The Bentonite Barrier - Swelling Properties, Redox Chemistry and Mineral Evolution

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

Analysis and Design of Passive Underwater Acoustic Identification Tags

The development of predeployed underwater infrastructures to aid in Autonomous Underwater Vehicle (AUV) navigation is of keen interest, with the increased use of AUVs for undersea operations. This work presents a class of passive underwater acoustic markers, termed Acoustic IDentification (AID) tags, which are inexpensive to construct, simple to deploy, and reflect unique, engineered acoustic signatures that can be detected by an AUV instrumented with high-frequency SOund NAvigation and Ranging (SONAR) systems. An AID tag is built of layers of materials with different acoustic properties and thicknesses such that a portion of the acoustic energy from an incident pulse from an AUV, for example, is reflected from each interface between two adjacent layers. In this manner, unique acoustic signatures can be generated, similar to an optical barcode. AID tags can be used therefore as geospatial markers to highlight checkpoints in AUV trajectories, or to mark areas of interest underwater. Numerical simulations of the acoustic signatures of two AID tag design iterations i.e. a horizontally stratified AID tag, and a hemispherically stratified AID tag, were experimentally validated using a sub-scale ultrasound setup. Furthermore, an energy based layer optimization strategy was proposed to maximize the strength of reflected AID tag signatures for different source frequency ranges. Subsequently, the detectability of AID tags in the proximity of strong interference such as a hard seabed or another AID tag was quantified, and the detection range of an AID tag was computed based on the standard SONAR equation. Finally, experimental results of hemispherical AID tags interrogated by high-frequency SONAR were presented to demonstrate AID tag performance in realistic deployment scenarios.Ph.D