At the potential nuclear waste repository at Yucca Mountain, Nevada, U.S.A., a large number of parallel emplacement drifts would be excavated and used for permanent geologic disposal of spent nuclear fuel and high-level radioactive waste. A stable drift is required to ensure safe waste isolation during the preclosure period which could span up to three hundred years. One of important design features is to evaluate impacts of potential seismic hazards on drift stability, so that specific design measures can be taken to mitigate seismic effects deemed significant. Seismic hazards include both vibratory ground motion and fault displacement. This paper examines the potential of earthquake damage to emplacement drifts due to both vibratory ground motion (shaking) and fault displacements (sheafing). The peak ground acceleration (PGA) up to 0.44g, the peak ground velocity (PGV) up to 47 crrds, and the maximum mean fault displacement of 100 cm are considered at the repository host horizon (RHH) during the preclosure period of the repository.
1.1 Geologic setting of the proposed repository The potential emplacement host horizon at Yucca Mountain lies primarily within the Thermal/Mechanical (T/M) unit called TSw2. A T/M unit is defined by grouping the lithostratigraphic units that have similar thermal and mechanical properties. The TSw2 unit, with its thickness ranging from approximately 175 to 230 m, is lithophysae (i.e., lithophysal cavities)-poor and consists of the middle nonlithophysal (Tptpmn), lower lithophysal (Tptpll) and lower nonlithophysal (Tptpln) zones. Other T/M units overlying the TSw2 are TSwl, PTn and TCw, as illustrated in numerical model configurations to be described later.
Both visual observation and field mapping from the Exploratory Study Facility (ESF) tunnel and alcoves as well as from the Enhanced Characterization of the Repository Block (ECRB) Cross drift reveals that the TSw2 rock mass consists of intact rock blocks separated either by frequent planar cooling joints or by infrequent discrete non-planar tectonic fractures. Some joints and fractures have a rather short trace length ranging from sub-meter to several meters.
1.2 Current emplacement drift layout
Emplacement drifts are intended to be circular tunnels and parallel to each other, with 5.5 m in diameter and 81 m in spacing. Each drift is about 1,000 m long. Depending on both the maximum drift temperature criterion and ventilation requirement, the total number of drifts will be over one hundred. The majority of the drifts will be excavated within the lithophysal strata. The overburden is approximately 300 to 400 m thick.