Alternatives to Geological Disposition
Geological disposition, as conceived by national programs worldwide, is the emplacement of current and future high-level radioactive waste (HLW) inventories in repositories that have been mined and constructed in deep formations. In conducting its study of the geological disposition option, the committee found three alternatives to this concept:
1. surface storage, either in-place where the waste was generated, or at consolidated locations;
2. alternatives that require advanced separation processes, including partitioning and transmutation (P&T) and disposal in outer space; and
3. geological alternatives to mined repositories, for example, seabed or borehole disposal.
The choice is not whether to put the waste in a repository or leave it on the surface for the order of 10,000 years. Rather, the choice is how and when to remove spent fuel from decommissioned reactors and where to put the fuel in order to assure safety and security. The major choice for nuclear programs in coming decades is how to assure the safe and secure control of HLW and spent nuclear fuel (SNF) while at the same time developing an option for geological disposal so that an ongoing commitment to active management can, at some future time, be brought to an end.
Geological disposition and surface storage (see Sidebar 1.3 ) are the only options that the committee found to be feasible now or in the foreseeable future, as described in this chapter. Furthermore, the committee found that moving from surface storage to geological disposition is a
societal, rather than a technical, decision for each country and that this decision need not be rushed. Geological disposition followed by closing the repository (geological disposal) is nevertheless the only permanent and final solution to the waste problem. The committee recommends that all countries maintain this option in their national planning.
This chapter begins with a discussion of surface storage, then compares its advantages and disadvantages with those of geological disposition, and finally summarizes the committee's views on these two options. This chapter also gives a brief overview of partitioning and transmutation, which could reduce but not eliminate the need for geological disposition, and the other alternatives.
Currently, most if not all, SNF and HLW are in surface (or near-surface) storage. Storage technologies have been used safely for half a century. 1 SNF storage, primarily at reactors, has not been seriously questioned on safety grounds. For example, the U.S. Nuclear Regulatory Commission has concluded that such storage is safe for at least the life of the reactor's operations, that is, for at least 40–60 years (USNRC, 1999). Assessment of the Swedish centralized storage facility, CLAB, showed that it could safely store SNF for 100 years or more (Söderman, 1997).
Safe storage in surface or near-surface facilities can be achieved by packaging SNF and HLW in suitably engineered structures or robust containers to assure that radioactive materials will not be released. The security of surface or near-surface storage can be achieved by restricting access of individuals and groups that might divert fissionable material for weapons use or use radioactive material for acts of terrorism. The technical community agrees that excellent safety and security have been achieved in most existing HLW and SNF storage facilities.
From a technical point of view, there has been no urgent need for final disposal facilities because of the recognized high level of safety of interim storage facilities, the relatively small volumes of long-lived radioactive waste from civilian programs, and the storage time needed to allow adequate cooling of the more radioactive waste before geological disposal can take place. (NEA, 1999b, p. 9)
As noted in the above quote, storage can be technologically advantageous by providing time for the wastes to cool thermally through radioactive decay, and societally advantageous, by allowing more time for delib-
1 The committee notes instances, especially regarding defense wastes in the United States and Russia, where storage conditions are not satisfactory. These have arisen primarily because the storage facilities, such as tanks, are well past their design lifetimes. Such examples illustrate the fact that storage facilities require continuing surveillance and maintenance.
erative decision making. Another 1999 Nuclear Energy Agency (NEA) review included the following regarding extended surface storage:
In virtually all countries, some period of interim surface storage to allow decay of radiation and heat generation has always been recognized to be necessary or valuable. This interim storage is often at a centralized location, but can also be at individual facilities. . . . More extended storage has been accepted as unavoidable because of delays in implementing disposal for both HLW and spent nuclear fuel. There are, in addition, arguments that favor extended (though not “indefinite”) surface storage. . . .
Postponement of disposal is advocated by some scientists and decision-makers who believe that more time is needed to prove the geologic disposal concept more completely and/or to allow public confidence to increase. (NEA, 1999a, p. 29)
Technologies are available now for HLW and SNF to achieve safe and secure storage, as well as for the transport and handling required to move waste from present locations to long-term storage facilities or repositories. However, public and political opposition to these activities is not necessarily less than that to geological disposition. Long-term storage will require continued funding for maintenance. This funding must be either from a continuation of nuclear power or by commitments from governments (Dejonghe, 1999).
Extended surface storage as an option has not yet been objectively addressed through performance assessment of likely scenarios (see Sidebar 7.1 ). The NEA has recognized this and recommended that such assessments be made (NEA, 1999c).
Sidebar 7.1: The Leave-in-Place Option
For use in the Yucca Mountain Environmental Impact Statement (EIS), the Department of Energy (DOE) examined only two leave-in-place scenarios, neither of which is likely:
1. societies and institutions similar to today exist, and storage facilities and waste forms are maintained and monitored for 10,000 years; and
2. institutional control is lost after 100 years.
For both scenarios the spent fuel and the HLW are kept at 77 existing utility and DOE sites stored in above-ground concrete modules or below-ground storage vaults. In scenario 1, facilities are repaired in 2060 and replaced every 100 years. Costs for this scenario are quite high. In scenario 2, facilities are repaired in 2060, maintenance stops in 2116, and memory of the purpose of the facilities is assumed to be lost. These scenarios include the three types of waste that are of greatest concern to the U.S. program: commercial SNF, SNF from DOE sites, and vitrified HLW from DOE sites. The commercial spent fuel canisters fail after 1100–5400 years; the DOE spent fuel canisters, after 800–1400 years; and the HLW canisters, after 500–1200
years (Walker, 1999a, 1999b). The DOE analysis begs the practical question of the longevity of the operations at the 77 sites themselves; it simply assumes that funds are available to support the activities specified in each scenario.
The proponents of surface storage do not argue that surface storage would be a permanent solution, as implied by the DOE EIS analysis scenarios, but rather that it should be examined as an interim approach that might be in place for one to perhaps several centuries. The DOE EIS analysis indicates no environmental or health problems through the first century of storage. It is this alternative of one to several centuries of delay that requires detailed, objective analysis.
SOCIETY'S TWO AVAILABLE DISPOSITION OPTIONS: GEOLOGICAL REPOSITORIES AND SURFACE STORAGE FACILITIES
For the purpose of this report, geological disposition (see Sidebar 1.3 ) is considered to be the emplacement of waste into a repository. The repository would remain open, and authorized access to the waste would be possible, for example, for inspection or retrieval. Permanent geological disposal would result from closing and sealing the repository. Surface storage is the de facto disposition option being used today in all countries with HLW or SNF. Both options require control by responsible societies and institutions until permanent disposal is achieved.
For the geological disposal option, long-term safety is provided by the geological characteristics of the site and the protection afforded by engineered barriers that are part of the repository design. The main features, events, and processes (feps, see Sidebar 6.3 ) influencing the safety of this waste disposal system are slow natural processes (such as groundwater movement, climate changes, and erosion), rapid natural processes (such as earthquakes and flooding), processes caused by waste disposal (such as groundwater thermal convection, radiation damage to the waste package, and changes induced by mining operations), and human intrusion. The main uncertainties in assessing future repository system behavior arise from uncertainties in these feps, for example, groundwater movement in fractured rock, permeability changes induced by movements in existing faults, temperature-dependent kinetics of groundwater-host rock physicochemical interaction, and long-term changes in population density (Laverov, 1999b). These are issues that the technical community believes do not pose insurmountable difficulties for geological repositories.
Achieving safety and security over decades, centuries, or millennia involves a different mix of uncertainties for surface or near-surface storage than for a geological repository. Facilities on the earth's surface are subject to weathering and a variety of other processes that eventually could breach
the HLW storage facilities and containers. In contrast, physical and chemical conditions deep underground are relatively stable and predictable, even on geological time scales. The behavior of future human societies will be critically important for surface storage. Human access will be much easier for a surface facility than for a geological repository. Surface storage involves uncertainties in whether future generations will continue to provide the resources to assure safety and security. Performance of a geological repository involves uncertainties in the geological setting and in the changes that may occur over time in engineered barriers, as well as the possibility of deliberate or inadvertent human intrusion.
Surface storage has many proponents who stress that such storage would be protective of public health and safety for several centuries. 2 Others argue that selecting surface storage only delays the inevitable decision on geological disposal. For example, according to T. Varjoranta, chairman of the European Radioactive Waste Regulators' Forum, “ . . . open-ended storing of the waste due to delaying the decisions necessary to obtain the far-reaching solution is, from the safety perspective, the worst option containing most uncertainties” (Varjoranta, 1999, p. 35). Ramon Gavela, Director of Science and Technology in the Spanish National Waste Management Company (ENRESA) commented that “[l]ong-term storage is more meaningful as an option if an advanced fuel cycle is chosen. . . . otherwise, it is simply a delay that increases the risk of repository disruption and creates the feeling that no final disposal solution exists” (Gavela, 1999, p. 113).
If surface storage is intended as a long-term alternative, the waste management strategy should address the effectiveness of controls to restrict access and assure containment integrity and the degree of confidence that can be placed in these controls. The preceding chapters (especially Chapter 4 , Chapter 5 and Chapter 6 ) describe the status of current national programs to accomplish siting, licensing, and construction of geological repositories. The pace of development was much slower than was anticipated when these national programs were established. The lack of public support for repositories, which has been largely responsible for the delays, is not likely to change rapidly. Even if repository programs do resume the planned schedules, the scheduled pace for waste emplacement assures that a substantial surface storage capacity will be needed for at least 50 years in the United States and even longer in some other countries.
The committee does not believe that societies have to make a final choice now. Surface storage has been advanced by many proponents as an interim measure for the order of a century or two. Irrespective of the planned duration of surface storage, the committee recommends that
2 About 300 years is a reasonable period for which waste facilities can be kept under societal control (IAEA, 1999).
geological disposal be pursued now as an option. The decision as to whether to implement this option, the timing, and the detailed technical choices in implementing geological disposal should be made, at an appropriate time and after appropriate deliberation, by the political leadership of each country with a responsibility for HLW.
While the societal choice remains open, steps can be taken to increase knowledge: studies on improving repository performance through modifying the waste form and changing the repository design; monitoring to determine whether there is a need to retrieve the waste for disposal elsewhere because the repository performance is not acceptable; and assessments of the desirability of retrieval because the waste has become valuable or another disposal alternative (site or technology) now appears preferable.
ALTERNATIVES TO GEOLOGICAL REPOSITORIES AND SURFACE STORAGE
During the past three decades many national and international organizations have examined alternatives for long-term management of HLW and SNF (BNWL, 1974; OTA, 1982; SKB, 1992; IAEA, 1995c). The discussion below gives the concept underlying each alternative, its advantages, and its limitations.
Partitioning and Transmutation
An approach that has been claimed to have the potential to change the future of geological disposal is partitioning and transmutation (P&T) of long-lived radionuclides to give wastes which have shorter half-lives and therefore do not present as serious a challenge to the isolation capacity of repositories. (NEA, 1999a, p. 29)
The P&T concept has an extensive history. Unlike the other options discussed below, P&T research is being investigated or pursued actively in a number of countries, including France, Japan, Russia, Sweden, and the United States. Most scientists believe that P&T might reduce the volume of long-lived HLW that would be sent to a repository, but that it will not remove the need for long-term HLW management (NRC, 1996b). Thus this option should be considered a supplement to, but not a substitute for, continued surface storage or geological disposition.
Any intense source of neutrons, such as a light-water reactor, a liquid metal reactor, a fast reactor (as in the U.S. Integral Fast Reactor [IFR]), or an accelerator-driven subcritical reactor, can accomplish transmutation of long-lived radionuclides. The physical requirements for neutron intensity and the energy requirements to achieve such intensity make it necessary to partition, or separate, the long-lived radionuclides to be transmuted from the uranium, the fuel rod cladding, and other components in SNF
and HLW. Partitioning is essentially the same as reprocessing spent fuel to recover plutonium and uranium, except that the goal includes separating long-lived fission products such as iodine-129 and technetium-99 as well as plutonium and other actinides. This step, which can lead to separated plutonium, is one reason for opposition to P&T.
Very high separation factors are required if the residue from partitioning is to be low enough in radioactivity to avoid being classified as long-lived waste requiring the same isolation as HLW. To achieve these very high separation factors, much more advanced and sophisticated reprocessing technologies than those available today are required. The reasons offered to support P&T are to make geological disposal safer and easier by reducing the volume of HLW, especially the long-lived radioactive constituents; to address plutonium management; and to extract valuable materials.
The National Research Council (NRC) report Nuclear Wastes: Technologies for Separations and Transmutation (NRC, 1996b), usually referred to as the STATS report, provided a comprehensive review of this approach as of the mid-1990s (see Sidebar 7.2 ). The STATS report concluded that removal of the actinides might allow four to five times as much waste to be emplaced in a given area of a repository and that removing the cesium and strontium could increase repository capacity by a factor of 10 to 40 (NRC, 1996b, p. 348). The STATS report also concluded, “The benefits of [P&T] (which is defined as enhanced reprocessing to recover essentially all radionuclides that would otherwise report to repository wastes) to [reduce] long-term repository risk can largely, if not totally, be achieved by employing basic reprocessing (i.e., recovery of about 99% of the uranium and plutonium in the spent fuel)” (NRC, 1996b, p. 349). Although it recommended further work on P&T, the STATS report is highly pessimistic that the need for a U.S. HLW repository can be eliminated: “It is to be emphasized that a [P&T] scenario in which the need for a repository is eliminated is considered to be highly unlikely if not absolutely impossible” (NRC, 1996b, p. 349).
In 1999, the U.S. Congress directed the Department of Energy to develop a “road map” for the accelerator transmutation of waste (ATW) (DOE, 1999a). This report highlights substantial technical issues, with the “key technical issues [being] (a) lifetimes of proposed materials and components, (b) reliability of components, (c) degree of partitioning and separations, and (d) quantification of long-lived radioactivity generated in operations, including spallation products” (DOE, 1999a, p. E-1). The report lays out a six-year, $280 million R&D program to assess the technical viability of ATW.
The report concludes that “ . . . ATW, if successful, on the first repository program, could reduce potential long-term radiation doses from repository wastes by a factor of about 10; however, a repository is still
required [for the United States] due to the presence of defense wastes, which are not readily treatable by accelerator transmutation of waste, and the long-lived radioactivity generated by ATW operations” (DOE, 1999a, pp. E-1, 2). “The inventory of fissionable materials from commercial spent fuel in the repository could be reduced by a factor of 1,000. . . . The 82,000 [metric tons] of chemically separated uranium could be suitable for nearsurface disposal at a low-level waste site or non-shielded storage for potential future use in advanced reactors” (DOE, 1999a, p. E-11). If a successful program can remove 99.9 percent of the actinides and 95 percent of the technetium-99 and iodine-129 from the spent fuel, then “ . . . the dose rate from the repository . . . is lowered by about four orders of magnitude out to 8,000 years after repository closure” (DOE, 1999a, p. 5-3). The improvement is about a factor of five after 20,000 years (DOE, 1999a, p. 5-3).
Should the initial R&D program be successful, the roadmap gives an estimate of “total life-cycle cost to treat 87,000 tonnes (t) of commercial spent fuel: approximately $280B [billion] ($2B R&D, $9B demonstration, and $270B post-demonstration design, construction, operation, and decommissioning).” The time period for treating the commercial spent fuel is 117 years (DOE, 1999a, p. E-2).
Sidebar 7.2: The STATS Report
The STATS report made the following recommendations for further research on P&T in connection with advanced nuclear reactors and for incorporating retrievability into repository design for Yucca Mountain (NRC, 1996b, p. 349).
The conclusion that P&T will not be sufficient to avoid the need for a repository was reaffirmed at the 1999 Irvine workshop by J. P. Shapira in his paper Is Partitioning and Transmutation (P&T) an Alternative to Geological Disposal?.
The ability of a P&T strategy to avoid any geological disposal lies in the possibility for [all] these wastes to be disposed in a shallow land burial disposal site (Shapira, 1999, p. 3).
. . . the DF [decontamination factor] needed to keep the total radiological capacity and the concentration limits for the waste packages low enough for a land disposal to be licensed, seems beyond the present and conceivable P&T technologies (Shapira, 1999, p. 9, emphasis added).
It appears to the committee that developments since the mid-1990s in P&T and ATW continue to support the position that partitioning and ATW require more R&D to determine whether their goals can be met. However, at least in some countries, even if the goals were to be met, a geological repository or ongoing surface storage appears to be needed eventually to handle some radioactive waste, including some from the partitioning and transmutation process.
Extraterrestrial disposal is conceptually straightforward—the waste material is simply given a one-way ride into outer space. Realistically, however, this option is not feasible due to scientific, technical, and economic factors. These include the energy required to boost payloads into space, the failure of launches and their consequences, and the tradeoffs between cost and safety (Rice and Priest, 1981).
Extraterrestrial disposal of HLW would require transport via spacecraft, with extreme attention to safety to avoid release of radioactive materials in the event of malfunction. Placing the waste in space near earth would not be sufficient. Orbits either around earth or around the sun in the inner solar system change over time periods that are short compared to the lifetime of waste components, so that a waste package placed in such orbits conceivably could return to earth. Most discussion of extraterrestrial disposal has therefore involved sending the HLW into the sun, which requires considerably more energy per pound than placing a payload into orbit, that is, larger rockets, and consequently, extremely high cost per pound of waste disposed. Although a technological approach has been described (Taylor, 1995), the formidable energy requirements for solar disposal imply that only the most dangerous waste components might warrant such expensive disposal. Accordingly, reprocessing or separation would be required, as in the case for P&T.
Geological Alternatives to Mined Repositories: Subseabed and Deep-Borehole Options
Approaches for isolating radioactive waste on earth by means other than geological disposition in mined, engineered repositories are discussed briefly in this section. These are variations on the basic concept of
geological disposal, in that the waste would be sequestered far below the earth's surface or they involve disposition in remote locations. In general, these approaches have about the same advantages and disadvantages as the geological disposition and surface storage approaches discussed in this report, and they have additional technical or legal constraints as noted in each discussion.
Emplacement in deep boreholes drilled from the surface is an approach to geological disposition that avoids the mined repository. Disposal in shallow boreholes was a normal practice at the “Radon” low-level waste disposal sites of the former Soviet Union, and it is still practiced in many former Soviet countries (IAEA, 1995a). Deep-borehole disposal at depths much greater than feasible for mined repositories has been studied in Australia, Italy, Russia (Khakhaev et al., 1995), Sweden, and Switzerland. Retrievability and sealing the boreholes (because they may be numerous) are greater technical challenges than for mined repositories (Juhlin et al., 1998; SKB, 2000).
Since deep-borehole disposal is so similar to geological disposal in mined repositories, it involves the same societal issues and long-term technical uncertainties. For large amounts of waste, drilling many deep boreholes from the surface is probably more expensive than a single mined repository. However, this variation of geological disposal may be suitable for countries with small waste inventories.
Subseabed disposal would result in waste emplaced tens or hundreds of meters deep in sediments or rock under the seabed, which itself is beneath several thousand meters of ocean. An international Seabed Working Group (SWG) conducted seabed disposal research under Nuclear Energy Agency auspices from about 1977 to 1987, and there is scientific support for this approach (NEA, 1988; Hollister and Nadis, 1998).
Two emplacement options are considered technically feasible. One option is dropping steel or titanium canisters of waste in missile-shaped penetrators from a ship. Field trials of about 100 penetrators showed that these free-falling devices would bury themselves up to 70 meters deep in seabed sediment. In the second option, waste-filled canisters would be lowered into predrilled boreholes. The requisite borehole drilling was demonstrated by the Deep Sea Drilling Project and Ocean Drilling Project of the U.S. National Science Foundation, although sealing the boreholes has not been demonstrated.
This approach has many technical advantages, including a very stable geological setting, dilution in the event of radioactivity release into a very large body of water, low cost to emplace the HLW, and very low probability of deliberate human intrusion. Retrieval might be possible, but only at a high cost. Such cost and the need for specialized retrieval equipment would discourage clandestine recovery attempts. However, research on seabed disposal has been discontinued as a matter of international policy.
Further consideration would, at minimum, require amendments to international treaties involving use of the seabed.
Small, uninhabited islands have been suggested in the past as alternative locations for geological disposal. This alternative would combine the advantages of both deep-seabed disposal and land-based repositories. The repository would be built by conventional shaft and drifts, starting from the island surface, but the disposal drifts would be situated below the ocean, away from the island. The rock type, which could be selected for such an option, would be similar to any on-land disposal (e.g., clay, granite) and thus provide the same degree of confinement as a land-based option. However, a major difference would be that the risk of unintentional human intrusion would be greatly reduced, and furthermore, any long-term small releases from the repository, which could occur both for a land-based repository and for such an under-the-ocean repository, would be immediately diluted in ocean water. Artificial islands built for this purpose have also been considered.
Direct injection of liquid waste or slurries of liquid waste and grout 3 is essentially a form of geological disposal that uses neither a waste package (e.g., a metal waste container) nor a mined repository. It was practiced in several countries for a variety of wastes, but it has generally been phased out. Clearly it is not applicable to SNF or vitrified HLW, which are solids.
The United States practiced direct injection of low-level liquid waste grouts under high pressure into a shale formation beneath the Oak Ridge, Tennessee site in the early 1970s. This process was abandoned due to uncertainties about how the grout flowed within the fractured shale. In 1972, an NRC study found the option of disposing of HLW at the U.S. Savannah River Site directly into crystalline bedrock beneath the site to be technically feasible. However the report cautioned that public approval for this option would be problematic (NRC, 1972).
For many years, the former Soviet Union injected intermediate-level liquid waste into the subsurface at sites such as Krasnoyarsk, Tomsk, and Dimitrograd. In these cases, the waste appears to have been contained between geological strata as intended (Parker et al., 1999, 2000). However, the approach is being phased out because it is not considered to be in line with better practices that include solidifying and packaging the waste (IAEA, 1995a).
CONCLUSIONS AND RECOMMENDATIONS REGARDING ALTERNATIVES
More advanced reprocessing, partitioning and transmutation technology, and disposal options such as deep seabed might be regarded
3 Grout is a cementitious material, for example, Portland cement that hardens into a solid material after being mixed with water.
favorably in future centuries. Science and technology will continue to advance, the availability of energy resources will change, and political institutions and public attitudes will evolve. Especially on a scale of centuries, the changes are nearly impossible to anticipate. Thus, options to benefit from advances should be kept open as far as possible. However, based on what is known today, several important conclusions can be drawn:
There does not appear to be any promising alternative to geological disposal for permanent isolation of HLW that avoids a need for ongoing active management.
The only two alternatives capable of assuring safety and security on the time scale of decades to a few centuries are geological disposition and continued surface storage.
More analyses of the alternatives and deliberation on choices for implementating them are needed, rather than rigid commitment to a schedule or a previously established plan for geological disposal. For example, the NEA has recommended that:
. . . the pros and cons of extended surface storage and of partitioning and transmutation should be objectively debated, since these two strategies have a strong body of support today. (NEA, 1999a, p. 6.)
Accordingly, the committee arrived at the following recommendations:
All countries with responsibility for radioactive wastes should be developing a geological disposal option, either for a repository within their own country or for a repository outside their country to be developed jointly with other nations. The questions of “when,” “how,” and “where” remain open.
Until HLW is emplaced underground as a way to achieve safety and security, countries must assure that adequately monitored surface storage facilities for HLW, including SNF, are available. New surface storage facilities also involve questions of when, how, and where, and such facilities can be expected to engender public controversy (see Chapter 5 ).
Options should be developed and maintained, with commitments made in small steps. As recommended by the NEA:
Wastes should not, as far as possible, be conditioned into a form which precludes taking advantage of future technological developments; sites should not be definitively nominated until options have been explored; designs should not be frozen too soon; commissioning, operation and closure of a repository should all be in small steps each of which allows commitments to be fully considered. (NEA, 1999a, p. 27)
The analysis and deliberation should look at incremental changes in how and when waste might be moved from reactors and other sources
into centralized storage and from storage to emplacement underground. The analysis must compare rigorously the leave-in-place option, for realistic time frames, with other alternatives.
The choice is not whether to put HLW and SNF into a repository or leave them on the surface for the order of 10,000 years. Rather, the choice is where to put them in order to assure safety and security. Although storage is not the final answer, it is a near-term necessity, and it will remain a necessity for many decades. Development of a site and the technology for geological disposal is a process that has taken decades already, and it may take many more for some national programs. Underground emplacement of HLW with retrievability is an alternative to surface storage, and neither surface storage nor such geological disposition precludes shifting to another disposition approach later.
The major choice for nuclear programs for coming decades is how to assure the safe and secure control of HLW and SNF—while at the same time developing an option for geological disposal so that an ongoing commitment to active management can, at some future time, be brought to an end. It is important not to prejudice the final steps in committing to geological disposal. Commitment to geological disposal will require both a robust technical basis and public understanding and acceptance of this commitment. The robust technical basis and public support may take some decades to achieve. In the meantime, other options for HLW disposition, or even disposal, may become available and may be preferred.