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Wyoming-type bentonite is one of the materials to be used as part of the Engineered Barrier System (EBS) in deep geological disposal facilities for the safe disposal of spent nuclear fuel.In the KBS-3 type repository, the canisters containing the spent fuel assemblies are surrounded by bentonite buffer. Therefore, it is necessary to understand the bentonite behaviour in shearing conditions for the long-term performance assessment of the canisters. Bentonite shearing could arise, for example, due to potential post-glacial earthquakes.The deviatoric stress at failure and the shear modulus of a compacted Wyoming-type bentonite were measured using different shearing tests. Tests were carried out on unsaturated samples at three laboratories. The shearing tests on unsaturated samples were unconfined and triaxial compression tests, hollow cylinder tests and simple shear tests. Unconfined and triaxial compression tests were also carried out on saturated samples. The failure was quite ductile when tests were carried out with confining pressure.The samples were compacted at different dry densities and degrees of saturation and tested at different confining pressures and strain levels to study their influence on the shear properties. Shear strength and shear modulus were studied based on models that consider the double porosity structure of the bentonites. Previously published triaxial tests and resonant column test results were also included in this study.In combination with these models, several alternatives for the definition of the constitutive stress were checked for the case of bentonite. A novel, approach, involving the results of mercury intrusion porosimetry in estimating the constitutive stress, was introduced.The shear strength calculated by the models showed a good agreement with the measured values. In some cases, unrealistic large tensile stresses were predicted. Furthermore, the models can assess the tendency of the shear modulus as a function of the shear strain, degree of saturation and dry density.& COPY; 2023 Elsevier Ltd. All rights reserved.

The shear modulus (G) of two different bentonites was measured by means of a resonant column apparatus. The samples were compacted at different dry densities and degrees of saturation and tested with different confinement pressures and strains levels for studying the influence of these parameters on the shear modulus. The results show similar tendencies in both bentonites: the shear modulus increases as the dry density increases and exhibits maximum shear modulus when degree of saturation is around 80%. An empirical equation, taking into account the microstructure of the clays, is used to evaluate the shear modulus at small strains as a function of dry density and degree of saturation. Although the values of the shear modulus measured are similar in both bentonites for a given stress and degree of saturation, there is difference in the elastic strain limit of the soil. Bentonite clay is going to be part of the Engineered Barrier System (EBS) in deep geological disposal facilities for the long-term confinement of spent nuclear fuel. In order to fully understand their long-term performance, their behaviour in shearing conditions should be assessed.

This paper describes the implementation of the Bodner–Partom model for the analysis of copper corrosion barrier in canisters containing spent nuclear fuel. The Bonder–Partom constitutive model implemented in CODE_BRIGHT finite element code is compared with experimental results for validation and with the Stealth finite difference code for verification. Deep geological disposal is the most reliable alternative for the spent nuclear fuel storage. This disposal is based on the multiple barriers principle. Different barriers will prevent the release of the radionuclides and one of the barriers is the canister that will contain the spent fuel. The canister will be constructed mainly in cast iron for giving mechanical protection to the spent fuel with a copper shell for protecting the cast iron against corrosion. Due to the high strength of metals, the canister components will keep it inside the elastic region during the hydration of the buffer, so it has not been necessary to implement more accurate constitutive models for the simulation. The canister is considered more as a boundary condition rather than a component to be analysed when the hydration and swelling processes are assessed but in case of relative displacements between the lips of joints intersecting a deposition hole because of an earthquake induced by the increase of vertical load due to the accumulation of ice during a glaciation or any other tectonic effect, not likely in Baltic shield, the stresses in canister could be high enough for reaching plastic conditions and in this case, it is considered necessary to use more accurate constitutive models for the simulation of the canister–buffer system. This work has been supported by a Consortium composed by SKB (Sweden), Posiva (Finland), GRS (Germany) and ANDRA (France). The members of the steering committee of this consortium are Anders Sjöland (Chairman, SKB), Mika Niskanen (Posiva), Oliver Czaikowski (GRS) and Antoine Pasteau (ANDRA).

Swelling clays like bentonite are proposed as a buffer in high level nuclear waste repositories. An experimental device useful for thermo-hydraulic analysis of the samples behavior is outlined from room temperature to 373 K. The temperature distribution is established by axial heating of cylindrical samples. The spreading of the water content distribution is also determined from sample sections at the end of the run. An analytical model is build for steady states coherent with the experimental measurements. The transient temperature distribution and the final water content are also outlined via a numerical model. From experimental measurements the thermal conductivity value of the bentonite is 0.471–Sr·1.15SrWm–1 K–1, where Sr is the bentonite degree of saturation (volume of water/volume of pores).

The paper presents a general methodology to perform backanalysis of laboratory tests involving thermohydraulic behaviour of bentonite in a systematic manner. The procedure is based on a maximum likelihood approach that defines a probabilistic framework in which error measurements and the reliability of the parameters identified can be estimated. The method is applied to the identification of some thermal and hydraulic properties of a bentonite specimen, using temperature and water content measurements as input data. Three basic cases have been considered: (a) thermal case, identifying the global thermal conductivity, λ e , and the global specific heat, c e ; (b) hydraulic case, identifying the tortuosity factor, τ , and the exponent of the unsaturated permeability law, n ; and (c) thermohydraulic case, in which τ , n , and the saturated thermal conductivity, λ sat , have been estimated. A numerical code that solves the coupled thermohydraulic equations has been used as main computational tool. A detailed description of the experimental device and a discussion on the tests results and on the parameters identified are also included. The values obtained are within the normal range of these parameters, but the method provides a systematic and consistent procedure to find the best parameters that reproduce the measurements for the selected models. Also the method gives an insight into the model structure, and allows detecting dependence and coupling between parameters.

The authors would like to thank the discussers for their interest in our paper and their valuable comments. The discussers have highlighted our conclusion by providing experimental data related to the difficulty in controlling total suction when hastening the equilibrium condition of vapour transfer by using a magnetic stirrer in their reference salt solution reservoir. Their data complement the approach followed by the authors using numerical results, even though they apply a different experimental procedure — the authors use a forced convection system to facilitate vapour transfer. Nevertheless, the authors agree with the discussers that these hastening procedures could possibly result in air pressure and temperature differences created along the constant mass circuit that would induce variations in the relative humidity, which is supposedly controlled by the reference salt solution reservoir. With this being stated, the authors would also like to present some experimental results to provide further evidence of what they concluded using numerical simulations. Dueck (2004) studied the influence of air pressure changes in a forced convection circuit of vapour — driven by an air pump — and their consequences on the applied relative humidity. Figure R1 shows a scheme of the experimental setup and the evolution of differential air pressures between two points of the circuit (upstream and downstream of the filter stones that transfer vapour to or from the soil). The point upstream of the filter is near the reference reservoir (‘‘ref’’ in the figure), while the point downstream of the filter is close to the aspiration branch of the air pump (‘‘p’’ in the figure) and is subjected to a pressure drop. After vapour equalization, the pump is turned on and air is forced through the circuit. At 67 min, the speed of the pump is rapidly increased and is then reduced from 78 min on. The consequences on the evolution of the relative humidity at the same two points of the circuit are shown in Fig. R2. As observed, downstream point ‘‘p’’ near the pump undergoes important relative humidity changes due to air pressure drop, while upstream point ‘‘ref’’ is buffered by the reference salt solution reservoir. The mass rate transfer under isothermal conditions of vapour by convection with a known air flow rate has been expressed in terms of the differences in mixing ratio (mass of vapour per unit mass of dry air) between the reference salt solution reservoir and the soil (see for example, Jotisankasa et al. 2007). As a consequence, an expression to account for the effects of air pressure variations on the relative humidity can be proposed based on the assumption under vapour equilibrium and isothermal conditions that the mixing ratio, x, set by the reference salt solution is also set at another point of the constant mass circuit. Assuming vapour and dry air as ideal gases and at the same temperature, the following expression is obtained for the mixing ratio: x = Mmwuv/ (Mmdauda) = 0.622 uv/uda, where Mmw and Mmda are the molecular masses of water and the dry air mixture, respectively; uda is the dry absolute air pressure; and uv is the vapour pressure. The following expressions are obtained between the relative humidity reference reservoir, hr ref, set by the salt solution, and another point of the circuit, hr p

The KBS-3H disposal alternative is composed by horizontally placed supercontainers comprising the canisters with the spent nuclear fuel surrounded in both drift axis and radial directions by compacted bentonite blocks (buffer) enclosed in a perforated shell. The different internal gaps between the supercontainer components and the one between the buffer blocks and the host rock have direct effects on the buffer behaviour. This paper presents a Thermo-Hydraulic (TH) Three-Dimensional (3D) numerical model developed to analyse a particular geometry assuming three different gap state conditions and providing results of the temperature, liquid pressure, and evolution of the degree of saturation. The material parameters, constitutive models, and assumptions made were carefully selected with regards to laboratory measurements reported in directly-related bibliography. The modelling settles the importance of understanding the groundwater flow through the rock mass and from fractures in the rock in order to achieve reliable predictions regarding buffer saturation, since it is known that the saturation times could range from few years to one thousand years depending on the hydrogeological conditions in the rock. The obtained results lead to full saturation times of 50 to 100 years. In addition to the rock hydraulic conductivity and fracture transmissivity, the saturation process was directly affected by the material properties of the buffer and gap presence between the buffer blocks and the host rock. Finally, in connection with thermal evolution, the thermal conductivity of repository components and the behaviour of air gaps in the buffer were key variables.

This paper describes the thermohydromechanical (THM) simulation of engineered barrier systems (EBS) for the final disposal of nuclear spent fuel in Finland. The bentonite barriers were simulated with the Barcelona Basic Model and the model was calibrated from laboratory tests. The evolution of gap closure and the presence of a fracture intersecting the disposal were analysed. The simulations were performed in 2D axisymmetrical geometries. Full 3D simulations were carried out in order to check the effect of the third dimension. The time required for the barriers to reach full saturation, the maximum temperature, deformations and displacements at the buffer–backfill interface and the homogenization of components both locally and globally are the main interests. The effect of rock fracture and the hydraulic conductivity of the rock are subjected to 2D sensitivity analyses.