Chapter One Yucca Mountain

When the 1987 amendments to the Nuclear Waste Policy Act were enacted, the Department was preparing to characterize three candidate sites to determine their suitability for a repository. The 1987 amendments directed the Secretary to characterize only the Yucca Mountain site. Since then our approach to site characterization has matured significantly as a consequence of what we have learned from scientific investigations of that site and from engineering design work. As we have gained in knowledge, we have gained a greater ability to apply the sophisticated performance assessment tools that we use to guide scientific studies and design work. Just as important, our insight into the nature of the task has grown. The result is a sounder--and simpler--conceptual framework for conducting site characterization and determining site suitability.

At the same time that our technical approach to site characterization was maturing, the policy framework and budgetary assumptions that governed it were evolving. Our 1994 Program Plan had fundamentally redirected site characterization activities to reflect the realities of the budget and to support an early evaluation of the technical suitability of the Yucca Mountain site. By Fiscal Year 1995, we had concluded that it would be prudent to make a comprehensive early assessment of the viability of licensing and constructing a repository at the site. Encompassing technical, cost, and management issues, the assessment will help policy makers better understand what would be required to proceed to licensing and construction, and it will help them decide whether it would be prudent to do so. The assessment will in no way replace the determination of site suitability; rather, it will help us prepare for it.

In reducing our Fiscal Year 1996 funding, Congress directed us to 1) focus on core scientific activities, 2) complete excavation of necessary portions of the Exploratory Studies Facility, 3) complete the scientific tests needed to assess repository performance, 4) complete repository and waste package designs, and 5) defer activities related to licensing.

Accordingly, in reallocating funds to accommodate the 40 percent reduction in overall program funding, we took into account both explicit congressional direction and the evolution in our own approach to site characterization. The result was that we reduced funding for site characterization by 33 percent, and we formally modified our site characterization plans to focus on three near-term objectives:

The first objective, as stated above, was formulated by our program. It was subsequently adopted by Congress in the Energy and Water Development Appropriations Act for Fiscal Year 1997. The second is programmatic, undertaken at our own initiative. The third assigns our target dates to statutory milestones. Together, they lead to the long-term goal of starting repository operations in 2010.

This schedule assumes that the Yucca Mountain site will prove suitable for a repository--an assumption that our site characterization program and the Nuclear Regulatory Commission's licensing process are designed to test rigorously.

Our revised approach will require only moderate funding increases in future years and will enable us to reach a license application for about $1 billion less than the December 1994 Program Plan required.

The determinations whether the Department should be issued licenses to construct and subsequently operate a repository at the Yucca Mountain site will be made by the Nuclear Regulatory Commission on the basis of its own regulations (10 CFR 60). But the prior determination whether the site is suitable and should be recommended to the President for licensing must be made by the Secretary of Energy. The Department published guidelines to govern the siting process, up to site recommendation, as a regulation in 1984.

These General Guidelines for the Recommendation of Sites for Nuclear Waste Repositories (10 CFR Part 60) were to provide the primary criteria for the evaluation and comparison of several possible repository sites in various geologic media. In 1987, however, Congress enacted the Nuclear Waste Policy Amendments Act, which directed the Department to study only the Yucca Mountain site. By directing the Department to study a single site, Congress rendered a number of the Department's original guidelines, which were designed to facilitate comparisons of different sites, inapplicable. In late 1995, in the Conference Report accompanying the Energy and Water Development Appropriations Act for Fiscal Year 1996, Congress directed the Department to focus only on those activities necessary to assess the performance of a repository at the Yucca Mountain site and to collect the scientific information needed to determine its suitability.

In keeping with this congressional direction, the Department has proposed to amend its siting guidelines to focus and clarify its regulatory review of the Yucca Mountain site. Under this proposal, a total system assessment of the performance of a specific repository design within the geologic setting of Yucca Mountain would be compared to the applicable regulatory standards to determine whether the site is suitable for development as a repository. We believe this approach will enable the Department to provide the public with a much clearer account of the process by which it determined whether the Yucca Mountain site is suitable for a repository, and the results of its determination.

Both the Nuclear Waste Policy Act of 1982 and the original guidelines anticipated a need for periodic updating to reflect changes in the repository program. The Department is not alone in having to reassess its regulations because of congressional direction. In the Energy Policy Act of 1992, Congress directed the Environmental Protection Agency (EPA) to prepare a new health and safety standard specific to a repository built at Yucca Mountain. In the same Act, Congress directed the Nuclear Regulatory Commission (NRC) to modify its repository licensing regulations. Any new guidelines issued by the Department will apply the new EPA standard and NRC regulations.

The Department is not proposing to change substantively the guidelines for comparative screening of potentially acceptable sites, for the nomination of sites as suitable for characterization, and for the recommendation of sites for characterization. The existing guidelines would be changed only as needed to permit them to accommodate the new language specific to Yucca Mountain. They can still be applied if Congress directs the Department to select other sites for study.

The Department is proposing, however, to add a new component to the existing guidelines that would govern the evaluation of the Yucca Mountain site alone. After more than a decade of research, scientists now have a better understanding both of the Yucca Mountain site and of geologic disposal. The Department has concluded that its assessment of the site's suitability should rely on a "system performance approach," that examines how specific design concepts of a repository system will work within the natural system at Yucca Mountain to meet public health and safety standards set by EPA and implemented by NRC.

Activities that support the viability assessment were the principal focus of ongoing scientific and engineering work at the Yucca Mountain Site Characterization Project in Fiscal Year 1996. The viability assessment is intended to be an interim step in the process leading to a site suitability determination in 2001. The revised Program Plan issued in May 1996 defined the essential components of the viability assessment and further refined the focus of scientific activities at Yucca Mountain. The components of the assessment are:

The viability assessment thus constitutes a point of logical convergence at which we can meaningfully appraise the prospects for geologic disposal at the Yucca Mountain site.

Taken together, what is learned from the first three tasks will provide a better understanding of repository design requirements, of repository performance in a geologic setting, and of the work remaining to prepare a license application. Together with a more precise cost estimate, this information will serve policy makers, program planners, and scientists and engineers, by focusing the final years of site investigations and facility design on the important uncompleted work and unresolved issues, and providing all participants with a common frame of reference for evaluating the project.

After the site viability assessment is completed, this body of work will serve as the foundation for the evaluation of site suitability. If the site proves suitable, the assessment will support development of the Environmental Impact Statement and the Secretarial site recommendation to the President.

The goal of submitting a successful license application to the Nuclear Regulatory Commission is central to the program's mission. The site viability assessment will also be an early and invaluable step on the path to a license application.

In Fiscal Year 1996, we identified major components of the license application plan. The Administration's budget for Fiscal Year 1997 sets forth this scope of work, a budget profile, and a schedule that--together with the updated regulatory framework described above and our revised Program Plan--can get us to a license application in 2002.

The goal of a repository is to dispose safely of the Nation's spent nuclear fuel and high-level radioactive waste. A primary assumption about geologic disposal is that water will serve as the principal means by which radionuclides will be transported to the accessible environment. A primary assumption about the Yucca Mountain site is that there is--and will be for many thousands of years into the future--very little available water in the rocks in and around the repository to dissolve radionuclides and transport them.

Our strategy for containing and isolating radionuclides within Yucca Mountain relies upon "defense in depth": a combination of engineered barriers, geologic features, and natural processes. This strategy governs our evaluations of probable repository performance, and it is by means of these evaluations that we further focus and narrow remaining work. The strategy for waste containment and isolation thus drives the scientific component of the site characterization program.

As our work has progressed, this basic strategy has not changed, but its application has evolved, and several developments have recently contributed to the need to update it. First, findings from scientific investigations and advances in design have permitted us to narrow the open questions about repository performance. Second, the mandated EPA revision of the radiation protection standard from a release-based to a dose-, or risk-based, standard requires more emphasis on modeling both the saturated zone that lies 1,000 feet beneath the repository and the biosphere.

The revised strategy we began formulating in Fiscal Year 1996 is based on the following assumptions: 1) Robust waste packages would provide near-complete containment of the radionuclides within the emplaced waste packages for thousands of years during the period of highest radionuclide inventory and temperature. 2) When the waste packages are breached, engineered barriers would retard migration of radionuclides. 3) The geochemical setting would further retard migration. 4) Those relatively few radionuclides that reached the water table would become significantly diluted by the time they reached the accessible environment. This defense-in-depth would limit the annual radiation dose to members of the public during the post-closure period to levels below the regulatory standard.

The strategy defines five system attributes as most important to the performance of natural and engineered barriers in containing and isolating radionuclides: 1) rate of water seepage into the repository, 2) waste package lifetime, 3) rate of release of radionuclides from breached waste packages, 4) radionuclide transport through engineered and natural barriers, and 5) dilution in the saturated zone below the repository. To assess the contributions that these key attributes make to system performance we have formulated hypotheses about them: one set of twelve hypotheses assesses performance under undisturbed conditions; one set of three assesses performance under conditions disturbed by such events as earthquakes and volcanism.

Rigorously testing these hypotheses will resolve the relatively few, but important, open technical questions about how significant features and processes of the natural geologic and hydrologic system influence the performance of engineered systems--especially the waste package. These issues primarily concern movement of water through Yucca Mountain, selection of materials for the waste package, and the need for and feasibility of using additional engineered barriers to enhance overall repository performance.

Thermal effects on the natural system over time are another focus of concern. As the waste emplaced in the repository generates heat and then gradually reaches ambient temperatures, the first four of the five key system attributes named above are likely to be affected. We will determine how this will alter their relative importance to system performance as a function of time.

We provided a summary version of our waste containment and isolation strategy to the Nuclear Waste Technical Review Board and the Nuclear Regulatory Commission in July 1996. A detailed version, to be completed in Fiscal Year 1997, will describe what we now understand about the five key system attributes, what further information we need about them, how we will gather that information, and how we will refine our models so that we can predict the performance of the attributes singly and in conjunction, under a range of conditions.

The Exploratory Studies Facility that we are constructing within Yucca Mountain gives scientists and engineers direct access to the setting within which a repository would be constructed. This permits them to observe and test subsurface geologic and hydrologic conditions and the engineering properties of the host rock and its response to construction activities. Although Fiscal Year 1996 budget cuts prevented us from optimizing our efforts and slowed our progress somewhat, we succeeded in completing more than three-quarters of the 5-mile underground, U-shaped tunnel loop ahead of schedule. The Tunnel Boring Machine was expected to exit the mountain at the South Portal of the facility in early 1997.

Excavation of the main drift of the Exploratory Studies Facility was completed and excavation of the south ramp was initiated in July 1996. Initial phases of construction for the Thermal Test Facility (Alcove 5) and Northern Ghost Dance Fault Alcove (Alcove 6) were completed. These alcoves allow scientific testing to proceed in the actual repository horizon where waste would be emplaced. In August 1996, a single-heater test was initiated in the Thermal Test Facility (Alcove 5) to measure the effects of heat and moisture in the actual geologic strata of the repository. The remainder of the alcove will be constructed in 1997 for a much larger drift-scale heater test.

Along with the Northern Ghost Dance Fault Alcove, the Southern Ghost Dance Fault Alcove will be constructed off the Exploratory Studies Facility main tunnel, to allow actual testing near the Ghost Dance Fault. Both alcoves are scheduled for completion in 1997. The results of these tests will be incorporated into scientific models and the total system performance assessment in support of the 1998 site viability assessment.

To ensure the continued safety, cost-effectiveness, and design adequacy of tunneling and underground construction, we established an Exploratory Studies Facility Board of Consultants. These outside experts visited the site in October 1995, December 1995, and March 1996 and assessed all aspects of the facility, including the adequacy of design, cost, safety, quality assurance, and construction methods. Their assessment was generally favorable. Their recommendations have been reviewed and are being implemented or considered for implementation.

To assess the performance of the key repository system attributes defined above, we use data from site characterization studies to build, test, refine, and confirm models of repository system features, processes, and events. In turn, what we learn from modeling informs the design of engineered barriers, and it tells us where we need more information from site characterization, and when we have enough. The information and analyses that shape modeling and design come from our core science program, which proceeds by way of surface and sub-surface site investigations and laboratory studies.

Fiscal Year 1996 brought the completion or near-completion of some investigations and increased confidence in the adequacy and significance of others. Our models of groundwater flow and radionuclide transport were made significantly more robust through incorporation of new data. Work to bound the estimate of volcanic hazards and work on the field-based portion of seismic hazards investigations neared completion. These modeling efforts will be key inputs to the total system performance assessment that will support the site viability assessment in 1998.

Fracture-fill materials and pore water samples collected in the Exploratory Studies Facility and from boreholes are being age-dated using chemical isotopes such as carbon-14, tritium, uranium-series, and chlorine-36. This age-dating tells hydrologists how long it takes water to move from the ground surface to the repository level, when the water moves through the rock layers, and what paths the water may take within these layers. Findings are consistent with the hypothesis that pore water currently in the potential repository horizon generally has been there for tens of thousands to hundreds of thousands of years. We also have evidence that some water has moved rapidly from the ground surface through fractures. While the volume of water associated with this rapid movement has not been determined, it is not expected to have a significant impact on repository performance. This evidence is consistent with the alternate hydrology model proposed in the 1988 Site Characterization Plan that stated that Yucca Mountain is an unsaturated, fast-draining site.

Air pressure testing and monitoring in the unsaturated zone and surface boreholes along the Exploratory Studies Facility main drift and north ramp have improved our understanding of air flow through the mountain. The effects of faults and differing geologic layers on air flow are being monitored and characterized, and the data are being compiled and used to calibrate the unsaturated zone flow model to constrain the physical properties of major hydrologic layers. This will allow us to better determine how water flows from the surface of the mountain to the potential repository horizon. Data collected by scientists working for Nye County, Nevada, have provided important input to this effort.

The unsaturated zone flow model has also been improved by advances in our understanding of pore-scale processes in natural fractures, which give us additional confidence in modeling groundwater flow in fractures. Observations of moisture distributions, including perched water occurrences, have been used to calibrate the unsaturated zone flow model. Findings have led to a refined unsaturated zone numerical flow model that credibly represents much of the hydrologic data collected to date and is being used to focus additional work. In Fiscal Year 1996, hydrologic testing continued in Alcove 2 (Bowridge Fault), and testing was initiated in Alcove 3 (upper contact nonwelded Paintbrush Tuff) and Alcove 4 (lower contact nonwelded Paintbrush Tuff).

Results from the unsaturated zone transport model completed in Fiscal Year 1995 suggest that sorption and matrix diffusion may be effective site-scale mechanisms for retarding radionuclide transport. Laboratory and modeling studies are providing a more robust characterization of potential radionuclide transport at the site. What we have learned recently has allowed us to reduce the number of radionuclides at issue to a few key radionuclides that are being studied in detail as part of our evaluation of post-closure site performance. These advances have been incorporated into the Fiscal Year 1996 iteration of the unsaturated zone transport model.

Testing has been initiated at the C-Hole testing complex to help characterize flow and transport in the saturated zone. Analysis of data from pumping tests has yielded local flow properties for the saturated zone. These data have been supplemented with hydraulic and geochemical data from aquifer tests at other boreholes to support development of a repository-scale model of flow and transport in the saturated zone.

Recent work on the hydrogeologic evaluation of the Death Valley regional groundwater flow system incorporated several improvements over previous versions. Geographic information systems and geoscientific information systems were used to develop, store, manipulate, and analyze regional hydrogeologic data sets describing various components of the groundwater flow system.

We updated the preliminary 3-D Geologic Framework Model and the surface-based geologic map of the repository block. Fracture data from surface and underground mapping were synthesized. Mapping in the Exploratory Studies Facility indicates that faults that appear major on the ground surface are not significant to construction.

We completed drilling a deep borehole, SD-7, to a depth of 2,669.62 feet (814.23 meters). This complements the approximately 40 boreholes drilled since our quality assurance program was approved in 1992, and an additional 40 boreholes drilled earlier. Drilling provides rock and water samples and permits analysis of air flow through the mountain. Models of air flow will help us predict the behavior of water vapor and gaseous radionuclides that result from the heat emanating from emplaced wastes.

Spent nuclear fuel is hot, and the up-to-70,000 metric tons of waste slated for disposal in a repository will give off a great deal of heat for a very long time. Understanding how the effects of that heat, coupled with natural processes at work in the site, will affect repository performance is essential to determining the site's suitability. To assess these impacts, we have designed thermal studies, colloquially referred to as heater tests, to be conducted within the Exploratory Studies Facility. These tests will provide information on rock mechanics, hydrology, and the chemistry of the formation under the influence of heat.
A peer review team of independent scientists was convened to review the in situ testing program that is planned to improve our understanding of coupled thermal-hydrologic-chemical-mechanical processes. Based on recommendations in the team's final report and on laboratory tests we have already completed, we refined our plans for the heater tests that will be used to validate our current site-scale process models.

The single-heater test we began in August 1996 in Alcove 5 (Thermal Test Facility) is a small-scale, shakedown test that will last about 9 months. The results will be used to refine a far larger and more complex drift-scale heater test that is scheduled for the fall of 1997 and will continue for at least 2 years.

Tools for Sound Decisionmaking: Expert Judgment, Expert Elicitation, Peer Review

The work of characterizing the Yucca Mountain site carries scientists not only into underground geological formations that have never before been explored but into intellectual territory that has never been mapped. Knowing what information it is important to gather, how to use that information, and how much information is enough, demands expertise and sound judgment.

As a continuing check on its own thinking, the Yucca Mountain Site Characterization Project uses several tools that are common to scientific endeavors, applying them to scientific areas that may be problematic or subject to varying interpretation:

Expert judgment is information provided by a technical expert based on opinion or on a belief based on reasoning.

Expert elicitation is a formal, highly structured, and well-documented process for obtaining expert judgments, usually of multiple experts. Typically, an expert elicitation is conducted to evaluate uncertainty. The uncertainty could be associated with the value of a parameter to be used in a model, the likelihood and frequency of various future events, or the relative merits of alternative conceptual models.

Peer review is typically a documented, critical review that evaluates the acceptability and adequacy of a particular form of research, performed by peers who are independent of the work being performed. Peer reviewers may comment on the validity of the assumptions, the appropriateness and limitations of the methodology and procedures, the accuracy of the calculations, the validity of the conclusions, and the uncertainty of the results and consequences of the work.

Examples of the project's application of these tools are the expert elicitation on probabilistic volcanic hazard analysis and the peer review that will be conducted on coupled processes related to thermal loading, described in this chapter. At OCRWM's request, the National Academy of Sciences has also conducted peer reviews of aspects of site characterization work. International collaborations on such tasks as modeling and performance assessment also contribute to our work.

During the past 2 years, we conducted an expert elicitation for a probabilistic volcanic hazard analysis. A panel of experts on volcanism from both within and outside the project assessed the probability of a volcanic event disrupting a repository at Yucca Mountain, and the panel quantified the uncertainties associated with its assessment. Four workshops and two field trips provided opportunities for technical discussions and interactions that ensured that the panel shared a common understanding of the issues. The panel was supplied with relevant data by the project, the State of Nevada, and the Center for Nuclear Waste Regulatory Analyses.

The final expert elicitation workshop was held in December 1995. While the panel members used a wide variety of approaches, the panel's mean probability estimate of a future volcanic event directly intersecting the repository was approximately a 1-in-70,000,000 chance per year. This estimate is virtually identical to that produced by project scientists, suggesting that the volcanic hazard is now well bounded by a wide range of scientific interpretations that yield reasonably consistent and defensible results.

During Fiscal Year 1996, field work to support the seismic hazards investigation approached completion. This work includes surface geologic mapping and construction of trenches across potentially active faults to help estimate the magnitudes and recurrence intervals of earthquakes. Data from these surface studies will be integrated with geophysical information to evaluate alternative tectonic models that will provide added confidence to our estimates of seismic hazards. A probabilistic seismic hazard analysis, to be conducted in Fiscal Year 1997, will be used to complete the evaluation of potential seismic hazards and to produce seismic design parameters for facilities that will be constructed at Yucca Mountain if it becomes the repository site.

To confirm our estimates of these hazards we will continue to monitor seismic activity in the Yucca Mountain area. Fifteen new digital seismic stations have been added to the Southern Great Basin Seismic Network. They are proving to be considerably more sensitive than the older analog recording stations, and they allow us to record seismic events that are well below the threshold for potential damage. Analysis of the data from these small events has increased our understanding of the earthquake potential of the region and has supported the development of our tectonic models.

The site viability assessment will evaluate the technological feasibility of those design elements critical to the performance of the repository and the engineered barrier system. Design work will focus on elements without regulatory precedent and will build on existing design, with emphasis on key technical questions that affect waste containment and isolation, performance, and cost.

These questions revolve around thermal management of the waste-generated heat, corrosion of waste packages, the role of supple-mental engineered barriers, and transport of radioactive wastes. In addition, the effort will address concepts for waste retrieval operations, performance confirmation requirements, safety systems, and other factors that significantly affect repository costs.

The Mined Geologic Disposal System Advanced Conceptual Design Report was completed in Fiscal Year 1996. It describes an integrated conceptual design for all important repository and waste package systems, structures, and components. It also incorporates results of the Total System Performance Assessment-1995. For some systems, structures, and components, this preliminary design will be sufficient to support the site viability assessment and a future license application for construction; for others, more detail will be required. Thus, the next task will be to determine what additional information is needed. The Advanced Conceptual Design also summarizes the concept of operations for the repository, including provision for a caretaker period during which waste could be retrieved.

Although the Advanced Conceptual Design Report necessarily makes assumptions about many specific factors, the repository concept it describes retains a great deal of flexibility. For example, operations would allow for adjustments in emplacement drift spacing if early emplacement information indicates the need.

Work continued to resolve issues concerning containment of radionuclides and to design waste packages that will meet the standard of providing substantially complete containment for at least 1,000 years. Investigations of engineered barrier systems are focusing on delaying contact between water and radioactive waste to minimize releases of radionuclides to the accessible environment and any exposure to the public.

A conceptual engineered barrier system design was incorporated into the Mined Geologic Disposal System Advanced Conceptual Design Report, which gives us a new reference design based on work to date. The report describes the design process; depicts the major repository and waste package configurations, components, and operational concepts; and identifies those areas of the design that require further refinement. The design in this report will serve as the initial basis for the design that will be used for the site viability assessment.

A thermal loading study re-examined several previously established thermal goals; it concluded that, given the assumptions at the time of the study, the planned thermal loading associated with the emplacement of 83 metric tons of uranium per acre may support meeting repository performance objectives for 10,000 years. Because of uncertainties associated with this issue, the repository plan for thermal loading provides sufficient flexibility to accommodate either high or low thermal loading concepts, and it can be revised up to the start of construction.

A backfill systems study that focused on thermal-hydrological calculations was completed. Sensitivity analyses conducted for this study identified more potentially beneficial performance attributes for backfill: one attribute minimizes liquid saturation; another minimizes liquid flux near the waste package. Analyses in support of the backfill systems study indicated that drift spacing wider than that provided for in the reference design could reduce relative humidity in rock and within the emplacement drift. This reduction could lower corrosion rates for waste canisters. The reduction in relative humidity is believed to occur because wider spacing encourages the run of condensate between drifts. The analyses also indicated that increased linear mass loading could reduce costs and variability in the near-field environment.

Several corrosion tests of materials that could be used to fabricate waste packages were conducted. Results from humid air tests showed that corrosion rates were substantially increased when surfaces were etched chemically or coated with salt. Electrochemical tests indicated that nickel-based and titanium-based alloys exhibited maximum corrosion resistance. Preliminary modeling shows that low-temperature oxidation will not significantly degrade the performance of thick corrosion-allowance materials for hundreds to thousands of years. This work is focusing initially on carbon steel, the principal candidate for the outer disposal container.

Further scientific and engineering information gained during construction and testing in the Exploratory Studies Facility, as well as results from surface-based and laboratory testing, will be incorporated into the designs. Scientific investigations and performance assessment activities will provide data and requirements for these designs. The site viability assessment will then identify the additional design detail needed to support the license application.

Tools for Scientific Forecasting: Performance Assessment

At what rate and by what routes will radionuclides be transported from a repository at the Yucca Mountain site to the accessible environment over the course of the next 10,000 years? Predicting physical behavior thousands of years into the future for a complex repository system comprised of properties that cannot be fully characterized in the present and that will change in time is a task containing many uncertainties.

But to tackle it, scientists have available a powerful tool: performance assessment. It synthesizes scientific and engineering data in conservatively bounded computer models to evaluate how well the particular repository designed for the Yucca Mountain site will contain and isolate waste under a range of conditions. The models are mathematical descriptions of the physical processes that affect containment and isolation, such as water flow in a geologic setting. Derived from data that have been acquired from site investigations and other sources, interpreted, and synthesized, these models can better predict performance, offering higher degrees of confidence, as the data stream grows more robust.

Total system performance assessments link together models that have been abstracted from lower-level process models. This modeling tool is the primary means by which we will ultimately determine the site's suitability and—if the site proves suitable—demonstrate its suitability in a licensing forum. But performance assessment is also invaluable as a working tool that we use iteratively, taking information from site investigations and design work to model performance, and then using the results of performance assessments to refine design work and more narrowly focus scientific investigations.

One important use of this modeling tool is to identify the significance of uncertainties in processes, models, and parameters for repository system performance, so that we can determine where further investigation is needed. In this respect, performance assessment is also a management tool that helps us determine where limited funds are best spent.

The total system performance assessment (TSPA) that will support the site viability assessment will be based on 1) an updated waste package and repository design, 2) a thermal strategy for managing the effects of heat emitted by spent nuclear fuel and high-level radioactive waste, and 3) process models of the site and the design that have been built from data resulting from laboratory and site investigations.

Focusing on the post-closure period, the TSPA will further refine past evaluations of repository performance under a range of undisturbed conditions and will update evaluations under conditions imposed by potentially disruptive events, such as earthquakes and igneous activity. The TSPA will also evaluate the possible range of performance encompassed by uncertainties in key factors such as ground- water flow, thermal effects, and corrosion. Treatment of uncertainty and the appropriateness of bounding conditions used in important natural and engineered system process models will be carefully examined.

Fiscal Year 1996 efforts focused on tasks necessary to prepare for this performance assessment: planning, sensitivity analyses, and prioritization of model development and site and design information needs. Groundwater travel time calculations were used to evaluate information about the water percolation expected in the repository horizon.

Performance allocation analyses were performed to evaluate the importance of several key hypotheses of our waste containment and isolation strategy. Results were used to prioritize the development and testing of process models and site and design activities needed to support the site viability assessment. Sensitivity analyses of process models evaluated the relative importance of input parameters, including their uncertainties.

We completed a plan outlining the performance assessment work required for the site viability assessment, including technical work, responsibilities, and schedules. Peer reviewers will examine the total system performance assessment, including previous iterations and the work leading up to and comprising the performance assessment that will support the site viability assessment.

Affected units of government are eligible under Section 116(c)(1) of the Nuclear Waste Policy Act to receive Federal financial assistance for review and monitoring of project activities. The Fiscal Year 1996 Energy and Water Development Appropriations Act did not expressly provide for these payments. The Department's Chief Financial Officer wrote to the cognizant congressional committees stating that the Department believes it is obligated to provide oversight funding. The chairman and ranking members of the Subcommittee on Energy and Water Development of the House Appropriations Committee wrote back strongly objecting to the Department's view.

No funds were granted to the affected units of government for Fiscal Year 1996 under this section of the Act, and lack of funding impaired their ability to review and monitor the project's activities. However, project staff regularly interacted with them by attending meetings and through phone calls and correspondence.

In September 1996, the State of Nevada sued the Department seeking funding that the State believes it is owed from Fiscal Year 1996 appropriations. At the end of Fiscal Year 1996, the lawsuit was pending before the Ninth Circuit Court of Appeals. In its December 1996 opening brief, the State claimed that the Department owes it $3.5 million.

Under Section 116(c)(3) of the Act, the Department continued to make Payments-Equal-to-Taxes to Nye County, the county in which the Yucca Mountain site is located. In Fiscal Year 1996, those payments totaled $5.4 million.

We continued our outreach programs by supporting public information, public education, and public participation in project-related activities. Our institutional activities centered on informing stakeholders and the public of our revised program strategy and the status of activities at Yucca Mountain. Information was provided to stakeholders, interested groups, and individuals in a cost-effective manner via the Yucca Mountain segment of OCRWM's Home Page, in correspondence, and in meetings.

We also completed a video entitled "Geologic Disposal of Nuclear Waste: Solving an Environmental Problem" that depicts various aspects of the nuclear waste program for general audiences. To help the public understand the geology of Yucca Mountain and the results of our site investigations, we developed and installed four major geology exhibits at the Las Vegas Science Center that invite public interaction using push button-activated video programs. We participated in 195 speaking engagements, reaching over 14,219 stakeholders through the Yucca Mountain Speaker's Bureau, and conducted 167 tours of Yucca Mountain, briefing 2,362 visitors on our site characterization progress.

A major opportunity for formal public involvement was the scoping process for the Repository Environmental Impact Statement, summarized below.

During 1996, the Yucca Mountain Site Characterization Project continued to monitor for environmental impacts, maintain compliance with necessary permits, support design reviews, conduct Environment, Safety, and Health (ES&H) worker training, and perform ES&H surveillances, assessments, and appraisals. We finished the year with a safety record that was 30 percent better than the comparable industry average, and 23 percent better than the safety records of other DOE sites.

The Nuclear Waste Policy Act requires that a final Environmental Impact Statement accompany the Secretarial site recommendation and the license application. We began the work of preparing the Statement by publishing a Notice of Intent in the Federal Register in Fiscal Year 1995 to initiate the public comment period on the proposed scope of the document. As part of the scoping effort, we conducted 15 public meetings across the Nation. The public comment period closed on December 5, 1995.

Because of Fiscal Year 1996 funding cuts, we deferred further action on the Environmental Impact Statement until Fiscal Year 1997, but we awarded a contract for its preparation, and we are now preparing a comment summary document. We now plan to publish the draft Environmental Impact Statement in 1999 and the final version in 2000.

OCRWM's international waste management activities involve cooperation with other countries and international organizations to exchange information and develop consensus on common issues. During Fiscal year 1996, we participated in bilateral agreements with Canada, Sweden, Switzerland, France, Japan, and Spain to support information exchanges, but due to funding constraints, we terminated cooperative research agreements with Canada, Sweden, and Switzerland that had served to develop information and technology relevant to characterization of the Yucca Mountain site. We continued limited interactions with the International Atomic Energy Agency and the Nuclear Energy Agency. Funding for international cooperative science and technology programs is expected to be minimal in future years.

The focus of our limited, ongoing international cooperative work is interpretation of site characterization data and performance assessment. This work involves continuing participation in two programs of the Nuclear Energy Agency: the Site Evaluation and Design of Experiments Group and the Performance Assessment Advisory Group. These groups work cooperatively to improve the state of the art in geosphere transport and two-phase flow characterization and modeling, and in performance assessment and modeling. OCRWM's participation in these activities strengthens the capabilities and defensibility of our models of natural processes that operate at the Yucca Mountain site.