The background，the preliminary technical strategy and the long-term plan for China¢s high-level radioactive waste disposal program are presented. China started research on HLW disposal in 1985，with a plan to build a national repository in the middle of the 21st Century. Beishan area，located in Northwest China¢s Gansu Province，is selected as the most potential area for China¢s HLW repository. During 1999–2004，preliminary site characterization evaluation has been performed，including surface geological，hydrogeological and geophysical investigation，and drilling of 4 boreholes (BS01，BS02，BS03，BS04) and in-situ tests in boreholes. Favorite results have been obtained from the site characterization campaign. Progresses are also made in studies on buffer/backfill materials，radionuclide migration and natural analogue.

In order to gain a better understanding of coupled thermo-hydro-mechanical(THM) processes in indurated clays being considered in various countries as host rock for the disposal of high-level radioactive waste，a heating experiment named HE-D is being conducted on the Opalinus clay at the Mont Terri Rock Laboratory in Switzerland. An electric heater of 5.4 m length has heated the clay rock over more than one year. The maximum temperature of 100 ℃ is reached at the heater/rock interface. During the experiment，temperature，pore water pressure，gas migration，and deformation in the clay rock have been monitored by means of more than 80 instruments installed in 24 boreholes. Thermal expansion of the heated rock was observed. In the heated region at distance about 1 m to the heater，the pore water pressure increased from about 1 MPa to 4 MPa. The responses of the clay rock to heating were simulated by coupled THM modeling. Preliminary results of the field measurements and the numerical calculations are presented.

A general formulation for the performance of coupled thermo-hydro-mechanical(THM) analysis is presented. A multi-phase and multi-species approach has been adopted for the development of the formulation. The analysis considers a porous material with three phases(solid，liquid and gas) and three species(mineral，water and air). The formulation is presented in terms of governing equations，constitutive laws and equilibrium restrictions. An extensive programme of coupled THM analyses simulating the behaviour of the near field in a scheme for the underground disposal of high level radioactive waste is presented. The analyses are able to take into account the main interactions between the various processes that occur in the engineered barrier and immediately adjacent rock associated with heating and hydration phenomena. Examination of the results leads to a series of significant conclusions concerning the thermal，hydraulic and mechanical aspects of the problem.

In Belgium，geological disposal in clay is the primary option for the final disposal of high-level radioactive waste(HLW). Repositories are designed on the basis of a multiple barrier system between the HLW and the biosphere. The Boom clay has been investigated for more than 20 years as the reference host rock. The most important decision was taken in 1980 to build an called high activity disposal experimental facility(HADES) in Boom clay formation at a depth of about 223 m at Mol site to investigate and demonstrate the feasibility of the disposal concepts and to provide reliable data on the performance of repository barriers(natural and engineering). Among numerous in-situ experiments carried out in HADES，several tests were designed to study the thermo-hydro-mechanical(THM) behaviour of the host rock as well as the engineered barrier system(EBS) including sealing and backfilling feasibility，such as CACTUS，ATLAS，BACCHUS，and RESEAL，etc.. Since 1995，the R&D(research and design) program has been oriented towards the large-scale feasibility and demonstration tests. Main achievements are the demonstration of the construction of the underground research facility(URF，shaft and gallery) using the industrial technique which gave an opportunity to extend the knowledge on the hydro-mechanical(HM) behaviour of host rock clay(CLIPEX project)，more specifically on the excavation damaged zone(EDZ) due to tunnelling works(SELFRAC project)，the realization of a large-scale heating and hydration mock-up test of a prefabricated bentonite called“OPHELIE”. Future work consists in realizing a large-scale in-situ heater test(PRACLAY experiments) that will start in 2006 for a period about 10 years. This paper will firstly give a brief description on the Belgian repository design for HLW，then a review on the performed tests regarding to the THM behaviour of the Boom clay and some EBS(in-situ and on surface mock-up)，finally a general presentation on the future large-scale in-situ heater test(PRACLAY experiments).

A model that reproduces the coupled deformation and two-phase flow(with special interest to gas migration) through a waste containment engineered barrier system has been developed. The simulation presented in this paper is novel because variable permeability interfaces that depend on deformation are introduced to model the contacts between backfill materials and solid rock or concrete structures. Calculated results of pressures，stresses and volumetric water contents are compared with measurements showing an encouraging agreement which permits to conclude that the model is quite accurate in representing the actual processes that take place in a large-scale in-situ test.

The purposes of the DECOVALEX project(development of coupled models and their validation against experiments) are focused on the various coupled thermo-hydro-mechanical(THM) and geochemical processes occurring in the near field of a high activity radioactive waste disposal facility，of a demonstration for the feasibility of the construction of engineered barriers，and of how they affect the role of performance assessment for radioactive waste disposal. Task_D of the DECOVALEX_IV project includes predictive analysis of the long-term coupled processes(up to 1×104 years) in two generic repositories——FEBEX type and Yucca Mountain Project type for comparison. To better understand the coupling THM processes and their influences on the system behaviours，this paper presents a rigorous treatment of the theory of non-isothermal flow and deformation in unsaturated porous media. The governing equations based on the conservation equations of mass and energy and the equations of equilibrium，Darcy¢s law，Fourier¢s law are derived by using a systematic macroscopic approach. Each of the three phases(solid，liquid and gas) is viewed as an independent continuum，endowed with its own kinematics. Four constituents(solid，water，vapour and dry air) are identified. Four separate but overlapping models are developed as follows：(1) deformation model for solid considering the fluid effective stress，thermal stress，and bentonite saturation swelling pressure；(2) flow model for water and gas considering the mass exchange between water and vapour in evaporation or condensation processes；(3) vapour molecular diffusion and advection model considering the relative movement of vapour to dry air in presumed homogenous pore gas phase；and (4) heat transfer model considering the non-equilibrium local thermal processes that three phases are not at a same temperature. Based on above equations，practical models are developed according to the given Task_D BMT(bench mark test) request，and the codes are programmed to simulate coupled THM processes in 1×106 years with Task_D BMT set-up. Simulation results are shown and verified in Task_D force meeting and the third workshop of DECOVALEX_IV in Kunming(2005)，China. There is a good agreement with results of participant teams from other countries.

Important attention has been devoted over recent ten years to the behaviour of highly compacted swelling clays that are considered as a possible component of engineered barriers for high activity nuclear waste isolation. Some experimental results obtained by running suction controlled experiments on compacted bentonites are interpreted in light of microstructure features. Water retention properties as well as transfer properties and swelling responses under suction changes are related to microscopic mechanisms. Some comments on the role of adsorbed cations are also made.

Within the framework of the on-surface preliminary heating simulation experimenting later instruments and equipment(OPHELIE) surface mock-up test—a Belgian reference repository concept for high level radioactive waste disposal，an exhaustive experimental program under oedometer conditions was carried out to characterize the thermo-hydro-mechanical(THM) response of the backfill material used. This material，pre-compacted in high-density blocks，was made up of 60% FoCa clay，35% sand and 5% graphite in a dry mass basis. Besides a brief description of the temperature and suction controlled equipment used，selected experimental results related to water retention properties，compressibility on loading at different total suctions and temperatures，as well as swelling capacity and collapsibility on soaking and heating，are also presented and interpreted.

Thermal gradients during the early，non-isothermal period of near-field evolution in a KBS–3 repository for nuclear wastes could alter the mineralogy of the bentonite buffer and cause the constituent clay particles to become cemented together by mineral precipitates. The present study evaluates a reactive-transport model of buffer cementation using published experimental results on the hydrothermal alteration of clay buffers embedded in crystalline rocks. Model predictions are qualitatively consistent with experimental observations of regions of the buffer exposed to the highest temperatures，which indicate precipitation of secondary phases(anhydrite ± amorphous silica(opal-CT) ± calcite) and alteration of Na-smectite to Ca-smectite ± saponite. The model does not predict extensive dissolution of quartz，kaolinite and feldspars in this region，however，which conflicts with observations from one of the field tests. This disagreement may be due to the fact that the hydrothermal transformation of high-charge smectites to low-charge smectites in kaolinite-smectite mixed layers cannot presently be accommodated in the model due to a lack of basic thermodynamic data. The otherwise good qualitative agreement overall between model predictions and experimental observations suggests that the model can presently be used to carry out sensitivity analyses of the potential for buffer cementation to occur in the KBS–3 near field using expected time-temperature and resaturation histories extending over time scales of hundreds for this system of years.

Deep geological disposal of radioactive waste is considered worldwide as the most safe and feasible method to protect human being and the environment for tens of thousands years. A number of disposal concepts have been developed in many countries on the basis of a multi-barrier system, which comprises the natural geological barriers and the engineered barriers. In order to prove the disposal concepts，a great number of full-scale experiments have been performed at underground research laboratories constructed in different geological formations(rock salt，crystalline，argillaceous and tuff rocks). Besides of the development of adequate techniques for construction，disposal operation and backfilling/sealing of repositories，very comprehensive knowledge about geological，hydrogeological，geochemical and geomechanical properties of the host rocks at potential repository sites as well as deep understanding of coupled thermo-hydro- mechanical-chemical(THMC) processes occurring in the multi-barrier have been achieved. Typical disposal concepts and the associated in-situ experiments are represented.

The paper summarises the studies performed in Spain as regards the selection and characterization of clays suitable for sealing and backfilling of radioactive waste repositories. This research began in the 1980s under the auspices of ENRESA，the Spanish agency for nuclear waste management，and started by a survey of apt clay deposits and suppliers. The characterization of the clays and the criteria followed for their further selection were those already accepted by the international community：mineralogical purity，retention properties，plasticity，low permeability，high swelling pressure and thermal conductivity. These initial studies resulted in the selection of two bentonite deposits，whose detailed characterizations were carried out by several laboratories. These included the determination of thermo-hydro-mechanical properties，and of the impact of pre-heating，temperature and addition of quartz on these properties. The high expandability and low permeability of these materials led to the modification of the available experimental techniques and to the design of new equipment. The Cortijo de Archidona deposit(Almería) was finally selected and the bentonite taken there has been the object of various research projects that have ended in this bentonite being one of the best characterised from the mineralogical，thermal，hydraulic，mechanical，geochemical and alterability points of view. Besides，the behaviour of this bentonite under the conditions of a repository has been studied at laboratory and natural scale.

The Radioactive Waste Management Funding and Research Center(RWMC) in Japan has been conducting (research and development) for the disposal of radioactive waste since its establishment in 1976. More recently，RWMC has focused on important sociological and technical issues for geological disposal，such as monitoring，remote-handling and site investigation technology. Among these issues，the evaluation of gas migration from a repository is important for the development of more reliable safety assessments. This issue has been studied in a large scale in-situ experiment，the“gas migration test(GMT) in engineered barriers”at Nagra¢s Grimsel test site(GTS) in Switzerland. GMT has focused on issues related to waste-generated gas migration through the engineered barriers in a silo-type disposal. A reduced scale EBS(engineered barrier system) consisting of a concrete silo with a permeable gas vent and bentonite/sand buffer has been emplaced and instrumented in a specially constructed silo cavern at the GTS. The concrete silo was 2.5 m in height and 2.5 m in diameter and was located in an approximately 4 m diameter shaft excavated from the upper cavern. After saturation of the EBS and water permeability tests，gas was injected into the silo during two main injection periods. After the first gas injection，repeat water permeability tests were performed to identify changes in EBS performance. The second gas injection used a cocktail of gas tracers which were sampled from multiple points in the EBS and geosphere. Following the end of gas injection，the EBS was carefully excavated and characterized using a range of techniques.

Attentions have been devoted in recent years to the behaviors of highly compacted bentonites used in engineered barriers for high level radioactive nuclear waste disposal. Througth a nationwide screening and comparison，Gaomiaozi(GMZ) bentonite deposit located in Xinghe County，Inner Mongolia，is considered as the first choice of Chinese buffer materials for high-level radioactive waste geological repository. A laboratory study has been carried out on compacted GMZ bentonite，including the measurement of soil-water characteristic curves(SWCC)，the environmental scanning electron microscopy(ESEM) test，and the mercury intrusion porosimetry(MIP) test. The osmotic technique and vapour equilibrium technique were used for controlling suction. Comparison between the SWCCs of GMZ bentonite obtained from the MIP test results and the measured data under confined conditions is also made. Water retention properties of the GMZ bentonites under different suctions are related closely with to microscopic mechanisms.

The deep geological disposal is regarded as the most reasonable and effective way to safely dispose high-level radioactive wastes(HLW) in the world. The conceptual model of HLW geological disposal in China is based on a multi-barrier system that combines an isolating geological environment with an engineered barrier system including the vitrified HLW, canister, overpack and buffer/backfill material. The bentonite is selected as base material of the buffer/backfill material in HLW repositories，due to the very low permeability and excellent retardation of nuclides from migration，etc. GMZ deposit is selected as the candidate supplier for buffer material of HLW repositories in China. Since 2000，systematic study was conducted on GMZ–1 that is Na-bentonite produced from GMZ deposit and selected as reference material for Chinese buffer material study. The mineral composition，basic parameters of GMZ–1 bentonite and thermal conductivity，hydraulic conductivity，unconfined compression strength as function of dry density and water content are presented. The swelling stress of GMZ–1 bentonite as function of dry density is also reported. GMZ–1 bentonite is characterized by high content of montmorillonite(about 75%) and less impurities. The adequacy understanding of property and long-term behavior in deep geological condition of GMZ–1 is essential to safe dispose the high-level radioactive wastes in China.

Safe disposal of high level radioactive waste is a challenging task facing the scientific and technological world. This paper introduces the latest progress of high level radioactive waste disposal programs in the world，and discusses the key scientific issues as follows：(1) the precise prediction of the evolution of a repository site；(2) the characteristics of deep geological environment；(3) the behaviour of deep rock mass，groundwater and engineering material under coupled conditions(intermediate to high temperatures，geostress，hydraulic，chemical，biological and radiation process，etc)；(4) the geochemical behaviour of transuranic radionuclides with low concentration and its movement with groundwater；and (5) the safety assessment of disposal system. Several large-scale research projects and several hot topics related with high-level waste disposal are also introduced.

高放废物；地质处置；地质；热–水–力耦合

Rock mass mechanics and seepage characteristics of the high level radioactive waste disposal candidate site—Beishan，Gansu Province，are studied with field joint survey，geostress test，high-pressure hydraulic test and rock mechanic test in laboratory. The study indicates that the most of rock mass joints in Beishan，Gansu Province，are with shear joint with steep angle of inclination. Distribution of joint¢s inclination direction can be effectively fitted by normal function，and the distribution of joint¢s aperture can be effectively fitted by negative exponential function. Rock of Beishan，Gansu Province，is of high density，low water content，low permeability，and low porosity. Rock near the ground and tonalite is not uniform，and the granite 300 meters below the ground is uniform with uniaxial compression strength and Young¢s module. The maximum principal horizontal stress is 17.52 MPa，and the minor principal horizontal stress is 11.12 MPa，which show that it is under a moderate geostress site. The coefficient of permeability of rock mass is about 10－7–10－5 cm/s with a low permeability. Codes of permeability tensor calculation and 3D joint simulation are programmed，where the seepage characteristic and 3D joint distribution can be visualized.

Geological disposal of radioactive wastes is a multi-disciplinary issue of importance for national interest. It stimulated many challenging scientific and technical issues，and at a higher level，presented a series of demanding requirements for a country¢s overall research and development programme，its implementation and engineering practice，about basic policies and legislature concerning nuclear energy，defense，waste management and environment. Rock mechanics and rock engineering are very important fields for geological disposal of radioactive wastes，and contribute significantly to the conceptual design，site investigation，engineering design and construction，operation and the long-term safety assessment of the waste repositories. It plays，therefore，a irreplaceable role in the research and development programme of geological disposal of radioactive wastes. In this paper，we first summarizes briefly the main steps about repository system，followed by the major demands for rock mechanics and rock engineering during feasibility study and site investigation，and the major international trends concerning these issues. The focus is placed on the coupled thermo-hydro-mechanical and chemical(THMC) processes and the current status of research in international communities. At the end，the progresses in research and development works in the field of radioactive waste disposal in China are presented；and possible future working directions are discussed.

In China，the issue of nuclear waste disposal was started in 1985，but without paying significant attention and therefore the progress has been quite limited. Recently，with the impressive economic boom in China，energy shortage has become a more and more serious problem. In this study，nuclear energy is considered as one of the alternative solutions. Obviously，the issue on nuclear waste disposal becomes important actuality. In this paper，the research state in China about the bentonite as engineered barrier material was first presented. As a whole，the selection of Gaomiaozhi(GMZ) bentonite as buffer material has been performed，and its physicochemical characteristics have been identified. No thermo-hydro-mechanical(THM) tests have been carried out yet. Considering that significant work has been performed over the world on other candidate bentonites，it seems to be of benefit to have an overview on the recent advances in order to better orientate the future research work on GMZ bentonite. The main aim is therefore to provide some helpful experience and perspectives for the study on geological disposal of nuclear waste in China. Firstly，the disposal method is presented by using bentonite-based engineered barrier. It corresponds to the widely accepted concept，and no further research seems to be needed on this question. Secondly，the selection criteria of bentonite，its geochemical properties，thermo-hydro-mechanical behaviors including the relationship between microstructure and macroscopic behaviour，as well as the joint behaviour and numerical analysis，influence of gas generation during radioactive waste disposal on the engineered barrier are introduced. Different examples are given to illustrate either the experimental principle or the main results obtained. As mentioned above，only the issue on bentonite selection has been dealt with in China；and other issues should be studied with energy. Finally，some recent achievements are summarized and some perspectives are given.

Salt rock is considered as a perfect medium for underground oil and gas storage. Normally，underground salt caverns for gas storage are formed by aqueous fusion method according to specific design. Presently，there is no underground natural gas storages operation in abandoned salt caverns in the world. Based on numerical results with finite element method(FEM) code ABAQUS，the creep behaviors of caverns and rock pillars and the range of creep damage of the cavern roof are approached. Furthermore，the working pressure for gas storage in abandoned salt caverns and location of casing shoes are discussed. The conclusions drawn from the study can provide useful reference to the practical engineering.

This paper firstly introduces the advance and development of international consortium of DECOVALEX (development of coupled thermo-hydro-mechanical(THM) models and their validation against experiments) project. A general summarization on the study of progress in mechanical-hydro-chemical coupling process in rocks is presented，followed by a detailed introduction to the progress of DECOVALEX Task_B and Institute of Rock and Soil Mechanics，Chinese Academy of Sciences(CAS). Efforts of CAS are focused on uniaxial and triaxial compression tests，meso-mechanical tests and CT tests under the coupled stress-chemical condition. Based on the influences of chemical corrosion on mechanical and failure properties of rocks，the fracture criteria of multi-crack with chemical corrosion are proposed. Furthermore，a new numerical model named elastoplastic cellular automata for modeling rock failure process is set up by CAS，in which heterogeneity and flaws in rocks can be easily considered，and some typical behaviors of rocks such as Kaiser¢s effect and Class II curve are successfully modeled using elastoplastic cellular automata. Another cellular automata model for modeling coupled hydro-mechanical process for porous materials is also proposed，and the influence of water pressure on mechanical properties of rock and its meso-mechanism are analyzed using this model. The above-mentioned advance greatly promotes the research on mechanical-hydro-chemical coupling process in rocks.