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Author: Charles McCombie
The chief obstacle to geologic disposal is the lack of public acceptance.
The technical maturity and development potential of technologies for the storage, reprocessing, and disposal of spent nuclear fuels differ greatly, as do the political and social issues they raise. The material flows and waste products are summarized in Figure 1 (see PDF version), normalized to one tonne of reactor fuel in a light-water reactor, closed-fuel cycle. All of the operations listed in Figure 1 produce radioactive wastes in solid, liquid, or gaseous forms. Although the greatest environmental challenges may be associated with wastes at the front end of this chain - namely, the millions of tonnes of mining and milling tailings that remain on or near the land surface of uranium-producing sites - most of the time, effort, resources, and public attention have been focused on the management of the low-volume, but highly hazardous wastes from the back end of the fuel cycle - spent nuclear fuel (SNF), if this is regarded as waste, or the vitrified high-level waste (HLW) and the accompanying long-lived transuranic wastes that remain after reprocessing.
The most controversial issue today is disposal of these waste streams. Deep geologic repositories are currently the only recognized feasible method of safe permanent disposal. This will be the main focus of this paper, but let us look at the three technologies in turn.
Storage of nuclear wastes, a long-established technology, is widely practiced today and will continue to be necessary for many decades to come. During storage, the key goal is to ensure the safety and security of stored HLW and SNF. Early on, SNF was stored primarily in water pools in well protected nuclear facilities (wet storage). Increasingly, however, SNF and HLW are being stored in strong, sealed, shielded containers (dry storage) or in vaults. Both wet and dry storage can provide safety and security. Dry storage is better suited for very long periods of time, although some wastes (e.g., SNF from U.K. magnox reactors [with magnesium alloy cladding]) must be kept under water for safety reasons. Although no new technical developments are necessary to ensure the safety of stored nuclear wastes, work continues on monitoring fuel creep, hydrogen migration, and so on. The recent increase in terrorist acts has also led to a reexamination of some security concerns, such as vulnerability to missile attack.
Some important policy issues must still be addressed, especially the duration of storage and the location of storage sites. When reactors or reprocessing plants were built, it was believed that the SNF or HLW would be removed (after limited storage to allow for cooling) and shipped to a disposal facility. The huge delays in implementing disposal facilities have meant that storage periods have been drastically extended. In some countries, geologic repositories are still so far off that no date can be predicted; in a few countries, not even the feasibility of geologic disposal has been accepted, thus making storage "indefinite." Additional storage can be located at reactor sites, provided that licenses can be amended, or at new centralized storage facilities, provided that these can be sited. But, communities that currently host stores of SNF or HLW, or that are being proposed as hosts, understandably do not wish to become de facto final repositories. As the delays in disposal continue and requirements for storage are increased, this controversy grows.
The situation in the United States concerning storage and disposal illustrates how policy issues can directly affect technical programs. Reactor operators who do not have sufficient on-site storage for SNF have initiated litigation against the U.S. Department of Energy (DOE), which failed to meet the 1998 target date for DOE acceptance of such fuel. This problem has also led to proposals for storage facilities run as private enterprises. To run disposal operations as efficiently as possible, it would be advantageous to have a large storage capacity close to the repository so that waste emplacement could be decoupled from DOE acceptance and could be planned to minimize thermal-loading problems. A recently published study by the National Research Council on repository staging suggests that storage facilities should be located adjacent to the Yucca Mountain repository (NRC, 2003). However, because of the political sensitivities associated with implementing a large storage area before a repository is available, this may not be a feasible policy option.
Civil reprocessing technology was developed to obtain unused uranium and plutonium for use in fast reactors. The technical challenges associated with this process have been solved, and large-scale reprocessing plants are in operation in France, United Kingdom (U.K.), Russia, and the United States; in other countries (e.g., Belgium and Italy), they have been operated as research facilities. France, the U.K., and Russia offer commercial reprocessing services that are being used or have been used by a number of countries (e.g., Japan, Germany, Switzerland, Italy, Spain, and Sweden); Japan is currently building its own commercial reprocessing plant.
Technical improvements are still possible, such as increases in separation efficiencies and reductions in emissions of radioactive substances during operation. There are bigger technical challenges, however. One is to develop an advanced reprocessing and fuel fabrication cycle that avoids having segregated plutonium at any stage. This could greatly reduce the risk of proliferation associated with reprocessing. Another technical challenge might be to develop a "second-generation" waste form to succeed the borosilicate glasses in which fission products are included. A great deal of work has been done on the development of ceramics and synthetic minerals. The question is whether the improved performance with these materials is necessary to increase the already high levels of safety predicted for repositories.
In practice, the main technical challenge associated with reprocessing may be the mundane challenge of reducing costs. The costs of reprocessing are so high today that they cannot be recouped either by the value of the recovered materials or by the savings on storage and disposal costs. Commercial repositories will break even only if the price of the uranium feed increases far beyond the current typical uranium prices of $20 to $40/kg. Subsidies for utilities willing to use mixed plutonium-uranium oxide (MOX) fuel would make MOX fuel more attractive commercially and could help reduce excess weapons-grade plutonium.
The policy decisions that led to the development of commercial reprocessing were based on conserving uranium resources by providing a supply of plutonium for fast reactors. This argument has been severely weakened over time by the slow growth of nuclear power, which has kept the price of uranium low. Moreover, because the deployment of breeder reactors has been postponed, the demand for, and therefore the value of, plutonium has been reduced. In fact, the excess plutonium already produced from reprocessing has become a liability (it is expensive to store and degrades in storage) and has created a potential risk of proliferation. The nuclear industry is using some plutonium in MOX fuels, although the costs are high and the uranium savings are only around 20 percent. Options being seriously considered include (1) reactors designed to burn excess plutonium; and (2) disposing of plutonium as a waste.
In summary, the key policy issues associated with reprocessing are the now weak resource-conservation arguments, increasing concerns about nuclear proliferation, and the economic costs associated with the low price of fresh uranium. Countries that currently follow a reprocessing strategy do so for one or more of the following reasons: