AbstractsChemistry

Bentonite is planned for use as a buffer material in high-level radioactive waste repositories, where safety assessment is performed for very long periods (100-1000 ka). This thesis focuses on the swelling of smectites in liquid water, and analysis of bentonite from field experiments at Äspö Hard Rock Laboratory, Sweden. Four field experiments were analyzed (Alternative Buffer Material experiment, ABM1, ABM2; Temperature Buffer Test, TBT; and Prototype) with focus on Fe- redox chemistry and formation of trioctahedral smectite. The techniques used were mainly synchrotron X-ray diffraction and X-ray absorption spectroscopy. In ABM1 and Prototype the Fe(II)/Fe-total ratio had increased. In TBT no significant increase in Fe(II) was found; instead the corrosion products were dominated by Fe(III). Formation of trioctahedral clays was found in the iron-bentonite experiments (ABM1, ABM2, TBT), but not in Prototype where the heater instead was of copper. In swelling experiments, Ca-Wyoming montmorillonite was shown to expand and partly form a four-water-layer hydrate at lower temperatures in water. This was studied in more detail, and the influence of divalent interlayer cation, temperature, layer charge, salt and irradiation was investigated. Among the investigated smectites, decreased temperature increased the crystalline swelling until ice was formed. Lower smectite layer charge increased the crystalline swelling. Increasing the Gibbs hydration energy of the divalent interlayer cation increased the crystalline swelling. Introduction of salt in the water partly dehydrated the montmorillonite at 20°C, but minimized the dehydration of montmorillonite upon ice formation at low temperatures (-50, -100°C), especially with CaCl2. It was found that in a gradient of ethylene glycol and water a 21 Å basal distance was formed in the montmorillonite, which was higher than in the pure liquids. I Sverige produceras en stor del av elektriciteten i kärnkraftverk och som restprodukt får man ett mycket farligt radioaktivt avfall som måste isoleras från människor i hundratusentals till miljoner år. Svensk Kärnbränslehantering AB (SKB) har i uppdrag att ta hand om detta avfall och har utvecklat metoden KBS-3. I denna placeras det använda kärnbränslet i en kopparkapsel som deponeras på cirka 500 meters djup i berget. Mellan kapsel och berg placeras kompakterad bentonitlera som kemiskt och fysiskt skyddar kapseln och dessutom fördröjer spridningen av eventuella radionuklider om en kapsel skulle gå sönder. Bentonitens svällande egenskaper kommer från dess innehåll av mineralet montmorillonit, vilket är ett svällande lermineral (smektit). Montmorillonit består av negativt laddade skikt staplade på varandra med positiva laddningskompenserande joner mellan varje skikt (huvudsakligen Na+ och Ca2+). Vid kontakt med vatten hydratiserar montmorilloniten genom att vatten går in mellan skikten, vilket gör att mineralet sväller. Denna svällning gör att lermineralet tätar och minskar…