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Page 85 6 Scientific and Technical Issues in Radioactive Waste Management Geological disposition of high-level radioactive waste (HLW) and spent nuclear fuel (SNF) is scientifically sound, but important challenges remain. This chapter discusses how scientists and technologists are addressing the challenge of analyzing the long-term behavior of a geological repository and endeavoring to present their results convincingly to other scientists, regulators, and concerned members of the public. The first part of the chapter describes the scientific basis on which repository behavior can be analyzed. The second part of the chapter describes the quantitative methodology, known as performance assessment, which has been developed for such analyses by the waste management community, and the measures taken to enhance confidence in its reliability. Emphasis is placed on the necessity for the methodology and for the results it delivers to be acceptable not just to scientists employing it, but also to a wider circle of interested and affected parties, including the broad scientific community, regulators, and the public. The third part of the chapter highlights the specific problems faced by a regulatory body in arriving at binding decisions in light of the unavoidable uncertainties remaining in the technical analyses. Finally, some examples are given of how policy decisions can ultimately affect technical issues in program implementation. The chapter ends with the committee's conclusions on the methodology and results of performance assessments for geological repositories. GEOLOGICAL DISPOSAL Practically, geological disposal does not represent a major construction challenge. All of the techniques required to build a repository, encap-
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Page 86 sulate the waste in a series of containers and barriers, emplace the waste, and close the repository are established or could be developed from established practices, and the associated cost is entirely compatible with the economics of energy production. A well-designed repository represents, after closure, a passive system containing a succession of robust safety barriers. These barriers are designed not solely to provide increasing levels of safety, but also to give increasing confidence that the overall system will remain safe even if individual barriers do not perform as well as they are designed to do. Our present civilization designs, builds, and lives with technological facilities of much greater complexity and higher hazard potential. In spite of these facts, there is a long-standing, intense debate on the feasibility of implementing “safe” repositories, that is, repositories that cause no harm to humans or to the environment. The reason for much discussion is that extraordinarily long time scales must be considered explicitly in the analyses of repositories. There are inevitable uncertainties in the models and data used for these analyses and in the nature of events that might occur far into the future. There are particular uncertainties due to the role of the geological medium in isolating waste placed into a repository. Earth scientists are accustomed to descriptive, deductive reconstruction of the past, but for the purpose of a repository, they must develop quantified, inductive assessments of future system behavior. These factors make it a challenging task to analyze reliably the future evolution of the system. In practice, these difficulties are mitigated by two important facts. First, the engineered barriers can partially compensate for uncertainties in the understanding of the geological medium. Second, a single exact prediction is not needed; rather, understanding the range of potential future changes and assuring that these do not present unacceptable risks is a more correct description of the challenge. Long-term uncertainties also arise in analyses for the disposal of nonradioactive toxic wastes and for managing fossil fuel reserves. All such analyses require good science that will illuminate the physics, chemistry, and other mechanisms that will dominate repository behavior over the long time scales involved. Nevertheless, the common perception is that for geological disposal specifically, one must be able to predict the future accurately—and it is beyond established engineering practices to predict accurately for many thousands of years how the waste and the repository will behave. It is also beyond established practice to predict accurately whether or not some of the radionuclides disposed in the repository may move through the geological formations and eventually come in contact with human beings and the environment in the future and cause them harm. As emphasized above, however, the challenge is not to accomplish these impossible tasks, but rather to assess the range of potential future behaviors with sufficient confidence to allow the appropriate societal decisions to be made.
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Page 87 Another important issue when evaluating the geological disposal option is physical security. Assuring security requires that safeguard controls over the fissile materials (particularly spent fuel or plutonium) be maintained in order to prevent their clandestine use for nuclear weapons development or to prevent the misuse of highly hazardous radioactive materials by terrorists (NAS, 1994). Disposal in deep sealed repositories is considered one of the most effective ways to prevent undetected recovery of nuclear material (Peterson, 1998). Although it is not impossible to reopen a repository and recover the stored material, such an action would require heavy equipment, long times, voluminous extraction of earth and sealing material, and high costs. SCIENTIFIC BASIS FOR MODELING The time scales of concern for deep disposal are so long that direct observations or measurements of temporal alterations in actual repository system components are of limited value (although much can be learned from studies of analogous systems existing in nature). Assessment of future repository performance must be based upon modeling of the physical and chemical processes involved. The basic considerations for modeling are the following: The laws of natural science that govern key processes such as corrosion, fluid flow, and mass transport do not change with time. However, our knowledge of these laws may be incomplete and may develop as more experience is gained. The issue is to know which laws apply during the relevant time scales so that we can assure proper application of current knowledge to be able to identify key parameters with sufficient accuracy. The retrospective geological database actually extends over very much longer time scales (billions of years) than the lifetimes of most radioactive elements. Although knowing the past does not mean that one knows the future, history does give some confidence-building information about geological processes and rates of change. Predictions of actual system behavior are not required. It suffices to provide conservative (or pessimistic) estimates of impacts that can reasonably be expected. However, this assumes that unknown effects—those neither expected nor accounted for in the analysis—will not materialize in significantly detrimental ways. The problem is to know whether the underlying concepts being used will result in a truly conservative estimate. As long as one can be accurate in assuring that the levels of release are low, precise estimates are not needed; even with some orders of magnitude of residual uncertainty, the calculated release may be clearly within defined safety goals or limits.
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Page 88 The major models that are needed for making these predictions can be grouped into three categories: (1) behavior of the waste package (i.e., the waste itself and its surrounding containment), (2) behavior of the host rock in the immediate vicinity of the waste, and (3) transport of nuclides from the waste package to the environment. These are described below. The Behavior of the Waste Package The approach used in modeling the waste package is to conduct short-term experiments (up to a few years) that bring the material (i.e., the canister, spent fuel, or solidified waste material) into contact with a leachant. The leachant is typically water containing additives so that its composition matches the expected composition of water within the repository. Corrosion or leaching models are fitted to the experimental results, and a model is used to extrapolate the results to very long time periods. Understanding of the interaction mechanisms (such as surface corrosion, pit corrosion, solubility, or coating of waste by insoluble films) has made very significant progress in the past 20 years and has benefited from progress in materials science. Furthermore, significant efforts have been devoted to studying ancient natural objects as analogues of the waste package material (e.g., volcanic glass, archaeological copper, bronze, or iron) to determine if the models can reproduce the inferred degradation. There is little experience, however, in modeling the behavior of modern materials derived from new compositions and fabrication methods. Quantifying the uncertainty of extrapolations with these models from short-term experiments to tens or hundreds of thousands of years is still a major challenge. Nevertheless, it is accepted by the scientific community that wastes encapsulated in glass or ceramic material can last for many thousands of years in a suitable geological environment and that containers can be designed and built with similar lifetimes. Other challenges are to predict the oxidation state of the leached radionuclides, and therefore their solubility (the effect of radiolysis being taken into account), and to predict the potential chemical forms of these radionuclides. Some of these chemical forms, or “species,” may have much greater mobility in the environment than others. The Behavior of the Host Rock in the Immediate Vicinity of the Waste Package (the “Near-Field”) The behavior of the host rock will be affected by the temperature of the waste package, the chemical composition (as a function of time) of the water contacting the waste package, the mechanical properties of the rock, and the water content and water velocity close to the waste package.
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Page 89 Conductive heat transport is probably the best-understood phenomenon. As long as conduction is the dominant heat transport mechanism, one can be confident of being able to predict temperatures in space and time. This is because the uncertainty and spatial variability of heat conduction parameters of both man-made material and natural geological media are small, and therefore the uncertainty in the temperature distribution as a function of time is small. Numerous heat transfer experiments have been made worldwide, including large-scale in situ tests in rock laboratories, for example, in Sweden (Stripa and Äspö), Belgium (Mol), Germany (Asse), France (Fanay-Augères), Spain (El Berrocal), Switzerland (Grimsel), and the United States Waste Isolation Pilot Plant [WIPP] (Brewitz et al., 1999). In the United States, a major, more complex heat transfer experiment at Yucca Mountain is in progress. It includes measurement of convective heat transport, water vaporization, and condensation in the geological medium, which are difficult to analyze. If a decision is made to allow temperatures in the volcanic tuffs to rise well above 100 °C, then there is considerable additional uncertainty because of the effects described above. Methodologies for hydrogeochemical modeling of the near-field environment have also been developed and tested. They are generally based on the measurement of the present composition of the water; on its potential evolution due to temperature increases; and on the nature of the host rock, the waste package, and possibly a surrounding buffer material. The roles of the buffer are to protect the waste physically and to restrict access of water and transport of radionuclides. Most often, clays and sometimes salt are used as a buffer material. At Yucca Mountain in the United States, the waste package, according to the current design, is surrounded by air, and possibly shielded by an “umbrella” constructed of titanium alloy, which would prevent drops of water from falling on the canisters (DOE, 1998b). Laboratory or in situ experiments are also being conducted to confirm the hydrogeochemical modeling. A large body of results (see, for example, NEA, 1999d, 2000c) gives confidence in the scientific approaches developed, although some residual uncertainties deserve more work, for example: Modeling the complex interactions between some wastes, waste forms, and buffers. These interactions occur for intensely radioactive waste from fuel reprocessing or if cement, bitumen, or degradable cellulosic material is present. The latter two materials are generally excluded from geological repositories because they may be flammable or support biological activity. Understanding the potential natural evolution of the chemical composition of the groundwater. In some cases, there already is spatial variability in
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Page 90 the observed composition, and it may be anticipated that changes will occur. The salinity is often variable (e.g., in Sweden), which is assumed to derive from past events during a glaciation or upward percolation of deep fluids. The oxygen content may also change with time, and there is evidence in the mineralogy of the rock that some oxidizing phases occurred. Predicting these changes is very uncertain and difficult. The mechanical behavior of the rock is a complex function of its physical properties, the natural in situ state of stress, the geometry of the openings, and the thermal and chemical evolution. Thermal stresses are particularly important to determine and have been studied extensively during in situ thermal experiments. Depending on the rock type, stress can produce additional fracturing, or increased plasticity and convergence of the openings. The transport and geochemical evolution of fluids in the near-field may seal or open fractures and thus locally change the permeability. The main issue in mechanical modeling is the evolution of the permeability of the host rock in the vicinity of the waste and, for those repositories that are backfilled or are constructed in a plastic rock (clay, salt), to assess the effectiveness of the natural sealing of the voids and discontinuities surrounding the waste. The natural evolution of the stress field due to tectonics, the probability and effects of earthquakes, and the effects of mechanical loading by ice during glaciation have also been studied in various programs. Historical records of earthquakes, plate tectonics, and theoretical modeling are used. The residual uncertainties are a function of the host rock (crystalline and volcanic rocks being much more sensitive to fracturing than clay and salt). The uncertainties potentially are significant and must be compensated for as far as possible by conservative repository design. Knowing the flux of water in contact with the waste package is necessary to model the chemical degradation of the waste. This flux will vary throughout the repository due to small-scale spatial variability of the rock. There have been attempts to estimate this variability (e.g., in granite in Sweden) by using stochastic discrete fracture flow models at the small scale (SKI, 1996). The validity of these models is still debated, and the data base on which they are built is site specific. The models depend on detailed observations of the frequency distribution of natural fractures, plus those generated by mechanical effects. In many cases, an average fluid flux or maximum fluid flux is assumed. During the construction of a repository, observations of the flux at the time of the opening of the deposition cavity can be made and, for emplacements in which the fluid flux would be high, discarded. Even so, the evolution of this fluid flux due to thermomechanical and hydrogeochemical mechanisms, and to tectonic evolution in the long term, is quite uncertain. Upper bounds based on judgment are generally used in evaluations of safety.
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Page 91 Transport of Radionuclides from the Near-Field Environment Radionuclide transport out of the waste form and repository (the near-field environment) through the more distant geological host medium (often called the far-field or the geosphere) to that part of the environment accessible by humans (the accessible environment) is probably the most uncertain area of modeling. The theory for transport of solutes in natural media is reasonably well developed, but actual measurements have yielded surprises in the magnitude of the travel distance and direction (see Sidebar 6.1 and Sidebar 6.2 ). These measurements force examination of whether the established theories apply in all cases. Even if classical behavior is assumed, transport models require detailed information on both near-field and far-field properties (i.e., properties of the geological medium between the repository and the accessible environment) of the repository system. Given the spatial variability of natural media, this is a formidable task. Clays and salt, which are relatively homogeneous media, generally have less spatial variability in physical and chemical properties than crystalline and volcanic rocks but are harder to characterize because low-permeability experiments are very difficult at large spatial scales. Moreover, radionuclide migration experiments in a medium that has been chosen because it should confine the waste for very long times can be conducted only over very short distances (e.g., meters) for periods of a few years. Such migration experiments have been conducted in several underground research laboratories (URLs), for example, at Mol (Belgium), Grimsel (Switzerland), Stripa and Äspö (Sweden), El Berrocal (Spain), and Fanay-Augères (France) (see, for example, Kickmaier and McKinley 1997; Brewitz et al., 1999). These experiments have very significantly improved understanding of radionuclide transport in geological media. It is, however, well known from experiments done elsewhere in more permeable media that transport parameters are scale dependent; consequently, the parameters measured at small scales cannot always be used to represent phenomena at larger scales (Matheron and de Marsily, 1980). The scale issue is related to the heterogeneity of natural media and, in particular, fractured media. Another difficulty encountered in modeling radionuclide transport in geological media is that of properly accounting for the numerous coupled hydrogeochemical interactions that take place among the solutes, host rock, and particulate or colloidal matter that may be present. Much has been learned about these complex interactions, mostly in laboratory experiments and also in some of the transport experiments done in URLs, as mentioned above. A large number of thermodynamic equilibrium constants for these interactions have been measured for the most important nuclides and the most common rock types. Studies of these interactions
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Page 92 are made more difficult by the spatial heterogeneity in rock properties, which so far has not been included in the models. Beyond the difficulties of measuring the properties of geological media at large spatial scales and understanding the complex coupled processes that occur therein, accurate representation of these properties and processes in the transport model remains a major challenge. This challenge is sometimes referred to as the conceptual model problem (NRC, 2000a). Simply stated, a transport model is only as good as the conceptualizations of the properties and processes that govern radionuclide transport on which it is based. If the model does not properly account for the physical, hydrogeochemical, and when appropriate, biological processes and system properties that actually control radionuclide migration in both the near- and far-fields of the repository system, then model-derived estimates of radionuclide transport are very likely to have very large—even orders-of-magnitude—systematic errors (see Sidebar 6.1 ). The present state of the art for transport modeling involves the use of many different kinds of numerical models to represent radionuclide migration in the environment. These models have large numbers of parameters that are difficult to estimate. Further, they are frequently based on highly simplified or even incorrect conceptualizations of the highly uncertain physical and chemical properties along the transport pathway. Special complications arise from the fact that in deep crystal systems very saline waters that contain methane and other gases are expected. Geochemical models for such fluids are not yet fully developed and transport considerations are very difficult. Only short-scale (tens of meters) direct validation of these models have been done. Attempts have also been made to use natural analogues to understand long-term behaviors of natural systems (see, for example, Miller et al., 1994; McKinley and McCombie, 1995; Smellie et al., 1997; EC, 1998). Analogues are natural or man-made systems that have existed for long times, whose characteristics can be measured today and whose evolution through time can be modeled using the same methodologies employed in safety assessments (see the following section). In contrast to laboratory experiments that have well-defined initial conditions but limited time scales, some analogue systems have evolved over time periods even longer than those envisioned for repositories. Although there are often inherent difficulties in specifying precisely the conditions at the outset and throughout the history of the analogue system, the study of relevant systems can provide useful data for the analyst, enhanced understanding for the scientist, and increased transparency for the public. It is, however, difficult to transpose the data inferred from the study of analogues to actual disposal sites, since the parameters of the models are site specific. Consequently, the use of environmental tracers (such as oxygen-18, deuterium, tritium, carbon-14, noble gases, and strontium iso-
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Page 93 topes) to provide a better understanding of transport at potential repository sites is a very active area of current research. The implication of the foregoing discussion of the scientific basis for modeling seems clear to the committee. To the extent that repository systems rely on geological media to provide a long-term barrier to radionuclide migration to the environment, it is essential that the fundamental system properties and processes that govern transport behavior be well understood and appropriately implemented in those models. In many disposal concepts, one approach to this challenge of understanding complex geology is to use other safety barriers to reduce the importance of the geological medium. A durable waste form and container, along with an effective, low-permeability buffer material around the waste packages, will reduce the performance requirements on the geology. However, long-term safety cannot be achieved and demonstrated without an adequate understanding of the geological structures and processes. Hence, gaining this understanding remains a major scientific challenge for repository development programs (see Sidebar 6.2 ). Sidebar 6.1: The Conceptual Model Problem The importance of accurate conceptualizations of subsurface properties and processes in radionuclide transport models can perhaps be best illustrated using two examples from the ongoing effort to clean up environmental contamination at U.S. national defense sites, which is being undertaken by the U.S. Department of Energy (DOE). Several major defense sites are located in the arid western United States, where the groundwater table is located tens to hundreds of meters below the earth's surface and the unsaturated zone above the water table is composed of highly heterogeneous rocks and sediments. When the sites were first established during and following the Second World War, it was thought that the arid climates and thick unsaturated zones would protect the groundwater from radioactive and chemical wastes discharged into the shallow subsurface. This is the same argument made to support a geological repository in a deep, fractured unsaturated zone at Yucca Mountain. Transport models seemed to confirm these initial predictions and further indicated that radionuclide transport through the unsaturated zone to the groundwater at these sites would not occur for hundreds or even thousands of years. Recent “surprise” discoveries of radionuclides in groundwater at two of these sites (Idaho National Engineering and Environmental Laboratory [INEEL] in east-central Idaho and the Hanford Site in eastern Washington), however, have prompted a reevaluation of this assumption. Now, similar data involving bomb-era radionuclides (chlorine-36) from the Yucca Mountain site provides further evidence that the observations at INEEL and Hanford may apply to a geological repository site (see Sidebar 6.2 ). At INEEL, recent monitoring of groundwater near a shallow land burial site (the Radioactive Waste Management Complex) containing radioactive and chemically hazardous waste confirmed the transport of low levels of plutonium and other con-
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Page 94 taminants through the unsaturated zone to the aquifer some 200 meters below. This migration was neither expected nor predicted from existing transport models, even though radionuclide travel time through the unsaturated zone has been the subject of intense debate at INEEL for almost four decades. In the 1960s, the National Research Council Committee on Geologic Aspects of Radioactive Waste Disposal visited the site and published a report on this issue (NRC, 1966). The report noted (p. 5) that The protection afforded by aridity can lead to overconfidence: at both sites it seemed to be assumed that no water from surface precipitation percolates downward to the water table, whereas there appears to be as yet no conclusive evidence that this is the case, especially during periods of low evapotranspiration and heavier-than-average precipitation, as when winter snows are melted. Indeed, since 1960, estimates for travel time through the unsaturated zone at INEEL have decreased by almost four orders of magnitude, as illustrated in the figure below. At the Hanford Site, billions of gallons of liquid waste containing millions of curies of radioactivity have been disposed in the ground on the central plateau (referred to as the “200 Area”) of the site. Initial field investigations of this site and subsequent transport models suggested that the 90-meter-thick unsaturated zone underlying the 200 Area would bind many of the released radionuclides, preventing their migration to groundwater (GAO, 1989, 1998). In 1997, however, DOE reported that cesium-137, technetium-99, and cobalt-60 had migrated deeper than expected, in some cases to groundwater (DOE, 1999b), along with some metals and chemicals. ~ enlarge ~ Figure 6.1 Changing estimates of travel time for measurable amounts of any mobile radionuclide to reach the water table beneath INEEL (NRC, 2000a)
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Page 95 The lack of accuracy of model predictions in these two examples can be attributed to incorrect conceptualizations of the hydrogeologic system at the sites, including improper simplifying assumptions, incorrect transport parameters, and overlooked transport phenomena. These problems reflect both an inadequate understanding of transport processes in geological media and poor implementation of current understanding in the transport models. The impact of this “conceptual uncertainty” is very difficult to determine in performance assessment (see discussion in text). This is well captured in a quote from Konikow and Ewing (1999) who, comparing long-term predictions of natural systems with a game of chance, remark: In hydrogeologic and geochemical systems, . . . we do not know the odds. In fact, we probably do not even know all of the rules of the game (or perhaps even which game we are playing); that is, for these natural systems there will be uncertainty in the conceptual models and in the complex non-linear coupling between models. Most national programs have responded to conceptual uncertainties in the modeling of transport in geological media by placing more reliance on engineered barriers. Within the geological setting, the engineered barriers can be well characterized and can be emplaced under quality-assured conditions. This increases confidence in the understanding of their behavior, although it does not remove all conceptual uncertainties associated with understanding the far-future evolution of the engineered barriers and does not obviate a need to understand the behavior of the geological media. Sidebar 6.2: Geological Barriers in Repository Systems The geological media surrounding a repository provides a barrier to migration of radionuclides to the accessible environment. The geological media may retard the movement of groundwater or bind (sorb) many of the radionuclides that escape from the repository; alternatively, the groundwater flow system itself may provide effective isolation from the accessible environment. The net effect is to increase groundwater travel times from the repository to the accessible environment and thereby allow time for radioactive decay to reduce radionuclide concentrations. Most repository designs utilize geological media as one of several barriers to radionuclide migration to the accessible environment. The Swedish design, for example, calls for a repository constructed in a fractured granite formation with a clay backfill below the groundwater table, with SNF encapsulated in copper canisters. The clay functions as a low-permeability buffer that greatly limits access of groundwater to the container and also retards any radionuclides that may be released from the wastes. The U.S. design, on the other hand, calls for the construction of a repository in the unsaturated zone in an oxidizing environment. The waste will be encapsulated in corrosion-resistant metal canisters, and the canisters will be surrounded by titanium drip shields to protect them from percolating water (DOE, 1998b). Figure 6.2 illustrates this engineered barrier option for Yucca Mountain, as well as the general concept of engineered barriers. Predicting the long-term behavior of water and radionuclide transport in the near- and far-field environments is a major challenge in all of these programs. In the U.S. program at Yucca Mountain, for example, it was initially assumed that
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Page 103 When waste management specialists are considering how to reach out to other communities to achieve a common understanding of key issues, it is useful to start asking the questions: “Who are the stakeholders; in which order, and in what way should they become involved?” (See also Sidebar 1.5 ). The committee has identified four general groups of stakeholders for this analysis. These include the general scientific and technical community, regulators, elected officials, and concerned citizens. How science and scientists interact with each of these groups is discussed below. The Scientific and Technical Community It seems clear that HLW disposal in any country cannot occur if the scientific and technical community does not support it. This community includes (1) scientists and engineers directly involved in the project who prepare the technical material that goes into the tools and models used in PA; (2) scientists and engineers involved in various review groups, at the local, national, or international level; and (3) members of the scientific and technical community not involved in the project, who may have some knowledge of the issue, or who have acquired the esteem and respect of their colleagues and the public because of the quality of their work. How does this scientific community arrive at its judgment? As in any field of science, the first step is that the scientists actually working on the project have the stature and reputation that brings respect for their work from colleagues. Agencies involved in the preparation of a repository project should be aware of this requirement, applying not just at the top scientific levels, but at all levels. Evaluation of the feasibility of HLW disposal is a scientific challenge, and the highest-quality and most up-to-date science needs to be used. Independent expertise, including international expertise, should be used to complement the work of inhouse staff. The findings of the scientific staff need to be published regularly in the open scientific literature. This should not be considered by the agency as a secondary aim, but as a major requirement of the work. Scientists involved in the programs must also have some opportunities for launching new studies in order to verify and develop new concepts. For instance, the examples concerning transport processes in Sidebar 6.1 and Sidebar 6.2 should motivate the examination of processes and mechanisms that were not anticipated when many national programs started one or two decades ago. It is necessary that new ideas be examined, which implies that scientists inside and outside the project can submit proposals and receive funding for performing work designed to disprove the prevailing conceptual model or to collect data that may support an alternative conceptual model. Of course, there must be a limit to the work that deserves to be funded; that decision, however, should not de-
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Page 104 pend on the responsible agency alone, but also on external agencies experienced in funding scientific research. Scientists with expertise relevant to a project must be able to express their views on the safety of the project to their peers, before the project enters the licensing phase. The National Research Council report on the Waste Isolation Pilot Plant (NRC, 1998), which came out about a month before the Department of Energy (DOE) applied to the Environmental Protection Agency (EPA) for compliance certification, was an excellent example of such an activity. Scientists will be more easily convinced by a formal argument (the “safety case”) that an engineered system is safe if the design of the system is robust. The concept of robustness can be applied to repository systems as follows (McCombie et al., 1991): A robust repository system has (1) simple geology, physics, chemistry, and design; (2) large safety factors; and (3) some degree of redundancy. A robust performance assessment is characterized by (1) being based either on well-validated, realistic models or else on clearly conservative models and data; (2) assuring that all potentially negative processes are analyzed; and (3) being relatively insensitive to parameter and conceptual model changes. For example, the robustness of the assertion that an engineered barrier is long-lived can be sought from archaeological evidence or from physical principles and short-term measurements. This approach led the Swedes and the Finns to select copper as the canister material (see Figure 4.1 ). Copper is known to be stable in reducing environments, both from the existence of native copper and from archaeological artifacts. Corrosion can also be estimated using the basic principles of metal corrosion thermodynamics. Both approaches provide confidence that this material will last for millions of years in favorable environments. The role of the geological barrier is then to maintain this favorable environment. The Regulators' Dilemma The first role of a regulator is to decide the rules for demonstrating compliance that the implementing agencies should obey; then the regulator has to establish for the remaining stakeholders the credibility of its decisions by making clear why these rules are necessary and sufficient to convince the regulator of repository safety. The regulator's second role is to decide if the license application, which is made by the implementing agency, meets these requirements. Both roles require that the regulator has scientific credibility and that the same rules as those described above for science at the implementing agencies apply also to regulators. This
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Page 105 includes the need for scientists at the highest levels, sufficient scientific staff, publications, room and funding for independent research, and expression of independent scientific views. In comparison to communicating with general scientists and the public, the interaction of regulators with implementers should be a more straightforward, technical task. This is not to say that the role of regulators is unimportant. Rather, experience shows that regulators should be in constant interaction with the implementing agencies. Regulators directly or indirectly interact with implementers with respect to the methods they should use to show compliance, the data they should collect, and the nature of the evidence they should provide. Furthermore, regulators have to understand the PA methodology very well. Only if regulators are convinced that the science behind the PA is good, and if the PA estimates of performance meet regulator-established requirements both quantitatively (i.e., meeting the numerical requirements in the regulations) and qualitatively (i.e., demonstrating in nonnumerical terms that the repository will be safe, using, for example, a convincing safety case), will regulators endorse and approve the licensing application. The ultimate application of the safety assessment methodology is in the preparation of a full safety case for licensing a repository. There are three categories of requirements: 1. The repository system itself must be based on a robust disposal concept, good engineering and technology, and a suitable site. 2. The safety analysis requires a convincing safety case (robust models based on sound data), proper regulatory framework, and transparent presentation at all levels. 3. The regulatory process depends on having a competent implementing body, a competent regulatory body, and proper communication among all stakeholders. Today, particularly with respect to nuclear activities, there is insufficient public and political trust to permit unquestioned acceptance of projects worked out only among technical experts in the implementer and the regulatory agencies. As in all highly technical initiatives, trust and confidence must be engendered in wider circles, and this requires that all experts be answerable to a wider audience. The special challenges of involving the public in nuclear and radioactive waste issues are dealt with in detail in Chapter 5 and Chapter 8 . Although no deep geological repository for long-lived wastes has yet been licensed through a standard procedure, 2 regulators and implementers are relatively confident that workable procedures have been, or can be, 2 The WIPP is a special case, certified by the EPA for limited types of DOE waste (see Sidebar 4.1 ).
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Page 106 developed. This is not to imply that there are no open issues. The following list provides some of the topics currently being discussed among regulators (NEA, 1997b): Time perspective—there are no scientific reasons for specific cutoff times such as 10,000 years—but are there policy reasons (NRC, 1995)? How can the safety of geological disposal be evaluated using the established regulatory concept of “reasonable assurance”? Should regulators require investigation of alternatives to geological disposal? What is the role of the regulator in site selection and choice of disposal method? Legal and regulatory issues—what are the roles of government agencies and regulators? What should the regulatory attitude be toward retrievability? How should human intrusion be treated when the future actions of society are unpredictable? How should the first 100-year-period risks (such as proliferation, transportation, and worker health) be weighted relative to the 10,000-year risks or to the maximum risks at any time? How can a reasonable decision be arrived at—setting up a decision process, participation, and a stepwise implementation process? The last item leads to the heart of the “regulator's dilemma”—how to organize a regulatory approach to enable solid and accepted regulatory decisions to be made in light of the uncertainties, some of which are in fact not resolvable. Sidebar 6.4 provides a summary of the committee's views on regulatory issues related to geological disposal. Sidebar 6.4: Summary of Committee Views on Regulatory Issues Related to Geological Disposal The following points summarize the committee's views of regulatory issues connected with geological disposal. 1. A “phased approach” to regulation of a deep geological repository, as expounded 10 years ago in the Board on Radioactive Waste Management's Rethinking High-Level Radioactive Waste Disposal (NRC, 1990), remains excellent advice. A key corollary is that the regulator must strive to avoid over-prescriptive rules too early in the overall multi-decade process of regulatory approval. A second corollary is that, in general, a “compliance” attitude and philosophy is an inappropriate way for the regulator to approach the major yes-or-no decision; the regulatory yes-or-no decision for a geological repository will always require a good deal of judgment, not merely a cookbook compliance-type finding. At some very fundamental level, the implementer is always responsible for showing that the site
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Page 107 is safe. Programs should be careful that a prescriptive regulatory approach does not induce a compliance attitude rather than a “safety” attitude. 2. Public involvement is essential in the process whereby the regulator arrives at the rules and regulations to be used in project approval. Public involvement should begin at the earliest phases of the rule-making process and continue throughout. This implies a fully open process, in which the public can challenge and comment on the approaches to be used by the regulatory body. 3. As stated explicitly in regulatory documents from many countries (USNRC, 1998; EPA, 1999), “proof” that the proposed geological repository meets any specific set of regulatory standards cannot be had in the ordinary sense of that word. This is because of the very long time frames involved, including but not limited to our inability to understand changes in human behavior over such a long period. 4. Even when a particular country's legal system requires quite prescriptive regulations for a deep geological repository, there is substantial room for flexibility at the level of detailed regulatory guidance, decision making, and inspection programs. 5. The regulatory body's ability to adopt and utilize a less prescriptive system that involves relatively more judgment is very much tied up with how much trust that body enjoys with the bread public. The more trust, the more deference is afforded the regulatory body to exercise judgment instead of relying on prescriptive yes-or-no findings, and the more likely is acceptance by the public of the regulator's decisions. 6. The corollary is that if a regulatory body has engendered mistrust and hostility among key sectors of the broad public, then it is typically forced into both more prescriptive and less flexible regulatory decision-making situations, and more conservative rules and decision criteria. This is because the political repercussions of any decision, where broad trust is absent, can become so uncomfortable for the regulatory body that it finds itself needing much more regulatory “margin” to make any decision. 7. There is inevitably a lot of uncertainty in any analysis of future repository performance, no matter how well it is done. This is true even of those parts of the analysis, such as the geological, hydrological, metallurgical, and chemical aspects, that ought to be more “science based.” This has led to suggestions that the key regulatory yes-or-no decision should in general be based on more than a single numerical figure of merit. The approach of relying on a single figure of merit could be supplemented by use of one or more quite different figures, including an overall judgmental criterion, namely, Does the safety case make good technical sense? 8. It is important to emphasize the value to every country of the principles incorporated in the Joint Convention on the Safety of Spent Fuel Management and on the Safety of Radioactive Waste Management (IAEA, 1997a). Internationally agreed-upon principles for radioactive waste management are discussed in Chapter 9 (see Sidebar 9.1 ). 9. Both the repository developer and the regulatory body should take a systems view of the analysis of future repository performance. Insights gained from a systems view are not just the sum of the insights from analysis of the details: understanding the “forest” is more than just a sum of understanding various individual “trees.” 10. How far into the future the specific regulatory compliance period should extend is primarily a public policy issue, not a technical issue that science can address.
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Page 108 11. The job of regulating a first-of-a-kind repository is intrinsically one of gradual learning and refinement. Nobody should expect that a regulatory body could put into place in 2000 a set of regulations that would govern for, say, 100 years. The phased approach also allows regulations to develop and to take account of new knowledge gained during the lengthy phases leading ultimately to closed and sealed repositories. 12. It is broadly agreed that while a regulator should require the developer to put into place a monitoring system to measure various relevant repository performance parameters, it is an incorrect regulatory approach to make such monitoring an essential feature of assuring safety (IAEA, 1997b; ANDRA 1998). Regulators do not believe that such monitoring could be relied on for long enough—centuries or millennia—to identify flaws in the repository's safety case, which will likely emerge (if at all) only centuries or millennia in the future. This is not to say that monitoring that does in fact detect a repository failure or surprising behavior should be ignored. 13. Finally, it is important to consider how the regulator can structure a program for communicating effectively with the public. The Link Between Scientific and Societal Responsibility In the past few years, more attention has been paid to the issue of increasing public involvement in the process leading to final disposal. In essence, there is pressure to structure a process that assures accessibility of all relevant information, gives adequate opportunity for public input and questions, retains options for as long as possible, and remains reversible should new evidence show that better options have become available. The most conspicuous result of these developments has been an intensive debate on monitoring repositories, on phased or stepwise implementation programs, and on measures needed to guarantee reversibility of steps—including retrieval of emplaced wastes. The technical community accepted the validity of these points rather grudgingly. Originally, it was often pointed out that no scientifically meaningful monitoring programs could be proposed and that retrieval was a scenario that could be excluded by proper planning and execution of disposal projects. Now it is widely accepted that “confidence monitoring” to address public concerns is a legitimate exercise, even if scientific considerations indicate that the probability of detecting a malfunction is negligible even for centuries or millennia. It is also accepted that retrievability, or more generally reversibility, which was always feasible in principle, can be made more straightforward (“enhanced”) without unacceptable negative impacts on repository safety (IAEA, 1997b). In short, there has been an increase in the readiness of insider “experts” to listen to public concerns and to work toward allaying them. This very fact should
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Page 109 contribute to enhancing the public's involvement in geological disposal decisions. Both the scientifically based performance assessment and the public processes associated with making choices about nuclear waste are complex and inherently uncertain. Those on both the technical and the public sides of this issue may wish that the other side were certain, but neither scientific understanding nor societal decision making are completely predictable. Importantly, there is a large potential for negative interaction between these two uncertainties. Scientists may wish to treat technical uncertainty through the calculation of risk. However, scientific uncertainty, or even the perception of scientific uncertainty, can easily cause public entities to fight a project. Public opposition can elicit a “bunker mentality” on the part of the implementer. The net result is a lack of true dialogue and the potential for suboptimal or even irrational decisions. Stirling (1999) refers to the tension between narrow notions of “sound science” and the “precautionary principle” 3 as a “dichotomy trap,” in which productive and creative “solution-oriented” thinking is impossible. Both public interest organizations and technical implementers should be enjoined to act responsibly. It is not responsible to obfuscate the potential risks of a geological repository when communicating with the public. Nor is it responsible to take irreversible action when the risk cannot be sufficiently quantified. On the other hand, it is not responsible to block any action designed to reduce significantly the risk to society, merely because some risk remains. Invoking the precautionary principle in this regard can cause great harm by preventing appropriate waste management. There is no path forward that is risk-free, including taking no action. It is important that society make wise choices concerning the wastes that already exist and require management. Rather than seeing the public need to be cautious as being in contradiction with a science-based management process, it is possible to view this caution as entirely consistent with sound scientific practice. In responding to intractable problems in risk assessment such as “ignorance” (“we don't know what we don't know”) and “incommensurability” (“we have to compare apples and oranges”), the role of precaution then becomes the same as the adoption of a stepwise approach in which difficult or irreversible decisions are taken only when commensurate with understanding. It is nearly impossible to quantify risk when we are also uncertain of the appropriate conceptual model underlying the phenomena of interest. The issues involved in gaining public acceptance under conceptual uncertainty include acknowledging the importance of ignorance and 3 The precautionary principle as a concept is a matter of intense debate in the risk community as well as in the international legal community. A recent summary describes its various formulations (Foster et al., 2000).
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Page 110 the subjectivity of the trade-offs and “framing assumptions” that necessarily condition any analysis. A rational response both to ignorance and to the implicit element of uncertainty in analysis is to broaden out the appraisal process systematically in such a way as to consider a wide array of different issues and options, contingencies, and possibilities and forms of effect, and to include a diverse range of disciplines and stakeholder groups, to provide for careful long-term monitoring, and to place the burden of persuasion on the developers rather than the regulators. These choices make sense from a technical perspective, and they also allow a constructive dialogue with the public. In a measured and incremental application of an approach, Stirling recommends that “science should be on tap, not on top” (Stirling, 1999, p. 29). There can be no simple analytical, instrumental, or institutional “fixes” for the complexities encountered in the management of technological risks. Policy making must obviously be based on available scientific information, but science on its own is not enough. In this way, it is possible to integrate technical and socioeconomic factors in the management of risk by examining different technological options and evaluating their flexibility, resilience, and diversity. The recognition and active accommodation of dissenting voices in the societal debate is simply an extension of that crucial principle of quality in the scientific process: organized skepticism. Transparency is an essential means to the end of full public engagement. Inter- and intradisciplinary scientific conflicts should not be concealed. The full implications of uncertainties should be acknowledged. Alternative assumptions and value judgments should be explored fully. Such an incremental approach, which makes sense from the technical standpoint, also offers a way to provide for the open-ended societal learning that is an essential quality of the successful waste management program. In this regard, there is a distinct difference between processes for deciding what to do and the process for doing what's decided. The two should not be confused. Societal acceptance will be more likely if the government's role is to facilitate the first process and not anticipate the result by sponsoring particular solutions. As the societal appraisal process illuminates the consequences of adopting different framing assumptions in interpreting the available science, then it becomes the role of the government to make incremental decisions and to be accountable transparently for the inevitable political and value-laden elements in such decisions. In addition, a stepwise program is scientifically beneficial since it allows decisions at each step to be commensurate with the status of the science base at that time. Moreover, the reduced pace of development that these interactive processes and deliberately phased schedules imply for implementation programs affords more time for building public and political participation in the decision. These gains are to be set against the unavoidable increases in time and resources needed to carry out reposi-
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Page 111 tory projects. Further aspects of the interaction with the public are discussed in Chapter 5 and Chapter 8. CONCLUSIONS The challenge of safely managing HLW and SNF is not only a scientific issue. Indeed, the key areas in which problems have arisen, and new approaches may be needed, are more concerned with societal issues. Scientists and technologists involved in waste management have recognized this increasingly in past years and have shown a readiness to become involved more directly in these issues. Nevertheless, scientists have a prime, continuing responsibility to try to identify, understand, and communicate to the public the technical issues that influence the general debate. The corresponding conclusions widely agreed to by the committee and, it believes, also by the scientific and technical communities, are summarized below. Science, Technology, and Performance Assessment Based on the preceding discussions, the following can be concluded: The necessity of modeling the long-term performance of a repository is universally recognized. Quantitative results from assessments provide a necessary input for decisions throughout disposal system development. The calculated results do not, however, provide hard criteria that obviate the need for human judgment. Safety assessments alone are not the only considerations governing the acceptability of any disposal facility. The feasibility of performing assessments of sufficient quality is accepted by technical experts within the waste management community. A somewhat lower level of confidence exists in wider scientific circles, and in limited segments of the public severe reservations are still expressed. Some of the remaining differences in views could be narrowed if assessors made clearer that their aim is not to analyze the future exactly, but rather to scope the range of potential future behaviors of the repository system and the consequences of the remaining uncertainties. Specific parts of the modeling chain for geological repositories will continue to be developed and refined. In particular, there is a critical need to focus explicitly on the conceptual models that underlie the calculations. It is these studies that will illuminate the physical and chemical principles that dominate the behavior of the repository. It is fundamentally necessary to identify the appropriate conceptual models before we define the appropriate parameters characterizing the site and then try to understand the statistical variability associated with these parameters. The common time scales for implementation of HLW repositories leave many years for potential improvements. These developments may ease the difficulties in
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Page 112 future licensing procedures; nevertheless, they will not result in perfect models that produce unquestionably accurate results. The requirements for human judgment and expert opinion will remain. The critical issue with respect to safety assessment is the required or achievable level of confidence in the results of the analyses. Neither a 100 percent level of safety nor 100 percent confidence in the reliability of the assessments is possible. This is a fact that is true also for every other comparable technical undertaking, and it is important to assure that unique, unattainable requirements to the contrary are not placed on radioactive waste disposal. The extensive technical efforts that are being put into specific, technical validation programs, centered around comparisons of calculations, experiments, and observations of analogue objects, should be complemented increasingly by further confidence-building measures. These include peer review, more formalized quality assurance, transparent documentation, large-scale demonstration experiments, and—of great importance—development of processes assuring open discussion among all involved parties. Confidence and Trust Confidence of the experts within the waste management community in the feasibility of safe geological disposal is documented in, for example, the collective opinions prepared by the OECD-NEA (NEA, 1991b). This confidence is not shared by sufficiently many members of the public and decision makers to allow rapid development of geological disposal projects. Performance assessments alone do not engender sufficient confidence in safety. Demonstration of sound scientific work (e.g., by transparency, peer review) and admission of indirect evidence (e.g., from natural or man-made analogues) are also necessary. The technical community now acknowledges the necessity of devising concepts and procedures explicitly aimed at raising public confidence. These include concepts for monitoring and retrieval of emplaced wastes and procedures for implementing repositories in a phased or stepwise manner. Maintaining a capability for reversibility of steps during the long processes leading to a closed and sealed repository is a major factor in enhancing public confidence. General Conclusions A phased or stepwise approach to implementation of repositories can offer a proper compromise between minimizing future burdens and maximizing future choice. Properly designed and sited repositories can
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Page 113 have a long period of monitored controls and enhanced retrievability before being converted into their final closed state. Adequately safe geological repositories can be implemented with various combinations of carefully selected host rocks, sites, and engineered barriers as long as decisions are taken commensurate with understanding. It will never be possible to demonstrate that any one site is the “safest” choice. Factors other than long-term safety will also determine or even dominate siting choices.
Representative terms from entire chapter: