Seepage into drifts with mechanical degradation

Area of Science:

• Geological Engineering
• Hydrogeology
• Nuclear Waste Management

Background:

• Seepage into underground drifts poses a long-term performance risk for nuclear waste repositories.
• Drift degradation, including rockfall, can be induced by stress-relief, seismic, or thermal events.

Purpose of the Study:

• To calculate seepage rates in unsaturated tuff units (Topopah Spring middle nonlithophysal and lower lithophysal) under various drift degradation scenarios.
• To assess the impact of percolation flux on seepage in degraded drift environments at Yucca Mountain.

Main Methods:

• Developed a heterogeneous drift-scale permeability model using field data.
• Calibrated hydrogeological parameters for the Topopah Spring middle nonlithophysal (Tptpmn) and Topopah Spring lower lithophysal (Tptpll) units.
• Simulated seepage using detailed degraded-drift profiles from rock mechanics analyses.

Main Results:

• Drift degradation showed no significant change to the seepage threshold (percolation flux at which seepage begins).
• The increase in seepage above the threshold due to degradation is primarily influenced by the geometry of rockfall-induced cavities, not the volume of rockfall.

Conclusions:

• Seepage behavior in Yucca Mountain drifts is robust against significant changes in the seepage threshold due to degradation.
• Cavity shape, rather than rockfall volume, is the dominant factor controlling increased seepage rates in degraded drifts.