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 bufferbackfill 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.Gold Open Access: This article is published under the terms of the CC-BY 3.0 licenseThe final disposal of nuclear spent fuel in crystalline bedrock is developing in Finland. Olkiluoto bedrock was proposed as the site for the repository (Posiva 2013). The disposal is based on the use of multiple release barriers, which ensures that the nuclear spent fuel cannot be released into the biosphere or become accessible to humans. The release barriers include the physical state of the fuel, the disposal canister, the bentonite buffer, the backfilling of the tunnels and the surrounding rock.The KBS-3V concept (Posiva 2013) consists of deposition holes spaced between 7.5 and 11 m of each other in backfilling tunnels excavated at a depth of approximately 400 -450 m and located at approximately 25 m from each other (Fig. 1). The spent nuclear fuel will be encapsulated in final disposal canisters made of cast iron and enclosed in a copper shell. These canisters will be placed in the deposition holes and surrounded with bentonite clay, which protects the canister from any potential jolt in the bedrock and slows down the movement of water in the proximity of the canister.

This paper describes an experimental programme and the corresponding modelling in order to characterise the hydromechanical (HM) behaviour of pellets. At least two levels of porosity can be distinguished in the pellets: microporosity and macroporosity. In general, the microstructure is associated with the active clay minerals, while the macrostructure accounts for the larger-scale structure of the material. In pellets, this concept can be somewhat different as a pellet arrangement creates a large-scale macropore structure. The Barcelona expansive model is proposed to reproduce the material response through infiltration and oedometer tests. In addition to expansive model parameters, a set of hydraulic laws considering the double-structure phenomenon is necessary to perform HM analysis. This paper also describes the effect of macroporosity on the intrinsic permeability changes. To illustrate the general objectives of this research, a full-scale thermohydromechanical calculation has been done to show the capabilities of the approach. The Barcelona basic model (for clays) and Barcelona expansive model (for pellets) have been used in this full-scale calculation.

The main objective of this thesis is to achieve an improved understanding of the thermo-hydro-mechanical processes and material properties that affect how the gaps and canister) behave during and after installation in the repository. The generated models and methodologies developed in this thesis have provided a deeper understanding of the THM processes taking place in a radioactive spent fuel disposal system and offered strategies for design improvement, material choice and optimization. The Thesis focuses on: - Material characterization (laboratory testing and numerical simulations of these tests), - Thermal dimensioning of repository (fixing canister and tunnel spacing, defining a power decay function for canister, adopting thermal boundary conditions), - 2D THM sensitivity analyses (developing a better understanding of the modelled system, several cases have been studied throughout the thesis), -3 D THM modeling (investigating the effect of variable gas pressure on the thermo-hydro-mechanical results). One of the main contributions of the thesis is to combine comprehensive and complex models to perform the calculations of a single deposition scheme: - BBM (Barcelona Basic Model) to represent clay buffer, BExM (Barcelona Expansive Model) to represent pellet-based components, combined with elasticity to represent rock and canister. - Porosity-dependent permeability and water retention curve (macro-porosity dependent in case of pellets using BExM). - Thermal conductivity depending of degree of saturation. - Gap-specific THM modelling under simplifying assumptions but capturing effects like thermal conductivity that may produce an early peak of temperature, or specific retention curve, which produces extreme, drying near the canister and gap closure that affects swelling pressure development. - Full scale 3D THM modelling with elasto-plastic parameters (BBM) is also an important contribution. The laboratory tests conducted for characterization of materials include: water retention curve tests, thermal conductivity tests, infiltration tests, oedometer tests and tortuosity tests. In general, satisfactory agreement between numerical and measured results is achieved. The majority of 2D sensitivity (fracture position, salinity of the inflow water, rock permeability, filling material between buffer and rock, artificial wetting of pellets etc.) cases show a behavior in safety margins in terms of temperature, density and stresses. A simplified 3D geometry has been adopted for THM calculations to check effect of third dimension. 3D calculations also include a sensitivity analysis. It has been shown that the full saturation of system components is delayed slightly when the air mass balance equation is considered, in other words, a variable gas pressure is taken into account. 3D THM simulations of full scale FISST test (a real scale in situ test will be performed in Onkalo research facility) is considered as a future work to validate, optimize and have better understanding the models and parameters used in the thesis. El objetivo principal de esta tesis es lograr una mejor comprensión de los procesos termo-hidro-mecánicos y las propiedades del materiales que afectan la forma en que se comportan los componentes del sistema de almacenamiento de los residuos nucleares (barrera arcillosa, relleno, pellets, rocas, huecos y contenedor) durante y después de la instalación en el depósito de residuos. Los modelos y metodologías desarrollados en esta tesis han proporcionado una comprensión más profunda de los procesos de THM que tienen lugar en un sistema de almacenamiento de residuos nucleares y ofrecen estrategias para mejorar el diseño, la selección de materiales y la optimización. La tesis se centra en: - Caracterización de los materiales (ensayos de laboratorio y simulaciones numéricas de estos ensayos), - Dimensionamiento del repositorio (fijación del espaciado entre túneles, definición de una función para la potencia de contenedor con el combustible gastado, adopción de condiciones de contorno térmico), - Análisis de sensibilidad 2D THM (desarrollando una mejor comprensión del sistema modelado, se han estudiado varios casos a lo largo de la tesis). -3D modelo THM (que investiga el efecto de la presión de gas variable en los resultados termo-hidromecánicos). Una de las principales contribuciones de la tesis es combinar modelos completos y complejos para realizar los cálculos de un esquema de repositorio único: - BBM (Barcelona Basic Model) para representar el comportamiento de arcilla no saturada, BExM (Barcelona Expansive Model) para representar componentes como pellets, combinado con modelos de elasticidad para representar roca y el contenedor. - Permeabilidad y curva de retención dependientes de la porosidad (macro-porosidad en el caso de pellets que usan BExM). - Conductividad térmica función del grado de saturación. - Un modelo bi-elastico para representar los huecos. El modelo captura efectos como la conductividad térmica que puede producir un pico temprano de temperatura o curva de retención específica, que produce un secado extremo cerca del contenedor y cierre de huecos, lo que afecta el desarrollo de la presión de hinchamiento. - El modelo de 3D THM a escala real con parámetros elasto-plásticos (BBM) es también una contribución importante. Los ensayos laboratorios realizados para la caracterización de materiales incluyen:ensayo de curva de retención, ensayo de conductividad térmica, infiltración, edómetro y el ensayo de tortuosidad. En general, se logra un acuerdo satisfactorio entre los resultados numéricos y medidos. La mayoría de las analisis de sensibilidad (posición de fractura, salinidad del agua, permeabilidad de la roca, materiales distintos de relleno, condiciones iniciales diferentes para los materiales) muestran un comportamiento en márgenes de seguridad en términos de temperatura, densidad y tensiones. Se ha adoptado una geometría 3D simplificada para los cálculos de THM para verificar el efecto de la tercera dimensión. Los cálculos 3D también incluyen un análisis de sensibilidad. Se ha demostrado que la saturación de los componentes del sistema se retrasa ligeramente cuando se considera la ecuación de balance de masa de aire, en otras palabras, se tiene en cuenta una presión de gas variable. Las simulaciones en 3D de THM del ensayo de FISST a escala real (se realizará un ensayo in situ en la instalación de investigación de Onkalo, Finlandia) se considera un trabajo futuro para validar, optimizar y comprender mejor los modelos y parámetros utilizados en la tesis.