SIDEBAR 2.2: The Planned Repository at Yucca Mountain
Yucca Mountain is located about 160 kilometers northwest of Las Vegas, Nevada, at the western edge of the Nevada Test Site (where testing of nuclear weapons was carried out). The area surrounding the site is sparsely populated and receives an average of 17.0 centimeters of precipitation per year. The mountain is made up of a volcanic ash, called tuff, which was deposited approximately 12 million years ago. The mountain has been under investigation for over 20 years as a potential host for the first mined geologic repository for spent nuclear fuel and high-level radioactive waste (HLW) in the United States, and the Congress has given approval for DOE to proceed with a license application to construct the repository. The proposed design would place the repository in a layer of welded tuff in the unsaturated zone, approximately 300 meters below the surface and approximately 300 meters above the water table (i.e., above the saturated zone).
The current design for the potential repository calls for spent nuclear fuel and high-level radioactive waste to travel to Yucca Mountain by truck or rail in shielded shipping containers. DOE has done only preliminary transportation studies, explicitly avoiding more detailed planning until after the site recommendation, which occurred in 2002. Once these materials arrive at the repository, they would be removed from the shipping containers and placed in double-layered, corrosion-resistant packages for disposal. The design lifetime of the disposal containers is required to be at least 1,000 years, and the current design utilizes an alloy (C-22) estimated to be corrosion resistant for at least 10,000 years. Rail cars would carry the canisters underground into the repository, and remotely controlled equipment would place the canisters on supports in drifts (side tunnels) off of a main underground tunnel. DOE is still exploring whether the plan should include backfilling the tunnels or ventilation should be maintained to keep the packages dry, and whether to keep the repository “hot” or “cold” (i.e., above or below the boiling point of water).
An 8-kilometer-long tunnel called the Exploratory Studies Facility has been bored through the mountain at the depth where a repository would be constructed. Several tests continue at the site to gather data on water flow through the medium, on the behavior of the rock when it is heated (as it would be by the waste), and on other unresolved technical questions.
Under the Nuclear Waste Policy Act, the law governing disposal of spent nuclear fuel and high-level waste, the first HLW repository in the United State will be allowed to accept no more than 70,000 MTHM of spent nuclear fuel and HLW until a second HLW repository is in operation. DOE has allocated space for 63,000 MTHM of commercial spent fuel and for 7,000 MTHM equivalent of DOE HLW and spent fuel. The 70,000 MTHM limit is not a technical capacity limit but a legislated limit.