ABSTRACT: A damage model has been developed to analyse the stability of a two-dimensional circular opening in rock subjected to both far-field loading and a time-dependent temperature pulse at the boundary of the opening. Time dependency in rock deformation under compression is modeled by considering an elastic body containing cracks that grow under compressive stresses due to subcritical crack growth. In order that the problem applies to the underground storage of nuclear waste at the proposed Yucca Mountain site, the thermal and mechanical properties for TSw2 tuff have been used. In addition, the far field boundary conditions in the model consider the tectonic setting at Yucca mountain, and the temperature profile is based on the placement of high level nuclear waste in a canister borehole. The damage model is initially used to determine the subcritical crack growth parameters for TSw2 tuff. Using these properties, the model is then run for 10,000 years under various scenarios of material strength, in-situ stress state, and temperature loading. It is found that under many realistic scenarios, borehole slabbing is predicted prior to 10,000 years.
1 INTRODUCTION The proposed underground excavations at the Yucca Mountain site include shafts, underground drifts, exploratory boreholes, and the boreholes that will house the waste canisters. In general, these underground excavations must remain stable during the period in which waste is transferred the site, and during some period following in which the waste could be retrieved if necessary. The ability of the Yucca Mountain site to contain high level waste for tens of thousands of years depends on the long-term integrity of the waste canisters, which in turn depends on the stability of the boreholes that house the waste canisters. The canister boreholes will be subjected to a large thermal pulse from the waste canisters. Based on the current design, the waste canisters will heat up the surrounding rock to a maximum temperature of 206C twenty years after emplacement, followed by a rock temperature above 100C for over 1000 years (DOE, 1988). This temperature pulse will result in a thermal stress in the rock, which, in conjunction with the pre-mining stress state, could cause slabbing of the walls of the boreholes. This could result in a reduced life for the waste canisters, by causing a contact between rock and canister, and increasing the likelihood of canister corrosion. A damage model has been developed to estimate the long-term stability of the canister boreholes. The model considers time-dependent microcrack growth in the walls of the borehole, finally resulting in the formation of a large-scale fracture. Recent experimental studies have shown that both time and rate dependent behavior in brittle rocks can be the result of rate-controlled processes acting at the tips of cracks where stress concentrations exist (Sano et al., 1981; Carter et al., 1981; Atkinson, 1984). This time-dependent crack growth occurs at values of the stress intensity factor below the fracture toughness of the material, and is referred to as sub-critical crack growth.