1. INTRODUCTION
Spent nuclear fuel and high-level radioactive wastes are the result of commercial power generation, nuclear weapons production, and other research and development activities. They have accumulated since the mid-1940s at sites now managed by the U.S. Department of Energy (DOE) and since 1957 at commercial reactors and storage facilities across the country. The responsible management and disposal of these materials is a critical part of the DOE mission to dispose of high-level radioactive waste and spent nuclear fuel from federal facilities, including the nuclear weapons program, as well as commercially generated spent nuclear fuel. The U.S. has evaluated methods for the safe storage and disposal of radioactive waste for more than 40 years. Many organizations and government agencies have participated in these studies: at the request of the U.S. Atomic Energy Commission (AEC), the National Academy of Sciences evaluated options for the disposal of radioactive waste on land in the 1950s (National Academy of Sciences Committee on Waste Disposal 1957). The AEC and its successor agencies, the U.S. Energy Research and Development Administration (ERDA) and the DOE, continued to analyze nuclear waste management options throughout the 1960s and 1970s. In cooperation with the U.S. Geological Survey (USGS), nationwide surveys were performed to identify potentially acceptable locations for disposal sites. During the late 1970s and early 1980s, the U.S. Nuclear Regulatory Commission (NRC) and the U.S. Environmental Protection Agency (EPA) performed numerous studies of disposal options and safety and began developing regulatory standards. An Interagency Review Group that included representatives of 14 federal government entities provided findings and recommendations on nuclear waste management to President Carter in 1979 (Interagency Review Group on Nuclear Waste Management 1979). After analyzing a wide variety of options, disposal in mined geologic repositories emerged as the preferred long-term environmental solution for the management of spent nuclear fuel and high-level radioactive waste. This consensus is reflected in the Nuclear Waste Policy Act of 1982 (NWPA) (42 U.S.C. 10101 et seq.), which established U.S. policy when it was enacted by Congress in 1982 (Public Law No. 97-425) and amended in 1987 (Public Law No. 100-203).Congressional Findings |
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| Section 111(a) of the NWPA (42 U.S.C. 10131(a)) contains seven Congressional findings for the repository program: | |
| (1) | Radioactive waste creates potential risks and requires safe and environmentally acceptable methods of disposal; |
| (2) | A national problem has been created by the accumulation of (A) spent nuclear fuel ; and (B) radioactive waste |
| (3) | Federal efforts during the past 30 years to devise a permanent solution have not been adequate; |
| (4) | While the Federal government has the responsibility to provide for permanent disposal , the costs should be the responsibility of the generators and owners |
| (5) | The generators and owners of high-level radioactive waste and spent nuclear fuel have the primary responsibility to provide for, and pay the costs of, the interim storage of such waste and spent fuel until accepted by the Secretary of Energy |
| (6) | State and public participation is essential in order to promote public confidence in the safety of disposal |
| (7) | High-level radioactive waste and spent nuclear fuel have become major subjects of public concern, and appropriate precautions must be taken to ensure that [they] do not adversely affect public health and safety or the environment for this or future generations. |
1.1 PURPOSE AND SCOPE
If the DOE decides to recommend the development of the Yucca Mountain site to the President, it is required by the NWPA to provide a comprehensive statement of the basis for the recommendation. This report is one part of a comprehensive suite of analyses and documents that the DOE is considering with respect to the possible recommendation. It describes the results of scientific and technical studies that have been performed to determine whether the Yucca Mountain site should be recommended for development as a geologic repository. It contains information that would be included in any site recommendation from the Secretary to the President, consistent with Sections 114(a)(1)(A), (B), and (C) of the NWPA (42 U.S.C. 10134(a)(1)(A), (B), and (C)). Section 1 introduces the report and provides background information on both the Yucca Mountain site and the site recommendation decision process set forth in the NWPA. Section 2 describes a preliminary design for a potential repository at Yucca Mountain, and Section 3 describes the proposed waste package designs and the waste forms to be disposed. Section 4 discusses data related to the safety of the Yucca Mountain site and describes how natural and engineered repository systems would work together to protect public health and limit the release of radionuclides to the environment. It explains the relationship between the waste form, the waste package, and the geologic medium at Yucca Mountain. It also describes analyses of the future long-term performance of a repository at Yucca Mountain. Section 5 describes analyses that evaluate the safety of the potential repository during operation and before final closure. The DOE has actively sought public involvement and participation in the consideration of the Yucca Mountain site. In May 2001, the DOE released an initial version of this report (DOE 2001a) to facilitate public comments on the consideration of the possible recommendation of Yucca Mountain. It was based on the technical information, and the draft regulations, available at the time of its preparation. Comments regarding the possible recommendation of the site were solicited during initial and supplemental public comment periods announced in the Federal Register, and at numerous public hearings conducted throughout the State of Nevada. Since the beginning of the public comment period, the EPA, NRC, and DOE have also released final versions of regulations relevant to Yucca Mountain. Therefore, this report has been updated to:
This Yucca Mountain Science and Engineering Report is based on an extensive foundation of scientific, engineering, regulatory, and programmatic research that analyzed the Yucca Mountain site, the potential repository and waste package designs, and other information. This report is supported by a comprehensive set of scientific and technical integrating documents that explain in detail the basis for the results presented. The major integrating documents include:
1.2 BACKGROUND INFORMATION
This section briefly describes the materials planned for disposal at Yucca Mountain. It also summarizes the evolution of and the rationale for the U.S. nuclear waste disposal program. All of the waste forms that would be transported to and received at the potential repository would be solid materials that would be stable in a deep, dry geologic repository. These waste forms fall into two categories: spent nuclear fuel and high-level radioactive waste. Some elements of this waste decay in only a few years; others are radioactive for hundreds of thousands of years (see Section 4.4.1.4). This waste must be safely contained and isolated to protect human health and the environment.1.2.1 Sources of Materials Considered for Disposal
Commercial nuclear power plants, which supply about 20 percent of the nation's electricity, produce spent nuclear fuel. The DOE manages several facilities that manufactured materials for nuclear weapons and, in doing so, produced spent nuclear fuel and high-level radioactive waste. The DOE also operates research reactors that produce spent nuclear fuel and has operated reprocessing facilities that produce high-level radioactive waste. Most of the waste from reprocessing was generated before 1982. Small quantities of high-level radioactive waste are produced in medical research that uses nuclear materials. In addition, the DOE owns small quantities of spent nuclear fuel from research reactors in foreign countries. This spent nuclear fuel will be returned to the U.S. for disposal to support nonproliferation goals. The total inventory of spent nuclear fuel and high-level radioactive waste in the U.S. could eventually exceed 100,000 MTHM or MTU, depending on the number of reactors that receive operating license extensions. MTHM and MTU are approximately equivalent terms for the once-through nuclear fuel cycle and can be used interchangeably for most of the material to be emplaced in the repository. The first repository is limited by the NWPA to 70,000 MTHM until a second repository is in operation. Section 3 of this report and Appendix A of the draft environmental impact statement (EIS) (DOE 1999a) describe the materials planned for disposal in a repository. The repository design summarized in Section 2 and analyzed in Section 4 is based on an inventory allocation that includes 63,000 MTHM of commercial spent nuclear fuel; 2,333 MTHM of DOE spent nuclear fuel; and 4,667 MTHM of DOE high-level radioactive waste.1.2.1.1 Commercial Spent Nuclear Fuel
As of December 1999, the U.S. had accumulated about 40,000 MTHM of spent nuclear fuel from commercial nuclear power plants (CRWMS M&O 1999a, Tables B-11 and B-12) This amount could more than double by 2035 if all currently operating plants complete their initial 40-year license period. Commercial spent nuclear fuel makes up most of the spent nuclear fuel that requires disposal. Spent nuclear fuel contains uranium-235 and uranium-238, relatively short-lived fission products such as strontium-90 and cesium-137, and relatively long-lived transuranic isotopes (i.e., isotopes with atomic numbers greater than 92) such as plutonium-239 and americium-243. Commercial spent nuclear fuel is now stored at 72 commercial nuclear sites in 33 states, where it will remain until a permanent repository, or an NRC-licensed storage facility, is in operation. These sites can only be decommissioned and used for other purposes after the spent nuclear fuel and other radioactive materials have been removed.1.2.1.2 U.S. Department of Energy Spent Nuclear Fuel
By 2035, the U.S. will have accumulated about 2,500 MTHM of spent nuclear fuel from reactors that produced materials for the nation's nuclear weapons program, from research reactors, from reactors on nuclear-powered naval vessels, and from reactor prototypes. These materials include commercial spent nuclear fuel that has been transferred to the DOE for research, project demonstration, or other purposes. An example is the core debris from the Three Mile Island-2 reactor. All of this spent nuclear fuel is the responsibility of the DOE. The majority is currently stored at sites in Idaho, South Carolina, and Washington (DOE 1999a, Appendix A, Tables A-16 and A-17). Under a negotiated agreement that involved the State of Idaho, the U.S. Navy, and the DOE, the DOE must remove all spent nuclear fuel from Idaho by the year 2035 (Public Service Co. of Colorado v. Batt, settlement agreement). In addition to the sites already mentioned, about 55 other facilities have small quantities of spent nuclear fuel and high-level radioactive waste. These sites include research reactors at universities, commercial research reactors, DOE laboratories, and commercial fuel fabrication plants. In most cases, DOE spent nuclear fuel and/or DOE waste stored at these facilities will be shipped to one of the larger DOE sites for drying, packaging or other treatment, and storage prior to transport to the repository for disposal.1.2.1.3 High-Level Radioactive Waste
Large volumes of high-level radioactive waste were created in the past when spent nuclear fuel was treated chemically (i.e., reprocessed) to separate uranium or plutonium isotopes that could be reused from the other elements in the fuel. The high-level radioactive waste left over from this process exists in both liquid and solid form; liquid wastes are stored in underground tanks at DOE sites near Hanford, Washington; Savannah River, South Carolina; Idaho Falls, Idaho; and West Valley, New York (DOE 1997a, p. 34). Liquid high-level radioactive waste will be vitrified (turned into glass) prior to shipment for disposal. In this process, the waste materials are mixed with other components of glass, melted, and poured into stainless steel canisters. The vitrified waste, typically in a form known as borosilicate glass, is leach-resistant and long-lived. Where there are agreements between the DOE and the states where the waste is stored, this high-level radioactive waste will be solidified and placed in about 22,000 canisters for future disposal in any permanent geologic repository for the disposal of high-level radioactive waste (DOE 1997b, Section 1.5.4). No liquid wastes will be received at or disposed in a geologic repository.1.2.1.4 Surplus Plutonium
The end of the Cold War has reduced the need for nuclear materials for weapons, which has resulted in the closure and cleanup of several weapons plants and the identification of a nominal 50 metric tons of surplus plutonium. The surplus plutonium associated with weapons production are no longer needed and must be safely disposed. Current plans call for some of the surplus plutonium, up to 33 metric tons, to be combined with uranium-238 to form a mixed-oxide fuel that would be used in commercial reactors, with later disposal of the commercial spent nuclear fuel at the potential Yucca Mountain repository. Up to approximately 17 metric tons would be immobilized in ceramic, placed inside stainless steel cans, and placed in canisters. These canisters will be filled with molten high-level radioactive waste glass, which will vitrify into a glass waste form around the stainless steel cans, as described above.1.2.1.5 Present Location of Spent Nuclear Fuel and High-Level Radioactive Waste
Spent nuclear fuel and high-level radioactive waste are presently stored in 39 states, as shown in Figure 1-2. During repository operations, waste would be consolidated for transport to the repository from 77 storage sites (72 commercial and 5 DOE [DOE 1999a, Section 1]). These storage sites are located in a mixture of urban, suburban, and rural environments.1.2.2 U.S. Policy: The Rationale for Geologic Disposal
U.S. policy on nuclear waste management has been developed by Congress through legislation and implemented by a succession of agencies, beginning immediately after World War II. The Atomic Energy Act of 1954 (42 U.S.C. 2011 et seq.) placed management of all aspects of the nation's nuclear programs under the jurisdiction of the AEC. Although the AEC initially focused on military applications, Congress soon realized it needed a broader mandate. Congress assigned the AEC major roles, which included continuing the nuclear weapons program, promoting the private use of atomic energy for peaceful applications, and protecting public health and safety from the hazards of nuclear power (Walker 2000). The AEC recognized that the nation needed a long-term strategy for managing and disposing of radioactive waste. On February 28, 1955, it reached an agreement with the National Academy of Sciences/National Research Council to establish a committee of leading scientists to evaluate methods of disposal and report their findings. The committee was also asked to recommend areas where research was needed (National Academy of Sciences Committee on Waste Disposal 1957, p. 8). The hazards posed by radioactive waste decline over time because of radioactive decay. Some radionuclides decay quickly and do not present a significant risk after a few decades, whereas others remain radioactive for thousands of years. Therefore, early studies of disposal options sought the most effective way to isolate waste long enough for the hazard to decline to levels that do not pose a significant risk to public health or the environment. The search led to geologic environments that have remained stable and are likely to remain so in the future. The committee evaluated a wide range of options and reached several conclusions and recommendations (National Academy of Sciences Committee on Waste Disposal 1957). One important conclusion was that a safe disposal facility for radioactive waste could be constructed at many sites in the U.S. However, the committee warned that many areas do not contain any likely disposal sites. The committee specifically noted that favorable conditions for disposal were not found along the Atlantic seaboard. The committee examined the best means for disposing of the liquid high-level radioactive waste that was already accumulating at sites in Hanford, Washington, and Savannah River, South Carolina. The scientists explored a variety of options and concluded that storage in tanks was, at the time, the safest short-term method (National Academy of Sciences Committee on Waste Disposal 1957, p. 6). However, tank storage of large volumes of liquid waste was not viewed as a permanent solution; deep disposal in cavities mined in salt deposits was suggested as the most promising possibility for a long-term solution. The committee noted that disposal would be much simpler if liquid waste could be converted to a solid form of "relatively insoluble character" (National Academy of Sciences Committee on Waste Disposal 1957, p. 1). A variety of options were considered feasible for disposal of solid wastes, including salt or dry mines or excavations in other geologic environments. The committee cautioned that the deep geologic disposal of waste would be a "special problem for each particular installation" (National Academy of Sciences Committee on Waste Disposal 1957, p. 77), and that "the education of a considerable number of geologists and hydrologists ... is going to be necessary" (National Academy of Sciences Committee on Waste Disposal 1957, p. 7). Between 1957 and the early 1970s, the need to find a safe method for waste disposal grew more urgent as the commercial nuclear utility industry expanded dramatically. However, little progress was made on a long-term solution. In 1970, the AEC tentatively selected a site for a repository in salt deposits near Lyons, Kansas. Under considerable political and technical fire, the Lyons site was abandoned two years later because of concern that nearby salt mine drilling had compromised the integrity of the geologic formation (The League of Women Voters 1993, p. 49). As the commercial nuclear utility industry continued to grow in the 1960s, the AEC was increasingly criticized because of the conflict between its roles of promoting peaceful uses of atomic energy and regulating the industry to protect public health and safety. Congress passed the Energy Reorganization Act in 1974 (42 U.S.C. 5801 et seq.), which divided the AEC into the ERDA and NRC. The NRC was assigned the regulatory role, with the responsibility to protect public health and safety, both for reactor operations and for the management and disposal of spent nuclear fuel and high-level radioactive waste. The energy crisis of the 1970s prompted Congress to pass the Department of Energy Organization Act in 1977 (42 U.S.C. 7101 et seq.), which combined the ERDA with other energy-related agencies to form the DOE. The act assigned to the DOE the responsibility for managing the nation's nuclear weapons programs and for managing the program for disposing the nation's nuclear waste. Since the middle of the 1980s, the environmental cleanup of the nuclear weapons complex has been a major focus of DOE efforts nationwide. After Congress created the DOE, waste management activities accelerated. The DOE initiated the National Waste Terminal Storage Program in 1977 to identify suitable sites and develop the technology to license, construct, operate, and close a repository. Site screening focused on areas with salt deposits and federal lands where radioactive materials were already present, specifically Hanford, Washington, and the Nevada Test Site. The Carter administration initiated an Interagency Review Group in 1978 to review national nuclear waste policy. The group recommended that the U.S. proceed with geologic disposal and proposed that the DOE study geologic settings other than salt deposits. The DOE decided to proceed with a strategy that relied on mined geologic disposal in 1980, as documented in Final Environmental Impact Statement Management of Commercially Generated Radioactive Waste (DOE 1980). The report evaluated many options, including disposal in deep boreholes, ice sheets, subseabeds, and space. Geologic disposal in different host rocks was also evaluated. The DOE's decision to move forward with geologic disposal was consistent with the original recommendation of the committee formed by the National Academy of Sciences/National Research Council and was later affirmed by numerous other evaluations, both in the U.S. and abroad. In a 1984 rulemaking known as the Waste Confidence Decision (49 FR 34658), the NRC found that mined geologic disposal was feasible and that spent nuclear fuel and high-level radioactive waste could be safely managed in a geologic repository. The NRC revisited the decision in 1990 (55 FR 38474) and again in 1999 (64 FR 68005), with similar conclusions. Congress enacted the NWPA in 1982 (it became law on January 7, 1983), which established a comprehensive policy for the disposal of the nation's commercial spent nuclear fuel and high-level radioactive waste. The NWPA directed the DOE to develop guidelines for site characterization. The Secretary considered various geologic media in which repositories could be located, including salt, volcanic rock (such as basalt and tuff), and crystalline rock (such as granite). The site selection process developed by the DOE is described in 10 CFR Part 960 (Energy: General Guidelines for the Recommendation of Sites for Nuclear Waste Repositories). In 1983, the DOE selected nine locations in six states to study as potential repository sites and performed preliminary environmental assessments of each. In 1986, the DOE published the results of these assessments, which documented the selection of five candidate sites from the original nine (DOE 1986a). The Secretary then recommended to the President that site characterization programs be undertaken at three sites: Yucca Mountain, Nevada; Hanford, Washington; and Deaf Smith County, Texas (DOE 1986b). The DOE began developing site characterization plans for each site and was preparing to begin site studies when Congress amended the NWPA in 1987. Concerned with rising cost projections for the simultaneous characterization of three sites, Congress directed the DOE to study only Yucca Mountain, to determine whether it was suitable for a repository, and to discontinue repository-related activities at the other sites and the work on the second repository program unless there is Congressional authorization. The concept of disposing of waste in the desert regions of the Southwest was first proposed by the USGS in the 1970s. In 1976, the director of the USGS identified a number of positive attributes in and around the Nevada Test Site that would make positive contributions to geologic disposal, including multiple natural barriers, remoteness, and an arid climate (McKelvey 1976). In 1981, a USGS scientist documented that water tables in the desert Southwest are among the deepest in the world, and the geologic setting includes multiple natural barriers that could isolate waste for "tens of thousands to perhaps hundreds of thousands of years" (Winograd 1981). In contrast to the strategy for isolating waste in salt or deep sites below the water table, waste could be disposed near the Nevada Test Site at relatively shallow depths, well above the water table. Following the initial USGS recommendation, the DOE sponsored investigations of the feasibility of disposal above the water table. Formal site characterization at Yucca Mountain began in 1986 and continues today. Recent experience in the U.S. has demonstrated that it is feasible to site and construct a repository for radioactive waste. In 1975, the ERDA selected a site in salt deposits near Carlsbad, New Mexico, to develop the Waste Isolation Pilot Plant for the disposal of transuranic waste. Transuranic waste contains radionuclides that are heavier than uranium; it emits less radiation and generates less heat than spent nuclear fuel. It is a by-product of the reprocessing of spent nuclear fuel and the use of plutonium in manufacturing nuclear weapons. Like high-level radioactive waste, transuranic waste remains hazardous for centuries and requires long-term isolation. In 1998, after more than 20 years of study, design, and analysis, the Waste Isolation Pilot Plant received the required permits from the EPA to receive and dispose transuranic waste. The plant opened in 1999 and has begun receiving waste from several DOE sites. With the end of the Cold War, many nuclear weapons have been disassembled and removed from the nation's stockpile, which has resulted in the accumulation of surplus nuclear materials. A repository is part of the strategy for managing and disposing these materials. Since the first scientific study in 1957, professional organizations that have looked at the nuclear waste problem have agreed that a geologic repository is the best approach for disposal. Scientists have widely agreed that waste encased in robust, long-lived waste packages and placed deep in stable geologic environments could be isolated from the biosphere for the long time periods necessary. As stated in a 1990 report from the National Academy of Sciences, there is "a worldwide scientific consensus that deep geological disposal, the approach being followed by the U.S., is the best option for disposing of high-level radioactive waste" (National Research Council 1990, p. vii). An international group of scientists issued a similar opinion in 1995 that affirmed the consensus for geologic disposal in a document entitled The Environmental and Ethical Basis of Geological Disposal of Long-Lived Radioactive Wastes: A Collective Opinion of the Radioactive Waste Management Committee of the OECD Nuclear Energy Agency (NEA 1995). The DOE published Viability Assessment of a Repository at Yucca Mountain (DOE 1998) in December 1998. This assessment summarized the results of site characterization, described preliminary repository and waste package designs, and presented the DOE analysis of the future performance of a potential repository located at Yucca Mountain. It concluded that Yucca Mountain remained a promising site for development as a repository. The Viability Assessment also described a program of studies to address remaining uncertainties before the Secretary would decide whether to recommend the site. Although there is a general scientific consensus in favor of geologic disposal, views differ on when to dispose of the waste and whether to dispose of it permanently. Many believe that the recoverable uranium-235 and other fissionable isotopes in spent nuclear fuel could be a future energy resource and should not be irreversibly disposed until their potential economic value is certain. Reprocessing spent nuclear fuel to reclaim the unused material is technically feasible but currently uneconomical in the U.S. Also, U.S. policy is to not encourage the civilian use of plutonium. Accordingly, the U.S. does not engage in plutonium reprocessing. Although reprocessing would reduce the amount of radioactive waste that requires disposal, it would not eliminate the need for disposal because reprocessing would generate additional waste materials that must be treated and disposed. Some experts advocate alternative technologies that might make geologic disposal easier. For example, accelerator transmutation of waste—bombarding waste with nuclear particles to convert it into material that is less radioactive or shorter-lived—has been proposed as an alternative waste management strategy. A National Research Council study found that transmutation is "technically feasible," but as a method to treat spent nuclear fuel it "would require many tens to hundreds of billions of dollars and require several decades to implement" (National Research Council 1996a, p. 81). Although transmutation would reduce the total radionuclide inventory to be disposed, disposal of large quantities of high-level radioactive waste would still be necessary. It has been suggested that a monitored retrievable storage facility, constructed at Yucca Mountain or elsewhere, would be preferable to geologic disposal at present. Although a monitored facility could safely isolate wastes for as long as it was properly maintained, this strategy has several disadvantages compared to a repository. It would not close the fuel cycle, and geologic disposal would still be required eventually. If the monitored facility were constructed at a location other than the same location as a repository, waste forms would have to be transported twice before final disposal, increasing the risk to workers and the public. Total program costs would be increased because of the need to construct, operate, and maintain two facilities. One way to accommodate these different views and still provide a permanent solution is to dispose waste in a manner that permits, but does not require, its retrieval. The NWPA (42 U.S.C. 10101 et seq.) requires that the DOE design a repository so that waste can be retrieved for any reason. NRC licensing regulations require that the DOE design a repository so that waste can be retrieved on a reasonable schedule starting at any time up to 50 years after waste emplacement begins, unless a different period is approved or specified by the NRC (10 CFR 63.111(e) [66 FR 55732]). Future generations would decide whether to keep the repository open and monitored or close it. To ensure that future decision-makers have some flexibility, the DOE will design the potential repository so it can be closed as early as 50 years or as late as 300 years after emplacement starts.1.3 DESCRIPTION OF THE SITE CHARACTERIZATION PROGRAM AND THE YUCCA MOUNTAIN SITE
In 1986, the DOE began a formal program of site characterization, as required by Section 113 of the NWPA (42 U.S.C. 10133). This program is described in Site Characterization Plan Yucca Mountain Site, Nevada Research and Development Area, Nevada (DOE 1988). The plan established a comprehensive set of studies to gather the information needed to evaluate the suitability of the site against EPA, NRC, and DOE regulations in effect at the time. After releasing the plan in draft form in January 1988 for review by the NRC, the State of Nevada, Congress, and other interested parties, the DOE issued the final plan in December 1988. Progress reports (e.g., DOE 1999b) describing ongoing site characterization activities and plans are issued semiannually. This report draws on numerous references that describe the investigations and results of the site characterization program in detail. Yucca Mountain Site Description (CRWMS M&O 2000b) is the most comprehensive.1.3.1 Site Characterization Investigations
The site characterization program has performed extensive surface-based tests and investigations, underground tests, laboratory studies, and modeling activities designed to provide the technical basis for an evaluation of repository performance. Figure 1-3 shows the location of the surface-based and underground test facilities at Yucca Mountain, including boreholes and underground excavations. The single largest effort of the site characterization program was the Exploratory Studies Facility, which provided access to the subsurface environment for exploration and testing along the entire north–south extent and at the proposed depth of the potential repository. Work on the Exploratory Studies Facility started in late 1992, and excavation with a tunnel boring machine began in September 1994 (Figure 1-4). The 8-km (5-mi) underground tunnel that is the main part of the Exploratory Studies Facility was completed in April 1997. A drift across the entire planned width of the potential repository, the Enhanced Characterization of the Repository Block (ECRB) Cross-Drift, was completed in October 1998. Additional subsurface niches and alcoves have been excavated to support specific tests. The suite of site characterization testing and analysis includes the following components:
1.3.2 Description of the Yucca Mountain Site
This section provides an overview of the location, geography, current population, and key geologic characteristics of the Yucca Mountain site. This description is provided as background information for Sections 2 and 3, which present a description of the proposed repository design (including information related to repository performance over a range of operating mode temperatures) and descriptions of the waste forms and waste packages. This section also provides a framework for the descriptions of hydrologic, geochemical, thermal, and other processes related to the performance of the potential repository contained in Sections 4.2, 4.3, and 4.4. Yucca Mountain is located on federal land in a remote area of Nye County in southern Nevada, approximately 160 km (100 mi) northwest of Las Vegas (Figure 1-5). If the site is recommended for repository development, sufficient land may be withdrawn to consolidate the control of the facilities and land needed to operate the repository. The draft EIS describes the potential withdrawal area (DOE 1999a, Section 3.1.1.3).1.3.2.1 Geography, Land Use, and Population
Yucca Mountain consists of a series of ridges extending approximately 40 km (25 mi) from Timber Mountain in the north to the Amargosa Desert in the south. The elevation at the crest of the ridges varies from approximately 1,800 m (5,900 ft) to 900 m (3,000 ft) above sea level. At the potential repository site, the crest of Yucca Mountain is 1,400 to 1,500 m (4,600 to 4,900 ft) above sea level. The western part of the potential repository site is a steep slope that rises approximately 300 m (1,000 ft) above the base of Solitario Canyon. On the eastern side, the mountain slopes gently to the east and is incised by a series of east- to southeast-trending stream channels. The elevation at the base of the eastern slope is approximately 350 to 450 m (1,100 to 1,500 ft) below the ridge crest. The site is in Nye County, which is bordered by Clark, Lincoln, White Pine, Eureka, Lander, Churchill, Mineral, and Esmeralda counties in Nevada and Inyo County in California. The federal government controls nearly all of the land in the region. The area needed for the potential repository encompasses land controlled by three federal agencies: the U.S. Air Force (Nellis Air Force Range), the DOE (Nevada Test Site), and the U.S. Bureau of Land Management (Figure 1-6). Except for a few scattered facilities constructed on the Nevada Test Site to support former U.S. nuclear propulsion and defense programs, there has been no development at the site. The land around Yucca Mountain will remain federally owned and, should the site be recommended, will be withdrawn from public use. The tracts of private land nearest to Yucca Mountain are to the south, in the Amargosa Desert. The closest year-round housing is at the intersection of U.S. Highway 95 and Nevada State Route 373, approximately 22 km (14 mi) south of the site. Active agricultural operations can be found approximately 30 km (19 mi) south of Yucca Mountain, in Amargosa Valley. There is one patented mining claim about 16 km (10 mi) south of the potential repository site. Scattered private lands are also present near the town of Beatty, 24 to 32 km (15 to 20 mi) west of the site. Because groundwater in the Yucca Mountain region flows toward the south, the potential for future exposure to radionuclides would be highest for populations living south of the site. EPA and NRC regulations (40 CFR 197.21(b) and 10 CFR 63.312 [66 FR 55732], respectively) for Yucca Mountain would direct the DOE to assume that the receptor is a person, known as the reasonably maximally exposed individual, with a diet and living style similar to the current residents of the Town of Amargosa Valley. The population density near Yucca Mountain is very low (CRWMS M&O 2000b, Section 2.3). There are no permanent residents within 22 km (14 mi), and Nye County as a whole averages only 0.6 persons per square kilometer (1.5 persons per square mile). Of the total population of 29,730 in Nye County, 68 percent live in the unincorporated town of Pahrump, 70 to 80 km (43 to 50 mi) south-southeast of Yucca Mountain. The major economic activities in the towns of Amargosa Valley and Beatty include agricultural and mining operations. Most of the other counties surrounding Yucca Mountain, including Lincoln and Esmeralda counties in Nevada and Inyo County in California, also have low population densities. The nearest large populations reside in Clark County, approximately 130 km (80 mi) southeast of Yucca Mountain.1.3.2.2 Geology
The Yucca Mountain site is located on the western boundary of the Nevada Test Site, where scientists have conducted geologic investigations since the 1950s. Studies related to nuclear waste disposal have focused on Yucca Mountain since the late 1970s and have included careful mapping of the rocks at the surface and in more than 10 km (6 mi) of tunnels below Yucca Mountain, along with the drilling and logging of numerous wells and boreholes. The characterization of the geology of Yucca Mountain provides a framework for understanding the future behavior of the potential repository. Section 4 describes data and analyses related to postclosure safety, including a discussion of processes such as water flow and thermal effects and how they would affect both the natural and engineered parts of the repository system. The following overview of the geology of the site describes the physical environment of the repository. It is based on the comprehensive description contained in Yucca Mountain Site Description (CRWMS M&O 2000b).1.3.2.2.1 Regional Geology
As shown in Figure 1-7, Yucca Mountain is located in the Basin and Range province of the western U.S., within the region known as the Great Basin. The Great Basin encompasses nearly all of Nevada and parts of Utah, Idaho, Oregon, and California. The Basin and Range, which extends into northern Mexico, draws its name from its characteristic, generally north–south aligned mountain ranges. These ranges are separated by basins containing thick deposits of sediment (mostly sand and gravel) derived from erosion of the adjacent ranges over millions of years. The structure of the Basin and Range has developed over a period of more than 30 million years. In the region of southern Nevada that includes Yucca Mountain, the pattern of mountains and valleys has formed over the past 15 million years from faults moving on one or both sides of the ranges (Fridrich 1999). The tilted, fault-bounded mountain ranges may extend more than 80 km (50 mi) in length and are generally 8 to 24 km (5 to 15 mi) wide. Relief between valley floors and mountain ridges is typically 300 to 1,500 m (1,000 to 5,000 ft), and valleys occupy between 50 and 60 percent of the total land area. Most modern tectonic activity (i.e., active faulting and volcanism) in the southwestern Great Basin occurs to the south, west, and northwest of Yucca Mountain. Among the most active areas in the region are the Furnace Creek–Death Valley fault zone, the Sierra Nevada front (i.e., the Owens Valley and Mammoth Lakes area), and the area north of the Garlock fault in the Mojave Desert. This domain includes modern basins and ranges with great structural relief, such as the Death Valley basin and the Panamint Range. The modern faulting and volcanic activity is caused by the continuation of the same tectonic extension that resulted in the formation of the entire Basin and Range. The crust on the western edge of the Great Basin (the Sierra Nevada) is gradually moving to the west relative to the eastern edge of the basin (the Wasatch Front in Utah). Regional Stratigraphy—Rocks and sedimentary deposits exposed in the region surrounding Yucca Mountain range in age from geologically old (Precambrian, or more than 570 million years old) in the mountains to geologically recent (Holocene, or less than about 10,000 years old) in the valleys. Understanding the distribution of rock types enables geologists to understand the geologic history of the area, which is fundamental to analyses of geologic hazards. Also, the characteristics of rock types below and around Yucca Mountain influence the regional flow of groundwater and would control where any potential releases of radionuclides from the repository system might migrate. Most of the Precambrian (older than about 570 million years) and Paleozoic (570 to 240 million years old) rocks in the Yucca Mountain region are sedimentary or metamorphic rocks that are not very permeable. The oldest Precambrian "basement" rocks are highly metamorphosed gneisses and schists that have been dated at about 1.7 billion years. These rocks are overlain by less metamorphosed sedimentary layers composed primarily of quartzite, siltstone, shale, and carbonate (CRWMS M&O 2000b, Section 4.2.2). Except for the carbonates described below, all of the Precambrian and Paleozoic units that underlie the volcanic rocks in the Yucca Mountain region are aquitards (i.e., rocks of low permeability that do not readily transmit water). In these units, water flow generally occurs only in strongly fractured zones. In contrast, there are several Paleozoic carbonate (limestone or dolomite) units that form important aquifers throughout southern Nevada (Winograd and Thordarson 1975). For example, the Spring Mountains, between Yucca Mountain and the Las Vegas Valley, are composed largely of carbonate rocks, which are a major source of recharge (i.e., rainfall that enters the flow system) to the regional groundwater system. Near Yucca Mountain, the most significant groundwater discharge occurs in carbonate rocks throughout the Ash Meadows area, which is about 50 km (30 mi) south-southeast of the potential repository. The aquifer from which this flow originates lies below the aquifers in the tuff units at Yucca Mountain. Knowledge of the location of groundwater recharge and discharge points, the direction of flow, and the relationship between the rock units and the groundwater system is important to analyses of potential future releases of radionuclides to the environment. Between about 14 and 7.5 million years ago (during the Miocene Epoch of the Cenozoic Era), a series of large-scale volcanic eruptions resulted in the formation of Yucca Mountain and the southwestern Nevada volcanic field (Sawyer et al. 1994), which consists of six major volcanic centers, or calderas (Figure 1-8). The Claim Canyon Caldera, just north of Yucca Mountain, was the eruptive source of the approximately 13-million-year-old rock units, known as the Paintbrush Group, that now form the mountain ridges at the potential repository site. Eruptions from the southwest Nevada volcanic field ended about 7 million years ago. More recently, small volume volcanoes (known as cinder cones) have erupted lava flows and volcanic ash to the west and south of Yucca Mountain (Crowe, Perry et al. 1995). Four cinder cones formed between about 1.3 and 0.7 million years ago in Crater Flat, west of Yucca Mountain. The latest volcanic episode created the Lathrop Wells Cone, about 16 km (10 mi) south of the potential repository site, about 80,000 years ago. This most recent volcanic activity in Crater Flat and at Lathrop Wells is described in Yucca Mountain Site Description (CRWMS M&O 2000b, Section 12.2), which provides the basis for the assessment of volcanic hazards in Section 4.3.2.1. Surficial deposits in the Yucca Mountain region provide a record of the evolution of surface processes and climate conditions over the past several hundred thousand years. Most surficial deposits in the region are composed of sands and gravels that are called alluvium if they are deposited by flowing surface water or colluvium if they originate from hill slopes as flows of debris. Eolian (wind-blown) deposits, such as sand dunes, are generally a minor component of the surficial deposits, except for a massive dune at Big Dune and sand ramps like those that flank Busted Butte southeast of Yucca Mountain. Southwest and south of Yucca Mountain, scientists have mapped minor spring and marsh deposits reflecting past, wetter climates. The ages of surficial deposits range from less than 1,000 years to more than 760,000 years, but most deposits exposed at the surface were deposited during the last 100,000 years. Determining the ages and distributions of these deposits is important to understanding the age and movement of faults in the area. A complete description of the results of studies of surficial deposits can be found in Yucca Mountain Site Description (CRWMS M&O 2000b, Section 4.4). The characteristics of surface deposits indicate that erosion in the Yucca Mountain region generally has proceeded slowly. Volcanic features are well preserved, and basic geologic relationships indicate that modern landforms (e.g., ridges and valleys) were already established by the time of eruption of the Rainier Mesa Tuff, approximately 11.6 million years ago. Near-surface carbonate deposits (sometimes called caliche or calcrete) occur in soils at shallow depths parallel to the surface and as fracture fillings. Evidence indicates that these deposits are pedogenic (i.e., related to the formation of soil) in origin, supporting the conclusion that past climate in the region has generally been similar to the modern semiarid to arid conditions, although some periods have been wetter. These types of deposits form in arid environments when downward infiltrating rainwater dissolves minerals present at the surface. Calcium carbonate is then precipitated in the soils when the infiltrating water evaporates or is taken up by plant roots.1.3.2.2.2 Site Bedrock Geology
The rocks that might host the potential repository are important to all aspects of repository design and performance. Figures 1-9 and 1-10 are a simplified geologic map and cross section modified from Day, Dickerson et al. (1998) that show geologic relations at a repository scale. In addition, stratigraphic, structural, and rock property data have been combined to form an integrated site geologic model (CRWMS M&O 1997a). This geologic model provides a common framework for developing the repository design and assessing the performance of the repository system. Measurements of the water level in boreholes at Yucca Mountain indicate that the water table is approximately 500 to 800 m (1,600 to 2,600 ft) below the ground surface. The potential repository would be located above the water table in the unsaturated zone. The ash-flow tuff layers discussed in this section lie mostly within the unsaturated zone. Site Stratigraphy—Yucca Mountain consists of successive layers of volcanic rocks (called tuffs), approximately 14 to 11.6 million years old, formed by eruptions of volcanic ash from calderas to the north. Individual layers of tuff thin from north to south. Most of these volcanic rocks are ash-flow tuffs of two types, welded and nonwelded, that formed when hot volcanic gas and ash erupted violently and flowed quickly over the landscape. As the ash settled, it was subjected to various degrees of compaction and fusion, depending on temperature and pressure. When the temperature was high enough, the ash was compressed and fused to produce a welded tuff—a hard, brick-like rock with very little open pore space in the rock matrix. Nonwelded tuffs, which occur between welded layers, are compacted and consolidated at lower temperatures, are less dense and brittle, and have a higher porosity (more open pore space in the rock). The composition of the rocks at Yucca Mountain ranges from rhyolite (a volcanic rock type with a chemical composition similar to granite) to dacite or latite. Dacite and latite are volcanic rock types with chemical compositions characterized by silica contents that are intermediate between rhyolite (high silica) and basalt (low silica). In the immediate vicinity of the potential repository, the stratigraphically highest volcanic unit present is the Rainier Mesa Tuff of the Timber Mountain Group. As shown in Figures 1-9 and 1-10, the Rainier Mesa Tuff, which is approximately 11.6 million years old, is found in only a few locations in the faulted valleys east and west of the crest of Yucca Mountain. It consists of nonwelded to partially welded rhyolitic ash flows that are up to about 30 m (100 ft) thick in this area. Beneath the Rainier Mesa Tuff, other volcanic rocks (known as pre-Rainier Mesa bedded tuffs) are also locally present. These tuffs are also nonwelded ash-flow deposits, and they range in thickness from 0 to approximately 60 m (200 ft). Most of the surface of Yucca Mountain above the potential repository location is composed of the volcanic rocks of the Paintbrush Group. The Paintbrush Group is composed of three distinct volcanic tuff layers that occur between the surface and the location of the potential repository: the Tiva Canyon welded tuff at the surface, the Topopah Spring welded tuff at the level of the potential repository, and an intervening layer of nonwelded tuffs. As a result of faulting over the last 13 million years, these layers are all tilted to the east about 10° (Figure 1-10). The Tiva Canyon Tuff is a large-volume, regionally extensive ash-flow tuff (Sawyer et al. 1994, Table 1) that has been dated at approximately 12.7 million years old. The thickness of the Tiva Canyon Tuff ranges from 50 to 175 m (165 to 575 ft); it is approximately 100 m (330 ft) thick near the potential repository site. A layer of nonwelded tuff underlies the Tiva Canyon Tuff near the site of the potential repository. This nonwelded layer includes two separate ash flows, the Yucca Mountain Tuff and the Pah Canyon Tuff. In the vicinity of the potential repository, the total thickness of the nonwelded units ranges from 30 to 50 m (100 to 165 ft). These nonwelded units contain few fractures, so they delay the downward flow of water below the surface. The lowermost unit in the Paintbrush Group is the Topopah Spring Tuff, which would be the host rock for the potential repository. The Topopah Spring Tuff was formed by an eruption about 12.8 million years ago and has a maximum thickness of about 375 m (1,230 ft) near Yucca Mountain. Based on surface mapping and studies of boreholes and underground exposures, the Topopah Spring Tuff has been subdivided into several layers according to chemical composition, mineral content, the size and abundance of pumice and rock fragments, and other variations in texture and appearance. An important characteristic of the layers is the presence and abundance of lithophysae, which are small, bubble-like holes in the rock caused by volcanic gases that were trapped in the rock matrix as the ash-flow tuff cooled. The average lithophysae range from about 1 to 50 cm (0.3 to 20 in.) in size, with a maximum size of about 1 m (3.3 ft) (CRWMS M&O 2000b, Section 4.5.3.1). Their nature, size, and abundance may affect the tuff's thermal, mechanical, and hydrologic properties. The lower and middle portions of the Topopah Spring Tuff have been divided into four layers according to the amount of lithophysae they contain. Because these layers are tilted, and the drifts in the potential repository would be approximately horizontal, the potential repository horizon crosses the lithophysal zones. Like the Tiva Canyon Tuff, the Topopah Spring Tuff is fractured throughout; these fractures provide the main pathway for water to flow through the rock unit (see Section 4.2.1). Beneath the Paintbrush Group, the Calico Hills Formation is a series of mostly nonwelded rhyolite tuffs and lavas that were erupted approximately 12.9 million years ago (Sawyer et al. 1994, Table 1). The formation thins southward, from a total thickness of about 290 m (950 ft) north of the repository block to 40 m (135 ft) south of it. Several characteristics of the Calico Hills Formation are important to repository performance. None of the tuffs of the Calico Hills are densely welded; therefore, they generally have higher matrix porosities than the Topopah Spring Tuff. Because the rock has higher ductility, the fractures that are common in welded tuffs are less common in the Calico Hills Formation. Surface, borehole, and Exploratory Studies Facility observations indicate that the highly fractured Topopah Spring Tuff may overlie tuffs of the Calico Hills Formation that have a lower fracture density (CRWMS M&O 2000b, Section 4.6.6.3). Analyses of surface and borehole samples (e.g., Bish and Chipera 1986, Table 2; Broxton et al. 1993, p. 1) show another important feature of the tuffs of the Calico Hills Formation: an abundance of zeolite minerals in the rock matrix and fractures. Zeolites are silicate minerals that have the ability to sorb (take up on their mineral surface and hold) radionuclides and other ions that might be transported in solution in water. The DOE's approach to ion exchange sorption is to quantify the extent of radionuclide-sorbent interaction, which does not require identifying the specific underlying processes of sorption (see Section 4.2.8.1). The zeolite minerals may also affect transport properties in another way: tuffs with high zeolite content have reduced matrix permeability, which will tend to focus water flow into any fractures that are present (CRWMS M&O 2000c, Section 3.6.3.1). The geologic units below the water table contain older volcanic rocks composed mainly of welded and nonwelded ash-flow tuffs. These older units can be up to 1,000 m (3,300 ft) thick below Yucca Mountain (CRWMS M&O 2000b, Section 4.5.4). The volcanic rocks are underlain by the Paleozoic limestones and dolomites described in Section 1.3.2.2.1. Near Yucca Mountain, the older volcanic rocks and the Paleozoic rocks lie deep beneath the surface, but they are found at much shallower depths (and even at the surface) to the south, where they are an important component of the hydrologic flow system. Selection of the Repository Location and Host Rock—The identification of a subsurface location for a potential repository was based on several factors, including the thickness of overlying rock and soil, the extent and geomechanical characteristics of the host rock, the location of faults, and the depth to groundwater (CRWMS M&O 2000d). Figure 1-11 shows the key features of the site that have controlled the siting of the repository, as described below. A repository would be sited deep enough to protect waste from exposure to the environment and discourage intentional or inadvertent human intrusion into the facility. Designers have specified a minimum overburden thickness of 200 m (650 ft) to ensure adequate protection from surface events. The host rock for a repository should be able to sustain the excavation of stable openings that can be maintained during repository operations and that will isolate the waste for an extended period after closure. In addition, the rock should be able to absorb any heat generated without undergoing changes that could threaten the site's ability to safely isolate the waste. The host rock should be of sufficient thickness and lateral extent to construct a repository large enough to support the design's intended disposal capacity. Moreover, the amount of suitable host rock should provide adequate flexibility in selecting the depth, configuration, and location of the repository. Studies to date have shown that the Topopah Spring Tuff has these features and characteristics. Experience gained from excavating the Exploratory Studies Facility demonstrates that openings can be excavated and maintained in the unit (Figure 1-12). The dense welding of the tuff originally occurred at temperatures of approximately 800°C (1,500°F); the results of laboratory and underground testing to date show that the heat added by the emplaced waste would not adversely affect the stability of the underground repository (CRWMS M&O 2000e). Faults could impact repository performance by affecting the stability of underground openings or by acting as pathways for water flow that could decrease waste package lifetimes and eventually lead to radionuclide release. No faults with significant displacement (i.e., movement of more than a few meters) occur within the area defined for emplacement. Detailed studies of the faults within the emplacement area indicate that they are not active; thus, they are considered to have an extremely low probability of being active in the future (CRWMS M&O 2000f, Section 2.1.1.3). The main potential repository emplacement area is bounded on the west by the Solitario Canyon fault, and on the east by the Ghost Dance fault. To mitigate any possible effects from fractures near faults (e.g., higher potential for water flow in fractures or less stable openings), emplacement drifts will be set back from faults. Because the potential repository is designed to take advantage of the performance characteristics of the unsaturated zone, separation from the saturated zone is an important component in selecting the repository elevation. The repository would be isolated not only from present-day groundwater levels but also from future fluctuations of the water table. Geologic evidence (CRWMS M&O 2000b, Section 9.4) shows that the water table has not been more than about 120 m (390 ft) higher than its present level over the past several million years, even during cooler and wetter climates. Figure 1-13 illustrates a conceptual repository layout that addresses the design and range of operating modes described in this document, superposed on a contour map of the known water table elevations in the vicinity of the potential repository. Details A through D of the figure indicate the elevations of the northernmost emplacement drifts of the upper and lower blocks. The northernmost emplacement drift in the upper block would be approximately 210 m (690 ft) above the present water table elevation at that location; the northernmost emplacement drift in the upper block is the closest emplacement drift in this layout to the present water table. The water table elevation in Borehole WT-24 can be used as a check on this observation. As indicated in Figure 1-13, WT-24 is located approximately 120 m (390 ft) in a northerly direction from the location of Detail A; the elevation of the water table at WT-24 was reported as 840 m (2,750 ft) (CRWMS M&O 2000g, Table 3). The northernmost emplacement drift in the lower block is approximately 265 m (870 ft) above the present elevation of the water table at that location. Even at the higher levels associated with a water table rise, the emplacement drifts would still be more than about 90 m (290 ft) above the highest projected water table elevation. Analyses of the potential for variation in the elevation of the water table have also considered the possibility that water table variations could be caused by tectonic, volcanic, or hydrothermal processes. In addition to the DOE (CRWMS M&O 2000b, Section 4.4.5), both the National Academy of Sciences/National Research Council (National Research Council 1992) and the NWTRB (1999a, pp. 19 to 21) have reviewed evidence regarding the hypothesis that tectonic or hydrothermal processes could cause large-scale variations in the water table, possibly compromising the performance of the potential repository. Each review found that the available evidence did not support the hypothesis that large-scale fluctuations in the water table (to the level of the potential repository) had occurred in the past. The National Research Council also evaluated the theoretical possibility that the water table could rise significantly in the future and concluded that large variations were unlikely. More recently, the NWTRB found that a review of information developed and presented after the National Research Council review did not significantly affect their conclusions (NWTRB 1999a, p. 20). The combination of factors described above resulted in the selection of the middle to lower portion of the Topopah Spring welded tuff as the potential repository horizon (Figure 1-11) (BSC 2001d). This section is densely welded, with variable fracture density and lithophysal content. Experience in the Exploratory Studies Facility (e.g., monitoring of excavation characteristics, rock bolt loads, deformation of portal girders, and strain magnitudes of steel sets) and design analyses indicate that stable openings can be constructed in the Topopah Spring Tuff (Figure 1-12). Also, the thermal and mechanical properties of the rock should enable it to accommodate the range of temperatures expected during repository construction and operation. The selected horizon is well below the surface and well above the water table. Finally, the potential repository development area is located between major faults, with setbacks to mitigate any potential effects. Faulting and Local Structural Geology—The distribution and properties of faults and fractures in the volcanic bedrock are important elements of the structural geology of a potential repository at Yucca Mountain. They control the hydrologic and rock-mechanical properties of the system and therefore may affect postclosure performance and design. The distribution and recurrence history of the faults also controls estimates of seismic hazard for the repository. Ground motion from earthquakes is one factor to be considered during the preclosure operation of surface facilities. Studies show that the effects of fault displacement in the repository after closure will not significantly affect performance (see Section 4.3.2.2). The evaluations of seismic hazard, and its potential effects on the preclosure and postclosure performance of the repository, are described in Probabilistic Seismic Hazard Analyses for Fault Displacement and Vibratory Ground Motion at Yucca Mountain, Nevada (Wong and Stepp 1998), Section 3 of the Disruptive Events Process Model Report (CRWMS M&O 2000f), and Preliminary Preclosure Safety Assessment for Monitored Geologic Repository Site Recommendation (BSC 2001f). As described in the Subsurface Facility System Description Document, repository emplacement drifts will be set back from known faults (CRWMS M&O 2000h, Section 1.2.2.1.5). The structural geology of Yucca Mountain is controlled by block-bounding faults spaced 1 to 4 km (0.6 to 2.5 mi) apart. These faults include (from west to east) the Windy Wash, Fatigue Wash, Solitario Canyon, Bow Ridge, and Paintbrush Canyon faults (Figure 1-14). The Dune Wash and Midway Valley faults are also block-bounding faults but differ from the others in that they have no evidence of Quaternary movement (within the past 2 million years). The block-bounding faults commonly dip 50° to 80° to the west, with scattered dips of 40° to 50° and 80° to 90°. Some left-lateral displacement is commonly associated with these faults (Simonds et al. 1995; Day, Dickerson et al. 1998, p. 8). In some fault zones, several Paintbrush Group rock types have been mixed within the most intensely deformed parts of the fault, indicating that faulting has structurally juxtaposed various subunits as displacement of the bedrock has occurred. This is most apparent in the Solitario Canyon fault system. Individual fault strands within these zones are highly brecciated (i.e., composed of angular, broken fragments of rock). Displacement between the block-bounding faults occurs along multiple smaller faults, which may intersect block-bounding faults at oblique angles. The Ghost Dance and Sundance faults are examples of smaller "intrablock" faults near the potential repository. The Ghost Dance fault trends in a north–south direction, and can be followed on the surface for 3.7 km (2.3 mi). The fault plane dips steeply to the west (75° to 85°). The displacement and amount of brecciation (the degree to which rocks adjacent to the fault are broken and deformed) varies considerably along its length (Day, Dickerson et al. 1998). The fault zone has a maximum displacement of approximately 27 m (89 ft) down-to-the-west offset. There is no demonstrable Quaternary displacement (movement in the last 2 million years). Mapping in the Exploratory Studies Facility has shown that the zone of brecciation near the fault at the depth of the potential repository is very narrow. The northwest-trending Sundance fault is the only named fault within the boundaries of the potential repository (Spengler et al. 1994; Potter et al. 1999). It can be traced for approximately 750 m (2,460 ft), and shows no evidence of activity during the past 2 million years. The northeast-side-down vertical displacement across the fault zone does not exceed 11 m (36 ft). Fracture Characteristics—The distribution and characteristics of fractures at Yucca Mountain are important because in many of the hydrogeologic units at the site, particularly the welded tuffs, fractures are the dominant pathways for water flow in both the unsaturated and saturated zones. By controlling where, and at what rates, water is likely to flow under various conditions, the fracture systems play a major role in the performance of the repository. The potential repository has been designed to capitalize on the free-draining nature of the repository host rock, which would promote the flow of water past the emplaced waste and limit the amount of possible contact between waste and water. This feature was one of the attributes originally recognized by geologists studying Yucca Mountain (Roseboom 1983). Fractures at Yucca Mountain are generally of three types: early cooling joints formed during the original cooling of the rock mass, later tectonic joints caused by faulting and rock stress, and joints due to erosional unloading. Cooling and tectonic joints have similar orientations, but cooling joints are smoother. Cooling joints form two orthogonal (at 90° angles to each other) sets of steeply dipping fractures and, in some areas, a set of approximately horizontal fractures. Four steeply dipping sets and one nearly horizontal set of tectonic joints have been identified. Fracture density (the number of fractures in a given volume of rock), connectivity (the number of fractures that intersect each other), and fracture hydraulic conductivity (the capacity of the rock to transmit water in fractures) are highest in the densely welded tuffs and lowest in the nonwelded tuff units. The Tiva Canyon and Topopah Spring welded units are characterized by well-connected fracture networks, whereas the Paintbrush nonwelded units and the Calico Hills tuffs generally do not exhibit connected fractures. In the lithophysal zones of welded tuffs, the degree of connectivity is intermediate because fractures may end in the void spaces rather than propagate through them. In all the geologic units, fracture density varies both vertically and laterally because of variations in tuff properties. Fractures related to faults may affect the hydraulic properties near fault zones and provide flow paths through hydrologic units that are otherwise not prone to fracture flow. Even nonwelded units, such as the Pah Canyon and Calico Hills tuffs, may allow water to move in fractured zones adjacent to faults. Based on observations at the surface and in the Exploratory Studies Facility, the zone of influence around faults in which fracture properties are modified may range from approximately 1 to 7 m (3 to 23 ft) (Sweetkind et al. 1997, p. 67). The width of the zone with increased numbers of fractures generally correlates with the amount of movement on the fault (i.e., faults with larger displacements have larger fractured zones). The amount of fracturing also depends on the rock type involved: nonwelded or partially welded tuffs can accommodate a greater amount of fault movement without fracturing than densely welded rocks. Integrated Site Model—A repository site would support stable excavations that can be safely maintained during the operating life of the facility, and it should have geochemical and hydrologic properties that contribute to waste isolation after closure. The welded tuffs in the unsaturated zone at Yucca Mountain were originally identified as potential candidates for hosting a repository (Roseboom 1983) because of their geologic, hydrologic, and geochemical characteristics. Site characterization has confirmed the basic assumptions about how a repository in the unsaturated zone would likely perform. The stratigraphic and structural data from the site have been combined with rock property and mineralogical results to build a three-dimensional integrated site model (CRWMS M&O 2000i). Data from boreholes, surface geological mapping, and geophysical surveys form the basis for the conceptual understanding of the geologic framework of the site. This framework was used to develop spatial models of the distribution of geological, geotechnical, hydrologic, mineralogic, and geochemical parameters. The integrated site model thus provides technical input to the design of the potential repository and to the models used to assess its future performance.1.3.2.2.3 Geomorphology and Erosion
Any potential geologic repository site would be selected and designed so that natural geologic and hydrologic processes do not compromise the integrity of the repository (DOE 1986c, Section 6.3.1.5). Because erosion of the rock overlying the potential repository could, in theory, threaten waste isolation, an evaluation of erosion rates over time and an assessment of the potential for future erosion have been performed. A wide variety of geologic evidence indicates that erosion at Yucca Mountain has occurred at very slow rates for millions of years and would not adversely affect the waste isolation capability of the site in the future (CRWMS M&O 2000b, Section 7.4). Geologic evidence indicates that the basic morphology (or shape) of Yucca Mountain was already formed about 10 million years ago. Studies of the tectonic evolution of the area (Day, Dickerson et al. 1998, pp. 17 to 19; CRWMS M&O 2000b, Section 4.6.3.3) demonstrate that most of the faulting occurred shortly before, during, and soon after the eruption of the tuffs that comprise Yucca Mountain. This period of intense tectonic activity began about 16 million years ago with faulting related to the extension of the Basin and Range, followed by eruption of the volcanic units below Yucca Mountain onto the Paleozoic and Precambrian basement. About 12.8 to 12.7 million years ago, the thick tuff units of the Paintbrush Group (including the Topopah Spring and Tiva Canyon tuffs) were erupted from calderas to the north and deposited in approximately horizontal layers. Movement along block-bounding faults then tilted the volcanic rocks 10° to 20° to the east and formed major topographic features, such as Solitario Canyon and Midway Valley. The Rainier Mesa Tuff of the Timber Mountain Group was erupted about 11.6 million years ago onto an irregular land surface that was already similar to modern Yucca Mountain (i.e., major ridges and valleys created by faulting already existed). Faulting continued after deposition of the Rainier Mesa Tuff but at greatly reduced rates: displacement on the block-bounding faults was up to several hundred meters before 11.6 million years but has been only a few meters in the last 10 million years. Over the past several million years, erosion in the Yucca Mountain region has been slow, with only minor effects on major landforms. Several lines of evidence have been considered in the analysis of potential erosion at or near Yucca Mountain. These include evidence related to:
1.3.2.2.4 Natural Resource Potential
A repository must isolate the spent nuclear fuel and high-level radioactive waste it contains from both people and the environment. In order to reduce the chance that future individuals or groups might inadvertently encounter waste while searching for other exploitable resources, sites with high potential for natural resources have typically been excluded from consideration. As part of the characterization of the Yucca Mountain site, therefore, the potential for economically valuable resources has been carefully evaluated. This section summarizes the results of that assessment. Resource potential is difficult to predict because it depends on many factors, including economics (i.e., supply, demand, and cost of production), the potential discovery of new uses for resources, and the discovery of synthetic materials to replace natural resources. Therefore, this evaluation is based on the present-day use and economic value of resources; it does not predict future market trends or undiscovered uses for resources. All common types of natural resources—including metallic minerals, industrial rocks and minerals, hydrocarbons (i.e, petroleum, natural gas, oil shale, tar sands, and coal), and geothermal energy—have been considered. In a general sense, Nevada contains abundant resources, ranking second in the U.S. in the value of nonfuel (i.e., excluding oil, gas, coal, and geothermal) mineral production in 1996 (Nevada Bureau of Mines and Geology 1997, Summary). Nevada leads the nation in the production of gold, silver, mercury, and barite. Additional metals, including copper, lead, zinc, iron, and such industrial materials as brucite, magnesite, clays, gemstones, gypsum, sand, gravel, and crushed stone are being or have been produced in Nevada. Small but economic oil deposits occur in Railroad Valley in east-central Nevada, and geothermal resources occur in California and northern Nevada within the Great Basin. Although economic gold mineralization is present in the region (most notably near Beatty), Yucca Mountain contains no identified metallic mineral or uranium resources (CRWMS M&O 2000b, Section 4.9). On the basis of detailed studies of geology, geochemistry, mineralogy, mineral alteration, and geophysical data and remote sensing, the Yucca Mountain site is considered to have little or no potential for deposits of metallic minerals or uranium resources that could be mined economically now or in the foreseeable future. Geological and geochemical comparisons between Yucca Mountain and metal mining districts in the region indicate substantial differences in the geologic and geochemical patterns observed for precious metals, base metals, and pathfinder elements (Castor et al. 1999, Section 6). Many industrial rock and mineral commodities occur in the Great Basin but not at Yucca Mountain. Although barite, clay minerals, fluorite, limestone, perlite, and zeolites have been identified in samples from Yucca Mountain, these occurrences are minor and at depths too great to be mined economically (Castor and Lock 1995, Section 7). It is possible that alluvial deposits at the site could be used as concrete aggregate for local construction and that some of the Tertiary tuff could be used as building stone. However, neither the alluvial deposits nor the tuff have any properties or features that would make them more marketable than other deposits readily available and closer to processing plants and end users (Castor and Lock 1995, Section 6.4.3.1). Nevada is not a large producer of oil or natural gas, although a few producing fields exist north of the Yucca Mountain region. Some of the conditions of source, reservoir, trap, and seal that characterize petroleum accumulations of the Great Basin are present to some degree in the Yucca Mountain area (French 2000, p. 39). Most evidence, however, indicates that the accumulation of oil or natural gas near the potential repository site is unlikely. It is extremely unlikely that tar sands, oil shale, or coal occur as economic resources at the Yucca Mountain site. If any of these resources were associated with Paleozoic marine rocks underlying the Tertiary volcanic rocks at Yucca Mountain, they would occur at depths greater than 1,800 m (5,900 ft). Extraction at these depths would not be economically feasible in the foreseeable future. There are no geothermal discoveries near Yucca Mountain, and there are no potential users located at or near the site. Chemical analyses of fluids throughout the area indicate that most waters are nonthermal in origin. Geophysical data, including gravity, magnetic, seismic, and heat flow data, failed to delineate any systematic structural evidence for a thermal anomaly. Compared with the physical attributes of geothermal systems that have developed in other parts of the Great Basin, no economically viable resources were identified within the Yucca Mountain area (Flynn et al. 1996).1.4 POSTCLOSURE PERFORMANCE
Since the National Academy of Sciences concluded that geologic disposal was feasible in 1957 (National Academy of Sciences Committee on Waste Disposal 1957), many scientists (e.g., de Marsily et al. 1977; Konikow and Ewing 1999) and reviewers of the U.S. repository program (NWTRB 1999b; NWTRB 2000; Budnitz et al. 1999) have recognized the difficulty associated with assessing repository performance over the long time frames necessary to protect public health and safety. Developing confidence in the long-term performance of a geologic repository is one of the greatest challenges faced by the DOE. Because of the uncertainty associated with assessments of performance for 10,000 years, there is no simple way to guarantee that the facility will function as modeled throughout the period of performance. In NRC's licensing rule, 10 CFR Part 63 (66 FR 55732), the NRC recognizes this irreducible uncertainty and clearly states that "proof" of performance cannot be produced in the ordinary sense of the word. Rather, the NRC, like the EPA, would require in licensing a "reasonable expectation" that the postclosure performance standards will be met. The DOE's approach to developing confidence in the safety of geologic disposal, known as the postclosure safety case, relies on multiple, independent lines of evidence. The first element of the safety case is a thorough and quantitative evaluation of the future performance of the repository, based on a comprehensive testing program that has evolved to address identified uncertainties and a repository design developed to complement the natural setting of the site. EPA and NRC regulations specify the method by which the DOE will analyze and demonstrate in licensing that a repository can safely isolate spent nuclear fuel and high-level radioactive waste (i.e., a total system performance assessment [TSPA]). The process used to develop the total system performance assessment for site recommendation (TSPA-SR), which is described briefly in Section 4.4 and in more detail in Total System Performance Assessment for the Site Recommendation (CRWMS M&O 2000a), includes analyses of all the processes expected to operate at the repository that could affect its ability to isolate waste. The supplemental TSPA described in FY01 Supplemental Science and Performance Analyses (BSC 2001a; BSC 2001b) and the revised supplemental TSPA models described in Total System Performance Assessment—Analyses for Disposal of Commercial and DOE Waste Inventories at Yucca Mountain—Input to Final Environmental Impact Statement and Site Suitability Evaluation (Williams 2001a) and Total System Performance Assessment Sensitivity Analyses for Final Nuclear Regulatory Commission Regulations (Williams 2001b) all use the same TSPA approach and method. All of these models and analyses also explicitly consider both disruptive events and alternative models that could result in unanticipated behavior (i.e., identify what could go wrong). The evaluation directly addresses uncertainty in both the DOE's knowledge of the site and in future conditions, and it includes numerical sensitivity analyses to test how the repository might perform if current or future conditions differ from those expected. Because the DOE recognizes that uncertainty about the future performance of the repository cannot be completely quantified or eliminated (i.e., models alone cannot capture all the uncertainties in natural systems), the safety case includes several additional measures designed to provide confidence and assurance that the repository will meet applicable radiation protection standards after it is permanently closed. These measures include:
1.4.1 Performance Assessment
As noted above, the regulatory requirements for a potential repository are based on quantitative assessments of the system's performance. For Yucca Mountain, performance assessment provides not only a means for estimating relative performance but also a framework for organizing and describing the site and the repository design. Performance assessment has been used as a management tool to integrate the scientific and engineering programs and to assess the importance and priority of various program activities, consistent with the overall goal of determining whether Yucca Mountain can safely host a repository facility. Performance assessment is a systematic method for evaluating repository system behavior over an extended time. Analysts build detailed mathematical models of the features, events, and processes that could affect performance. Then they incorporate the results of these detailed models into an overall model of the repository system, called the "total system performance assessment model." This integrated model is used to assess how the natural and engineered elements of a waste disposal system would work together over the long period required to isolate wastes. Sections 4.2, 4.3, and 4.4 present the results of these analyses, which are more fully described in Total System Performance Assessment for the Site Recommendation (CRWMS M&O 2000a). Supplemental analyses are described in FY01 Supplemental Science and Performance Analyses (BSC 2001a; BSC 2001b), and Total System Performance Assessment—Analyses for Disposal of Commercial and DOE Waste Inventories at Yucca Mountain—Input to Final Environmental Impact Statement and Site Suitability Evaluation (Williams 2001a), and Total System Performance Assessment Sensitivity Analyses for Final Nuclear Regulatory Commission Regulations (Williams 2001b). Performance assessment models are probabilistic (i.e., they consider the likelihood that the system will behave in a certain way) because of the nature of the processes and systems being analyzed. The natural system is heterogeneous both in space and time; the processes simulated in the models are also variable in space and time. For these reasons, performance assessment uses a probabilistic approach that directly incorporates evaluations of the variability of site properties and a range of possible process behaviors that could occur into the estimates of the future performance of the repository. Performance assessments provide one means to identify which uncertainties about the behavior of a disposal system are significant and which are not, and which elements of the repository design are most important to performance. This helps focus efforts to improve the design and the defensibility of performance analyses. Performance assessments are refined iteratively during the course of developing, evaluating, and improving a repository design. The DOE has also conducted analyses of potential barriers to radionuclide migration to identify and evaluate the performance contribution of natural features of the geologic setting and design features of the engineered barrier system (see Section 4.5). At Yucca Mountain, these multiple barriers may contribute to confidence in the performance of the repository by providing defense in depth: several elements of the natural and engineered systems contribute to the isolation of waste by functioning independently to limit possible releases. Section 4.4 presents a summary of the results of the performance assessment calculations performed for the Yucca Mountain site and found in Total System Performance Assessment for the Site Recommendation (CRWMS M&O 2000a) and subsequent analyses (BSC 2001a; BSC 2001b; Williams 2001a; Williams 2001b). The TSPA-SR results show that for a 10,000-year period, the calculated dose in the nominal scenario is zero (CRWMS M&O 2000a). A supplemental TSPA model yielded a calculated dose of approximately 2
10-4 mrem/yr (BSC 2001b, Section 5.1). A revised supplemental TSPA model calculates a dose of 1.7 10-5 mrem/yr (Williams 2001a). The small doses in the supplemental TSPA model result from the inclusion of a few early failures of waste packages due to improper heat treatment. The supplemental TSPA model projected a peak mean dose of 2 10-4 mrem/yr for the higher-temperature operating mode and 6 10-5 mrem/yr for the lower-temperature operating mode over a 10,000-year period. Over this same period, the revised supplemental TSPA model projected a peak mean dose to the reasonably maximally exposed individual of 1.7 10-5 mrem/yr for the higher-temperature operating mode and 1.1 10-5 mrem/yr for the lower-temperature operating mode for the nominal scenario (BSC 2001b, Section 4.1.3; Williams 2001a, Section 6, Table 6).
1.4.2 Importance of Repository System Components to Long-Term Performance
The various components of the repository system contribute to performance in different ways. Certain components of the system are more important to performance than others, as shown by performance assessment sensitivity studies (e.g., Section 4.4.5). Considerations of safety margin, defense in depth, insights from analogues, and expert judgments also help identify the importance of repository components to performance (see Section 4.1). Knowledge of how components of the system affect performance can provide insights on how design or operating mode features could be developed in a manner that could contribute to long-term performance or mitigate potentially adverse conditions. Ongoing evaluations over the range of operating modes could lead to enhanced understanding of how the repository system components could contribute to long-term performance.1.4.3 Addressing Uncertainty in Total System Performance Assessment
Even though the Yucca Mountain site represents a fairly simple geologic/hydrologic system conceptually—a relatively dry site consisting of fractured volcanic rock hundreds of meters above the water table—it has complexities that are difficult to model but could be important over long periods of time. Uncertainties exist in the understanding of both natural processes (e.g., infiltration of water at the surface, percolation in fractures and rock matrix, disruptive processes of earthquakes and volcanism) and the ways in which the engineered system will perform when exposed to the environment (e.g., seepage into drifts, thermal processes related to the heat generated by the waste, corrosion of waste packages). Capturing those uncertainties and understanding their impacts is critical to understanding how a repository might behave in the future. Accommodating uncertainties in the assessment of the performance of a potential repository at Yucca Mountain means recognizing that uncertainties exist and explicitly identifying those that may be important to performance. The DOE's approach to dealing with uncertainties is described in Sections 4.1.1.2 and 4.4.1.2. Some, but not all, of those uncertainties can be quantified; that is, scientists can and have collected data that allow them to estimate the probability that a variable will assume different values over the spatial and temporal scales of an operating repository. Those uncertainties that can be quantified can be incorporated directly into performance assessment results. Approaches to address design-related uncertainty concerns through consideration of a range of thermal operating modes and the effects on performance across the range of temperatures are described in Sections 2.1.5 and 4.4.5.1.2. Studies have been performed to enhance the understanding of the environmental conditions associated with the lower-temperature ranges of the operating modes. Supplemental performance assessment analyses and sensitivity studies have been conducted to evaluate the performance of lower-temperature operating modes. The analyses incorporate the results of other efforts to quantify uncertainties and extend the applicable range of the process models. Of particular interest are analyses that address performance-related responses of the design and operating mode, considering temperature-sensitive parameters and coupled thermal-mechanical-chemical-hydrologic processes. This approach is intended to ensure that the performance evaluations appropriately consider the potentially detrimental and potentially beneficial aspects of the repository's performance over a range of operating modes encompassing temperatures above and below the boiling point of water. Results of these evaluations are described in FY01 Supplemental Science and Performance Analyses (BSC 2001a; BSC 2001b). There are also uncertainties that cannot readily be quantified (e.g., the possibility that the models used for compliance analyses do not include, or accurately simulate, processes that may be important to performance). The DOE has made a substantial effort to identify, characterize, and mitigate the potential impacts of these unquantified uncertainties. They have been identified and characterized through detailed consideration of the features, events, and processes that might affect repository performance. The principal mechanism for mitigating unquantified uncertainties is the multiple lines of evidence provided by the postclosure safety case, as described in Section 4.1. In particular, design (safety) margin and defense in depth provide a degree of confidence independent of the results of TSPA analyses.1.4.4 Time Frame for Performance Analyses
EPA standards and NRC licensing regulations relevant to the Yucca Mountain disposal system contain postclosure performance standards that would apply during the first 10,000 years after repository closure. For this reason, performance assessment results have been presented on plots that extend for 10,000 years, as shown in Section 4.4. The EPA's 40 CFR 197.35 and NRC's 10 CFR 63.341 (66 FR 55732) also specify that the DOE should calculate the peak dose that would occur after 10,000 years but within the period of geologic stability, and present the results in the EIS. Although no regulatory standard applies to the results of this analysis, the EPA and NRC noted that the peak dose calculations would complement the 10,000-year performance assessment results as an indicator of long-term performance. These analyses, which have been performed out to 100,000 years and 1 million years (see Sections 4.4.2.2 and 4.4.2.4, respectively), provide additional confidence in the 10,000-year results.