Silicic tuffs of the southern Great Basin and basalts of the Columbia River Plateau are under investigation as potential host rocks for high-and intermediate-level radioactive wastes. Nonwelded and partially welded tuffs may contain major amounts (>50%) of the zeolite minerals clinoptilolite, mordenite, and analcime. Densely welded tuffs and some basalt flows may contain clinoptiiolite as fracture filling that limits the permeability of these rocks-The cation exchange properties of these

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... minerals allow them to pose a formidable natural barrier to the migration of cationic species of various radionuclides in aqueous solutions. However, these minerals are unstable at elevated temperatures and at low water-vapor pressures and may break down either by reversible dehydration or by irreversible mineralogical reactions. All the breakdown reactions occurring at increased temperature involve a net volume reduction and evolution of fluids. Thus, they may provide a pathway (shrinkage fractures) and a driving force (fluid pressure) for release of radionuclides to the biosphere. These reactions may be avoided by keeping zeolite-bearing horizons saturated with water and below about 85°C. This may restrict allowable gross thermal loadings in waste repositories in volcanic rocks. If this constraint is exceeded, the reactions may provide both a pathway (shrinkage fractures) and driving force (evolved fluid) for release of radionuclides to the biosphere.