In situ pneumatic testing at Yucca Mountain

Abstract

The Yucca Mountain site in Nevada is a potential site for a geologic high-level nuclear waste repository. The proposed repository location is in the unsaturated zone in the mountain. Fluid flow through the fractured tuff repository rock is the potential transport mechanism for radionuclides from nuclear waste to the environment. This flow is believed to be predominantly confined to the fractures, which means that an understanding of how fracture systems transport fluids is essential to determining how fast and in what quantities fluids will travel through the rock. As part of an ambient testing program for the site characterization project at Yucca Mountain, the unsaturated zone is under investigation to examine the fluid flow and transport characteristics of the fractured tuff. The size and number of connections between fractures are characteristics that affect flow and how flow can travel in networks of fractures. To determine these potential flow paths in the fracture system, site investigators use automatically controlled packer assemblies to seal clusters of boreholes drilled into the rock, enabling injection of air into specific locations while monitoring pressure responses at other locations. Tests have been performed both before and after major excavations near the test locations, enabling study of the mechanical effects of construction on local permeability. To date, an estimated 3500 separate air injections have been undertaken and nearly a quarter of a million pressure-response curves have been logged in the study. The number of tests lends itself to visualization and statistical examination of the flow connections and the distributions of permeability in the rock mass.