Evolution of clay seals in deep boreholes: Case with compacted smectite-rich clay in air-dry and prewetted forms contained in perforated tubes. Theoretical modeling and experimental study
Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
Solid high level radioactive waste (HLW) and spent nuclear fuel are isolated from the environment by being stored in underground repositories. Boreholes from the ground surface into the host rock need to be sealed by installing clay/concrete plugs for preventing quick migration of possibly released radionuclides from the repository. Clay seals are installed where the rock is poor in fractures and has a low hydraulic conductivity, while concrete will be cast where the boreholes are intersected by fracture zones. The risk of unstable conditions in the early stage of their maturation should be taken into consideration. For the concrete seals, erosion before stiffening, and dissolution and loss of the cement component at later stages can weaken them. For the clay, dispersion and channeling of the early formed clay seals are the greatest risks. The thesis deals with evolution of clay seals in deep boreholes, focusing first on the determination of the rate of homogenization of clay plugs of standard type, i.e. with the clay blocks prepared by compaction of air-dry clay granules as well as granules pre-saturated to about 100 %. A second aim was to investigate the risk of piping of the clay formed between the perforated tube and the borehole walls by hydraulic gradients in the vertical direction of the borehole through flow tests for identifying critical states for piping and erosion. A third issue was to investigate the chemical interaction of clay seals and concrete cast on them in simulated boreholes.A fourth issue was to develop conceptual models for the maturation of clay seals and to make an attempt to work out theoretical physical models of the maturation process. The study demonstrated, in conclusion, that the maturation rate of the prewetted compacted clay was slower than the initially compacted air-dry sample and also that for clay gel with initial density of saturation 1050kg/m3 the critical hydraulic conductivity that causes erosion/piping is below 10 m/m (meter water per meter flow length).Finally, the report recommends that further study should concern: 1) Long-term behavior of borehole seals, and 2) Performance assessment analyses should be extended to consider a complete list of relevant features, events and processes, and to incorporate more detailed modeling of the performance of single and interacting boreholes.
Place, publisher, year, edition, pages
2012. , 36 p.
Technology, Swelling pressure, Air dry/Prewetted clay block, Smectite-rich clay, clay gel
IdentifiersURN: urn:nbn:se:ltu:diva-53267Local ID: a4c58912-dfa3-4b19-93ab-83916ef44e5eOAI: oai:DiVA.org:ltu-53267DiVA: diva2:1026641
Subject / course
Student thesis, at least 30 credits
Civil Engineering, master's level
Validerat; 20120613 (anonymous)2016-10-042016-10-04Bibliographically approved