Site characterization studies performed include seismic stability, volcanism, hydrology, and radioisotope transport within the host rock. DOE recently published its "viability assessment" of the Yucca Mountain site. The viability assessment summarizes the scientific information that has been collected at the site over the past 15 years and estimates how well a repository would isolate wastes from the human environment. The viability assessment is not a site-suitability decision. However, based on the viability assessment, DOE believes that Yucca Mountain remains a promising site for a geologic repository, and that work should proceed to support a decision in 2001 on whether to recommend the site to the President for development as a repository.

Contents

Site Characterization Studies
    Seismic Stability
    Volcanic Activity
    Climate Change
    Hydrology
    Radioisotope Transport

Estimating Risk
    The Scientific Trap
    Expert Elicitation
    Human Intrusion
    Nuclear Criticality

Why Not Rethink the System?
Congressional concerns
    Storage Versus Disposal
    Transmutation
    Program Funding
    The Decision

Since nuclear materials began to be used to produce electricity and make nuclear weapons, high-level nuclear waste and used or "spent" nuclear fuel has been accumulating. As of December 1998, the United States had accumulated 38,500 metric tons of spent nuclear fuel from commercial nuclear power plants. A typical large nuclear reactor produces 20 to 30 metric tons of highly radioactive spent fuel per year, an amount that totals about 2,000 metric tons annually for the entire U.S. nuclear power industry. If all currently operating nuclear power plants complete their licensed life cycle, total accumulated used fuel could more than double. In addition, the production of nuclear weapons has left a legacy of high-level radioactive waste that was created when nuclear fuel was treated chemically to separate uranium and plutonium.

Congress has been involved with the question of disposal of used nuclear fuel and high-level radioactive waste for many years. Although legislation has been enacted, the issue is still contentious. The Nuclear Waste Policy Act of 1982 and the subsequent Nuclear Waste Policy Amendments Act (NWPAA) of 1987 provide the legislative framework under which the Department of Energy (DOE) must evaluate potential sites for the geologic disposal of high-level nuclear waste.¹ In enacting the NWPAA, Congress also created the Nuclear Waste Technical Review Board as an independent establishment within the executive branch to provide a source of independent expert advice to DOE and the Congress on technical issues and to review DOE's efforts to implement the nuclear waste program.

The Nuclear Waste Policy Amendments Act of 1987 designated Yucca Mountain as the only candidate site that the Department of Energy may investigate for a permanent spent nuclear fuel and high-level nuclear waste repository. If the site proves unsuitable, DOE must return to Congress for further instructions. The major issues cover the spectrum of possibilities: whether the site characterization work, thus far, has indicated that the site is unsuitable (as the State of Nevada and others contend); whether the data, thus far, indicate that the site is viable (as DOE and others contend); or whether the current process itself may be doomed to failure by attempting to "prove the impossible" or to state its suitability with a level of certainty that is not possible (as some outside experts suggest).

Yucca Mountain is located in southern Nevada about 160 kilometers (100 miles) northwest of Las Vegas. The Yucca Mountain site was selected as a candidate based on a number of geological and environmental reasons. The mountain might best be described as a ridge, about 29 kilometers (18 miles) long, jutting several hundred meters above the surrounding land. It is in an arid region of mountain ranges and valleys, with sparse vegetation, low rainfall-less than 16 centimeters (6 inches) a year, and limited population. The site is located in the Great Basin, which does not have surface drainage outside the basin. Yucca Mountain lies adjacent to and partly within the Nevada Test Site where hundreds of underground nuclear tests have been performed.

The design of the proposed repository at Yucca Mountain includes more than 160 kilometers of drifts (tunnels) approximately 300 meters (980 feet) below the surface and spread over about 5.7 square kilometers (2.2 square miles). Congress has limited the amount of high-level ² nuclear waste that can be put into the repository to 70,000 metric tons, which will consist of 62,000 metric tons of spent fuel ³ in approximately 25,000 waste packages and 8,000 metric tons of defense high-level waste in approximately 15,000 waste packages. Plans call for sealed containers of high-level waste to be placed in boreholes within the underground repository by a shielded transporter and covered with rock or more shielding. The thermal loading of the repository (from the heat released by radioactive decay) would be controlled by mixing the ages of the spent fuel loaded into each waste package and by appropriately spacing the waste packages in the repository. The design of the repository and waste package currently proposed by DOE calls for maintaining the temperature of the host rock surrounding the waste packages above the boiling point of water for 300 years or more to drive ground water away from the containers.

The proposed repository horizon is within the Topopah Spring welded tuff,⁴ which is overlain by the Paintbrush nonwelded tuff and underlain by the Calico Hills nonwelded tuff These tuffs dip eastward about 10 degrees and are displaced along widely separated faults. The distance below the repository to the regional water table ranges from 400 meters (1,300 feet) on the southwest side to 170 meters (560 feet) on the northeast side.

There are a number of criteria against which the site and design must be evaluated. A multi-barrier approach is to be used in the design of the geologic repository. The primary barrier is the host rock, which in this case is welded tuff; and the secondary barrier is the engineered barrier system. The engineered barrier system is to provide substantially complete containment of the radionuclides for 300 to 1,000 years. The host rock plus the engineered barrier system must meet radionuclide release limits that would meet standards being written by the Environmental Protection Agency (EPA). The Energy Policy Act of 1992 (P.L. 102-486) made a number of changes in the nuclear waste regulatory system, particularly that EPA must issue new environmental standards specifically for the Yucca Mountain repository site. General EPA repository standards previously issued and subsequently revised (40 CFR 191) no longer apply to Yucca Mountain. DOE and the Nuclear Regulatory Commission (NRC) had raised the concern that some of EPA's general standards might be impossible or impractical to meet.

Other regulatory criteria against which the site and design must be evaluated include NRC's regulations at 10 CFR 60 and DOE's at 10 CFR 960. In addition, NRC will modify its current requirements and criteria, as necessary, to be consistent with standards specific for Yucca Mountain when issued by EPA. The NRC has approved development of a new regulation (10 CFR 63) to contain site-specific requirements for a possible Yucca Mountain repository.

In the license application for the site, DOE must project performance of the proposed geologic repository for 10,000 years or longer after permanent closure and must consider the full range of scenarios that may affect repository performance during that time. Because this time frame is much greater than that of any previous civil-works project, unique geoscience considerations are required. Since one must predict the fate of the repository into the distant future, the undertaking is necessarily full of uncertainties. From a scientific viewpoint, no geologist can predict with absolute certainty the behavior of a particular area for the next 10,000 years. However, one can project with informed confidence the most likely geologic behavior of that region, and bound that projection with some probability range. whether this will eventually meet regulatory needs and satisfy public perceptions remains to be seen.

NRC evaluates and licenses commercial facilities that handle radioactive materials, including the proposed Yucca Mountain repository. During licensing, all interested parties (state and local agencies, environmental groups, industry, and individuals) are guaranteed the right to participate and are allowed access to any and all documents relating to a facility's licensing. Typically, licensing a nuclear power plant may require the review of 10,000 pages of information. It has been estimated that licensing a final repository will mean making as many as 25 million pages of information available to interested parties.⁵ Much of this information will involve geologic/geotechnical aspects and risk analyses of the site including seismic stability, volcanic activity, hydrology, host rock integrity, geoengineering, possibility' of future human intrusion, modeling techniques, and regulatory requirements.

Yucca Mountain has been investigated as a potential repository since 1977. At this point, it is probably fair to say that it has the most extensively studied geology of any place on Earth. DOE issued the Site Characterization Plan (SCP) for the Yucca Mountain site in December 1988.⁶ The SCP consisted of eight chapters. The first seven discussed the current understanding of the technical characteristics and features of the site and a preliminary conceptual design of the repository. Chapter eight of the SCP described, in general, the tests and analyses that DOE would conduct during the site characterization and the rationale behind them. The site characterization process would be the basis for DOE's decision on the technical suitability of the Yucca Mountain site in accordance with DOE's siting guidelines in 10 CFR 960. If DOE determines the site is suitable for spent nuclear fuel and high-level waste disposal, it must then demonstrate to the NRC that the site meets regulatory licensing criteria

In 1996, DOE released a revised program plan.⁸ The revised program plan officially introduced a new program milestone, the viability assessment. It was announced that the viability assessment would be completed by September 30, 1998, and would include four components;

· The preliminary design concept for the critical elements for the repository and waste package;

· A total system performance assessment, based upon the design concept and the scientific data and analysis available by 1998, describing the probable behavior of the repository in the Yucca Mountain geological setting relative to the overall system performance standards;

After missing its initial deadline, the viability assessment was released on December 18, 1998.⁹

The viability assessment is not a site-suitability decision. Its purpose is to present the results of DOE's Yucca Mountain Site Characterization Project thus far and identify the critical issues that need to be addressed. DOE would still need to make a technical and scientific decision on site suitability.

Based on the viability assessment, DOE believes that Yucca Mountain remains a promising site for a geologic repository and that work should proceed to support a decision in 2001 on whether to recommend the site to the President for development as a repository. Uncertainties remain about key natural processes, the preliminary design, and how the site and design would interact. To address these uncertainties, DOE plans to improve the preliminary design, complete critical tests and analyses, and prepare a final environmental impact statement. Assuming a favorable decision is reached, DOE then has to satisfy NRC licensing requirements, and if that proceeds favorably, the site would hopefully be open by 2010.

The advantages of Yucca Mountain as a potential repository site that are highlighted in the viability assessment include its location, semiarid climate, and deep groundwater table. The site is about 160 kilometers (100 miles) northwest of Las Vegas, Nevada, on unpopulated land owned by the federal government and adjacent to the Nevada Test Site where more than 900 nuclear weapons tests have been conducted. Water is the primary means by which radioactive elements could be transported from a repository, and Yucca Mountain is located in a desert environment averaging less than 16 centimeters (6 inches) of rainfall per year. Groundwater is on average about 300 meters (nearly 1,000 feet) below the planned repository and flows into a closed regional basin rather than into any rivers that would reach the ocean.

An additional report, submitted by six outside experts hired by DOE to peer review the Total System Performance Assessment, raised some doubts about the conclusions presented in the viability assessment. ¹⁰ The Peer Review Panel's report faults the department's current model for predicting the repository's behavior, which takes into account everything affecting the movement of radioactive elements out of the fuel rods and into the distant environment over millennia.¹¹ Further, the Peer Review Panel does not think that there are sufficient data to model as convincingly as DOE appears to claim. In particular, the Panel's report calls for more work on the behavior of the cladding that encases the fuel rods and on the behavior of the radioactive material once it leaks out, as it eventually must. Considering the questions that remain unanswered, the panel of experts casts some doubt on DOE's ability to make a final decision in 2001. The Peer Review Panel's report states that in recognition of its limitations, decisions based on the total system performance viability assessment should be made cautiously.

Although the performance assessment reveals no show stoppers, in order to be considered suitable, the site, along with appropriate engineered barriers, would have to have a high probability of providing long-term waste isolation. Several areas of scientific investigation and data analysis, critical to a decision of site suitability, need to be addressed further. These include continuing to test the site and candidate waste package materials, and evaluating alternative repository designs that could reduce the possible radiation doses to people living near the site thousands of years in the future.

Evaluating alternative repository designs is a consideration that the Nuclear Waste Technical Review Board recommended in its review of the Viability Assessment. After stating that neither its review of the Viability Assessment nor its reviews of the program have identified any features or processes that would automatically disqualify the site thus far, the Board went on to state: "The Board believes that DOE should give serious consideration to alternatives to the Viability Assessment reference design, including changing from a high-temperature design to a ventilated low-temperature design (e.g. below the local boiling point of water)." ¹² With regard to a low-temperature design, the Board points out that lower temperatures would significantly reduce the uncertainty associated with coupled thermal-hydrological and thermal-geochemical processes, and for a given environment, the chances of degradation of corrosion-resistant waste package materials would be reduced significantly.

The tectonic framework of the region is complex. Yucca Mountain lies within the Walker Lane shear zone, a broad zone of deformation that extends from northeast California to Las Vegas, Nevada, and is typified by large-scale strike-slip faulting. Yucca Mountain is bounded by several Quaternary¹³ faults, and is composed of a series of north-trending structural blocks that have been tilted eastward along west-dipping, high-angle, normal faults. The proposed repository is within one of these structural blocks. DOE has attempted to determine estimates of the slip rates, recurrence intervals, and probable cumulative offset in 10,000 years on Quaternary faults in the site area and surrounding region. Slip rates for faults at Yucca Mountain are low, varying from 0.0001 millimeter per year to 0.04 millimeter per year. Some of these faults have experienced displacement in the past 11,000 years. The average time interval between surface displacement events varies from 13,000 to 100,000 years or more.

Displacements during prehistoric earthquakes tentatively appear not to have exceeded a few tens of centimeters on individual faults, although the possibility that several faults might have ruptured simultaneously during a single, relatively large regional earthquake cannot as yet be ruled Out. Of particular concern is the Ghost Dance Fault, which cuts through the repository area. In the opinion of the Technical Review Board, these faults and displacement relationships do not necessarily imply that the site is unsuitable. Suitability should be judged on the basis of potential risk (the likelihood of adverse consequences such as the release of radionuclides to the environment), not on the potential occurrence of natural phenomena alone.¹⁴ In this light, the Board suggested that it would be wise to assume that relatively large local seismic events may occur during both the pre- and post-closure periods of the repository and to investigate the engineering and safety consequences of such events.

Concerns over future seismic activity (ground motion) and fault displacement (ground rupture) fall into two time frames: pre-closure and post-closure. During the period before the repository is permanently sealed, ground motion or rupture associated with a nearby large earthquake could cause damage to surface facilities. In the post-closure period, hypothetically, fault displacement within the geologic block could cause possible failure of the canisters or possibly change the groundwater level. However, geologic stress in crustal rock is normally relieved by displacement along existing fault planes. Repository and surface facilities would be designed to withstand earthquakes, as are modern tunnels, buildings, and power plants in seismically active areas. For this reason, DOE concludes that there is little likelihood that earthquakes would significantly affect the long-term performance of a repository. The Nevada Agency for Nuclear Projects in the Office of the Governor disagrees, however, citing independent investigations¹⁵ that suggest that the rate of ground deformation in the Yucca Mountain area is more than 10 times greater than DOE investigators had previously determined. If correct, they contend that this has potential implications of increased frequency and magnitude for future seismic and volcanic events.¹⁶

The potential for igneous activity and volcanism at the proposed repository is another concern. Regional volcanism can be divided into two stages: silicic and basaltic. Yucca Mountain is within the Death Valley-Pancake Range volcanic zone of the southern Great Basin in a series of rhyolitic¹⁷ ash-flow tuffs. These silicic tuffs and lavas were erupted from several large caldera complexes that were active in the region from about 15 million to 7.5 million years ago. It is, in fact, in these tuffs that the proposed repository itself would be emplaced. Major explosive caldera eruptions have not occurred in southern Nevada for at least 7.5 million years. Based on geology of similar systems in the Great Basin, it appears that the silicic volcanic cycle is complete and will not recur. Therefore, this type of volcanic activity is not considered likely to affect the integrity of the repository.

However, more recent small-volume basaltic volcanism is present in the vicinity of Yucca Mountain. One million years ago, one volcano erupted as close as 10 kilometers away. Another, called the Lathrop Wells cinder cone is within 15 kilometers of the proposed repository and erupted a small volume of ash within the past 140,000 years, perhaps as recently as 20,000 years.¹⁸ Other cones in Crater Flat are dated at about 1.2 million years. Basaltic volcanism, although not likely to occur, is considered to be the most credible intrusive scenario during the post-closure period. This type of activity is characterized by the intrusion of dikes from magma bodies at depths of 16 to 32 kilometers (10 to 20 miles) to form small volcanoes or cinder cones. However, experts have concluded that the chance of future volcanic activity disrupting the site is negligible (see section on expert elicitation). Again, the Nevada Agency for Nuclear Projects has disagreed, maintaining that future volcanic disruption of the Yucca Mountain site is a critical concern relative to the safety of a repository at Yucca Mountain.

Climate and its changes over time directly affect the waste isolation system's performance at Yucca Mountain. Precipitation and surface weather conditions ultimately control the infiltration of water into and through the mountain. Studies of past climates show that climate oscillates between glacial and interglacial periods. Paleoclimate records indicate that the current interglacial period is hotter and dryer than earlier interglacial periods. In contrast to the current climate, periods of more extensive glaciation have dominated the long-term climate for most of the past 500,000 years. Glacial periods, characterized by colder and wetter conditions, have prevailed over approximately 80% of that time. Interpretation of paleoclimatic records provides a rationale to link elements of the hydrologic system with the climate history that drives them. Although projecting future climate involves many uncertainties, future climate variability seems likely to fall within the bounds of past climate variability. Studies thus far have enabled scientists to establish the range of past climates that have affected the site.

For assessing the performance of the proposed repository over the long term, the viability assessment presented models depicting three climate states: the present-day climate, characterized by hot, dry conditions and low infiltration; long-term average climate with conditions typical of glacial periods with much higher precipitation and infiltration; and "superpluvial" climate, representing the wettest, coldest conditions present over the past several hundred thousand years and characterized by very high precipitation and infiltration. while the effect of climate change on projections of repository performance is significant, the Peer Review Panel believed that the approach taken by the viability assessment is reasonable, but considered the analysis of infiltration based on increased precipitation to be less clear. ¹⁹

The potential repository at the Yucca Mountain site is proposed to be located in dry, unsaturated rock, on average about 300 meters above the water table. Locating the repository in the unsaturated zone is advantageous because of the anticipated very long groundwater travel times from the repository to the water table and the limited availability of moisture to corrode the waste packages and transport the radionuclides. Thus, the suitability of this site for a repository is highly dependent on the perturbations of the water table, amount of water present and in contact with the waste package, and groundwater travel time.

Groundwater travel time estimates have recently come into question. The discovery of traces of chlorine-36, an isotope created by nuclear tests in the Nevada desert in the 1950s, in fracture coatings in and below the repository level indicated that rainwater seeping through cracks and fractures had carried the isotope through 250 meters of rock in less that 50 years. This is much faster than scientists had predicted, and has caused DOE to develop a new model depicting more rapid flow through fractures in the unsaturated zone.

The water table at Yucca Mountain lies between 500 and 700 meters (1,600 to 2,300 feet) below the surface. Except where water can move through interconnected fractures, the percolation flux, or rate at which water moves down through the unsaturated zone at Yucca Mountain (on the order of 10 millimeters per year), is small in comparison to the groundwater flux (on the order of 1 meter per year) in the saturated zone below Yucca Mountain. ²⁰

After years of intensive study by hydrologists, a vast quantity of information has been obtained about the region surrounding the Yucca Mountain site. This information indicates to most geologists and hydrologists that drastic changes in the water table are unlikely. However, there is some disagreement.

In January 1997, Szymanski, now with the State of Nevada Nuclear Waste Project Office, presented additional material to the Nuclear Waste Technical Review Board (NWTRB). This material (11 reports), along with material submitted by the Nevada Attorney General's office, was reviewed by the NWTRB, and, because it contained some new information, the Board contracted with four outside scientists considered highly knowledgeable in that area to help evaluate it. The Board concluded that the new material does not make a credible case for the assertion that there has been ongoing, intermittent hydrothermal activity at Yucca Mountain or that large earthquake-induced changes in the water table are likely there. Consequently, it determined that the material does not significantly affect the conclusions of the 1992 NAS report.²²

In yet a further study, a Russian geologist, Yuri Dublyansky, who previously served as a consultant for the State of Nevada, found minerals in veins and fluid inclusions in veins that he claimed indicated geologically young and recurrent hydrothermal activity. In his interpretation, more pulses of hydrothermal activity may be expected within the next 10,000 years.²³

Geochemical studies have been undertaken of the radionuclide sorption characteristics of the various tuffs that make up the Yucca Mountain geologic block. This is an important aspect of the ability of the proposed site to provide a suitable geologic barrier to the long-term movement of radionuclides to the outside environment. Zeolites ²⁴ and other potentially sorbing minerals in the tuffs could provide important characteristics in meeting EPA and NRC safety requirements.

Retardation factors are used to quantify the potential of specific radionuclides to sorb, or bind, onto specific mineral surfaces. For a given porous material saturated with water containing a concentration of a specific radionuclide, a retardation factor can be determined in a laboratory experiment. The net effect of sorption is to retard the velocity of the radionuclide relative to that of the water.

Plutonium sorbs very effectively on many mineral surfaces and, thus, has a very high retardation factor. Technetium and iodine, on the other hand, are essentially unretarded. Thus, the predicted travel time of plutonium is several orders of magnitude longer than that of technetium and iodine. Neptunium, another critical radionuclide, has a retardation factor in the midrange.

Geophysical analysis and computer modeling techniques have an important role in the assessment of long-term repository isolation. In response to public concerns about safety, however, geoscientists and geophysical models are being asked to predict the detailed structure and behavior of sites over thousands of years. In addition, this process is being conducted to comply with a highly detailed schedule of technical specifications mandated in advance. In the view of the Board on Radioactive Waste Management of the National Research Council, this approach is poorly matched to the technical task at hand. "The United States appears to be the only country to have taken the approach of writing detailed regulations before all of the data are in. As a result, the U.S. program is bound by requirements that may be impossible to meet." ²⁶ The Board goes on to state, "Because one is predicting the fate of the HLW [high-level radioactive waste] program into the distant future, the undertaking is necessarily full of uncertainties. In this sense the government's HLW program and its regulation may be a 'scientific trap' for DOE and the U.S. public alike, encouraging the public to expect absolute certainty about the safety of the repository for 10,000 years and encouraging DOE program managers to pretend that they can provide it." ²⁷

Estimating risk is one of the biggest difficulties in making a technical and scientific decision on site suitability. The problem is in trying to link science and societal choices because safety is, in part, a social judgment, not just a technical one. Several methods have been developed in attempts to make this linkage. Risk analysis produces an estimate, not a prediction, and estimates vary m quality. Risk analysts can only discuss the likelihood of various outcomes and, at best, present risks as statistical probabilities. Risk analysis is a tool for evaluating what is known about things that cannot be known with certainty.²⁸ It is, however, only a means of making a tradeoff, and the actual tradeoff is determined by the context in which the decisionmaker is making the choice.²⁹

One approach used to estimate the likelihood or risk of occurrence of some future event in regard to Yucca Mountain recognizes that well-qualified scientists, acknowledged by their colleagues to be "experts" in their fields, could hold very different ideas about volcanic eruption, for example. The same could be said for experts in the fields of climatic variation or seismic source characteristics and ground motion estimation. Attempts to sample and quantify this range of diverse expert opinion, therefore, make use of multiple experts.

Expert elicitation is a method that draws together a panel of experts, carefully assesses their views for the uncertainties, then mathematically combines their risk estimates along with the accompanying uncertainties. This method was recently applied to an assessment of the volcanic hazard of the proposed Yucca Mountain repository. A panel of 10 experts was assembled and each member was interviewed for 2 days for the best estimates of the locations and frequencies of expected eruptions, and the accompanying uncertainties for each parameter. Those parameters were then plugged into a chain of calculations leading to the probability that a magma conduit would cut through the repository. The 10 experts' resulting probabilities, and associated uncertainties, were eventually combined into an aggregate probability. The panel concluded that the probability of a volcano erupting through the repository during the next 10,000 years is about 1 in 10,000. The 90% confidence interval runs from 5 chances in 1 million to 5 chances in 10,000.

Throughout, the experts were asked to put aside their favorite hypotheses and instead act as impartial evaluators of various theories. In the end, only one-third of the uncertainty in the final estimate was due to the conflicting opinions among experts, while two-thirds stemmed from the uncertainties perceived by each expert. In fact, whether Lathrop Wells was 20,000 or 140,000 years old didn't matter as much as the fact that a dozen volcanoes had penetrated the area at unpredictable intervals of 100,000 to 3 million years.

In the field of seismic hazard analysis, the process of expert elicitation is called Probabilistic Seismic Hazard Analysis (PSHA). PSHA involves estimation of how often a particular ground-motion amplitude exceeds a chosen threshold at some place of interest. In one view, "what is happening is that diverse expert assessments have become surrogate data and that such a mapping of experts into data must surely be an imperfect science." ³⁰ While perhaps imperfect as science, it is necessary and important to insure that the elicited and aggregated expert assessment define the body and range of informed scientific opinion in explicit and meaningful ways amenable to quantitative analysis. This process has been applied to seismic and faulting displacement hazards at Yucca Mountain.

Expert elicitation has also been applied to assessments of the flow of moisture within the saturated and unsaturated zones at Yucca Mountain. These assessments focused on data inputs, modeling approaches, and properties and processes that affect the flow of water and the transport of any released radionuclides within either zone.

"There is no scientific basis for predicting the probability of inadvertent human intrusion over the long times of interest for a Yucca Mountain repository." ³¹ This statement is part of the view of the NWTRB that intrusion analysis should not be required and should not be used during licensing to determine the acceptability of the candidate repository. Yet the potential for inadvertent or deliberate human intrusion is recognized as one of the risks involved in creating a long-term high-level waste repository

Many scenarios involving human intrusion during the 10,000-year period following closure at Yucca Mountain can be imagined. Among those that could adversely affect groundwater flow systems are:

· groundwater withdrawal,
· extensive irrigation,
· subsurface injection of fluids,
· underground pumped storage,
· military activity, and
· construction of large-scale surface water impoundments.

However, the most realistic scenario may be the possibility of prospective exploratory drilling penetrating the site and releasing radioactivity. Yucca Mountain exhibits few characteristics that would make it an attraction for future generations to drill or otherwise explore for gold, hydrocarbons, or other materials. The natural-resource potential of the area has been investigated, and groundwater is the only known resource at the site.

A nuclear criticality occurs when sufficient quantities of fissionable materials come together in a precise manner and the required conditions exist to start and sustain a nuclear chain reaction. The waste packages would be designed to prevent a criticality from occurring inside a waste package. In addition, it is very unlikely that a sufficient quantity of fissionable materials could accumulate outside of the waste packages in the precise configuration and with the required conditions to create a criticality. If, somehow, an external criticality were to occur, analyses indicate that it would have only minor effects on repository performance. Both the viability assessment and the outside peer review panel agree that nuclear criticality is not credible.

The question of safety must be examined in the context of related risks and consequences. In addition, it should be tempered with the realization that there will always be some residual uncertainties in predicting such things as the long-term behavior of a repository. As the report of the National Research Council points out, science simply cannot "prove" (in an absolute sense) that a repository will be "safe" as defined by EPA standards and NRC regulations. Uncertainty does not mean, however, that there is significant risk. What it does mean is that there is a range of possible outcomes and an adequate risk assessment and management plan must accommodate residual uncertainties and provide a reasonable assurance of safety. Further, this risk assessment has to address such questions as whether it is safer to leave radioactive waste where it is, mostly at nuclear reactor sites near cities, or put it into an underground repository in some sparsely populated region. At present, there is a fairly strong worldwide consensus that the best, safest long-term option for dealing with high-level radioactive waste is geologic isolation.

The issue for Nevada is that the proposed repository would be located "in my backyard" (provided, you can prove with an acceptable degree of certainty that my backyard would be safe). Thus, any hypothesis concerning geological and hydrological conditions at Yucca Mountain (no matter how unlikely that hypothesis may seem to be) will have to be given serious consideration so that it can be 'proved' or 'disproved' according to the present system. This process is time consuming and expensive. It also raises public expectations. A policy that is perceived by the public to promise to anticipate every conceivable problem or to assume that science will shortly provide all the answers is bound to fail. The National Research Council's Board on Radioactive Waste Management argued that "A more realistic--and attainable-goal is to assure the public that the likelihood of serious unforeseen events (serious enough to cause catastrophic failure in the long-term) is minimal, and that the consequences of such events will be limited." ³³ This is generally the concept of the viability assessment, and, within the constraints of the standards and regulations that are imposed, this is the direction in which DOE's civilian nuclear waste program appears to be going.

However not everyone agrees that it will be possible to assure the public 'beyond a reasonable doubt' that Yucca Mountain will provide a safe repository; at least, there is a question of whether DOE can do this under the present system. As the Peer Review Panel that was formed to provide a formal, independent evaluation and critique of the Total System Performance Assessment - Viability Assessment pointed out: "With the benefit of hindsight, the Panel finds that, at the present time, an assessment of the future probable behavior of the proposed repository may be beyond the analytical capabilities of any scientific and engineering team. This is due to the complexity of the system and the nature of the data that now exist or that could be obtained within a reasonable time and cost. ³⁴

The National Research Council's Board on Radioactive Waste Management proposed an alternative institutional approach--an approach that is more flexible and experimental. That approach acknowledges the following premises:

· Surprises are inevitable in the course of investigating any proposed site, and things are bound to go wrong on a minor scale in the development of a repository.

· If the repository design can be changed in response to new information, minor problems can be fixed without affecting safety, and minor problems, if any appear, can be remedied before damage is done to the environment or to public health.

This approach would start with the simplest description of what is known and meet problems as they arise, rather than trying to anticipate in advance all the complexities of a natural geological environment As site characterization proceeded, increased knowledge could be incorporated into the design. As the Board on Radioactive Waste Management points out, "This approach uses a scientific approach and employs modeling tools to identify areas where more information is needed, rather than to justify decisions that have already been made on the basis of limited knowledge." ³⁵

On the other hand, a centralized storage facility would reduce concerns about storing spent fuel at reactor sites, many of which are located near metropolitan areas. Recently there has been opposition at reactor sites where utilities have tried to add on-site dry-storage capacity. In general, while the NWTRB supports developing an interim storage facility, the Board states that there is no compelling technical or safety reason to move spent fuel to a centralized storage facility in the next few years. The NWTRB also points out that there are technical, operational, and fiscal advantages of having such a storage facility located at an operating repository site, and recommends that the construction of a federal storage facility be deferred until after a decision has been made about the suitability of the Yucca Mountain site for repository development.

Another concern that Congress has raised is doubt that a permanent repository could be built at Yucca Mountain because DOE may be facing a scientifically impossible task to prove for licensing purposes that any repository could safely contain nuclear waste for 10,000 years or more. This concern is occasionally expressed in terms of support for transmutation as an alternative. Transmutation is the process of bombarding nuclear waste with neutrons in an accelerator to reduce radiation levels by changing the atomic structure of the waste nuclides. Currently, the DOE is seeking $4 million for transmutation research. Some experts have expressed doubt that transmutation would provide enough waste reduction benefits to justify its high costs.³⁸ However, as cost offsetting measures, constructing an accelerator for transmutation of nuclear waste and tritium production for the U.S. nuclear weapons program has been discussed.³⁹

The funding mechanism for the nuclear waste program would be modified by the bills introduced in the 106th Congress. Currently, nuclear utilities must contribute to the Nuclear Waste Fund a fixed annual fee of a tenth of a cent (one mill) per kilowatt-hour generated. Nuclear utilities and state officials have filed lawsuits to force DOE to begin taking nuclear waste from reactor sites as it had contracted to do beginning in 1998. Thus far, the courts have ruled against DOE, but have not required it to begin accepting nuclear waste. Instead the plaintiffs and defendants have been ordered to work out a settlement involving compensation. The settlement is still subject to future litigation.

In consonance with the need for Congress to consider funding for the program, DOE's viability assessment attempted to project the total cost of the proposed repository through its entire life cycle. Total life cycle costs for the entire waste management system include the following elements:

The total of estimated future costs is $36.6 billion, in constant 1998 dollars. (The additive total of the elements above differs due to rounding.) In addition to the fee nuclear utilities pay to fund the disposal of wastes from their plants, the federal government uses tax revenues to pay for the disposal of radioactive waste from the nations's defense programs.

Disposing of nuclear waste is a case where, if one does nothing, the waste will go away. However, that is not relevant since it will take many millennia for the radioisotopes with long half-lives to decay. The question of providing for a safe, long-term repository will not go away, and Congress will ultimately be involved in resolving that issue.