EXECUTIVE SUMMARY
I. INTRODUCTION
Commercial electric power generation, nuclear weapons production, the operation of naval reactors, and research and development activities produce spent nuclear fuel and high-level radioactive waste. These radioactive materials 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, the National Academy of Sciences evaluated options for land disposal of radioactive waste in the 1950s. The U.S. Atomic Energy Commission and its successor agencies, the U.S. Energy Research and Development Administration and the DOE, continued to analyze nuclear waste management options throughout the 1960s and 1970s. In 1979, an Interagency Review Group that included representatives of 14 federal government entities provided findings and recommendations to the President. After analyzing a range 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 was reflected in the Nuclear Waste Policy Act of 1982 (NWPA), which established the U.S. responsibility and policy for the disposal of spent nuclear fuel and high-level radioactive waste. Congress established the framework for addressing the issues of nuclear waste disposal in the NWPA and related statutes and designated the roles and responsibilities of the federal government and the owners and generators of the waste. Congress assigned responsibility to:II. Sources of Materials Considered for Disposal
By statute, the DOE is responsible for the safe, permanent disposal of spent nuclear fuel from commercial nuclear power plants. The DOE must also dispose of large quantities of DOE-owned high-level radioactive waste from the production of nuclear weapons and smaller quantities of spent nuclear fuel from weapons production reactors, research reactors, and naval reactors. The NWPA limits the amount of spent nuclear fuel and high-level radioactive waste that can be emplaced in the nation's first geologic repository to 70,000 MTHM until a second repository is in operation. The materials that may be disposed at Yucca Mountain include about 63,000 MTHM of commercial spent nuclear fuel; about 2,333 MTHM of DOE spent nuclear fuel; and about 4,667 MTHM of DOE high-level radioactive waste. All the waste forms transported to and received at a repository would be solid materials. No liquid waste forms would be accepted for disposal. Figure 1 shows the current locations and types of waste that would be emplaced at a repository. As of December 1999, the United States had generated about 40,000 MTHM of spent nuclear fuel from commercial nuclear power plants. This amount could more than double by 2035 if all currently operating plants complete their initial 40-year license period. By 2035, the United States will also have about 2,500 MTHM of spent nuclear fuel from research reactors, naval reactors, reactor prototypes, and reactors that produced nuclear weapons materials. The majority of this spent nuclear fuel is stored at DOE sites in Idaho, South Carolina, and Washington. In addition, liquid waste from nuclear weapons production programs is stored in underground tanks at the same DOE sites. This high-level radioactive waste will be mixed with silica sand and other constituents, melted together, and poured into stainless steel canisters. Once the glass solidifies (vitrifies), the canister would be sealed, loaded into a transport cask, and shipped to a repository. Vitrified waste is resistant to dissolution and would stay intact for thousands of years. Some of the material that would be disposed in a repository would come from surplus plutonium resulting from the production and decommissioning of nuclear weapons. A nominal 50 metric tons of surplus plutonium must be safely dispositioned. Current plans call for some of the surplus plutonium to be combined with uranium to form fuel that would be used in commercial reactors. The resulting spent nuclear fuel would be disposed as commercial spent nuclear fuel. Some of the surplus plutonium would be immobilized in ceramic and placed in canisters for repository disposal. These canisters will be filled with molten high-level radioactive waste glass, which will vitrify into a glass waste form.III. Geology of the Yucca Mountain Site
The Yucca Mountain site is located on federal land adjacent to the Nevada Test Site in Nye County, Nevada, about 160 km (100 mi) northwest of Las Vegas. The mountain consists of a series of ridges extending 40 km (25 mi) from Timber Mountain in the north to the Amargosa Desert in the south. The water table at Yucca Mountain is approximately 500 to 800 m (1,600 to 2,600 ft) below the surface of the mountain at the potential repository location. The zone of soil or rock below the ground surface and above the water table is called the unsaturated zone. The underground facility would be located in the unsaturated zone, about 200 to 500 m (660 to 1,600 ft) below the surface and, on average, about 300 m (1,000 ft) above the water table. The deep water table and thick unsaturated zone at Yucca Mountain result from the low infiltration rate of surface water due to low annual rainfall and high rates of evaporation and transpiration (the process by which water vapor passes from soil into plants, then into the air). The potential repository would be located in volcanic rock, called tuff, that was deposited by a series of eruptions between approximately 11 and 14 million years ago. The characteristics of the volcanic rock have been studied in underground excavations and boreholes, and by geologic mapping of the surface. Mapping and other studies show that faults are present in the vicinity of Yucca Mountain. The location, timing, and amount of movement on these faults have been characterized as part of the DOE's seismic hazard analysis. The location of the underground facility was identified using several factors, including the thickness of overlying rock and soil, the characteristics of the rock that would host the repository, the location of faults, and the depth to groundwater. The facility would be sited deep enough underground to prevent waste from being exposed to the environment and to discourage human intrusion. The host rock for a geologic repository must be stable enough to sustain excavated openings during repository operations. The rock must also be able to absorb heat generated by the spent nuclear fuel. The Topopah Spring Tuff rock unit, in which the underground facility would be constructed, exhibits these characteristics.IV. Repository Design
The DOE has developed a design for a Yucca Mountain disposal system that could give future generations the choice of either closing and sealing the underground facility as early as allowable under NRC regulations or keeping it open and monitoring it for a longer time period. The design for the potential repository would not preclude the option for future generations to make societal decisions to monitor the repository for up to 300 years before making decisions to close the underground facility. Figure 2 is a conceptual illustration of the proposed facilities and structures that would make up a potential repository. It shows the facilities as they would appear after construction. In general, the operations that would be performed include:V. Natural Barriers
The barriers important to waste isolation are broadly characterized as natural barriers, associated with the geologic and hydrologic setting, and engineered barriers, discussed in the following section. The engineered barriers are designed specifically to complement the natural system in prolonging radionuclide isolation within the disposal system and limiting their potential release. The natural barriers at Yucca Mountain include:VI. Engineered Barriers
The components of the engineered barrier system are designed to complement the natural barriers in isolating waste from the environment. The repository design includes the following engineered barriers: the waste package, the waste form, the drip shield, and the emplacement drift invert. Figure 3 depicts waste packages within an emplacement drift. The engineered barriers would contribute to waste isolation by (1) using long-lived waste packages and drip shields to keep water away from the waste forms and (2) limiting release of radionuclides from the engineered barriers through components engineered for optimum performance in the expected environment. Waste Packages—Waste packages would have a dual-metal design containing two concentric cylinders. The inner cylinder would be made of Stainless Steel Type 316NG. The outer cylinder would be made of a corrosion-resistant, nickel-based alloy (Alloy 22). Alloy 22 would protect the stainless steel inner cylinder from corrosion, and Stainless Steel Type 316NG would provide structural support for the thinner Alloy 22 cylinder. Corrosion tests have been performed and are continuing in a variety of thermal and chemical environments to provide additional information on the corrosion rate of Alloy 22. Numerous analyses of the expected performance of the waste package and associated uncertainties have been performed. Tests and analyses indicate that Alloy 22 would last considerably longer than 10,000 years in the range of expected repository environments at Yucca Mountain. Each waste package would have outer and inner lids at each end of the cylinder. The outer (closure) lids would be made of Alloy 22, and the inner lids would be made of Stainless Steel Type 316NG. The loading end of the waste package has a third flat closure lid made of Alloy 22, which would be placed between the inner lid of stainless steel and the outer lid of Alloy 22. The flat closure lid provides an extra barrier against a potential release caused by cracks and corrosion in the closure weld areas. The basic waste package design is the same for all the waste forms. However, the sizes and internal configurations vary to accommodate the different waste forms. Figure 3 illustrates several common internal designs, including two for different types of commercial spent nuclear fuel and one for high-level radioactive waste and DOE spent nuclear fuel (a codisposal package). Waste Form—The materials that would be disposed at Yucca Mountain include spent nuclear fuel and vitrified high-level radioactive waste. Both of these waste forms are solid materials that will degrade very slowly in the unsaturated environment of the repository. Spent nuclear fuel primarily consists of heavy metal oxides of uranium, plutonium, and other radionuclides. High-level waste consists of a vitrified borosilicate glass containing radionuclides. The release of radionuclides from the waste form will further be limited by the low solubilities of most radionuclides in the oxidizing environment of the unsaturated repository. Drip Shields—Drip shields would be installed over the waste packages prior to repository closure. The drip shields would divert any moisture that might drip from the drift walls, as well as condensed water vapor, around the waste packages to the drift floor. All the drip shields would be the same size, so one design could be used with all the waste packages. The drip shields would be made of titanium, which would provide corrosion resistance and structural strength. They are designed to divert moisture around waste packages for thousands of years. Tests continue on drip shield materials to assess how well current data and models can be extrapolated over long periods of time. The drip shields would also maintain their function in the event of expected rockfalls, as the emplacement drifts degrade over time. Drift Invert—The invert includes the structures and materials that would support the pallet and waste package, the drift rail system, and the drip shield. It is composed of two parts: the steel invert structure and the ballast (or fill) that consists of granular material. Following closure, one function of the granular material in the invert would be to provide a layer of material below the waste packages that would slow the movement of radionuclides into the host rock. Water is not expected to accumulate and flow beneath the drip shields, so the most likely way radionuclides could move is by diffusion (where dissolved or suspended particles migrate slowly from zones of high concentration to zones of low concentration) through thin films of water on the granular material.VII. Site Characterization Data and Analyses
During the site characterization program, the DOE has performed extensive surface-based tests and investigations, underground tests, laboratory studies, and modeling activities designed to provide the technical information necessary for the evaluation of long-term repository performance. The site characterization program has evolved in response to advancements in scientific understanding, changes in regulatory requirements, and changes in program requirements, such as changes in design requirements for the potential repository. Yucca Mountain Science and Engineering Report describes the data collected during site characterization and explains the DOE's understanding of the processes that could affect the ability of the potential repository system to isolate waste. Computer models of the hydrologic, geochemical, thermal, and mechanical processes that would operate in the repository system over time have been developed from the data collected. These process models have been used to develop an overall total system performance assessment (TSPA) model that evaluates how the potential repository may behave for 10,000 years. Analyses have also considered uncertainty in model results, and have evaluated alternative conceptual models to assess the extent to which the results of performance assessment depend on the details of the underlying process models.VIII. Processes Important to Long-Term Repository Performance
The processes important to the repository's performance after it is closed (postclosure) include those that control the movement of water through the geologic setting. These processes begin with precipitation, as rain and snow, at the surface, a fraction of which infiltrates into the mountain. This net infiltration would move through the unsaturated zone to the level of the emplacement drifts, then downward through the unsaturated zone to the saturated zone. Within the saturated zone, water would move laterally away, where it could eventually reach the accessible environment. Within the underground facility, water moving past the engineered barriers would be affected by the physical and chemical processes associated with heat from the emplaced waste. These processes could eventually corrode waste packages, degrade the waste form, and dissolve some of the waste. Only after all of these processes have occurred could radionuclides move out of the repository. To analyze the possible future performance of the repository, the DOE has studied many of the physical and chemical processes that would act on the repository system's barriers. The results of these studies are provided in Yucca Mountain Science and Engineering Report, along with an analysis of how the parts of the system would work together to isolate waste. The following subsections present a summary of the interdependent processes that may affect the repository's ability to isolate waste. Figure 4 illustrates the processes that were considered and modeled for the TSPA. Certain disruptive events that could affect these processes are also considered in the following discussion. The treatment of uncertainties associated with these processes is addressed in a later section. Unsaturated Zone Flow—Because of the present-day arid climate at Yucca Mountain and the surface processes of runoff, evaporation, and transpiration, the amount of water available to contact and transport radionuclides is expected to be small. However, the availability of water may increase as a result of wetter climates that may occur in the future. The higher-temperature operating mode described in this report uses the heat produced by the waste to effectively limit the potential for contact between water and waste packages for hundreds to thousands of years. The pillar area between the emplacement drifts would be maintained below the boiling point of water to promote water drainage through the cooler portions of the rock pillars. Both the higher-temperature and lower-temperature operating modes would also take advantage of natural processes that would divert water around drift openings. Most water moving in the unsaturated zone flows through fractures. Water flowing in narrow fractures will usually remain in them rather than flow into large openings, such as drifts, because of capillary pressure in the fractures. Thus, capillary forces and water flow in unsaturated zone fractures would limit seepage into openings, allowing most water to move past (not into) the emplacement drifts. If water does seep into an emplacement drift, most of it would flow down the drift wall to the floor and drain without contacting either the drip shields or the waste packages. Models of repository performance have evaluated both higher- and lower-temperature operating modes.Many processes have been studied during the evaluation of the Yucca Mountain site. Important processes considered in the unsaturated zone flow models include:
IX. The Attributes of Safe Disposal
The Yucca Mountain disposal system can be described in terms of five key attributes that would be important to long-term performance: (1) limited water entering waste emplacement drifts; (2) long-lived waste package and drip shield; (3) limited release of radionuclides from the engineered barriers; (4) delay and dilution of radionuclide concentrations by the natural barriers; and (5) low mean annual dose considering potentially disruptive events. These attributes are summarized below. The first four reflect the interactions of natural barriers and the engineered barriers in prolonging the containment of radionuclides within the repository and limiting their release. The fifth attribute reflects the likelihood that disruptive events would not affect repository performance over 10,000 years. Limited Water Entering Emplacement Drifts—The climate at the Yucca Mountain site is dry and arid with precipitation averaging about 190 mm (7.5 in.) per year. Little of this precipitation percolates into the mountain; nearly all of it (above 95 percent) either runs off or is lost to evaporation or transpiration, thereby limiting the amount of water available to seep into the underground facility. For the higher-temperature operating mode described in Yucca Mountain Science and Engineering Report, a thermal management strategy was developed that would take advantage of the heat of the emplaced wastes to drive the limited water that exists away from the emplacement drifts. The heat generated by the waste would dry out the rock surrounding the drift and decrease the amount of water available to contact the waste packages until the wastes have cooled substantially. Drainage of water in the rock pillars between drifts would be encouraged by keeping much of the pillar rock between the drifts below the boiling temperature of water. As long as emplacement drift walls remain at temperatures above the boiling point of water, no liquid water can remain in the underground facility, and very little can remain in the underground structure. For the lower-temperature operating modes (and for higher-temperature operating modes after the waste packages have cooled below the boiling point of water), the heat associated with waste will still cause evaporation in and near the emplacement drifts. This process would limit the amount of water in the rock near the waste packages. In lower-temperature operating modes, the waste packages would be exposed to water sooner. Because the rock would eventually cool in any operating mode, there does not appear to be a significant difference in the amount of water to which the waste packages would eventually be exposed. The repository design also takes advantage of the mechanical and hydrologic processes that divert water around emplacement drift openings in the unsaturated zone. Because of capillary forces, water flowing in narrow fractures tends to remain in the fractures rather than flow into large openings, such as drifts. If any water reached an emplacement drift, it could flow down the drift wall to the floor and drain without contacting the drip shields or waste packages. Thus, the natural and engineered features of a Yucca Mountain disposal system will combine to limit the potential for water to enter the emplacement drifts. Long-Lived Waste Package and Drip Shield—To further reduce the possibility of water contacting waste, the DOE has designed a robust, dual-wall waste package with an outer cylinder of corrosion-resistant nickel-based metal, Alloy 22. Alloy 22 was selected because it will remain stable in the geochemical environment expected in the potential repository. In the higher-temperature operating mode, the repository environment would be warm, with temperatures at the waste package surface initially rising above the boiling point of water. Waste package surface temperatures are expected to gradually decrease to below boiling after a period of hundreds to thousands of years, depending on the waste package's location in the repository and other factors discussed previously. In lower-temperature operating modes, the waste packages would be exposed to water earlier. Chemically, the environment is expected to be at near-neutral pH (mildly acidic to mildly alkaline) and mildly oxidizing. Because most corrosion would occur only in the presence of water, and because highly corrosive chemical conditions are not expected, both the titanium drip shield and the Alloy 22 outer barrier of the waste package are expected to have long lifetimes. Limited Release of Radionuclides from the Engineered Barrier System—Because of the characteristics of the natural system, the drip shields, and the waste packages, the DOE does not expect water to come into contact with the waste forms for more than 10,000 years. Even if water were to penetrate a breached waste package before 10,000 years, several characteristics of the waste form and the repository would limit radionuclide releases. First, because of the warm temperatures, much of the water that penetrates the waste package would evaporate before it could dissolve or transport radionuclides. Neither spent nuclear fuel nor glass waste forms will dissolve rapidly in the expected repository environment. Although the performance of the cladding as a barrier may vary because of possible degradation, it is expected to limit contact between water and the waste. The component of the engineered barrier system below the waste package, called the invert, contains crushed tuff that would also limit the transport of radionuclides into the host rock, as discussed under Engineered Barrier System Transport. Delay and Dilution of Radionuclide Concentrations by the Natural Barriers—Eventually, components of the engineered barrier system will degrade, and small amounts of water will contact waste. Analyses indicate that this is not likely to occur within the first 10,000 years following repository closure. Even then, features of the geologic environment and the repository would limit radionuclide migration to the accessible environment and slow it by hundreds to thousands of years. Processes that could be important to the movement of radionuclides include sorption, matrix diffusion, dispersion, and dilution. Rock units in both the unsaturated zone and the saturated zone contain minerals that can adsorb many types of radionuclides (i.e., radionuclides would attach to and collect on the mineral surfaces). As water flows through fractures, dissolved radionuclides can diffuse into and out of the pores of the rock matrix, increasing both the time it takes for radionuclides to move through the geologic setting and the likelihood that radionuclides will be exposed to sorbing minerals. Dispersion and dilution will occur naturally as potentially contaminated groundwater flows and mixes with other groundwater and reduces the concentration of contaminants. Low Mean Annual Dose Considering Potentially Disruptive Events—Yucca Mountain provides an environment in which hydrologic and geologic conditions important to waste isolation (e.g., a thick unsaturated zone with low rates of water movement) have changed little for millions of years. Analysts have identified and evaluated a wide variety of potentially disruptive processes and events that could affect the performance of the Yucca Mountain disposal system. These range from extremely unlikely events, such as meteor impacts, to events that are likely to occur, such as regional climate change. Although the probability of volcanic activity in or near the Yucca Mountain site is low, volcanic activity was a consideration in TSPA in the disruptive scenario case. Performance assessment results to date show that potentially disruptive events are not likely to compromise the system performance.X. Uncertainties in Data and Models
Quantitative assessments of the long-term performance of the disposal system consider a comprehensive set of features, events, and processes that may have an effect on that performance. The features and characteristics of the site and geologic setting are incorporated into conceptual and numerical models. The likelihood of occurrence and consequences of processes and events that may affect repository performance are evaluated, then incorporated, as appropriate, into the numerical models. Although the DOE continues to evaluate ways to reduce uncertainties in repository performance models, uncertainties will always remain because of the long time frames over which the system performance must be assessed, the natural variability in features and processes at the site, and limitations on the amount of data that can be collected. Features, events, and processes are generally represented probabilistically in a performance assessment to address this inherent uncertainty and variability. Numerous analyses have been performed to help the DOE understand the extent to which the results of total system performance assessments are robust (not likely to change significantly as new information is gathered in the future). These include analyses to assess the degree of realism in current process models, to quantify key uncertainties, and to improve the understanding of conservatism in the models and in performance assessment results. The DOE has also evaluated whether uncertainties (especially modeling uncertainties that are not easily quantified) can be reduced further by operating the repository at lower temperatures.Because uncertainty cannot be eliminated, the DOE's approach to building confidence in analyses of repository performance relies on multiple lines of evidence. Collectively, these multiple lines of evidence are known as the postclosure safety case. Elements of the safety case include:
XI. Performance Assessment Results
The numerical (quantitative) evaluation of postclosure repository performance is an important part of demonstrating that a Yucca Mountain disposal system can be constructed that would protect public health and safety. The DOE has completed a TSPA to evaluate the system performance. TSPA is a numerical calculation, based on the process models described above, that forecasts when and to what extent radionuclides released from a Yucca Mountain disposal system might reach the accessible environment and expose human beings. The results are typically displayed as a graph of dose rate plotted in millirem per year against time (10,000 years). The total system analyses are presented in three scenarios: nominal (the scenario that uses the features, events, and processes expected to occur), disruptive (the scenario that uses possible but unlikely events with potentially harmful consequences), and stylized human intrusion (the scenario proposed by the NRC in which someone drills through a waste package 100 years after the repository had been permanently closed). As noted earlier, the DOE does not expect a human intrusion during the compliance period. The analysis is included, however, to illustrate the disposal system's resilience. Analyses were completed to evaluate the potential importance of each feature, event, or process at the site to system performance and included or excluded each from TSPA analyses, as appropriate. Performance assessments for the initial 10,000-year period after closure have been completed in a manner consistent with the EPA radiation protection standards and NRC licensing regulations. In addition to these assessments, the peak mean annual dose to the reasonably maximally exposed individual beyond the 10,000-year time period was also calculated to see if dramatic changes in the performance of the disposal system could be anticipated beyond 10,000 years. These results are described in Section 4.4. Nominal Scenario—The DOE has assessed the repository's performance for the nominal case. The TSPA-SR model projected no dose over a 10,000-year period. A supplemental TSPA model projected a dose to the reasonably maximally exposed individual of 2 × 10-4 mrem/yr, while a revised supplemental TSPA model projected a dose of 1.7 × 10-5 mrem/yr over a 10,000-year period. Both doses were calculated based on the higher-temperature operating mode. The doses calculated by the supplemental and revised supplemental TSPA models would be reduced by approximately one-third if an annual water demand of 3,000 acre-ft was used, consistent with final NRC regulations (10 CFR 63.312). Disruptive Scenario—The primary disruptive event considered in these analyses is igneous activity. The TSPA models evaluated two igneous disruptions: a volcanic eruption that intersects drifts and brings waste to the surface; and an igneous disruption that damages waste packages and exposes radionuclides for groundwater transport. The probability of igneous disruption is extremely low (the mean annual probability is about one chance in 60 million per year of occurring). The TSPA-SR model projected a probability-weighted mean dose from igneous activity of 0.08 mrem/yr over a 10,000-year period, and both the supplemental TSPA model and the revised supplemental TSPA model projected a dose to the receptor of 0.1 mrem/yr over 10,000 years. Human Intrusion Scenario—The DOE has assessed the consequences of a stylized human intrusion into the repository at 100 years after closure, pursuant to the proposed NRC regulation. The results of this analysis show that the peak mean dose is approximately 0.01 mrem/yr over a 10,000-year period. Consistent with final NRC regulations at 10 CFR 63.321, the DOE analyzed the period of time that would be necessary for waste packages to degrade sufficiently that a human intrusion could occur without recognition by a driller. The DOE subsequently found that human intrusion would not occur for more than about 30,000 years. Therefore, no doses related to human intrusion would occur within 10,000 years. The dose from a human intrusion at 30,000 years is analyzed and presented in the final environmental impact statement (EIS).XII. Evaluation of Thermal Operating Modes
The performance assessment presented in Yucca Mountain Science and Engineering Report considers both lower- and higher-temperature operating modes. The repository design is flexible: it can be operated in a range of modes that would allow the temperature and humidity in the underground environment to be varied. Analyses of the effects of the higher-temperature operating mode indicate that higher temperatures would effectively limit the potential for contact between water and waste packages for time periods of hundreds to thousands of years. However, during that period, the interaction of rock, water, and heat in and near emplacement drifts may affect rock properties and water chemistry in complex ways that cannot be fully captured in the models. The complexity of these processes introduces uncertainty into the analyses. For this reason, the DOE has completed extensive analyses to determine whether the complexity of the associated process models could be reduced. As part of these investigations, the DOE has analyzed the performance of the repository over a range of thermal operating modes. The results indicate that the repository's performance is similar over range of operating modes, encompassing above- and below-boiling conditions. The DOE expects the repository design and operating mode to be refined as the project evolves. This design evolution will be based on a process that includes (1) refining specific design requirements and performance goals to recognize performance-related benefits that could be realized through design and (2) enhancing components of the design to best achieve the performance-related benefits.XIII. Preclosure Safety Assessment
A potential repository at Yucca Mountain would be designed and operated in a manner that would limit worker and public exposures to radiation. A preclosure safety evaluation has been conducted to evaluate the performance of the Geologic Repository Operations Area during the preclosure period. The safety evaluation plays a key role in identifying design features and controls that are important to safety, and is a primary input to the quality assurance classification process. Results indicate that a repository could be operated such that radiation doses to the public and workers would be below EPA and NRC regulatory limits. To begin the safety evaluation, the DOE systematically identified and examined a range of potential hazards and the event sequences such hazards could cause, as well as their likelihood and consequences. From this examination, the DOE identified structures, systems, and components important to safety that would be relied upon to protect the public and workers. These structures, systems, and components are those engineered features of the repository whose function is to (1) reasonably ensure that spent nuclear fuel and high-level radioactive waste can be received, handled, staged, emplaced, and retrieved without exceeding regulatory limits; or (2) prevent or reduce the impact of Category 1 and Category 2 event sequences (events the repository would be designed to withstand) that could potentially lead to exposure of individuals to radiation. The structures, systems, and components were then classified in grades according to their importance to safety to ensure that appropriate quality assurance controls are implemented during the repository's operational lifetime. The DOE has developed a preclosure safety test and evaluation program to verify that structures, systems, and components are designed as specified and perform as required.XIV. Performance Confirmation and Monitoring
A performance confirmation program established to monitor and confirm that the Yucca Mountain disposal system is performing as expected was initiated during site characterization and will continue to permanent closure. The focus of the performance confirmation program is to gather and analyze data on natural processes and engineered barriers performance that will affect repository performance after closure and to evaluate their impacts. Subsurface facilities, including performance confirmation drifts and alcoves, would be constructed to facilitate monitoring of the underground facility and the performance of the engineered barrier system. Primary testing and monitoring activities will include seepage monitoring to evaluate flow of water into excavations and confirm expected waste package environment; in situ waste package surface temperature monitoring to infer cladding temperatures; rock mass monitoring to confirm the conceptual understandings and numerical simulations of coupled processes considered in performance assessments; and other activities that focus on providing an increased understanding of processes important to repository postclosure safety. Performance confirmation and monitoring activities would continue throughout the preclosure period, which could be extended up to 300 years.