DISSOLUTION and transport of radionuclides by ground-water are the main threats to the effective isolation of radioactive wastes by burial in deep geological formations. Calcine and glass are two solid forms which have been studied in this way for radioactive waste disposal. Long term release rates, based on extractions of radionuclides by simulated groundwater at 25°C and atmospheric pressure, are projected to be very low from borosilicate glass, the most favourable candidate waste form. We present here evidence that contact with pressurised water at the 200-400°C temperatures that could occur near the waste in the early years of storage rapidly alters the borosilicate glass and promotes interactions between waste and both shale and basalt repository wall rocks. The specific examples are the formation of a uranyl silicate (weeksite) directly from hydrothermal alteration of the glass and the reaction of caesium and sodium from calcine with aluminosilicates in basalt and shale to form the caesium sodium aluminosilicate, pollucite.
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