METHODOLOGY FOR ANALYSIS OF FRACTURE REACTIVATION IN RESPONSE TO THERMALLY INDUCED STRESSES AT YUCCA MOUNTAIN

Fractures at Yucca Mountain in southwestern Nevada are important to the performance of a potential high level nuclear waste repository because they would affect drift degradation and impact saturated and unsaturated flow. In particular, fracture propagation influences drift degradation by connecting fractures to form rock blocks surrounding the emplacement drifts. These rock blocks could fall on the drip shields and waste packages, potentially damaging the engineered barrier system and eventually filling the drift with rock debris. Over the lifetime of the repository, fractures will be subjected to complex and evolving local stress fields caused by a combination of (i) regional tectonic stress, (ii) lithostatic load, (iii) excavation-related stresses, (iv) perturbations from nearby fault slip, and (v) thermally induced stresses. Significant thermal loading is expected to result from the radioactive decay of the emplaced nuclear waste. Knowledge of the fracture network and thermal and stress conditions under which fractures propagate to produce rock blocks is therefore important to performance assessment of the proposed repository.

Here, we discuss a new approach for evaluating the effect of thermally-induced stresses on fractures at Yucca Mountain. This methodology evaluates detailed three-dimensional, synthetic fracture models using numerical models of the evolving three-dimensional stress fields. The synthetic fracture models are constructed using detailed fracture data from the site. Modeled stresses include the effects of heating of the repository and advance of this thermal pulse through the rock volume surrounding the drifts. The calculation and display of resolved stresses (normal and shear) as well as slip tendency on fractures are then used to evaluate the stability of the drift walls. This methodology identifies fracture orientations and fracture locations that are likely to reactivate (slip), thereby forming rock blocks. The methodology of combining detailed stress information and detailed fracture information can also be used to investigate flow behavior in the fractured rocks by calculating dilation tendency.

[This abstract is an independent product of the CNWRA and does not necessarily reflect the views or regulatory positions of the U.S. Nuclear Regulatory Commission.]