Management of High-Level Waste: A Historical Overview of the Technical and Policy Challenges
The National Research Council (NRC) has provided scientific and technical analyses to inform policy decisions related to the disposal of nuclear waste since the 1950s. One of the NRC's earliest reports on this subject, The Disposal of Radioactive Waste on Land (NRC, 1957), was among the first technical analyses of the geological disposal option, and it marked the beginning of a four-decade effort by the U.S. government to identify a disposal site for commercial spent fuel and defense waste (collectively referred to here as high-level waste [HLW]), including the effort that is now underway at Yucca Mountain, Nevada. Repository programs are underway or planned in several other countries as well, most notably Belgium, Canada, Finland, France, Germany, Japan, Sweden, Switzerland, and the United Kingdom. Some of these programs deal with transuranic and long-lived low-level wastes as well as HLW.
In 1988, the Board on Radioactive Waste Management convened a study session with experts from the United States and abroad to discuss U.S. policies and programs for managing the nation's spent fuel and high-level waste. The board's follow-up report, Rethinking High-Level Radioactive Waste Disposal (NRC, 1990), provided a broad assessment of the technical and policy challenges for developing a repository for the disposition of HLW. The board noted in the report (p. 2) that
“There is a strong worldwide consensus that the best, safest long-term option for dealing with HLW is geological isolation…. Although the scientific community has high confidence that the general strategy of geological isolation is the best one to pursue, the challenges are formidable.”
This worldwide consensus of the technical community was documented as well by international bodies. The clearest examples are the Collective Opinions published by the Nuclear Energy Agency (NEA) of the Organization for Economic Cooperation and Development together with the International Atomic Energy Agency and European Union (NEA, 1991). The views expressed were that geologic disposal can be done ethically and safely. It was also
recognized that considerable technical work—in particular in the area of site characterization—remained to be done.
In pursuing geological disposal, national programs in several countries have encountered a number of challenges. These include understanding the nature and rates of geological processes, predicting long-term environmental change, predicting repository and waste package performance, and predicting long-term human behavior for the purposes of risk estimation. Those national programs that have made or are in the transition from research to repository development confronted a challenge that has both technical and sociological overtones: Simply put, many members of the public do not believe that geological isolation can be proven to be a safe, long-term waste disposal solution. They doubt the ability of experts to predict future changes or to make the right decisions to protect public health and are therefore reluctant to relinquish control of the waste, even though the continued management of this waste may be a burden on future generations. These public doubts and objections typically come to the fore when a disposal program moves into the task of identifying specific repository sites. General misgivings about safety suddenly crystallize into specific opposition.
Many national programs are seeking improved methods for addressing these challenges. Some countries have begun a phased approach to repository development while others have suspended their programs. In the United States, for example, the U.S. Department of Energy is considering a plan that would provide the capability to delay for centuries, or even indefinitely, the physical closure of a repository at Yucca Mountain, leaving the final decision on closure to a future generation. In Canada, a federally appointed panel has recommended that the government postpone the search for a repository site until broad public acceptance of the geological isolation approach has been achieved. In Germany, there has been intense public opposition to moving spent fuel to an interim storage facility near Gorleben, the site of the German candidate repository, and the decision on whether to proceed with development of a repository at Gorleben has been delayed. In the United Kingdom, a House of Lords committee conducted a full-scale inquiry into the management and disposal of radioactive wastes after a government decision to cancel a proposed rock laboratory project at Sellafield. Spain has halted its siting program, and in Holland all work on specific disposal projects has been formally suspended for a hundred years.
In the midst of this general picture of forced or planned delays in repository programs, there are some counter examples. In the United States, the Waste Isolation Pilot Plant (WIPP) has gone into operation as the first deep repository designed for long-lived transuranic wastes. A community in Finland has agreed to host a repository and application has been made for a siting permit. The low-and intermediate-level waste program in Sweden is advancing.
What is High-Level Nuclear Waste? A Brief History
In 1955, the U.S. Atomic Energy Commission requested that the National Academy of Sciences consider the possibilities of disposing of high-level radioactive waste in quantity within the continental limits of the United States. This request led to a conference at Princeton in 1955 and the subsequent
report, The Disposal of Radioactive Waste on Land (NRC, 1957). The problem posed to the National Academy of Sciences at that time was primarily the disposal of fission products from the reactors used in weapons manufacture. These wastes were being stored at the Hanford, Washington, Oak Ridge, Tennessee, and Savannah River, South Carolina sites. The major concerns were Cs-137 and Sr-90. It was expected that the liquid or slurry waste could be disposed of directly, and containment was envisioned as necessary for a period of 600 years (about ten half-lives of these isotopes). The report recommended that geological formations such as salt deposits held promise for providing the needed isolation, because water would not be able to pass through these deposits and provide a pathway by which hazardous radioactive isotopes could come into contact with humans and other living things.
In the mid-1950s, the use of nuclear reactors for commercial power generation was just beginning. Most parties involved with nuclear power at that time envisioned that commercial power reactor fuel would be reprocessed to recover uranium and plutonium, using technology similar to that which had been used for the military reactors. The fuel for the first-generation light-water reactors was uranium enriched in the fissile U-235 isotope to about 3–4%, compared to the 0.7% (by weight) of this isotope in natural uranium deposits. High-grade uranium ore was scarce and enrichment using the gaseous diffusion technology was deemed expensive, so this low-enriched uranium fuel was considered expensive and limited as the basis for a large nuclear industry in the future. Many experts expected that breeder reactors producing plutonium from the non-fissile but plentiful (99.3% by weight) isotope U-238 would become the dominant technology as civilian use of nuclear power plants expanded during the next century. Reprocessing would be used to recover plutonium for recycling into light water reactor fuel and for breeder reactors when these were developed. Radioactive waste was expected to consist mainly of fission products, with Cs-137 and Sr-90 viewed as the most hazardous, as they were for the weapons program waste.
As the civilian nuclear power industry has developed over the past four decades, this picture has changed. The composition of high-level nuclear waste is now very different than what was considered in the 1957 National Research Council report. The main difference is that the United States and a number of other countries with nuclear programs now plan to dispose of spent nuclear reactor fuel as high-level radioactive waste, without reprocessing. This usage is called a “once-through” fuel cycle.
Reprocessing of civilian power reactor fuel is being carried out by other countries such as France and the United Kingdom, but the United States decided during the late 1970s not to support reprocessing of civilian reactor fuel. This decision was motivated by concerns that a large commerce in recovered plutonium could increase proliferation of nuclear weapons. However, economic factors have also favored a shift to once-through use of uranium fuel without reprocessing. Advances in techniques for locating high-grade uranium ore deposits have made uranium less expensive, and a large worldwide capacity has developed for uranium enrichment. The end of the Cold War led to additional availability of enriched uranium released from military programs. At the current time the availability of low-cost enriched uranium fuel makes once-though use of this fuel significantly less costly than recovery and recycling of plutonium for
commercial power plant fuel. However, many parties continue to believe that recovering and stockpiling plutonium for future use is an appropriate strategy for meeting future energy needs.
Spent nuclear fuel contains substantial quantities of plutonium and other transuranic elements produced through multiple capture of neutrons by the fertile isotope of uranium, U-238. Over the past forty years the nuclear industry has increased the residence time and therefore the burn-up of low-enriched uranium fuel rods. This improvement in fuel burn-up reduces the cost of nuclear electricity generation, but it also results in spent fuel with greater concentrations of long-lived transuranic radioisotopes. Neutron capture and beta decay transform U-238 into Pu-239. Additional neutron captures transform Pu-239 into Pu-240 and Pu-241. Reactors optimized for manufacture of plutonium for nuclear explosives minimized the creation of Pu-240 and Pu-241 by irradiating U-238 target assemblies for a relatively short time. Pu-241 decays to Am-241, and Am-241 decays to Np-237, a very long-lived radioisotope that is more soluble in water than is plutonium.
Isotopes such as Np-237 and the very long-lived fission products such as Tc-99 and I-129 were not discussed in the 1957 report. The most recent performance assessments of the proposed U.S. repository at Yucca Mountain indicate that releases of Np-237 into slowly-moving groundwater could provide the largest doses of radiation to nearby humans, who might use such water for subsistence farming, at a time tens to hundreds of thousands of years in the future. Risk assessments made in the United Kingdom show that nuclides like Cl-36 and Tc-99 can contribute substantially to peak long-term doses from intermediate level waste.
The United States and Russia are now determining how to manage an inventory of plutonium from nuclear weapons that have become surplus as the result of disarmament agreements. One proposed method is disposal in a geological repository, perhaps after placing fission products in proximity to the plutonium (e.g., by immobilizing plutonium in vitrified high-level waste) to make it as theft resistant as spent nuclear fuel. Another proposal is to irradiate plutonium in the form of mixed-oxide fuel in commercial reactors, and to dispose of it along with other spent fuels (NRC, 1994).
The composition of high-level radioactive waste has therefore changed considerably from what was contemplated in the 1957 report. The composition could change further if surplus weapons plutonium from military programs is also to be considered as high-level waste to be placed in geologic repositories.
The Quantity of High-Level Waste to be Managed
Quantities of spent fuel are usually described in terms of the heavy metal equivalent, where the heavy metal is uranium (or other actinides such as thorium or plutonium). The amount of commercial spent fuel in the United States in 1995 was 32,000 metric tons. By 2020 the amount is projected to be 77,100 tons. This projection for the quantity of spent fuel depends on the usage pattern of existing commercial reactors. It assumes no new reactor construction, no extension of existing plant operating licenses, and that all plants will run to the end of their current licenses (Ahearne, 1997). A similar estimate for the spent fuel from all reactors operating or
under construction worldwide in 1995 is 447,000 metric tons (McCombie, 1997).
The quantity of high-level waste from military programs is more difficult to estimate, because of the diverse forms of the reprocessing waste and the quantities of fuel from naval reactors that use highly enriched uranium fuel. Measured in radioactivity units, the 1995 estimate of the U.S. military HLW is about 3% of that in U.S. commercial spent fuel (Ahearne, 1997).
Assuring the Safety of a Geological Repository for High-Level Waste
The summary of the 1957 NRC report states, “Unlike the disposal of any other type of waste, the hazard related to radioactive waste is so great that no element of doubt should be allowed to exist regarding safety” (NRC, 1957, p. 3). Assuring that any release of radioactivity from a repository will fall within acceptable limits has continued to be a major focus of discussion in the planning of geological repositories in the United States and in other countries.
The problem of assuring safety was initially considered by many of those involved in development of geological repositories to be closely linked to the selection of rock type. The 1957 report found salt deposits to be promising, but did not recommend salt deposits without qualification. For disposal of liquid waste as contemplated at that time, the report noted problems with salt such as the structural weakness of salt cavities, and the report recommended a research program for further investigation. The report noted as the “second most promising method” the formation of “a silicate brick or slag which would hold all elements of the waste in virtually insoluble blocks” (NRC, 1957, p. 5). Thus, the 1957 report considered, as an alternative to finding a favorable geological setting, the development of an engineered barrier that would prevent radioactive materials from migrating into the geological environment from the original containment structure.
More than four decades later, many national programs have carried out extensive investigations of a variety of geological settings as candidates for geological repositories. Many of these programs have included extensive research in an underground laboratory to investigate transport pathways by which radioisotopes from the waste might be able to migrate into the biosphere, and so come into contact with humans and other living organisms. In most cases, the concern has been with water as a transport pathway. Water is essentially ubiquitous in underground environments, and most radioisotopes can under certain conditions dissolve into groundwater. Transport via groundwater is the main concern for how safety might be compromised, since radioactive materials from the repository may then come into contact with humans or other living organisms in the biosphere. In a few cases there has been concern that radionuclides in gaseous form might escape to the atmosphere through fractures in the rock.
The flow of water (and more generally, fluids, including gases) has been a focal area in the geological sciences and geoengineering. Hydrology and geohydrology have taken on additional practical importance in recent decades. Making relatively precise predictions about the flow of groundwater and other fluids is difficult, even in relatively homogeneous media such as salt or clay. In
rocks containing fracture systems such as granite or volcanic tuff, it is possible to determine only the general character of groundwater transport. However, more detailed knowledge of the rock and the fracture systems may be needed to predict the degree of connection between a source area and a target area of concern, and the quantity and timing of flow from the source to the target. Even for rock types in which the extent of fluid flow appears very low, it is difficult to assure conclusively that conditions that permit flow will not occur in the future.
As research has proceeded in national programs, in particular in those proposing fractured host rocks like granite or volcanic tuff, more emphasis has developed on the use of engineered barriers as an aid to assuring adequate waste isolation. Waste is placed inside a container that acts for a long time as a complete barrier to the flow of water (and gases). The container is often surrounded by an impervious clay-type buffer that restricts flow of groundwater to the waste and transport of released radionuclides from the waste. Assuring the integrity of this engineered barrier system for time periods that are greater than those known for any engineered structures is an important safety question.
Most national repository programs now plan to rely on both the geological setting and engineered barriers to achieve safety. This design philosophy is often described as “defense in depth,” or reliance on multiple barriers. Crucial aspects of the geological setting may be to maintain geochemical conditions that inhibit degradation and corrosion processes, to avoid geochemical conditions that facilitate the dissolution and transport of key radionuclides, and to avoid mechanical stresses that would compromise the engineered barriers.
A difficulty with both geological settings and engineered barriers is that safety cannot be demonstrated by direct observation, because of the very long time periods required. Changes in climate may bring about changes in water table levels and flow regimes, and in high-latitude countries such as Sweden and Finland loading of ice from renewed glaciation is considered. Earthquakes, volcanism, and a variety of other geological processes may affect the ability of the site to contain waste. Generally, these issues must be addressed by modeling what would happen under the altered circumstances.
While some metals such as copper have been known to resist significant corrosion for long time periods, most modern materials have not been shown to maintain their integrity over a long period of time. Borosilicate glass and certain types of metal alloys are expected to be highly corrosion resistant under the conditions in the geological repository, but their performance over very long periods cannot be demonstrated directly. A major concern with engineered barriers is whether the barriers might fail as the result of a defect in manufacturing, such as a poor weld that escaped detection. Thus, for engineered barriers, as well as for the capabilities of the geological setting to isolate the waste from fluid transport, indirect methods of proof are the best that science can offer. Models based on short-term observations in laboratories and short-duration field experiments as well as other considerations such as defense in depth and natural analogs must be the basis for assuring that a geologic repository can meet its safety objectives.
If the time scale of concern is set by the lifetime of actinide isotopes such as Pu-239 and Np-237, assurance of safety requires modeling of release and transport processes over time periods that are unprecedented in human
experience. The National Research Council report on the Technical Bases for Yucca Mountain Standards (NRC, 1995) recommended that compliance with a standard for the release of radioactivity be determined at the time of peak risk, whenever that occurs. A challenge of great magnitude thus confronts both those parties seeking to construct a geological repository (i.e., the national program managers) and those parties responsible for evaluating the safety of proposed repositories (i.e., the national regulatory authorities, or other responsible organizations as designed by the national government). Indirect means must be used, because the time scales are so long that experimental methods cannot be used to confirm directly predictions of the repository system or even of its components. The models and methods being used involve state-of-the-art scientific and analytical procedures from a large array of scientific disciplines.
The 1990 NRC report, Rethinking High-Level Radioactive Waste Disposal, reaffirmed deep geological disposal as the best option for disposing of high-level radioactive waste. It called into question the direction of the U.S. program during the 1980s and noted that the prescriptive approach being taken was
“…poorly matched to the technical task at hand. It assumes that the properties and future behavior of a geological repository can be determined and specified with a very high degree of certainty. In reality, however, the inherent variability of the geological environment will necessitate frequent changes in the specifications, with resultant delays, frustration, and loss of public confidence. The current program is not sufficiently flexible or exploratory to accommodate such changes.” (NRC, 1990, p. vii).
There has thus been considerable evolution in the conceptualization of the high-level waste problem from 1957 to 1990 and to today. In 1957 the isotopes Sr-90 and Cs-137, having intermediate half-lives and presenting a clear radiation hazard, were seen as requiring “no element of doubt” in the containment of HLW for about 600 years. Prescriptive approaches for geological disposal continued into the 1990's, although the period of time for which the waste was perceived as hazardous increased some 100-to 1000-fold.
During the 1990s, experience in many national programs has demonstrated the need for flexibility and for accommodating the inherent limitations in the information that scientific investigation can provide about the geological setting and the durability of engineered barriers. The design and the assurance of safety for geological repositories must be a carefully considered exercise in making decisions in the face of scientific uncertainty. Current scientific and technical knowledge might be interpreted as leading to pessimism that every “element of doubt” regarding safety of HLW can be eliminated. But extensive analysis of uncertainties may allow the doubts to be resolved. Francis Bacon, a founder of the scientific method, said in 1605:
“If a man will begin with certainties, he shall end in doubts; but if he will be content to begin with doubts he shall end in certainties.” (Bacon, 1605).
Today it is understood that the design and the assurance of safety for geological repositories must be a carefully considered exercise in making decisions in the face of scientific uncertainty.
Achieving Public Acceptance of High-Level Waste Repositories.
The magnitude of the technical challenges in designing and constructing a geological repository for HLW and assuring its safety is perhaps comparable to that of other large-scale and potentially hazardous engineered systems, such as the development of coal mines, bridges, or a new generation of commercial aircraft. A radioactive waste repository may face special challenges since it may be perceived as involving involuntary risks to members of the general public. Best technical efforts are made to model the system, to assure an appropriate amount of conservatism in the design, and then to improve the design as experience is acquired.
There are many areas in our highly technological society at the end of the 20th century where the public is willing to leave the assurance of safety to the technical experts. Both empirical experience and social science research on public attitudes suggest that this is not the case with radioactive waste disposal. There are a number of interrelated reasons that might be advanced for why public acceptance poses such a formidable challenge. First, the time scales for damage are long, much longer than for many other safety issues. It might be hypothesized that people might not care about adverse impacts that could occur in the far distant future. However, there is much research indicating that nuclear facilities and in particular, nuclear waste repositories, are viewed by many among the public with great concern (Weart, 1988; Flynn et al., 1995; Easterling and Kunreuther, 1995). Furthermore, research shows that underlying that concern is very strong negative imagery (Weart, 1988; Flynn et al., 1995), and the representation that damage from radioactive waste to human health, to ways of life, and to the environment may be widespread and severe.
There are many indications that publics neither understand nor trust the expert community on radioactive waste issues. For the non-specialist, the array of applicable scientific data and proposed methods is so complex as to be virtually incomprehensible. One critic of the U.S. program describes it as follows:
“To an outsider, the issue of what to do with high-level radioactive waste introduces a morass of obscure jargon and abstruse questions. An almost measureless bulk of documents, data, and technical reports describes the technology of nuclear waste management.” (Jacob, 1990, p. 164).
This lack of trust in experts and public concerns caused by the proliferation of conflicting expert opinions is not a phenomenon peculiar to radioactive waste disposal. Today, similar attitudes are perceivable in other controversial areas such as genetic modification of foodstuffs.
Many social and policy scientists believe that the U.S. geological repository program and many other national programs are overbalanced in terms
of too much emphasis on technological research, and too little emphasis on institutional innovation and supporting social science research. Again citing the same U.S. Critic,
“While everyone can appreciate that complex, highly sophisticated engineering is required to safely store nuclear materials for thousands of years, few have appreciated the political requirements necessary to design and implement such a solution. While vast resources have been expended on developing complex and sophisticated technologies, the equally sophisticated political processes and institutions required to develop a credible and legitimate strategy for nuclear waste management have not been developed.” (Jacob, 1990, p. 164).
Finally, there is the problem of perceived inequity. HLW is concentrated, so that the waste produced over many decades in a large nuclear industry may fit into one site, with a surface area on the order of a few square miles. While many citizens may enjoy the benefits of nuclear electricity, a far smaller number will be located near the single disposal site, deliberately placed in a relatively unpopulated area. Those who live immediately adjacent to the site may enjoy employment or other benefits from the proximity of the repository. Their neighbors, further away, may feel they have the stigma of living near an undesirable facility with no offsetting benefit. Social scientists have termed this response pattern a “doughnut effect” (Easterling and Kunreuther, 1995). It is easy for these “neighbors” to perceive that they were not treated fairly, especially if they did not participate directly in the choice of the disposal site.
This issue of fairness across current populations is referred to as intragenerational equity. The issue of fairness towards future inhabitants, intergenerational equity, is also important in waste disposal. Current generations benefit from nuclear technologies, which produce wastes. Should these generations attempt to reduce future burdens by disposing of wastes now, even if small potential releases are predicted for the far future?
As noted in the first section of this paper, many national HLW programs have made or are considering changes to deal with the myriad issues raised by the need for public acceptance. A review for the U.S. Secretary of Energy noted the gravity of the problem in the United States (DOE, 1993). Studies in connection with programs in Canada, France, Japan, Spain, and the United Kingdom have reached similar conclusions on the importance of public understanding and acceptance for geological repositories (Bataille, 1993; Aoyama, 1999). In a democratic society, public acceptance is an essential part of the political decision process. Excellence in geological science and engineering may be necessary, but not sufficient, to assure that a geological repository program will be successful.
Social scientists have identified a number of principles that apply to gaining public acceptance of national programs. First, participation in the decision process leading to a geological repository should be open, and to the extent possible given the complexity of the science, the process should be transparent. One method for accomplishing this goal is extensive review of the repository program by independent scientific experts. A second principle is that
of staged decision making: the selection of the technologies and the site, the specific design, construction, emplacement of waste, and closure of the repository should be established as a sequence of steps, each taken after careful study and regulatory approval with public participation. Commitment to a site and to technology choices then occurs slowly. In the event of perceived weakness, the process can be slowed down and modifications made to repository design. In a more extreme case, the site selection process can be restarted.
Alternatives to a Geological Repository
A considerable amount of study has been given over many years to alternatives to geological HLW disposal. Among those that have received the most consideration are disposal in the seabed and in outer space, transmuting long-lived isotopes, and continued storage (DOE, 1980).
Disposal of HLW in the deep seabed has been proposed by oceanographic experts and carefully evaluated by the international scientific community (Hollister and Nadis, 1998). This alternative offers excellent prospects for isolation of the waste in areas of the ocean floor that have been highly stable on a time scale of hundreds of millions of years. The major problem for this alternative is achieving the international political consensus needed to place waste in a location not under the sovereignty of a single nation.
Sending HLW into the sun also has been considered. While the required technological capability is not yet available, it is foreseeable within the next century. The space transport system would need to provide adequate protection so that in the event of a failure, waste would not be dispersed onto the Earth. The high cost of such transport would imply a need to separate HLW into components, and only the most long-lived and hazardous materials (e.g., plutonium, neptunium) might merit this alternative (Taylor, 1995).
Transmutation of plutonium, other actinides, and long-lived fission products can be accomplished with further irradiation in nuclear reactors or in accelerator-driven subcritical reactors. This class of alternatives has been studied by the National Research Council (NRC, 1996). The 1996 NRC report “found no evidence that applications of advanced separations and transmutation technologies have sufficient benefit to delay the development of the first permanent repository for commercial spent fuel” (NRC, 1996, p. 2). Several countries are continuing investigations on partitioning and transmutation technologies and whether they could be deployed on the scale needed to make a significant reduction in the inventory of spent fuel (NEA, 1998). Reprocessing of spent fuel is an essential aspect of these technologies.
As described previously, reprocessing rather than “once-through” use of nuclear fuel was the original concept for the commercial nuclear power industry at the time of the 1957 NRC study on nuclear waste. However, transmutation for waste management purposes would involve more extensive recycling and destruction of plutonium and other transuranic actinides than contemplated in a “closed” fuel cycle optimized for generation of electricity. In addition, the potential improvement in repository performance from transmutation of these materials in commercial spent fuel will be limited by the amount of such materials that will be present in high-level waste from defense nuclear activities.
An alternative to geological disposal, especially for the next century, is continued storage in surface facilities. This approach is not a true alternative, rather, it allows postponing the implementation of a permanent solution. Storage methods might be pool storage or dry cask storage at the site of the nuclear reactor where the fuel was irradiated, or dedicated facilities might be constructed at other locations. The concept of such “monitored retrievable storage” has been extensively studied in the United States and other countries. Sweden, Finland, Germany, and Switzerland have commissioned central storage facilities for spent fuel from electric utilities. The technologies for pool and dry cask storage are established and available, and most experts do not see assuring the safety of such storage is a problem on the time scale of a hundred years. The cost of ongoing surface storage becomes very high if such storage facilities have to last for hundreds of centuries, requiring them to be rebuilt on a regular basis. There are also significant risks inherent in the assumption that society will continue to manage such stores.
Disposal vs. Disposition
Many discussions of the need for geological repositories have taken the position that current generations have an ethical imperative to avoid imposing the burden of ongoing surveillance and management, and of ultimate disposal, on future generations. One statement of this view is as follows:
“those who generate the wastes should take responsibility, and provide the resources, for the management of these materials in a way which will not impose undue burdens on future generations.” (NEA, 1995, p. 13).
At a time when many considered that assurance of the safety of geological repositories would be easily accomplished and demonstrated, this objective was widely accepted. As difficulties have emerged both with making the technical case for repository safety and with bringing the public to view proposals as acceptable, the feasibility of this objective has been increasingly questioned. The argument is predicated on acceptance that a sealed repository represents less of a future burden than controlled wastes for which one is still free to choose a disposal option. Many parties within the international HLW community are now reconsidering the merits of a strategy of ongoing monitoring and possible retrieval, as opposed to a program that involves closure of a repository and absence of planned activities thereafter.
Several factors motivate ongoing study and monitoring rather than expediting repository closure as soon as the HLW can be emplaced. First, the response of the geological site and of the engineered barriers to the waste may lead to adjustments in the containment system. Second, provisions for ongoing monitoring may be viewed by publics as preferable, so that if very unlikely failures in containment or unexpected events should occur at the repository, effective and timely action can be taken to avoid releases of radioactivity into the biosphere. Third, retrievability may become desirable in the future because of the energy value of components such as plutonium in spent fuel.
The counter arguments are that safe disposal is already now feasible and that delaying closure for long times presents a greater hazard. For example, operational expertise and funding in the future are not guaranteed. Retrieval from a closed geological repository remains in principle possible for very long times.
In planning this workshop we have chosen to use the word “disposition” (an arrangement or plan for disposing) instead of “disposal” (emplacement without intent to retrieve) to describe the desired objectives of national programs. Disposition allows opportunities for ongoing management of HLW in a geological repository, as opposed to conceiving a repository only as a facility that will be filled with waste and then sealed. It may be appropriate to consider strategies for extending the time between emplacement of waste and closure of a repository, and to regard an underground repository in a deep geological formation as a monitored, retrievable HLW storage facility, until sufficient confidence in its safety can be developed and the repository closed.
Safety Against Human Intrusion
In this discussion paper we have waited until this point to introduce what some regard as the most challenging aspect of the planning of a geological repository: assuring safety against the future actions of human society. This is, in fact, a problem with any material requiring long-term isolation. While the problems of assuring adequate isolation of HLW against intrusion and transport of radionuclides by groundwater are formidable, geological and climatic processes are relatively well understood compared to what may happen to human society on a time scale of hundreds to thousands of years. For the reasonable assurance of safety, consideration must be given to what humans might do that could compromise the integrity of the repository and release radioactivity that could damage human health or the well-being of other living species.
Human intrusion into a repository and consequent release of radioactivity could happen inadvertently. It should be expected that humans will continue to drill into and mine the earth to search for and recover a variety of materials that a future society may find valuable. These may include groundwater; oil, gas, or other energy sources (including geothermal); fertilizers for agriculture; minerals and other raw materials for manufacture; and gems for personal adornment. It is difficult to anticipate just what a future society might seek from materials underground. It is also difficult to anticipate how effective warning signs or other measures might be in alerting future miners and drillers that they could encounter HLW at a repository site established centuries or millennia before their time, or if such a warning would be necessary.
The NRC report on Yucca Mountain standards (NRC, 1995) and many regulatory agencies have taken the position that it is appropriate to evaluate safety in terms of the consequences of an inadvertent intrusion using current drilling technology. Examining the radioactive release from drilling a hole that intersected one container of HLW gives insight into how serious an intrusion event might be, even if the details of future intrusion with unknown technology are difficult to foresee. It is not possible to determine the frequency with which future human intrusion could occur, although in picking the repository site there
are advantages to avoiding locations that have obvious potential for energy resources or valuable minerals.
In 1995, NEA published a consensus position that deliberate intrusion should not be covered in repository safety analysis (NEA, 1995). Most of the repositories being contemplated for current national programs could easily be opened by a future generation using today's mining technology. In some cases, mining technology used hundreds or even thousands of years ago might be adequate. The energy value of components in spent fuel, or materials in the engineered barriers, or a variety of other reasons might motivate a future society to dig into a repository, despite the hazard from the radioactivity, which will slowly diminish as the HLW ages.
The most serious threat from deliberate intrusion may be the recovery of materials to make nuclear explosives. In this case, the visibility of such intrusion may be important. An advantage of seabed disposal is that retrieval of HLW from beneath several miles of ocean could only be accomplished with large, specialized surface vessels whose purposes could easily be ascertained via satellite surveillance. Reopening of a geological repository on land might be carried out in such a way that it could only be detected through on-site inspection. Relatively few and very remote repository sites offering easier surveillance possibilities may discourage unauthorized intrusion.
The risk of intrusion, including both inadvertent intrusion and deliberate intrusion, such as an effort by terrorists to release radioactivity or to steal waste material, is a concern for both waste storage and underground disposal. An advantage of both centralization and of placing waste storage underground is that access can be more easily controlled.
The Evolving Balance between International and National Efforts
Most of the planning of HLW management has been though individual national programs. Most nations with commercial nuclear power reactors have a nuclear waste program or have plans to establish such a program in the near future. The concept underlying these national programs is that a nation with commercial nuclear reactors will retain its own HLW (spent fuel if once-through or the waste residue if reprocessing is used) and will dispose of this waste in its own territory. The United States and several European nations are far along in planning HLW repositories, but no country has yet approved a design and commenced construction, although the United States has started operation of a geologic repository for long-lived transuranic waste (WIPP). The national HLW programs regularly share technical data and the results of ongoing scientific research, and many scientific experts participate in multiple national programs. International conferences are held regularly, and international organizations such as the NEA, the International Atomic Energy Agency (IAEA) and the European Union (EU) promoted consensus-building on appropriate principles and technology for a HLW program.
Several factors suggest that it may be appropriate to increase the extent of international cooperation on HLW. Some countries, especially small and densely populated nations, face considerable difficulty in locating a suitable site for a
geological repository. There have been purely commercial proposals in the past for disposal in China or on Pacific islands. One proposal for an international repository is now being developed that could offer a site in a large, essentially uninhabited region of Australia with highly stable geology. Many small countries could be eager customers for an international HLW repository, should such a facility be developed that would allow countries to place HLW outside their own borders under credible international supervision and standards. The end of the Cold War and the financial difficulties in the former Soviet Bloc have left some of these nations with legacies of nuclear materials but little funding to manage these materials. Assuring the safety of materials that could be used for nuclear explosives is particularly important. A proposal has been made to locate a facility in Russia that could provide secure storage for spent fuel from Asian nations and also for nuclear materials from the Russian military program. Other proposals may emerge for cooperation or consolidation of national programs into international activities.