Thermal expansion measurements were conducted as a function of confining pressure on welded specimens of Topopah Spring Member tuff recovered from borehole USW SD-12 at Yucca Mountain, NV. Each specimen was tested at confining pressures between 1 and 30 MPa over a nominal temperature range of 25 to 250 ºC. On several specimens, the higher confining pressure thermal cycles were performed first to inhibit thermal effects, such as cracking, that occur at lower confining pressures in other rock types.
The coefficient of thermal expansion for welded tuff increases with temperature. At temperatures below 100 C the mean coefficient of thermal expansion range from 7.7 to 10.8 x 10-6 C-1.
An integral part of the licensing procedure for the potential nuclear waste repository at Yucca Mountain, Nevada involves the prediction of in situ characteristics of the tuff for the emplacement of radioactive waste containers. The data used in modeling the thermal and mechanical behavior of the repository rock require a detailed knowledge of the rock properties. In particular, the thermal expansion, thermal conductivity, heat capacity, elastic constants and strength as a function of porosity, temperature, and pressure for the welded tuff in the proposed repository horizon are required. Previous studies show that thermal expansion of low porosity, crystalline rocks depends not only on temperature, but also confining pressure. Wong and Brace, (1979) and Heard and Page (1980) report that the coefficient of thermal expansion for granite increases with temperature and decreases with confining pressure. Decreases in the mean coefficient of thermal expansion for Westerly granite of as much as 40% are observed as the confining pressure increases from 7.6 to 55.2 MPa on specimens heated to 300 0C (Wong and Brace, 1979). Heating, even to moderate temperatures, produces damage in the form of microcracks in granites. The formation of microcracks in granite specimens during thermal cycling is due to the differential thermal expansion of quartz and feldspars along grain boundaries. The increase in microcrack porosity results in a decrease in the elastic constants and strength and an increase in permeability at low confining pressures. Summers et al. (1978) show that thermal cycling to 400 0C increases the permeability of granite specimens, presumably due to enhanced microcrack populations. Johnson et al. (1978) and Simmons and Cooper (1978) observe that thermal cycling under ambient conditions, augments the microcrack density in granites at temperatures as low as 75 0C. In general, the porosity in granite is characterized by low aspect ratio microcracks. The porosity in the Topopah Spring member welded tuff, in contrast, is characterized by relatively large spherical to elliptical lithophysae and vapor-phase altered zones. These differences in pore geometry produce significantly different pressure dependencies for the elastic constants of tuff and granite (Haupt et.al., 1992; Martin and Haupt, 1994; Price et al., 1994). The effect of pressure on thermal expansion has not been studied on tuff from the potential repository horizon. Therefore, it is important to assess the role of the pore geometry on thermal expansion under appropriate repository conditions.