Physical and chemical properties of zeolite minerals occurring at the Yucca Mountain Site.

摘要

Silica tuffs at Yucca Mountain, Nevada, are under investigation as potential host rocks for isolation of high and intermediate level radioactive wastes from commercial nuclear power generation. Non- welded and partially welded tuffs at this site may contain major amounts (up to 90%) of the zeolite minerals clinoptilolite, mordenite, and analcime. Densely welded tuffs which form the proposed repository horizon and some of the minor basaltic dikes at the site contain clinoptilolite or heulandite as fracture filling which limits the permeability of these rocks. The cation exchange properties of these zeolites allow them to form a natural sorptive barrier to the migration of radionuclides that may move as soluble cationic species in ground water. However, these zeolites are unstable at elevated temperatures and may breakdown either by dehydration at low water-vapor pressures or by mineralogical reaction to more stable phases at higher water-vapor pressures and at relatively low temperatures (80--100(degree)C). All the breakdown reactions occurring with increasing temperature result in significant volume reductions 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 if heat-generating wastes are emplaced in zeolite-rich horizons. The breakdown reactions and the sorptive properties both vary strongly with zeolite crystal chemistry, and for any given structure, with the exchangeable cations present in that structure. Thus an improved knowledge of the crystal chemistry of these zeolites, particularly clinoptilolite which has not been thoroughly studied due to a scarcity of adequate crystals, will allow improved models of radionuclide migration and a better evaluation of any potential hazards posed by breakdown of these highly temperature-sensitive minerals at the Yucca Mountain site. 38 refs., 5 figs., 2 tabs.