Time-dependent borehole stability under mechanical and thermal stresses: Application to underground nuclear waste storage

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.