AbstractsEarth & Environmental Science

Transport of water, vapour, heat and solutes in concrete for storing radioactive waste

by true

Institution: TDX
Year: 2016
Posted: 02/05/2017
Record ID: 2077809
Full text PDF: http://hdl.handle.net/10803/393885


Cementitious materials are used as barriers in radioactive waste storage. Hence, transport processes in this kind of material are important. The objective of this thesis is to improve the understanding of transport of water (liquid and gas), heat and solutes in concrete. This is applied in the concrete from the low-and intermediate- level radioactive waste facility at El Cabril (Southern Spain). To do so, the following methodology was used. First, evaporation tests in concrete columns have been simulated in order to obtain thermo-hydraulic parameters. The conceptual model considers unsaturated liquid flow and transport of vapour and energy. The retention curve was estimated from relative humidity and gravimetric water content at the end of the test. Relative permeability, thermal conductivity and tortuosity factor for vapour diffusion were obtained by calibrating the numerical model. Results show that the vapour diffusion is the dominant water transport process above an evaporation front and liquid advection is dominant below it. Second, numerical models simulating the processes that take place inside the concrete cells have been carried out. Temperature and relative humidity measured by sensors in the cells and thermo-hydraulic parameters from laboratory tests have been used. Results show that temperature oscillations outside the cell create a temperature difference between the two sides of an air gap between the concrete containers and the wall of the cell. Water rises from the phreatic level into the wall of the cell through capillary rise. Water evaporates at the hot side (wall of the cell in summer and containers in winter) and diffuses as vapour from the hot to the cold side. Condensation is produced at the cold side. Consequently, water runs off to the drain. In order to avoid this phenomenon, various scenarios have been studied. Third, a laboratory-scale tracer test in concrete has been carried out using a high entry pressure. The conceptual model considers matrix diffusion between a mobile pore domain, were water can flow, and an immobile zone with only diffusion. Three geometries have been compared, considering the immobile zone as slabs, spheres or tubes. Porosity of the mobile zone and characteristic time were estimated by calibrating the model results to the measured breakthrough curves of deuterium and bromide. The calculated values show that the characteristic time depends on the geometry, and a similar porosity of the mobile zone was estimated for all geometries. Bromide behaviour could not be reproduced even when linear retardation was applied. Finally, reactive transport models of concrete has been applied in order to study the changes in mineralogy produced during the performance of the tracer test. All minerals are considered in the immobile zone. The cement paste consists of alite, belite, gypsum, calcite, C-S-H gel, portlandite and ettringite. The aggregates are composed of quartz. Overall model results show mineral dissolution of alite, belite, gypsum and quartz and precipitation of C-S-H…