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Notes: Abstract Whether damage is localized or delocalized in a composite, the composite's fracture toughness when localization occurs can be controlled to a much greater extent than hitherto exploited by properly choosing the composite's internal geometry. Delocalization and high toughness are both favoured by building in systematic defects and lock-up mechanisms. Widespread defects make available arbitrarily many sites at which energy may be absorbed by non-linear behaviour. Lock-up mechanisms cause local hardening following local damage, which drives subsequent damage to initiate elsewhere, possibly leading to damage delocalization. In brittle-brittle composites, these mechanisms may be the best hope for achieving toughness values similar to those of alloys. Illustrations are taken from recent research into woven composites with three-dimensional reinforcement and new work on model composites.

Notes: Abstract Uranium tailings are generated as solid and liquid wastes in uranium mining/milling operations. Since most of the uranium deposits in the world have low grades, millions of tonnes of such wastes are produced annually. Often, the uranium tailings are locally disposed of, using sites with suitable conditions to construct tailings basins. The main concern during the operation of a disposal site is the presence of radium in the liquid phase which overflows from dewatered tailings. This barium is precipitated by adding proper chemical reagent. The potential environmental hazards of uranium tailings arise when the disposal site is abandoned after the decommissioning of the uranium mill. Huge amounts of solid waste as small particles of depleted ore remain in place. Top soils are usually for soil stabilization and for controlling radon emissions. Hence water infiltration through uranium tailings presents a potential hazard to underground aquifers. Water plays a dual role by triggering a sequence of reactions and by carrying contaminants away from the wastes sites. The situation is more serious in a country like Canada, where the precipitation rate is higher than the evaporation rate and the water table is about one metre beneath the ground surface. For permanent disposal of uranium tailings, either the hazardous constituents of these wastes have to be isolated from the percolating water, or the subsoils must have acceptable sealing and sorptive characteristics. The first approach is effected by blending proper solidifying reagents with the tailings in order to totally block them from the leaching effects of aggressive pore water. Solid waste treatment may prove costly in additive usage and the blending operation. The leaching of toxic constituents, however, becomes effective so long as the tailings are in a loose form. This is because of the acid-generating properties of the pyrite, a substantial constituent (up to 8% wt) of the tailings. Two types of contaminants tend to leave the solid and migrate to the pore water. These are the heavy metals and the radioactive elements, both of which are hazardous for their toxicity and persistence. The sorptive and neutralizing capacities of the underlying soils have to be carefully examined and measures have to be taken to stop the progression of acidic interstitial water. This paper focuses on three aspects of crucial importance to the permanent disposal of uranium tailings. These are: leaching in uranium tailings, sorption on natural geological barriers, and radium immobilization.