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BACKGROUND: Underground research laboratories (URLs) provide a window on the deep biosphere and enable investigation of potential microbial impacts on nuclear waste, CO2 and H2 stored in the subsurface. We carried out the first multi-year study of groundwater microbiomes sampled from defined intervals between 140 and 400 m below the surface of the Horonobe and Mizunami URLs, Japan. RESULTS: We reconstructed draft genomes for > 90% of all organisms detected over a four year period. The Horonobe and Mizunami microbiomes are dissimilar, likely because the Mizunami URL is hosted in granitic rock and the Horonobe URL in sedimentary rock. Despite this, hydrogen metabolism, rubisco-based CO2 fixation, reduction of nitrogen compounds and sulfate reduction are well represented functions in microbiomes from both URLs, although methane metabolism is more prevalent at the organic- and CO2-rich Horonobe URL. High fluid flow zones and proximity to subsurface tunnels select for candidate phyla radiation bacteria in the Mizunami URL. We detected near-identical genotypes for approximately one third of all genomically defined organisms at multiple depths within the Horonobe URL. This cannot be explained by inactivity, as in situ growth was detected for some bacteria, albeit at slow rates. Given the current low hydraulic conductivity and groundwater compositional heterogeneity, ongoing inter-site strain dispersal seems unlikely. Alternatively, the Horonobe URL microbiome homogeneity may be explained by higher groundwater mobility during the last glacial period. Genotypically-defined species closely related to those detected in the URLs were identified in three other subsurface environments in the USA. Thus, dispersal rates between widely separated underground sites may be fast enough relative to mutation rates to have precluded substantial divergence in species composition. Species overlaps between subsurface locations on different continents constrain expectations regarding the scale of global subsurface biodiversity. CONCLUSIONS: Our analyses reveal microbiome stability in the sedimentary rocks and surprising microbial community compositional and genotypic overlap over sites separated by hundreds of meters of rock, potentially explained by dispersal via slow groundwater flow or during a prior hydrological regime. Overall, microbiome and geochemical stability over the study period has important implications for underground storage applications.

Abstract Background Underground research laboratories (URLs) provide a window on the deep biosphere and enable investigation of potential microbial impacts on nuclear waste, CO2 and H2 stored in the subsurface. We carried out the first multi-year study of groundwater microbiomes sampled from defined intervals between 140 and 400 m below the surface of the Horonobe and Mizunami URLs, Japan. Results We reconstructed draft genomes for > 90% of all organisms detected over a four year period. The Horonobe and Mizunami microbiomes are dissimilar, likely because the Mizunami URL is hosted in granitic rock and the Horonobe URL in sedimentary rock. Despite this, hydrogen metabolism, rubisco-based CO2 fixation, reduction of nitrogen compounds and sulfate reduction are well represented functions in microbiomes from both URLs, although methane metabolism is more prevalent at the organic- and CO2-rich Horonobe URL. High fluid flow zones and proximity to subsurface tunnels select for candidate phyla radiation bacteria in the Mizunami URL. We detected near-identical genotypes for approximately one third of all genomically defined organisms at multiple depths within the Horonobe URL. This cannot be explained by inactivity, as in situ growth was detected for some bacteria, albeit at slow rates. Given the current low hydraulic conductivity and groundwater compositional heterogeneity, ongoing inter-site strain dispersal seems unlikely. Alternatively, the Horonobe URL microbiome homogeneity may be explained by higher groundwater mobility during the last glacial period. Genotypically-defined species closely related to those detected in the URLs were identified in three other subsurface environments in the USA. Thus, dispersal rates between widely separated underground sites may be fast enough relative to mutation rates to have precluded substantial divergence in species composition. Species overlaps between subsurface locations on different continents constrain expectations regarding the scale of global subsurface biodiversity. Conclusions Our analyses reveal microbiome stability in the sedimentary rocks and surprising microbial community compositional and genotypic overlap over sites separated by hundreds of meters of rock, potentially explained by dispersal via slow groundwater flow or during a prior hydrological regime. Overall, microbiome and geochemical stability over the study period has important implications for underground storage applications.

The study presents the results of the research on geodynamic and geological conditions of the Enisei site (Krasnoyarsk Krai), chosen for the construction of an underground research laboratory. The laboratory is being built at a depth of 500 m to assess the suitability of the rock mass for burying high-level radioactive waste. The rocks consist of weakly fractured gneisses, granites, and dikes of metadolerites. Field observations were conducted on bedrock outcrops. They included the determination of rock mass quality indicators, measurement of rock fracturing, and a rating classification of stability using N.Barton's method. GNSS observations were also made to monitor surface deformations. These data were used to develop a three-dimensional structural model, including lithology, fault disruptions, intrusive bodies, elastic-strength properties of rocks, and the sizes of zones influenced by faulting. It will serve as a basis for boundary conditions and the construction of three-dimensional variational models of stress-strain states, identifying zones of concentration of hazardous stresses, and planning in situ geomechanical experiments in underground mines of the laboratory. The obtained values of the modified QR index for the main types of rocks allowed their classification as stable and moderately stable, corresponding to strong and very strong rocks on Barton's scale and the massif rating according to geomechanical classification.

In deep hard rock excavation, stress plays a pivotal role in inducing stress-controlled failure. While the impact of excavation-induced stress disturbance on rock failure and tunnel stability has undergone comprehensive examination through laboratory tests and numerical simulations, its validation through in-situ stress tests remains unexplored. This study analyzes the three-dimensional stress changes in the surrounding rock at various depths, monitored during the excavation of B2 Lab in China Jinping Underground Laboratory Phase II (CJPL-II). The investigation delves into the three-dimensional stress variation characteristics in deep hard rock, encompassing stress components and principal stress. The results indicate changes in both the magnitude and direction of the principal stress during tunnel excavation. To quantitatively describe the degree of stress disturbance, a series of stress evaluation indexes are established based on the distances between stress tensors, including the stress disturbance index (SDI), the principal stress magnitude disturbance index (SDIm), and the principal stress direction disturbance index (SDId). The SDI indicates the greatest stress disturbance in the surrounding rock is 4.5 m from the tunnel wall in B2 Lab. SDIm shows that the principal stress magnitude disturbance peaks at 2.5 m from the tunnel wall. SDId reveals that the largest change in principal stress direction does not necessarily occur near the tunnel wall but at a specific depth from it. The established relationship between SDI and the depth of the excavation damaged zone (EDZ) can serve as a criterion for determining the depth of the EDZ in deep hard rock engineering. Additionally, it provides a reference for future construction and support considerations.

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The in situ stress state within fault zones is technically challenging to characterize. At the Bedretto Underground Laboratory in the Swiss Alps, the breakouts observed in an array of eight inclined boreholes penetrating a fault zone offer a unique opportunity to characterize the fault‐associated spatial stress variations. Synthesizing multiple geophysical logs, natural geologic structures intersecting these boreholes are identified, revealing a hierarchy of a major fault zone along with secondary structures. Within the boreholes, breakout rotations occur over multiple scales, spanning individual fractures and the entire major fault zone. We first estimate and rule out the effect of the fracture‐induced anisotropy on the breakout rotations, which are attributed mainly to the stress variations. Based on the stress field around a circular borehole and Mohr‐Coulomb failure criterion, the observed breakout azimuths are used to invert the stress information. Results show that the stress field outside the fault zone features a stress ratio (quantifying the relative stress magnitude) of about 0.9, an inclined overburden stress (inclination: 12°∼18°), and a maximum horizontal principal stress (SHmax) oriented N100∼120°E. Within the fault zone, a substantial reduction of the stress ratio and complicated stress rotations are constrained, likely induced by the stress drop on local fractures. As a result, less critical stress state inside the major fault zone is expected. Our work provides a semi‐quantitative estimation of the in‐situ stress variations around fault zones in the absence of direct stress measurements, which is beneficial to a number of scientific and engineering applications. Plain Language Summary: Fault zones exert appreciable influences on fluid flow, earthquake processes and subsurface engineering by modifying the local stress state. Due to the structural complexity of the faults and limited stress measurement techniques, we lack a comprehensive understanding of how stress magnitude and orientation change around faults. This study presents an elaborate geomechanical study at the Bedretto Underground Laboratory for Geosciences and Geoenergies, Switzerland. We propose a workflow to describe the changes of stress magnitude and orientation within a fault zone according to the observations in an array of boreholes penetrating it. Our results provide new insights into the spatial relationship between the stress state and fault structure. Key Points: Spatial stress variations within a fault zone can be indicated by the breakout rotations thereinBreakout rotations occur at different scales, indicating significant stress heterogeneity across scalesIt is inferred that the two horizontal stresses become more isotropic, accompanied by complex stress rotations within the fault zone [ABSTRACT FROM AUTHOR]

Copyright of Uranium Mining & Metallurgy is the property of Uranium Mining & Metallurgy Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The mechanical behavior of host rock for a deep geological repository of high-level radioactive waste plays a key role in ensuring the isolation function of host rock as a natural barrier under the multi-field coupling environment. For a better understanding of granite in China's Beishan pre-selected area for geological disposal of high-level radioactive waste, a series of investigations were carried out on in-situ stress field of rock mass at depth, strength and deformation characteristics of rocks under different stress and temperature conditions, and rock boreability and adaptability to Tunnel Boring Machine (TBM) technology. The results indicate that Beishan granite shows typical characteristics as a hard and brittle rock with a quite low permeability, and it is favorable to geological disposal. Meanwhile, a new rock mass suitability evaluation system was proposed, and the rock mass mainly composed of Beishan granite was proven to be suitable for geological disposal. Besides, the constructability of Beishan granite at engineering scale was tested and verified through field tests in the Beishan Exploration Tunnel (BET). Here, we summarize the main outcomes of rock mechanics research on Beishan granite in the past years and introduced the current progress of Beishan underground research laboratory (URL) for geological disposal. [ABSTRACT FROM AUTHOR]

Copyright of World Nuclear Geoscience is the property of World Nuclear Geoscience Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The paper presents the results from upgrading the satellite geodetic network of the geodynamic test area of the Nizhne-Kansky massif using bedrock pin geodetic centers in 2021–2022. A brief review is given of the historical experience of using bedrock pin centers in developing satellite geodetic networks at geodynamic test sites, in particular, in areas where underground research laboratories are located. Based on this experience, the design and technology of installing rock geodetic centers were developed and implemented. The design of bedrock pin centers makes it possible to minimize the centering error and ensure immobility of GNSS equipment throughout the entire measurement session. The results of expanding the geodetic network in connection with the start of construction of an underground research laboratory in Krasnoyarsk krai are presented. The form of finite elements of the modernized geodetic network is analyzed. It is shown that construction of new GNSS stations had a positive effect on the network geometry. As a result, the network has increased the number of close-to-equilateral triangles (finite elements), which will have a positive effect on the accuracy in calculating the deformations of the Earth's surface when processing geodetic measurement results and when forecasting the long-term stability of the geological medium in the study area. [ABSTRACT FROM AUTHOR]

There has been growing interest in using underground locations for applications in various fields, including research. In Poland, for several years, attempts have been made to build an underground laboratory. For this purpose, selecting an appropriate location requires a detailed analysis of the level of natural radioactivity. The present study presents detailed characteristics of the natural background radiation in close vicinity to shaft L-VI of the Lubin mine, at the depth of 910 m (2275 m w.e.). The in situ measurement of the photon flux in the 7–3150 keV energy range was equal to 8.08 ± 0.90 cm−2s−1, and the gamma-ray dose rate of 0.070 ± 0.010 µSv/h with the highest contribution from 40K and 214B isotopes. The thermal neutron flux measured using helium counters was equal to 4.2 ± 0.9 × 10−6 cm−2s−1. The radon concentration in the air measured with the RAD7 monitor showed low values ranging from 0 to 15.3 Bq/m3. Laboratory measurements of rocks using alpha and gamma spectrometry techniques showed a significant variation in the concentration of 226Ra and 234,238U isotopes, and the highest concentration values were recorded for shales. The 40K, 234,238U and 226Ra isotopes make the greatest contribution to the natural radioactivity of analyzed rocks. [ABSTRACT FROM AUTHOR]

Multi-stage hydraulic stimulation experiments were conducted at the Bedretto Geothermal Testbed (Switzerland) at a depth of over 1 km to study seismo-hydromechanical processes at the hectometer scale. Eight intervals with a total length of 206 m were stimulated within a densely monitored, fractured granitic rock volume. Geological characterization shows that all intervals contain NE-SW striking, steeply dipping fractures, but the transmissivity and natural inflow vary between intervals. Reactivation pressures estimated from injectivity increases during stimulation indicate shear reactivation rather than hydraulic jacking, suggesting reactivation of pre-existing non-filled fractures. Overall, the seismo-hydromechanical response to stimulation is heterogeneous and indicates channelized flow. • Multi-stage hydraulic stimulations performed in a fractured crystalline rock volume. • Novel hectometer-scale at >1km depth reveals heterogeneous rock mass response. • Stimulation intervals contain steeply dipping NE-SW striking non-filled fractures. • Most stimulations cause a strong injectivity increase at comparable pressures. • Reactivation pressures can be explained by shearing of pre-existing fractures. [ABSTRACT FROM AUTHOR]

The construction of an underground research laboratory (URL) precedes the construction of a repository for geological disposal of а radioactive waste. The article considers the types of laboratories, their main objectives and tasks. Based on a comparative analysis between the sites selected as prospective for geological disposal in Bulgaria, the Sumer site in the Sumer Formation is proposed for further investigation as an alternative for URL construction. [ABSTRACT FROM AUTHOR]

Copyright of World Nuclear Geoscience is the property of World Nuclear Geoscience Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The aim of the work is the substantiate the development of a GNSS deformation monitoring network at the site of the underground research laboratory, for estimating the safety of underground isolation of radioactive waste in the Nizhne-Kansk massif. The article overviews the international experience of GNSS observations of recent crustal movements in the vicinities of underground research laboratories location (Switzerland, Finland, Sweden). The general information and main results of research works during 2010–2020 in the Nizhne-Kansk geodynamic test area are described. The article presents the prospects for extending the geodetic network in connection with the start of construction of the underground research laboratory. Based on the Delaunay triangulation we have analyzed the triangular finite elements of the existing and planned geodetic network. It is shown that the location of new points will significantly affect the geometry of the network. As a result, the number of near-equilateral triangles in the network will increase, which has a positive effect on increasing the accuracy in determining deformations of the Earth's surface when processing geodetic measurements. [ABSTRACT FROM AUTHOR]

The measurement of the stress field and its natural heterogeneity is a common challenge in the field of geophysics and underground engineering. This study presents findings from an extensive mini-frac campaign conducted at the Bedretto Underground Laboratory (BedrettoLab) in the Swiss Alps. Six dedicated stress measurement boreholes, spanning 500 m along the Bedretto Tunnel, were utilized to prepare for meso-scale hydraulic stimulation experiments in a granitic rock volume. A detailed analysis of the induced hydraulic fracture trace morphology was carried out using acoustic televiewer logging data, with an interpretation based on the local geological context. The results show that the far field orientation of the maximum horizontal stress (S Hmax) is on average N112°E and the overburden stress can be assumed to be a principal stress direction. However, the en-echelon shape of the observed hydraulic fracture traces suggests that there is a slight inclination of the greatest principal stress away from the vertical stress direction. The mini-frac test intervals along boreholes inside the enlarged niche of the BedrettoLab indicate a counterclockwise rotated horizontal stress by up to 37° and potentially stronger inclination of the greatest principal stress from the vertical stress direction. The most likely causes of this stress field rotation, along with a considerably larger scatter in stress magnitudes, appear to be the larger laboratory niche dimensions compared to the tunnel and the presence of natural fractures that are preferentially oriented for slip in the far field stress state. Although the uncertainties and variability of the stress field around the BedrettoLab are not yet fully understood, our results are useful for strategic planning and design of meso-scale stimulation experiments in the rock volume. Furthermore, our detailed analysis procedure contributes to advancing the understanding of mini-frac tests in similar geological settings. • Mini-frac campaign in six boreholes spanning 500 m along a tunnel in the Swiss Alps. • Analysis of fracture trace morphology to estimate stress field orientation. • Consistent far field stress orientation is perturbed within a larger tunnel niche. • Natural fractures and excavation effects cause stress field perturbation. [ABSTRACT FROM AUTHOR]

The deep geological disposal is an internationally recognized method for the disposal of high level radioactive waste. As an essential facility, the construction safety of underground research laboratory (URL) is very important. Taking the China's first URL in Beishan, Gansu Province as engineering background, considering the influence of arranged joints in rock mass, an elastic–plastic damage numerical method for fractured rock mass is presented and the corresponding subprogram in Flac3D is developed. Based on the numerical calculation of underground research laboratory construction, the optimized excavation scheme of URL is obtained and the distribution rules of surrounding rock displacement field, stress field and plastic damage zone are studied. The research result can provide important guidance for the construction of Beishan URL. [ABSTRACT FROM AUTHOR]

The evaluation of Thermo-Hydro-Mechanical (THM) coupling behavior is important for the development of underground space for various purposes. For a high-level radioactive waste repository excavated in a deep underground rock mass, the accurate prediction of the complex THM behavior is essential for the long-term safety and stability assessment. In order to develop reliable THM analysis techniques effectively, an international cooperation project, Development of Coupled models and their Validation against Experiments (DECOVALEX), was carried out. In DECOVALEX-2015 Task B2, the in situ THM experiment that was conducted at Horonobe Underground Research Laboratory(URL) by Japan Atomic Energy Agency (JAEA), was modeled by the research teams from the participating countries. In this study, a THM coupling technique that combined TOUGH2 and FLAC3D was developed and applied to the THM analysis for the in situ experiment, in which rock, buffer, backfill, sand, and heater were installed. With the assistance of an artificial neural network, the boundary conditions for the experiment could be adequately implemented in the modeling. The thermal, hydraulic, and mechanical results from the modeling were compared with the measurements from the in situ THM experiment. The predicted buffer temperature from the THM modelling was about 10 °C higher than measurement near by the overpack. At the other locations far from the overpack, modelling predicted slightly lower temperature than measurement. Even though the magnitude of pressure from the modeling was different from the measurements, the general trends of the variation with time were found to be similar. [ABSTRACT FROM AUTHOR]

Bukov Underground Research Facility (Bukov URF) is designed to operate as a test site to assess the properties and behavior of the rock mass analogous to selected seven candidate sites in the Czech Republic. It is situated at a depth of about 600 m beneath Earth's surface, which corresponds with the proposed storage depth of the final locality for the national deep repository of high-level radioactive waste. Bukov URF, the construction of which has started in 2013, is situated in the southern part of the Rožná uranium deposit, about 40 km NNW from Brno. The rock mass is composed of relatively monotonous rock sequences mainly represented by migmatized biotite paragneisses up to stromatic migmatites, amphibole-biotite to biotite-amphibole gneisses, and amphibolites. Based on the analysis of physical and mechanical properties, intact rocks in the studied area exhibit a high to very high strength and may be only locally affected by metasomatic alterations. The research activities focus on a complex geological and geotechnical characterization of the rock mass of interest, which is necessary for further in situ research. This paper briefly describes the main results of the current stage of geotechnical exploration and research works made there by the Institute of Geonics of the CAS. These works especially include: determination and evaluation of physical and mechanical properties of the rocks taken from the drift walls and from the boreholes during driving, calculation of rock mass quality based on selected index rock mass classification systems, determination of stress and strain state of the rock mass using the methods of hydrofracturing, Goodman Jack, strain gauge probe measurements, long-term periodical tensometric and convergence measurements, and evaluation of the effects of technical and mining-induced seismicity on the rock mass of interest. [ABSTRACT FROM AUTHOR]

This paper evaluates the modifications created in the concrete of the FEBEX shotcreted concrete plug after 13 years in the Grimsel Test Site conditions. During this time the concrete interacted with granite groundwater and also with bentonite porewater at the concrete/bentonite contact. Three long cores and 6 small cores from different parts of the concrete plug were evaluated. Mechanical performance was not modified during this time but hydraulic conductivity increased. The main transport mechanisms involved in the alteration of the concrete were groundwater flow from the host rock to the concrete and diffusion at the concrete/bentonite interface. Leaching occurred in the concrete parts near the host rock due to the action of granite water with further portlandite dissolution. The joint action of granite groundwater and bentonite porewater has caused many changes to the concrete matrix which was located at a depth lower than 5 cm from the bentonite-concrete interface. In the first centimetre C-S-H was significantly altered, incorporating elements like Al, S and Mg which change the initial microstructure by loss of compactness. The ettringite content was very high along the length of the concrete plug due to the shotcreting technique which made use of accelerator additives that caused the formation of ettringite. An increase in the ettringite content is also shown near the bentonite barrier. Therefore, sulphate diffused from the bentonite into the concrete, causing the massive formation of new ettringite. Chloride also diffused from the bentonite barrier deeper into the concrete by up to 4–5 cm from where the formation of Friedel's salt was detected. [ABSTRACT FROM AUTHOR]

The in situ FEBEX experiment performed at the URL in Grimsel (Swizerland) was dismantled after 18 years of operation. Interface samples between bentonite and a shotcreted concrete plug that was constructed in a second operational phase have been studied after 13 years of interaction. Mineralogical and geochemical characterization of samples have been performed by XRD, SEM-EDX, TG and FTIR techniques in addition to determinations of major ions by chemical analysis of aqueous extracts, δ 18 O and δ 13 C stable isotopes both in concrete paste and bentonite, and exchangeable cations in bentonite. Low mineralogical alteration impact was observed in bentonite that is only affected by a few millimeters. A large accumulation of Mg was observed at the bentonite side of the interface precipitating as silicates in various forms. In addition, heterogeneous carbonation was observed at the interface, but mostly affecting the concrete side. Migration of aqueous species occurred, being the most relevant the diffusion of chloride and sulfate from bentonite to concrete, in agreement with Part I of this study. Chloride advanced more into the concrete, while sulfates reacted to form ettringite, which has an evident alteration impact at the very interface (<0.5 mm rim) within the concrete. The ionic mobility has also redistributed the exchangeable cations in bentonite, increasing the content in Ca 2+ and Na + , compensated by a decrease in Mg 2+ . The results presented in this paper complement those presented in Part I, focusing on the alteration of concrete by the bentonite and the granite groundwater. [ABSTRACT FROM AUTHOR]

Secure disposal or storage of nuclear waste within stable geologic environments hinges on the effectiveness of artificial and natural radiation barriers. Fractures in the bedrock are viewed as the most likely passage for the transport of radioactive waste away from a disposal site. We utilize ground penetrating radar (GPR) to map fractures in the tunnel walls of an underground research tunnel at the Korea Atomic Energy Research Institute (KAERI). GPR experiments within the KAERI Underground Research Tunnel (KURT) were carried out by using 200 MHz, 500 MHz, and 1000 MHz antennas. By using the high-frequency antennas, we were able to identify small-scale fractures, which were previously unidentified during the tunnel excavation process. Then, through 3-D visualization of the grid survey data, we reconstructed the spatial distribution and interconnectivity of the multi-scale fractures within the wall. We found that a multi-frequency GPR approach provided more details of the complex fracture network, including deep structures. Furthermore, temporal changes in reflection polarity between the GPR surveys enabled us to infer the hydraulic characteristics of the discrete fracture network developed behind the surveyed wall. We hypothesized that the fractures exhibiting polarity change may be due to a combination of air-filled and mineralogical boundaries. Simulated GPR scans for the considered case were consistent with the observed GPR data. If our assumption is correct, the groundwater flow into these near-surface fractures may form the water-filled fractures along the existing air-filled ones and hence cause the changes in reflection polarity over the given time interval (i.e., 7 days). Our results show that the GPR survey is an efficient tool to determine fractures at various scales. Time-lapse GPR data may be essential to characterize the hydraulic behavior of discrete fracture networks in underground disposal facilities. [ABSTRACT FROM AUTHOR]

Nuclear power plant (NPP) Krško has a license to operate until 2023, and under the current agreement between the Republic of Slovenia and the Republic of Croatia, countries are bound to dispose one half of radioactive waste produced during the operation time and after decommissioning of NPP each. Safe long-term management of high level radioactive waste and spent fuel represents one of the most important issues of the modern world. The best way to provide practical demonstration of repository’s safety, which will be one of convincing arguments in the process of licensing future repository, is developed underground research laboratory (URL). Existence of URL open to international co-operation would certainly improve the international recognition and credibility of Croatian programme, as well as allow dissemination of scientific research results to a broader scientific community (the paper is published in Croatian).

A detailed scientific study is required for the disposal of high-level radioactive wastes because they generate extremely high heat during their half-life period. Although, several methods have been proposed for the disposal of nuclear wastes, deep underground repository is considered to be a suitable option. In this paper, field investigation has been done near to Bhima basin of peninsular India. Detailed fracture analysis near the borehole shows very prominent maxima of fractures striking N55E coinciding with the trace of master basement cover metasediment fault. Physico-mechanical properties of rocks have been determined in the laboratory. The host rock chosen is granite and engineered barrier near the canister is proposed to be clay. A thermo-hydro-mechanical (THM) analysis has been done to study the effect of heat on deformations, stresses and pore-pressure variation in granite and clay barriers. For this purpose, finite difference method has been used. Suitable rheological models have been used to model elastic canister and elasto-plastic engineered barrier and host rock. It has been found that both temperature and stresses at any point in the rockmass is below the design criteria which are 100C for temperature and 0.2 for damage number. [ABSTRACT FROM AUTHOR]

Summary The Excavation Damaged Zone (EDZ) surrounding a drift, and in particular its evolution, is being studied for the performance assessment of a radioactive waste underground repository. A specific experiment (called CDZ) was designed and implemented in the Meuse/Haute-Marne Underground Research Laboratory (URL) in France to investigate the EDZ. This experiment is dedicated to study the evolution of the EDZ hydrogeological properties (conductivity and specific storage) of the Callovo-Oxfordian claystone under mechanical compression and artificial hydration. Firstly, a loading cycle applied on a drift wall was performed to simulate the compression effect from bentonite swelling in a repository drift (bentonite is a clay material to be used to seal drifts and shafts for repository closure purpose). Gas tests (permeability tests with nitrogen and tracer tests with helium) were conducted during the first phase of the experiment. The results showed that the fracture network within the EDZ was initially interconnected and opened for gas flow (particularly along the drift) and then progressively closed with the increasing mechanical stress applied on the drift wall. Moreover, the evolution of the EDZ after unloading indicated a self-sealing process. Secondly, the remaining fracture network was resaturated to demonstrate the ability to self-seal of the COx claystone without mechanical loading by conducting from 11 to 15 repetitive hydraulic tests with monitoring of the hydraulic parameters. During this hydration process, the EDZ effective transmissivity dropped due to the swelling of the clay materials near the fracture network. The hydraulic conductivity evolution was relatively fast during the first few days. Low conductivities ranging at 10 –10 m/s were observed after four months. Conversely, the specific storage showed an erratic evolution during the first phase of hydration (up to 60 days). Some uncertainty remains on this parameter due to volumetric strain during the sealing of the fractures. The hydration was stopped after one year and cross-hole hydraulic tests were performed to determine more accurately the specific storage as well as the hydraulic conductivity at a meter-scale. All hydraulic conductivity values measured at the injection interval and at the observation intervals were all below 10 –10 m/s. Moreover, the preferential inter-connectivity along the drift disappeared. Specific storage values at the observation and injection intervals were similar. Furthermore they were in agreement with the value obtained at the injection interval within the second hydration phase (60 days after starting hydration). The graphical abstract synthesizes the evolution of the hydraulic/gas conductivity for 8 intervals since the beginning of the CDZ experiment. The conductivity limit of 10 –10 m/s corresponds to the lower bound hydraulic definition of the EDZ and it is demonstrated that EDZ can be sealed. This is a significant result in the demonstration of the long-term safety of a repository. [ABSTRACT FROM AUTHOR]

The performance and safety assessment and technology demonstration are the main objectives of research programs for feasibility studies for deep geological repository of radioactive waste. In this context, the French national radioactive waste management agency (ANDRA) started to develop the Meuse/Haute-Marne underground research laboratory (URL) at Bure, nearly 300 km East of Paris. The host formation consists of a Callovo-Oxfordian claystone found between 420 and 550 m below ground, overlain and underlain by poorly permeable carbonate formations. One of the major concerns related to performance assessment is the excavation-induced fractures which may provide groundwater preferential pathway for radionuclide migration. The extent of the fractures possibly acting significantly in the radionuclide migration is known as the excavation damaged zone (EDZ). A scientific study on the EDZ characterization is performed at the main level of the URL (−490 m). Observations such as structural analysis on core, overcored resin-filled samples, geological survey of the drift face and sidewalls, were made to better understand the fracture network characteristics, extent and its generation. Pulse and constant head test hydraulic conductivity measurements were performed with multi packer system to estimate the extension of the EDZ hydraulic conductivity. Fractures exhibited high transmissivity near the excavation walls, but farther from the exaction walls, shear fractures showed hydraulic conductivity values reflecting values of undisturbed or slightly disturbed rock mass condition. The major findings in terms of geometry and properties of excavation-induced fractures are discussed in detail in this paper. For example, it is observed that the shape of the fracture network depends on the orientation of the drift in relation to the orientation of the in situ stress field. [ABSTRACT FROM AUTHOR]

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During the long-term stability assessment of underground disposal repositories buried in sedimentary formations, one of crucial scientific problems is to determine parameters of site formations. The analysis of response of the pore pressure due to atmospheric loading provides a possible way of accurate estimation of formation parameters. In this paper, an analytical solution of response of the pore pressure to atmospheric loading was derived starting from the Biot's poroelasticity, and its corresponding numerical solutions were developed by finite element methods for 1D and 2D situations. With comparison to the analytical solution, the experimental results show that the numerical schemes sound reasonable to implement the assessment of response of the pore pressure in horizontal multilayer sedimentary formations. In particular, it is effective for a 2D model over large area with consideration of background horizontal groundwater flow. A highlight of this work is to construct a reasonable approach to capture responses of the pore pressure to atmospheric loading, and at the same time to provide the feasibility of integrated analysis of well monitoring data of pore pressure variance in Horonobe Underground Research Laboratory (URL), Hokkaido, Japan. [ABSTRACT FROM AUTHOR]

Abstract: Hydraulic conductivity of the Callovo-Oxfordian argillaceous rock ranges between 10−14 and 10−12 m/s. In spite of these low values, long term ‘pumping tests’ covering a range of head differentials have been performed in several boreholes in the Andra Meuse/Haute-Marne Underground Research Laboratory (URL). Specifically designed experimental setups made it possible to monitor water production flow rates ranging from 0.5 to 50mL/day, during more than 4years. Long term pumping tests provided an alternative to the pressure test method for evaluating hydraulic conductivities at the −430m and −505m depth levels in the URL. The obtained values were close to 0.8×10−13 m/s and 1.3×10−13 m/s, respectively. The lowest flow rate measured under well-established hydraulic head boundary conditions was close to 2.5mL/day. It corresponds to a calculated hydraulic gradient value less than 120m/m. This result indicates that water can flow in the Callovo-Oxfordian with such a hydraulic gradient value. [Copyright &y& Elsevier]

Abstract: Analysis of a 1.15 km deep apatite fission track (AFT) thermochronology profile at the Underground Research Laboratory (URL), in the southwestern Canadian Shield suggests two Phanerozoic heating and cooling episodes indicating significant, previously unsuspected, Phanerozoic heat flow variations. Phanerozoic temperature and heat flow variations are temporally associated with burial and erosion of the Precambrian crystalline shield and its overlying Phanerozoic successions, which are now eroded completely. Maximum Phanerozoic temperatures occurred in the late Paleozoic when the geothermal gradient is estimated to have been ~40–50 °C/km (compared to a present day gradient of ~14±2 °C/km) and the sedimentary cover was ~800–1100 m thick. Our thermal history models, confirm regional stratigraphic relationships that suggest that the Paleozoic succession was completely eroded prior to beginning of Mesozoic sedimentation. A second heating phase occurred during Late Cretaceous–Paleogene burial when the geothermal gradient is estimated to have been ~20–25 °C/km and the Mesozoic and Cenozoic succession was ~1200 to 1400 m thick. The Phanerozoic thermal history at the URL site shows a pattern similar to that inferred previously for the epicratonic Williston Basin, the centre of which lies several 100 km to the west. This implies a common regional thermal history for cratonic rocks underlying both the basin and the currently exposed shield. It is suggested that the morphotectonic differences between the Williston Basin and the exposed shield at the URL are due to a dissimilar thermomechanical response to a common, but more complicated than previously inferred, Phanerozoic geodynamic history. The two Phanerozoic periods of variations in geothermal gradient (heat flow) were coeval with epeirogenic movements related to the deposition and erosion of sediments. These paleogeodynamic variations are tentatively attributed to far-field effects of orogenic processes occurring at the plate margin (i.e. the Antler and the Cordilleran orogenies) and the associated accumulation of cratonic seaway sedimentary sequences (Kaskaskia and Zuni sequences). [Copyright &y& Elsevier]

Abstract: Andra has conducted a very comprehensive investigation on the in situ stress field within an argillite formation interbeded between two stiffer limestone formations, for the development of its underground research laboratory. A series of hydraulic tests has been conducted in boreholes located in the vicinity of the future site. Borehole and shaft convergence measurements were also carried out to analyse the mechanical behaviour of the rock. Throughout the stress measurement campaign, several borehole images have shown breakouts or induced fractures, that have been analysed in terms of in situ stress field, both in the argillite and in the limestone layers. All these reliable data highlight the high dependency of the state of stress on the rheology of the formation. A high ratio between the two horizontal principal stress components – ranging from 1.3 to 2.0 – has been observed in the limestones, whereas the maximum horizontal principal stress component has been found to be close to the lithostatic stress in the clay formation at the main level of the laboratory; i.e. at the 490m depth. [Copyright &y& Elsevier]

Abstract: During decommissioning of the tunnel sealing experiment (TSX) at AECL’s Underground Research Laboratory, many samples were taken from the clay bulkhead (70% bentonite and 30% sand) to determine microbial occurrence and viability (culturability) in this clay seal. All samples taken from the TSX showed culturable populations of heterotrophic aerobic and anaerobic bacteria, sulphate-reducing bacteria (SRB) and facultative anaerobic nitrate respiring and nitrate-reducing bacteria. Aerobic and facultative anaerobic populations were significantly higher in the various interface environments. This was likely due to the presence of more water and larger pore spaces at the interface locations, especially at the clay–rock (shotclay) interface, and to persisting aerobic conditions at these locations. Heterotrophic anaerobic and SRB populations were almost constant with no significant differences between the interface locations and the bulk clay. This could suggest that the heterotrophic anaerobes and SRB survived in a metabolically inactive but resistant state because the conditions in the clay were still essentially aerobic. Samples taken in the backfill region of the seal (10% bentonite, 90% sand) showed higher SRB and a noticeable decrease in heterotrophic aerobes and facultative anaerobes. This suggested that moisture, space and nutrient conditions in the backfill region were conducive to microbial activity, causing a reduction in O2, followed by a decline in aerobic and an increase in anaerobic populations. Concurrently, higher temperatures in the backfill region of the seal during the second phase of the TSX may have had negative effects on the aerobic and facultative anaerobic populations, while stimulating the anaerobic (seemingly hardier) species. These results suggest that microbial culturability (and in situ activity) in clay-based materials is controlled by a combination of local redox conditions, suitable pore space and moisture availability. In clays the latter two factors are difficult to address separately because they are mutually dependent. In order to reduce or minimize microbial activity in clay-based materials, it is crucial to restrict both pore space and moisture availability as much as possible. This can be achieved by using very high dry densities (>1.8Mg/m3) or, more generally applicable, very high effective montmorillonite dry densities. [Copyright &y& Elsevier]

Abstract: In November 1999 Andra began building an Underground Research Laboratory (URL) on the border of the Meuse and Haute-Marne departments in eastern France. The research activities of the URL are dedicated to study the feasibility of reversible, deep geological disposal of high-activity, long-lived radioactive wastes in an argillaceous host rock. The Laboratory consists of two shafts, an experimental drift at 445m depth and a set of technical and experimental drifts at the main level at 490m depth. The main objective of the research is to characterize the confining properties of the argillaceous rock through in situ hydrogeological tests, chemical measurements and diffusion experiments. In order to achieve this goal, a fundamental understanding of the geoscientific properties and processes that govern geological isolation in clay-rich rocks has been acquired. This understanding includes both the host rocks at the laboratory site and the regional geological context. After establishing the geological conditions, the underground research programme had to demonstrate that the construction and operation of a geological disposal will not introduce pathways for waste migration. Thus, the construction of the laboratory itself serves a research purpose through the monitoring of excavation effects and the optimization of construction technology. These studies are primarily geomechanical in nature, though chemical and hydrogeological coupling also have important roles. In order to achieve the scientific objectives of this project in the underground drifts, a specific methodology has been applied for carrying out the experimental programme conducted concurrently with the construction of the shafts and drifts. This methodology includes technological as well as organizational aspects and a systematic use of feedback from other laboratories abroad and every scientific zone of the URL already installed. This methodology was first applied to set up a multi-purpose experimental area at 445m depth. Then the setting up of the experimental programme at the level 490m was improved from the knowledge acquired during installation of the drift at 445m. The several steps of the underground scientific programme are illustrated by presenting three experiments carried out in the underground drifts. The first experiment was carried out from the drift at 445m depth, from end of 2004 to mid 2005. This experiment aimed at setting up an array of about 16 boreholes to monitor the geomechanical changes during and after construction of the shaft between 445 and 490m. The second experiment was set up in the drift at 445m depth, and also at the main level at 490m depth. It consisted in determining the composition of the interstitial water by circulating gas in one borehole and water of a known composition in the other. The evolution of the composition of both water and gases enabled us to test the thermodynamic model of the water/rock interactions. The third example is related to the testing of a concept of interruption of the EDZ through a cross-cut slot technology. The concept, which was tested successfully at Mont Terri (Switzerland), has been transposed and adapted to the URL site conditions. The results will be used for developing a concept for drift sealing. [Copyright &y& Elsevier]

Abstract: Andra, the French National Radioactive Waste Management Agency, is constructing an underground test facility to study the feasibility of a radioactive waste disposal in the Jurassic-age Callovo-Oxfordian argillites. This paper describes the processes, the methods and results of a scientific characterization program carried out from the surface via deep boreholes with the aim to build a research facility for radioactive waste disposal. In particular this paper shows the evolution of the drilling programs and the borehole set up due to the refinement of the scientific objectives from 1994 to 2004. The pre-investigation phase on the Meuse/Haute-Marne site started in 1994. It consisted in drilling seven scientific boreholes. This phase, completed in 1996, led to the first regional geological cross-section showing the main geometrical characteristics of the host rock. Investigations on the laboratory site prior to the sinking of two shafts started in November 1999. The sinking of the shafts started in September 2000 with the auxiliary shaft completed in October 2004. The experimental gallery, at a depth of 445m in the main shaft, was in operation by end 2004. During the construction of the laboratory, two major scientific programs were initiated to improve the existing knowledge of the regional hydrogeological characteristics and to accelerate the process of data acquisition on the shales. The aim of the 2003 hydrogeological drilling program was to determine, at regional scale, the properties of groundwater transport and to sample the water in the Oxfordian and Dogger limestones. The 2003–2004 programs consisted in drilling nine deep boreholes, four of which were slanted, to achieve an accurate definition of the structural features. [Copyright &y& Elsevier]

Abstract: Incremental development of a rock mechanics, rock fracturing, and excavation stability knowledge base has been ongoing at Atomic Energy of Canada Limited''s (AECL''s) Underground Research Laboratory (URL) since shaft excavation commenced in 1982. Excavation response experiments conducted as part of shaft construction recorded displacements, shaft convergence, stress changes, and microseismic events in the rock as excavation progressed. An excavation response test at the 240 Level of the URL involved similar monitoring, as well as pore pressure measurements, of a horizontal tunnel excavated through a subvertical water-bearing fracture. These precursor studies led to a series of experiments in the more highly stressed rock at the 420 Level of the URL to investigate the formation of rock damage around tunnels, and to assess the factors that influence the stability of excavations. The first of these experiments was the Mine-by Experiment, an excavation response study involving a mechanically excavated cylindrical tunnel in a pre-instrumented rock volume. The Heated Failure Tests were subsequently conducted in the same rock volume to assess the influence of thermal loading on damage development. These investigations were complemented by studies of borehole breakouts in adjacent excavations. The Excavation Stability Study, which involved excavating ten tunnel segments of different geometry to assess the effects of excavation design on stability and damage development, followed these experiments. The Tunnel Sealing Experiment, focused on developing sealing technologies, also provided insight into the excavation response of the rock mass. The excavation response experiments at the URL culminated in the Thermal-Mechanical Stability Study (TMSS), a comprehensive study to link characterization, numerical modeling, monitoring, and design of underground excavations. This paper provides an overview of the various experiments and studies leading up to the TMSS, highlighting the advances in our fundamental understanding of rock mechanics related to underground excavations, and in our means of designing stable underground openings with minimal excavation damage. [Copyright &y& Elsevier]

Two large-scale sealing experiments were conducted at Atomic Energy of Canada Limited's Underground Research Laboratory at Lac du Bonnet, Manitoba. The rate of water uptake in densely compacted sand–clay buffer materials proposed for use in a deep geologic repository for spent nuclear fuel was monitored. The buffer–container experiment examined the influence of heat on the performance of a large mass of buffer. Temperatures, water contents, and total and hydraulic pressures within and surrounding the installation were monitored for approximately 2.5 years. Local groundwater pressures increased as a result of rising temperatures. Water uptake and redistribution occurred in the buffer due to drying shrinkage close to the heater and counter-acted swelling due to an increase in water content near the rock–buffer interface. The isothermal test (ITT) allowed natural groundwater uptake from the surrounding rock mass under isothermal conditions. It was monitored for a period of 6.5 years and is the first, and longest running test of its kind yet conducted in the world. During its operation, the ITT (for as yet unconfirmed reasons) experienced a 35% decrease in the rate of water supply relative to that measured prior to experiment installation. This decrease impacts on the time required for saturation to be achieved.Key words: buffer, bentonite, underground research laboratory, instrumentation.Deux essais d'étanchéité à grande échelle ont été conduits au Laboratoire de recherche souterrain de l'Énergie atomique du Canada au lac du Bonnet, Manitoba. On a mesuré la vitesse d'absorption d'eau dans des matériaux tampons formés de sable et argile densément compacts devant être utilisés dans un dépôt géologique profond pour le combustible nucléaire irradié. L'expérience tampon–conteneur a permis d'examiner l'influence de la chaleur sur la performance d'une large masse de tampon. Les températures, teneurs en eau, pressions totales et hydrauliques à l'intérieur et autour de l'installation ont été mesurées durant environ 2,5 années. Les pressions de l'eau souterraine locale ont augmenté à cause de l'augmentation de la température, une absorption et une redistribution se sont produites dans le tampon à cause du séchage et du retrait près des points chauds d'une part, et de l'action inverse du gonflement dû à l'augmentation de la teneur en eau près de l'interface roc–tampon d'autre part. L'essai isothermique a permis l'absorption de l'eau souterraine naturelle venant du massif rocheux dans des conditions isothermiques. Des mesures ont été prises durant une période de 6,5 années, et cet essai est le premier et le plus long essai de ce type à être réalisé jusqu'à ce jour dans le monde. Au cours de son opération, cet essai a subi, pour des raisons encore non confirmées, une diminution de 35 % de la vitesse de son alimentation en eau par rapport à celle mesurée avant la mise en place de l'expérience. Ceci a des impacts sur le temps requis pour atteindre la saturation.Mots clés : tampon, bentonite, Laboratoire de recherche souterrain, instrumentation.[Traduit par la Rédaction] [ABSTRACT FROM AUTHOR]

For underground engineering with complex structure, it is important to carry out reliability analysis to ensure engineering safety. This research takes China's first underground research laboratory (URL) for the geological disposal of high-level radioactive waste (HLW) in Beishan area as engineering background. Firstly, based on a large number of drilling core data of Beishan site, the optimal probability distribution model of the rock mechanical parameters is established by statistical analysis. After that, a typical underground structure of the URL, which consists of a shaft and three intersecting tunnels at the depth of −560 m, is selected as research object. The reliability indexes of the structure are calculated by Response Surface-Monte Carlo method. The results show that after excavation, most of the reliability indexes of the structure are above 4.2. The structure is safe as a whole. Due to the interaction between different tunnels/shaft, the minimum reliability index appears at the intersecting corner of the tunnels/shaft, which indicates that the intersecting areas are the weakest area of the structure. The reliability calculation results are basically in good agreement with the geo-mechanical model test results, which verify the validity of the reliability calculation results. [ABSTRACT FROM AUTHOR]