TESTING AND MODELING OF SEEPAGE INTO UNDERGROUND OPENINGS IN A HETEROGENEOUS FRACTURE SYSTEM AT YUCCA MOUNTAIN, NEVADA

摘要

Seepage is an issue of some importance for the proposed long-term geologic disposal of high-level radioactive waste at Yucca Mountain. Seepage into the tunnels in which the waste will be emplaced can mobilize the waste allowing it to enter the accessible environment through natural springs or man-made wells in neighboring valleys down gradient. A testing program was initiated to characterize the process of seepage into tunnels in Yucca Mountain in the same stratigraphic unit as the proposed waste emplacement tunnels. The seepage characterization data are .used to create predictive models to assess the risk and magnitude of seepage under future conditions. Off the main underground tunnel at Yucca Mountain, several niches, or short tunnels about 10 m long, were excavated to be used for seepage characterization. Three boreholes were drilled about 0.6 m above the crown of the niche and are labeled UL, UM, and UR in Figure 1. Air-injection testing was carried out in the boreholes above the niches. Intervals were isolated every 0.3 m along the boreholes by means of an inflatable packer system. The tests were analyzed to infer the fracture system permeability of the rock. After the air-injection tests were completed, liquid-release tests (LRTs) were initiated in some of the same intervals that were used for the air-injection tests. In this way, intervals with very low permeability could be identified, and LRTs in these intervals initiated at lower rates. LRTs of short duration were performed at Niche 3650, the first niche to be tested. A capture system was set up beneath the drift ceiling, as shown in Figure 1, so that potential seepage could be weighed and compared to the mass of water injected into the rock during that test. The seepage data for these tests included only the time of seepage initiation and the total mass of water that seeped. Multiple injection rates were tested in many of the intervals in order to capture the rate of injection below which no seepage occurs.