties are linked with the bentonite swelling capability. Transformations of indigenous microorganism communities from Wyoming-type bentonite and the Finnish repository site groundwater and their effects on the bentonite structural iron over five years were studied in repository relevant anoxic and oxic slurry conditions. Active sulfate reduction (0.06 nmol mL−1 day−1) was detected in the anoxic microcosm waters after a year, however after two years sulfate reduction was not active anymore. Microbial numbers determined by quantitative PCR in the bentonite slurry of both experiment types supported the finding of decrease of overall microbial activity after a year of incubation that was not maintained anymore by the dissolving organic carbon from the bentonite. Regular electron donor additions (final concentration of 2 mM for formate and acetate each, three times per year) activated the microbiome resulting in increasing numbers of bacterial 16S rRNA gene copies and sulfate reducers (dsrB gene copies) as well as detection of sulfide in the water phase of both experiment types. After 4.9 years the structural iron in the fine portion of the montmorillonite had become completely reduced in all microbial microcosms and minor smectite illitization was detected especially in anoxic microcosms. Dominating bacterial groups at the end of the experiment were mainly known sulfur/sulfate and iron reducers. Archaea and fungi constituted a minor part of the microbiomes. In originally oxic microcosms, the bacterial 16S RNA and dsrB gene copy numbers were lower than in the anoxic experiment but started to significantly increase after the electron donor additions. Microorganisms originating from the repository environment could reduce the bentonite structural iron in a few years to an extent likely to affect the bentonite swelling ability if sufficient amounts of suitable electron donors are available in localized areas where bentonite is not at high density and pressure in the geological disposal.

Analyzing of the hydration crack evolution in compacted GMZ bentonite with consideration of technological void ratio

Technological voids widely exist around the bentonite blocks in the deep geological repository. After groundwater seeps into the voids, bentonite nearby expands to fill up the voids and completes the self-sealing gradually. The self-sealing ability is of highly importance, which prevents the technological voids from playing a preferential channel for groundwater migration and thus threatening the safety of the canister accommodating the high-level radioactive nuclear waste (HLW). So far, much of the research focus has been on the hydro-mechanical properties of bentonite after self-sealing, while the process itself is neglected. But actually, a cracking phenomenon will pop up in the interim of sealing process. In this paper, the dynamic evolution of self-sealing on compacted bentonite with different technological voids and sample thicknesses was observed by using a transparent observation apparatus. Digital image processing method combined with fractal theory was utilized to process, analyze and quantify the images captured. The results show that the self-sealing process contains the lifetime of the hydration crack, including its appearance, development and extinction. The technological void ratio evidently affects the time require for closing stage compared with the cracking stage. But in the range of 10–25%, there is no clear correlation between the technological void ratio and the geometry parameters or complexity of crack network. The fractal dimension introduced to quantify the complexity corresponds well with the visible image result, showing the feasibility and credibility of box-counting method. The cracking and closing mechanisms are also investigated.

An analytical solution of temperature near nuclear waste canister under three-dimensional heat transfer condition

The temperature distribution is an important issue for the design and safety assessment of the repository for disposing high-level radioactive nuclear waste (HLW). In order to analyze the heat conduction property near the HLW canister under three-dimensional heat transfer condition, a solution to the temperature distribution is obtained by combining the following two parts: (1) The ratio of temperature between the finite length heat source (FLHS) and infinite length heat source (ILHS), which are obtained by integrating the solution of the point source heat transfer model. (2) An analytical solution to the temperature distribution near HLW canister with two-dimensional heat transfer for double-layered media in the lateral direction. Then, the solution of temperature near the HLW canister under three-dimensional heat transfer condition is verified by checking against the existing ones for a special case in the literature. Finally, a parametric study is performed to investigate the temperature performance in the buffer layer and surrounding rock with different parameters.

Thermo-mechanical characterization of shale using nanoindentation

Shale can be a potential buffer for high-level radioactive nuclear wastes. To be an effective buffer while subject to waste heat, shale's mechanical response at elevated temperature must be known. Many researchers have experimentally characterized the mechanical behavior of various shales at different length scales in adiabatic conditions. However, its mechanical performance at elevated temperatures at the nano-scale remains unknown. To investigate the temperature dependency of nanomechanical properties of shale, we conducted both experimental and numerical studies. In this study, we measured mechanical and fracture properties of shale, such as hardness, elastic modulus, anisotropy, and fracture toughness from 25 °C up to 300 °C at different bedding planes. Statistical analysis of the results suggests that hardness and fracture toughness significantly increased at temperatures from 100 to 300 °C; while, temperature does not have a significant impact on elastic modulus. Data also shows that the bedding plane orientations have a substantial impact on both mechanical and fracture properties of shale at the nano-scale leading to distinct anisotropic behavior at elevated temperature below 100 °C. Additionally, we numerically investigated the mechanical performance of the shale samples at room temperature to gain an insight into its mechanical response through the thickness. Numerical results were validated against the experimental results, confirming the simulation can be used to predict shale deformation at the nano-scale or potentially be used in multi-scale simulations.

An analytical solution of temperature in a nuclear waste repository

The temperature distribution is an important issue for the design and safety assessment of the repository for disposing high-level radioactive nuclear waste (HLRW). In order to analyze the heat conduction property near the HLRW canister accurately, the solution of temperature distribution is divided into two parts. Two mathematical models were established respectively, and the corresponding analytical solutions were obtained in terms of the heat conduction equations in the buffer layer and surrounding rock. Then, by the superposition method, the solution of total temperature is obtained and verified by checking against the existing ones for a special case in the literature. Finally, a parametric study is performed to investigate the temperature performance in the buffer layer and surrounding rock with different parameters.

Swelling pressure of compacted MX80 bentonite/sand mixture prepared by different methods

Compacted bentonite/sand mixture have been considered as possible sealing/backfilling material in the deep geological disposal of high-level radioactive nuclear waste. The swelling pressure of the compacted bentonite/sand mixture is of significance in the design of the geological repository, which requires good consistency between data obtained by laboratory and field measurement. In this work, a series of swelling pressure tests were performed on compacted MX80 bentonite/sand specimens prepared by methods commonly adopted in laboratory (As-compacted, Transferred) and those mimicking the real block manufacturing process (Trimmed, Inserted). Results shown that with identical dry density (especially when the dry density was larger than 1.70 Mg/m3), largest swelling pressure was obtained in specimens prepared by method Inserted, followed by method As-compacted, Transferred and Trimmed. The distinct difference between the swelling pressure could be largely attributed to the effects of residual post-compaction lateral stress. More interestingly, specimens prepared by methods Trimmed and Transferred followed a similar swelling pressure-dry density relationship. From this point of view, method Transferred without causing mass loss, change of bentonite content and possible technological voids effect was recommended in lieu of method Trimmed for specimen preparation in laboratory.

Cracking and sealing behavior of the compacted bentonite upon technological voids filling

The technological voids, which are inevitably formed during the construction of the deep geological repository for high-level radioactive nuclear waste, have negative influence on the hydro-mechanical properties of the engineered barrier. In this study, hydration tests along with image acquisition were carried out on compacted GMZ bentonite samples with simulated technological voids to investigate the self-sealing behavior and research emphasis was placed on the cracks appearing during the hydration. Results showed that with the presence of technological voids, the self-sealing process of the hydrating sample can be visually divided into cracking and closing stages, both of which were affected by the sample preparation process, the void width, and the sample height. The hydration cracks occurred at the early phase was considered as tensile cracks induced by the failure of tensile strength against the tensile stress induced by the difference of swelling strain. At the end of hydration, though the cracks were closed with the sealing of technologicalvoids, the distributions of water content and dry density along the sample diameter were found in a decreasing and an increasing trend with the increasing distance from the technological voids, respectively. By applying the image processing method, the cracking and sealing behaviors were further studied with the reciprocal verification of digital images and grey scale profiles. A good agreement between the grey scale profiles and digital images was found and the quantifiable grey values could accurately reflect the color difference on the samples which was difficult to be distinguished by the naked eye, showing the applicability and superiority of image processing technology.

Active source seismic imaging on near-surface granite body: case study of siting a geological disposal repository for high-level radioactive nuclear waste

In order to research whether it is suitable to set a geological disposal repository for high-level radioactive nuclear waste into one target granite body, two active source seismic profiles were arranged near a small town named Tamusu, Western China. The study area is with complex surface conditions, thus the seismic exploration encountered a variettraveltimey of technical difficulties such as crossing obstacles, de-noising harmful scattered waves, and building complex near-surface velocity models. In order to address those problems, techniques including cross-obstacle seismic geometry design, angle-domain harmful scattered noise removal, and an acoustic wave equation-based inversion method jointly utilizing both the and waveform of first arrival waves were adopted. The final seismic images clearly exhibit the target rock’s unconformable contact boundary and its top interface beneath the sedimentary and weathered layers. On this basis, it could be confirmed that the target rock is not thin or has been transported by geological process from somewhere else, but a native and massive rock. There are a few small size fractures whose space distribution could be revealed by seismic images within the rock. The fractures should be kept away. Based on current research, it could be considered that active source seismic exploration is demanded during the sitting process of the geological disposal repository for nuclear waste. The seismic acquisition and processing techniques proposed in the present paper would offer a good reference value for similar researches in the future.

A double-layered model for near-field temperature in a nuclear waste repository

The temperature distribution is an important issue for the design and safety assessment of the repository for disposing high-level radioactive nuclear waste (HLRW). In order to analyze the heat conduction property near the canister containing HLRW, in this paper two kinds of double-layered model are established respectively for the unit cell scale and repository scale, and the corresponding analytical solutions are obtained in terms of the heat conduction equations for the buffer layer and surrounding rock by the separate variable method. Then, the proposed analytical solutions are verified by checking against the existing ones for a special case in the literature. Finally, a parametric study is performed to investigate the temperature behavior in the buffer layer and surrounding rock with different parameters.

Experimental Investigation on Drying and Wetting of GMZ Bentonite Using X-Ray Tomography and Volumetric Digital Image Correlation Technique

To investigate the shrinking and swelling properties of Gao-Miao-Zi (GMZ) bentonite, which has been considered as engineering barriers for high-level radioactive nuclear waste disposal in China, drying and wetting tests were carried out. The microstructure of the material at each hydric state was recorded using X-ray tomography (X-CT). The 3D images of the material were analyzed using digital volume correlation (DVC) technique, and the full-field strains at mesoscale (i.e., in the order of clay aggregate) during drying and wetting were quantified. The results show that the GMZ bentonite exhibits notable swelling characteristics during wetting and the swelling strain of the material is up to 8% at mesoscale. The full-field strain in 3D of the material is heterogeneous, anisotropic, and irreversible during drying and wetting. The corresponding deformation mechanisms are discussed. Two different swelling mechanisms were also identified: traction occurs in the interface between the matrix and inclusions during swelling; pre-existing fissure closes during swelling.

Evolutionary characteristics of mode-I fracture toughness and fracture energy in granite from different burial depths under high-temperature effect

For urgent solutions to existing major engineering problems such as deep mining and deep burial of high level radioactive nuclear wastes, exploring the mechanical behaviors of rocks under the synergistic effect of the burial depth of rocks and temperature is of great significance. The mode-I fracture characteristics of granite specimens in the semi-circular bending (SCB) test under the effects of different temperatures and burial depths were experimental investigated. The test results showed that the mineral compositions and fracture characteristics of granite both varied significantly at different burial depths in Daliuhang Gold Mine in Yantai, Shandong Province, China. The high-temperature effect would remarkably weaken the influence of the burial depth on the fracture characteristics of granite; moreover, increasingly growing temperature gradually became a dominant factor influencing the fracture characteristics of granite. As the temperature rose from 20 °C to 900 °C, the peak strength, fracture toughness and fracture energy of granite at different burial depths all declined with the temperature rise. Additionally, a significant quadratic polynomial function relationship was found in temperature with peak strength and fracture toughness. Additionally, it was macroscopically found that the fracture energy linearly increased with the fracture toughness, implying that the specimens with larger fracture toughness need to consume more energy during the failure. However, the specific proportions and rates of energy consumption differed in different fracture stages of granite.

Three‐dimensional thermal analysis of the repository for high‐level radioactive nuclear waste

One of the key issues of the dimensioning design and safety assessment of the repository for deposing the high-level radioactive nuclear waste is the temperature distribution. A three-dimensional layered model was established for analyzing the heat conduction near single waste canister. Applying the Laplace transform and finite Fourier Sine transform upon the governing equations of heat conduction, the solutions in the Laplace domain were obtained. The solutions in the Laplace domain were numerically inverted into semi-analytical solutions in the time domain by the Crump method, and the temperature distribution was obtained near the canister at the center of one disposal panel. The initially estimated value of the canister spacing was determined according to the temperature distribution and design criterion. Finally, the obtained temperature distribution was used to investigate the influence of relevant parameters on the canister surface temperature. The results show that the present model is reliable by comparing with the existing analytical model. The peak canister surface temperature is significantly affected by the tunnel spacing, canister spacing, thickness of buffer layer, and thermal conductivities of buffer material and surrounding rock.

Chemical forms of rhodium ion in pure water and nitric acid solution studied using ultraviolet-visible spectroscopy and first-principles calculations

We have investigated chemical forms of rhodium (Rh) ion in nitric acid solution by using spectroscopic and first-principles methods, in order to develop the disposal of high-level radioactive nuclear liquid wastes (HLLW). The electronic structure of Rh complexes in both pure water and nitric acid solutions was investigated using ultraviolet-visible (UV-vis) absorption spectroscopy. Comparison with many-electron calculations indicates that Rh ion forms an octahedral bidentate complex coordinated with three nitrate ions, [Rh(NO3)3], in both pure water and nitric acid solutions. This is consistent with thermodynamic prediction. Since the overall feature of UV-vis spectra of the Rh complex remained unchanged in a range of 0-3 M nitric acid solutions, the structure of the Rh complex was found to be stable in the range. In addition, since no significant changes in UV-vis spectra of Rh complex were observed under γ-ray radiation, the Rh complex, [Rh(NO3)3], can be expected to be stable in actual HLLW.

Canister spacing in high-level radioactive nuclear waste repository

One of the core problems of waste canisters layout in high-level radioactive waste repository is the evolution of the temperature field. On basis of the layered thermal analysis model for single waste canister, the expression of temperature increment at any location in surrounding rock in the repository was obtained by the superposition principle. The initially estimated value of the canister spacing (CS) was determined according to the temperature design criterion. Finally, the influence of relevant parameters on the canister surface temperature (CST) was analyzed. The results were drawn as follows: (a) Taking thermal conductivities of 2.4 and 2.8 W/(m × K) for the rock as examples, the appropriate CS is 12.2 and 13.5 m under the tunnel spacing of 40 m, respectively. (b) The greater the CS, the greater the thermal conductivity of bentonite and rock, the smaller the CST would be. (c) The thicker the buffer layer, the less the heat flux inside the canister would spread out.

Investigation on gas migration in saturated bentonite using the residual capillary pressure technique with consideration of temperature

Determination of parameters for description of gas migration in saturated bentonite is of great importance for the design and construction of artificial barriers in the geological repository for the disposal of high-level radioactive nuclear waste. In this paper, temperature-controlled gas injection tests were conducted on initially water-saturated bentonite specimens using the residual capillary pressure (RCP) technique. Effective gas permeabilities in low injection pressures and the gas breakthrough pressures at temperatures 20, 40 and 60°C were obtained. Results show that: (i) for all the temperatures tested, the intrinsic water permeabilities (kin) range between 3.2 × 10−20 and 5.72 × 10−20 m2, furthermore, those values increase with rising temperature. Meanwhile, the intrinsic water permeabilities derived from the steady state tests are lower than those obtained from the non-steady state tests; (ii) for all the temperatures tested, the effective gas permeabilities (keff) corresponding to the viscous gas flow before gas breakthrough range between 4.81×10-24 and 2.74×10-22 m2, with slight fluctuations with time. The maximum effective gas permeabilities measured at the occurrence of gas breakthrough on the initially water-saturated bentonite specimens extend from 2.27×10-18 up to 3.32×10-17 m2 and increase as temperature increases, while the time required for gas breakthrough decreases as temperature increases; (iii) for all the temperatures tested, the gas breakthrough pressures measured on the initially water-saturated bentonite specimens vary from 2.74 to 4.08 MPa, while the residual capillary pressure differences, also denoted as snap-off pressures (Psnap-off), range from 0.2 to 0.38 MPa. Additionally, the gas breakthrough pressures and the residual capillary pressures decrease as temperature increases.