MASSIVE SILICA REDISTRIBUTION BY DIFFUSION IN POROUS FRACTURED TUFF

The Department of Energy (DOE) continues to use the current pre-disposal hydrologic structure within Yucca Mountain, Nevada to assess the long-term safety of a proposed nuclear waste repository. However, heat generated by long-lived isotopes in the nuclear waste will raise more than 1/2 the total mountain volume to > 40°C and 90% water saturation in tuff pores for 40,000 years (DOE Viability Assessment, 1999). The extended moist heating of the metastable tuff minerals will allow infiltrating rainwater to redistribute silica to permanently alter the mountain’s hydrologic behavior. We report here on experiments that measured how much silica can dissolve and diffuse from matrix pores to a flowing fracture at 40°C in the most geochemically resistant tuff from the mountain (devitrified Topopah Springs welded,TSw).

10 x 6 x 2 cm blocks of TSw tuff were cut, polished, and ultrasonically cleaned. Split pairs of blocks were reassembled with Teflon spacers to create fractures with apertures of 20, 50, 100, and 200 microns, spanning estimates for natural fractures in the TSw tuff. A self-contained, isothermal refluxing apparatus was used to maintain a steady flow of 40°C condensate water through each fracture. Fracture effluents reached silica concentrations consistent with cristobalite saturation in the pore water within a few hours. After several hundred days, however, the silica concentration in the fractures rose to levels corresponding to pore water concentrations exceeding saturation with respect to silica glass (150 ppm SiO2). These dissolution rates and silica concentrations cannot be explained by dissolution from silica polymorphs in the tuff, but are consistent with recently published estimates of feldspar/water interactions for highly disordered feldspars (see Packer and Matyskiela, this conference). As a result of tuff's large pore surface area of metastable silicate minerals, in a 1 hour rainstorm, infiltrating rainwater can transport 10,000 times as much silica in a 40°C TSw tuff fracture as in an identical fracture flowing through granite. Safety assessments of a Yucca Mountain repository need to take account of the hydrologic changes that could be created by tuff's unique ability to redistribute silica.