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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]

Like other nuclear countries, Lithuania is facing the problem of spent nuclear fuel (SNF) disposal from RBMK 1 1 “RBMK” is a Russian acronym for “Channelized Large Power Reactor”. -1500 type reactors at the Ignalina NPP (~22,000 of the SNF assemblies). The application of probabilistic and deterministic methods for uncertainty assessment and important parameter ranking is of high importance in order to build confidence in the assessment results on radionuclide migration and indicates directions for further uncertainty reduction. This study is devoted to the uncertainty and sensitivity analysis of radionuclide migration through the engineered barriers of a conceptual geological repository for RBMK-1500 SNF disposal. The analysis was performed using computer codes AMBER (United Kingdom) and MATLAB (Sweden). The analysis showed that the uncertainty of defect enlargement time had no significant impact on 129 I and 226 Ra peak flux from the engineered barriers yet it was an important parameter for time dependent mean flux of 129 I. Sensitivity analysis revealed that in the case of 129 I peak flux, further investigations should be oriented to more precise determination of diffusion coefficient in bentonite, instant release fraction and SNF dissolution rate. In the case of 226 Ra, efforts to reduce the uncertainty of SNF dissolution rate, sorption coefficient in bentonite and equivalent flow rate should be made. [ABSTRACT FROM AUTHOR]

Approximately 22 600 spent nuclear fuel (SNF) assemblies originating firm the RBMK-1500 reactor of the Ignalina nuclear power plant in Lithuania need to be managed and disposed of safely. Generic investigations of RBMK-1500 SNF disposal options in Lithuania were initiated. This paper presents insights on RBMK-1500 SNF disposal in crystalline rocks gained during participation in the International Atomic Energy Agency Coordinated Research Project "The Use of Numerical Models in Support of Site Characterization and Performance Assessment Studies for Geological Repositories," as well as in the Lithuanian Science Development Program. The research was focused on the analysis of disposal behavior of different SNF types under generic geological conditions and for a one-canister defect scenario with two different corrosion rates. A comparison of peak fluxes from the near field for Lithuanian RBMK-1500 and Swedish boiling water reactor SNF revealed differences that are not directly proportional to the differences in SNF inventory. [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]

There are two RBMK-1500 type graphite moderated reactors at the Ignalina nuclear power plant in Lithuania, and they are under decommissioning now. The graphite cannot be disposed of in a near surface repository, because of large amounts of 14 C. Therefore, disposal of the graphite in a geological repository is a reasonable solution. This study presents evaluation of the 14 C transfer by the groundwater pathway into the geosphere from the irradiated graphite in a generic geological repository in crystalline rocks and demonstration of the role of the different components of the engineered barrier system by performing local sensitivity analysis. The speciation of the released 14 C into organic and inorganic compounds as well as the most recent information on 14 C source term was taken into account. Two alternatives were considered in the analysis: disposal of graphite in containers with encapsulant and without it. It was evaluated that the maximal fractional flux of inorganic 14 C into the geosphere can vary from 10 − 11 y − 1 (for non-encapsulated graphite) to 10 − 12 y − 1 (for encapsulated graphite) while of organic 14 C it was about 10 − 3 y − 1 of its inventory. Such difference demonstrates that investigations on the 14 C inventory and chemical form in which it is released are especially important. The parameter with the highest influence on the maximal flux into the geosphere for inorganic 14 C transfer was the sorption coefficient in the backfill and for organic 14 C transfer – the backfill hydraulic conductivity. [ABSTRACT FROM AUTHOR]