An Example of Geologic Storage
Background on Yucca Mountain?
Location: located in Nye County in the southern part of Nevada, 160km NW of Las Vegas. Yucca consists of layers of volcanic ash approximately 12.7 to 12.9 million years old. Yucca Mountain has been identified by the U.S. as the primary means by which high-level nuclear waste will be disposed. Scientific and political considerations and testing are underway. If constructed, tt would be the world’s first geologic repository for high level waste. (Dyer & Peters, 2004).
Image courtesy of: United States Nuclear Regulatory Commission
What progress has been made?
History of Yucca Mountain and its testing:
Yucca Mountain was selected as a potential site for the disposal of spent fuel after it was compared to several other potential sites and found to offer the best possibilities. Other sources suggest that Yucca Mountain was initially considered as a test site largely because of its proximity to the Nevada Test Site where nuclear weapons have been tested and developed. The area is thought to be already contaminated with nuclear waste and radioactivity that additional waste in the form of spent fuel would cause few problems. Other research shows that the area is in fact uncontaminated. (Bryan, 1987).
Nearly 20 years of testing have been performed at the site, characterizing the area, and examining and considering how natural processes could affect the ability to safely store large volumes of nuclear waste at this facility. Yucca Mountain has passed public and environmental safety tests mandated by the US Nuclear Regulatory Commission. The site was recently reviewed by the US Secretary of Energy who found the plans acceptable and recommended the site to President George W. Bush. Congress approved Yucca Mountain as site for repository development on July 9, 2002; the bill was signed by President Bush on July 23, 2002. These acts gave the DOE license to complete its site characterization tests. The DOE is now in the process of lobbying to obtain a license from the US Nuclear Regulatory Commission to construct the repository and working on plans for the development of a transportation system that would bring the waste to the site. (Ewing & McFarlane, 2002).
While this act has been approved by Congress, the decision to pursue development of Yucca Mountain, Nevada has expressed discontent at having a nuclear waste facility located in the state. They submitted a Notice of Disapproval to Congress shortly after the passage of the bill in 2002. (Ewing & McFarlane, 2002)
In spite of the recommendation from the Secretary of Energy, the Nuclear Waste Technical Review Board expressed concern over the performance estimates and technical research performed at Yucca Mountain. They claim the assessments, while detailed are not comprehensive enough to ensure that Yucca Mountain is a long-term storage facility. (Ewing & McFarlane, 2002).
Elements of Yucca Mountain
Different elements of the Yucca Mountain project are being studied by several different laboratories and facilities.
Basics:
The nuclear waste would be stored in canisters buried 1000m underground, and 1000m above the water table in the mountain. The waste would be in a repository that could be accessed via two tunnels (Whipple, 1996). The Ghost Dance Fault line divides the repository in two, so it is possible that the repository would be split into two to avoid contact with this fault line. (Whipple, 1996). The two most pressing challenges are to create canisters that will protect and isolate the waste for many years and to evaluate the consequences of the high heat generated from the waste on the surrounding area.
Engineered Barrier System to Contain Nuclear Waste:
The development of an engineered barrier to contain the nuclear waste at Yucca Mountain has been studied and researched by Lawrence Livermore Laboratory in collaboration with the Department of Energy. The goal is to create a canister that would last up to 10,000 years. The current proposal is to use an outer barrier made of alloy 22, which contains 60% nickel, 22% chromium, 13% molybdenum, and 3% tungsten. This alloy is highly resistant to corrosion and fracture. The inner barrier is made from a specific type of stainless steel to add additional strength. A barrier outside the alloy 22 made of Titanium protects against water and rock fall in the repository. (Lawrence Livermore National Laboratory, Yucca Mountain Project)
Picture of the nuclear waste disposal canister:
Image courtesy of Lawrence Livermore National Laboratory, Yucca Mountain Project. http://ymp.llnl.gov/ebs.php
Location of Burial:
The canisters would be buried 1000m underground and 1000m above the water table at the base of the unsaturated zone to minimize contact with water.
Image taken from Richardson, JA. 1997.
How does water and fluid at the site impact storage capabilites?
Yucca Mountain’s area is very dry: Annual precipitation is only 170mm, and little of this moisture goes into the ground. 95% runs off or is lost to evaporation of transpiration. This makes for a very good site because there is very little water that could run into the ground and reach the nuclear waste. A large fear is that the nuclear radioisotopes could contaminate ground and drinking water.(Dyer & Peters, 2004)
There are two types of ash present at Yucca Mountain that have important ramifications for fluid percolation: welded tuff and non-welded tuff. Welded Tuff is hard and bricklike and has low porosity such that any fluid present must flow through fractures. Non-welded tuff exists between the welded layers, is brittle and less dense and has a higher porosity and fewer fractures. Fluid flow in these layers is primarily through the matrix. The DOE conducted tests to study the surface, drilling deep boreholes to monitor changes in the crust and ground over long periods of time. (Dyer & Peters, 2004)
Lawrence Berkeley Laboratory has been responsible for creating the Exploratory Studies Facility to conduct hydrological and thermal-hydrological-chemical coupled process testing at Yucca Mountain. Similar studies have also been conducted by Los Alamos National Laboratory.
Two primary tests are conducted to assess how liquid water might interact with the nuclear waste: (Dyer & Peters, 2004)
Percolation Flux
Seepage
Percolation Flux is the flow of water to the repository. This influences drift seepage and the likelihood of radionuclide transport. A value for percolation flux is derived from various parameters including the presence of certain ions and chlorides, environmental isotopes, perched water occurrence, heat flow and fracture fillings.
Seepage: this is the flow of water into an underground opening. Research on the effect of seepage at Yucca Mountain has been examined through water release studies.
The fluid flow through Yucca Mountain has been tested. Water containing tracers is released in the lithophysal rock and any water that seeps through is collected below at the non-lithophysal layer approximately 20m below. These rates of seepage are conducted under various conditions of heat and humidity for the most precise results. To date, no seepage has been observed. (Dyer & Peters, 2004).
Various different regions have been tested to determine their response to the presence of fluid, their heat tolerance and geologic properties. Since radioactive waste generates such high heat, the rock around the disposal area would have to be stable under these conditions. (Whipple, 1996).
Near Field Environment Characterization Project:
This project, carried out at Lawrence Livermore National Laboratories aims to examine the long term effects of the heat generated from the nuclear waste on the near-field environment. By performing tests on the effects of the waste on moisture, temperature, corrosion of metal, gas and vapor pressure, deformation, and microbial survival, it is hoped to gain a better understanding the changes that might occur in the surrounding rock. Computerized models have been used to predict potential fractures and their effects in the rock. Tests are still underway. (Lawrence Livermore National Laboratory, Yucca Mountain Project)
Seepage Collection System:
Image taken from Dyer, JR & Peters MT. 2004.
Development of a Transportation System:
A transportation system would need to be created to safely transport the nuclear waste to Yucca Mountain. Most of the US’ nuclear utilities are in the Eastern US, while Yucca Mountain is in the west. The two possible transportation systems are rail and highway. The preference is to ship by rail because this system has already been in use for many years to ship radioactive materials across the country. A connection to Yucca Mountain would need to be built into the existing rail system. It is estimated that construction of this branch would cost between $300 million and $1 billion. Additional research and funding would be needed for the development of safe casks to transport the waste in, and a maintenance facility. (Dyer & Peters, 2004).
Criticism:
The nuclear waste would be placed in the unsaturated zone where it is free from interaction with fluid, but subject to oxidizing conditions.
The potential for large-scale earthquakes, fault movement, movement of groundwater, nuclear activities at a nearby test site and volcanic action could interfere with the safety and stability of this storage site.
Nevada has expressed deep concern with the placement of a nuclear waste disposal facility in the state. The area around Yucca is full of wildlife, and Nevada is one of the fastest growing states in the nation, a trend that might change with the placement of a waste disposal facility. The facility would not provide many job opportunities for local workers, and many businesses indicate that this facility would decrease their desire to build a business in that county. How Nevada would be compensated for these factors is unknown.
In conclusion, Yucca Mountain has received an extraordinary amount of political support but scientists are not convinced that Yucca Mountain is a viable means of storing high level nuclear waste. The U.S. should not be pursuing Yucca Mountain as the only viable disposal strategy, especially given the finding that Yucca Mountain does not have the capacity to store all the nuclear waste currently awaiting disposal and the waste that will be generated in the future. A greater collaboration between scientists and politicians is needed.
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References
Bryan, R.H. (1987). The Politics and Promises of Nuclear Waste Disposal: The view from Nevada. Environment, 29, 8, 14-38.
Bodvarsson GS, Boyle W, Patterson R, Williams D. 1999. Overview of scientific investigations at Yucca Mountain—the potential repository for high-level nuclear waste. Journal of Contaminant Hydrology, 38, 3-24.
Dyer, JR & Peters MT. (2004). Progress in permanent geologic disposal of spent nuclear fuel and high level radioactive waste in the United States. J. Power and Energy, 218, 319-334.
Ewing, R.C. & Macfarlane, A. (2002). Yucca Mountain. Science, 296, 659-660.
Lawrence Livermore National Laboratory, Yucca Mountain Project. http://ymp.llnl.gov/nearfield.php
Los Alamos National Laboratory: http://ees.lanl.gov/archive/yuccamtn.shtml#2
Richardson JA. 1997. United States high-level radioactive waste management programme: current status and plans. Proc Inst Mech Engrs, 211, 381-391.
Whipple CG. 1996. Can nuclear waste be stored safely at Yucca Mountain. Scientific American, 274, 6, p. 72.