Studies of altered vitrophyre for the prediction of nuclear waste repository - induced thermal alteration at Yucca Mountain, Nevada

Nuclear waste emplacement in devitrified volcanic tuff at Yucca Mountain will raise the temperature of surrounding rock for a geologically significant period of time. This study evaluates the susceptibility of an underlying 50-ft-thick vitrophyre to thermal alteration by examining alteration that occured in the rock as it cooled after deposition. A 10{sup 0}C temperature rise should have no mineralogical effects on the vitrophyre, but an increase of 60{sup 0} or more is likely to result in alteration. Expected mineralogic changes in the vitrophyre caused by this amount of thermal loading include crystallization of zeolites and smectite. Alteration will be concentrated of zeolites and smectite. Alteration will be concentrated in a thin interval near the top of the vitrophyre and along fractures. Adsorbed water and water in preexisting hydrous minerals and in glass may contribute to hydrothermal alteration of underlying vitrophyre. Bulk porosity change would be slight and local porosity increase would probably be restricted to the upper part of the vitrophyre. Although some fracture filling could occur, such a minor sealing effect would be balanced by development of secondary porosity. Zeolites and smectite, newly-crystallized along fluid flow paths below the waste repository, could provide an enhanced sorptive barrier to radionuclide migration. 21 references, 3 figures