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As in any other industry, nuclear energy results in the accumulation of some waste, which needs to be managed safely and responsibly due to its radiotoxicity. In the case of highly radioactive waste, geological disposal in stable rock is considered a broadly accepted solution. For the evaluation of the long-term safety of a geological repository, the assessment of radionuclide transport needs to be carried out. Radionuclide transport through engineered and natural barriers of the repository will highly depend on the barriers' transport-related properties, which will be determined by coupled thermal, hydraulic, chemical, mechanical, biological, and radiation processes taking place in those barriers. In this study, the thermo-hydro-chemical (THC) state of bentonite was analysed considering CO2 gas diffusion and temperature-dependent solubility in water. Reactive transport modelling of bentonite under non-isothermal conditions was performed with the COMSOL Multiphysics software (v6.0), coupled with the geochemical solver Phreeqc via the iCP interface. The modelling demonstrated that the consideration of chemical processes in bentonite had no significant influence on non-reactive Cl− transport; however, it would be important for other radionuclides whose sorption in porous media depends on the porewater pH. Based on the modelling results, changes in the bentonite mineralogical composition and, subsequently, porosity depend on the partial CO2 pressure at the bentonite–granite boundary. In the case of low CO2 partial pressure at the bentonite–granite interface, the calcite dissolution led to a slight porosity increase, while higher CO2 partial pressure led to decreased porosity near the interface. [ABSTRACT FROM AUTHOR]

The geological disposal of high-level radioactive waste is the final step in the nuclear fuel cycle. It is realized via isolating the high-level radioactive waste in the geological environment with an appropriate system of engineered barriers. Radionuclides-containing materials must be isolated from the biosphere until the radioactivity contained in them has diminished to a safe level. In the case of high-level radioactive waste, it could take hundreds of thousands of years. Within such a long timescale, a number of physical and chemical processes will take part in the geological repository. For the assessment of radionuclide migration from a geological repository, it is necessary to predict the repository's behavior once placed in the host rock as well as the host-rock response to disturbances due to construction. In this study, the analysis of repository barriers (backfill, concrete, inner excavation disturbed zone (EDZ), outer EDZ, host rock) thermo–hydraulic–mechanical (THM) evolution was performed, and the scope of gas-induced desaturation was analyzed with COMSOL Multiphysics. The analysis was based on modelling of a two-phase flow of miscible fluid (water and H2) considering important phenomena such as gas dissolution and diffusion, advective–diffusive transport in the gaseous phase, and mechanical deformations due to thermal expansion of water and porous media. The importance of proper consideration of temperature-dependent thermodynamic properties of water and THM couplings in the analysis of near-field processes was also discussed. The modelling demonstrated that such activities as 50 years' ventilation of the waste disposal tunnel in initially saturated porous media, and such processes as gas generation due to corrosion of waste package or heat load from the waste, also led to desaturation of barriers. H2 gas generation led to the desaturation in engineered barriers and in a part of the EDZ close to the gas generation place vanishing soon after finish of gas generation, while the host rock remained saturated during the gas generation phase (50–100,000 years). Radionuclide transport properties in porous media such as effective diffusivity are highly dependent on the water content in the barriers determined by their porosity and saturation. [ABSTRACT FROM AUTHOR]

Heat load from high-level radioactive waste (HLW) packages will result in elevated temperatures around the disposal tunnel. Differences in thermal expansion between rock and water will induce the redistribution of stresses. The assessment of the thermo-hydromechanical (THM) regime is necessary to evaluate the potential for fracture development. For this purpose, it is important to evaluate the nature and extent of induced strains, and their impact on rock permeability, which, subsequently, is important for radionuclide transport. This paper presents the modeling activities of the Lithuanian Energy Institute performed in the framework of the European joint program EURAD and the analysis of the influence of temperature on clay-based material behavior. Within this study, different stress conditions and material properties (isotropic, anisotropic) were analyzed with a thermo-poroelastic material model for the Callovo-Oxfordian clay host rock in the 100 m × 100 m domain. The heat load on the clay rock comes from a tunnel with a radius of 1.25 m. The overall THM response of the clay host rock system to the heat load is performed with the COMSOL Multiphysics (Burlington, MA, USA) software. The THM response near the HLW disposal tunnel was analyzed in terms of temperature, pore pressure, displacements, and stresses, and the results are presented in this work. Besides the impact of anisotropy, the effect of hydraulic conditions at the tunnel boundary was also analyzed. The modeling results revealed that anisotropy in stress and properties had an impact on the hydro-mechanical response of the material even during excavation and waiting phases. The analysis also showed that the water drainage condition on the tunnel boundary had no effect on the thermal state around the tunnel, but it had a significant impact on the hydro-mechanical response. [ABSTRACT FROM AUTHOR]

Radioactive waste disposal, as the final step of the open nuclear fuel cycle, is an important process to protect humans and the environment from harmful effects of ionising radiation. Approaching the construction of the geological repository, the understanding and predictability of the behavior of engineered barrier material becomes more important than ever. Therefore, a number of research studies are being focused on the experimental and numerical analysis of the engineered barrier material state and behavior under repository conditions. Engineered barrier material will be in contact with the host rock and waste packages, and its properties and behavior will be governed by complex and coupled thermo-hydro-mechanical processes. This paper presents the modeling activities of the Lithuanian Energy Institute, performed in the framework of the H2020 project BEACON (Bentonite Mechanical Evolution). The numerical model, developed in COMSOL Multiphysics (Burlington, MA 01803, USA), was applied for the modeling of experiments, performed by Ecole Polytechnique Federale de Lausanne (EPFL, Switzerland), on granular MX-80 bentonite in the odeometer cell. The hydromechanical behavior of a compacted bentonite sample was analyzed under different conditions: hydration with groundwater under confined volume conditions and hydration under free swelling conditions and subsequent mechanical loading. Model outcomes (swelling pressure, saturation, dry density, and void ratio) were compared to the available experimental data. The modeling results were in line with the analyzed experimental data. [ABSTRACT FROM AUTHOR]

Coupled physical processes will take place in a multibarrier disposal system for spent nuclear fuel and high-level radioactive waste. The knowledge of these processes (thermal, hydraulic, mechanical, chemical, microbiological, etc.) as well as the scope and scale of their interactions is fundamental for the safety assessment of a disposal facility. Numerical modeling is an important component in the process of acquiring and deepening the knowledge of coupled processes, while experimental evidence isimportant for model validation. This article will present a hydro-mechanical model developed by the Lithuanian Energy Institute (LEI) in the framework of H2020 project BEACON (Bentonite Mechanical Evolution). The non-linear elastic model developed in COMSOL Multiphysics (Burlington, MA 01803, USA) was applied to predict the swelling behavior of large-scale oedometer experiments (MGR) performed by Research Centre for Energy, Environment and Technology (CIEMAT, Spain). In these experiments on bentonite hydration at isochoric conditions, a sample was made of two layers of calcium bentonite (FEBEX type) having initially different hydro-mechanical characteristics: one layer made of pellets and the other of a compacted block. Satisfactory agreement between the modeling results and the experimental data were obtained, especially for water intake and sample saturation. [ABSTRACT FROM AUTHOR]

As in other nuclear countries, the operation of the Ignalina nuclear power plant in Lithuania has led to the accumulation of around 22 thousand assemblies of spent nuclear fuel (SNF). The development of geological disposal program involves an iterative assessment of the system safety supported by scientific research on radionuclides migration and related processes. This study focused on the application of Contribution to the Sample Mean (CSM) and Contribution to Sample Variance (CSV) methods to complement the uncertainty and sensitivity analyses of the time-dependent flux of I-129 from the engineered barriers of a conceptual disposal facility for RBMK-1500 SNF (RBMK is abbreviation of "High Power Channel-type Reactor" (in Russian)). The analysis was performed using a MATLAB platform (8.0.0.783 (R2012b), MathWorks, MA, USA). The mean and variance ratios derived from CSM and CSV plots were applied to estimate the effect of reduced uncertainty range on mean flux and its variance, and the uncertainty analysis was also complimented. Increasing the lower bounding value of defect size enlargement time range to 4.6 × 104 years would lead to a lower mean flux until 5 × 104 years after repository closure. Later on (up to 1 million years after repository closure), the only reduction of the upper bounding value of the SNF dissolution rate range would affect a decreased mean flux. [ABSTRACT FROM AUTHOR]