Norbert Maes

Concrete in Engineered Barriers for Radioactive Waste Disposal Facilities: Phenomenological Study and Assessment of Long Term Performance

The paper aims to highlight recent developments at the Belgian Nuclear Research Centre SCK.CEN in experimental and numerical study of the coupled physical-chemical behaviour of concrete subject to chemical degradation. The discussion mainly focusses on three interlinked research projects covering novel experimental methods to study the alteration of hydraulic and transport properties during carbonation and calcium leaching, a pore scale numerical model to capture microstructural changes due to the above degradation processes and a generic multiscale model aimed at determining evolution of the properties of a macrostructure over the long term. The paper also describes supplementary continuum scale numerical studies concerning concrete-clay interactions and geochemical impact on the physical structure of concrete. Preliminary findings from these studies show encouraging results such as the development of novel leaching, water permeability and diffusion apparatus, a robust pore scale model based on Lattice-Boltzmann method and a mesoscale study focused on the importance of interfacial transition zones on the effective diffusivity for linear and nonlinear diffusion problems.

Evolution of Microstructure and Transport Properties of Cement Pastes Due to Carbonation under a CO2 Pressure Gradient—A Modeling Approach

Most carbonation models only account for diffusion as the main transport mechanism rather than advection. Nevertheless, in the case of concrete used for underground waste disposal facilities, concrete may be subjected to a high hydrostatic pressure and the surrounding environment may contain a high dissolved CO2 concentration. Therefore, a combination of diffusion and advection should be taken into account. This is also the case in accelerated carbonation where a high CO2 pressure gradient is applied in which advection in the gas phase has a significant contribution to the carbonation process. This study aims at developing a model to predict the evolution of the microstructure and transport properties of cement pastes due to carbonation under accelerated conditions in which a pressure gradient of pure CO2 is applied. The proposed model is based on a macroscopic mass balance for carbon dioxide in gaseous and aqueous phases. Besides the prediction of the changes in transport properties (diffusivity, permeability), the model also enables to predict the changes in microstructure. Data from accelerated tests were used to validate the model. Preliminary verification with experimental results shows a good agreement.

Current Concerns on Durability of Concrete Used in Nuclear Power Plants and Radioactive Waste Repositories

Nuclear power, to most of us, is mystic and somehow scary, and despite its drawbacks, is still playing an important role in the world wide energy supply. However concrete, without mystery as the most widely used materials in construction, is used as a major constituent for nuclear facilities such as radioactive waste repositories and nuclear power plants. Concrete is the only practical material offering a number of advantages including sufficient shielding against the dangers of radiation, good compressive strength, low cost, easy building, and retention of radionuclides limiting their dissipation. The assessment of the long-term durability of such concrete structures is of utmost importance and urgently needed as our knowledge on concrete durability beyond the basis of an expected several decade service life is limited. Within its service environment, these structures undergo chemical degradation processes which are very slow but they significantly change the physical integrity and the chemical conditions of the structures with the passage of time. Current issues on durability of these concrete structures include alkali-silica reaction, delayed ettringite formation, leaching, carbonation, etc. which might be magnified under severe/accelerated conditions (high temperature, radiation, moisture, cyclic loading, and acidic environments). These degradations induce an evolution of the microstructure, cracking and changes in transport properties of concrete which are still unclear due to the limited experimental timeframe available to capture these processes. This paper presents an overview on these concerns with the focus on the long-term chemical degradation aspect and presenting a case study on Ca-leaching.