The mission of the Used Fuel Disposition (UFD) Campaign of the US Department of Energy (DOE) Office of Nuclear Energy is to identify alternatives and conduct research and technology development to enable storage, transportation, and disposal of used fuel from existing and future nuclear fuel cycles. As part of this mission UFD is developing analysis capability and general experimental data related to geologic disposal options. This report presents the rationale for focusing modeling and experimental efforts on mined, geologic repositories in three media (salt, clay/shale, and crystalline rocks), and the use of deep boreholes in crystalline rocks.
Preference for Geologic Disposal
Geologic disposal has been the recommended option for permanent management of spent nuclear fuel (SNF) and high-level waste (HLW) for 54 years. The National Academy of Sciences (NAS) reported that deep geologic disposal (in salt formations) was the most promising method to explore for disposing of HLW in 1957. NAS reaffirmed that position in 1966 and 1970. More recently in 2001, NAS concluded that after 40 years of study, “geologic disposal remains the only scientifically and technically credible long-term solution available to meet safety needs without reliance on active management” and there is overwhelming international consensus on geologic disposal as the preferred option.
Consideration of Salt, Clay/Shale, and Crystalline Rocks
In the 1970s and early 1980s, the United States evaluated a number of geologic media. The Atomic Energy Commission (AEC), predecessor to DOE, gave disposal in salt priority based on an NAS recommendation in 1957. Sited in 1974, DOE developed the Waste Isolation Pilot Plant (WIPP) for disposal of defense transuranic waste in bedded salt in southern New Mexico. WIPP opened in 1999.
By 1974, AEC had also identified shale, volcanic tuff, and crystalline (igneous/metamorphic) rock as potential media in addition to salt. In the 1980 Environmental Impact Statement on Management and Disposal of Commercially Generated Radioactive Wastes (Generic EIS), DOE included repository concepts in salt, shale, crystalline rocks, and basalt. While searching for candidate sites for the first repository under the Nuclear Waste Policy Act of 1982, DOE selected three salt dome sites, four bedded salt sites, one basalt site, and the volcanic tuff site at Yucca Mountain. While searching for a site for the second repository in crystalline rocks in the mid 1980s, DOE also evaluated the feasibility of sedimentary rock and concluded clay/shale was still a favorable medium. In addition, alluvia in the United States and carbonate rocks in Canada have been considered for low- and intermediate-level waste.
By 1984, the United States had concluded that many types of geologic media were feasible for radioactive waste disposal, especially, if used with engineered barriers in addition to the natural barrier to create a robust disposal system. Hence, many types of media could be examined by the UFD Campaign. However, the UFD Campaign is not conducting in-situ experiments to develop properties of each geologic medium: rather, the UFD Campaign plans to the extent possible to use data available from underground research laboratories reported in the literature. Salt, clay/shale, and crystalline rocks are the most frequently considered geologic media in the international community. Crystalline repository concepts have been evaluated in Switzerland and Japan. Sweden and Finland have selected crystalline sites and are preparing licenses. Clay/shale disposal concepts have been evaluated in France, Belgium, and Switzerland. Finally, Germany continues to investigate disposal of heat-generating SNF and HLW in salt.
Furthermore, the UFD is not selecting a geologic medium for disposal, but rather is selecting a set of geologic media for further study that spans a suite of behavior characteristics that impose a broad range of potential conditions on the design of the repository, the engineered barrier, and the waste. Salt, clay/shale, and crystalline rocks represent a reasonable cross-section of behavior. Salt and clay/shale represent sedimentary rocks with different degrees of strength/cavity stability/mining experience, heat resistance/thermal conductivity, and radionuclide adsorptive behavior. Crystalline rocks (along with the US extensive experience with volcanic tuff) represent igneous rocks that differ from salt and clay/shale in deformation behavior/strength, importance of the waste package to the disposal system performance, and coexistence of economic resources and, thus, prevalence of boreholes and their associated hazards. Crystalline rocks are also the primary basement rock to consider for deep borehole disposal described below.
Preference for Mined Repositories
Two thorough reviews of available options for management of HLW and SNF were conducted in 1974 and 1976 that considered surface storage in near-surface burial sites and disposal in deep mined repositories on the continent or islands, deep boreholes, seabed, geologic cavities coupled with rock melt, well injection, ice sheets, and space. By 1979, the Interagency Review Group for Nuclear Waste Management, formed the year before by President Carter with representatives from 14 federal agencies, concluded that mined, geologic repositories were a promising method for disposal of SNF and HLW.
A year later in the 1980 Generic EIS mentioned above, DOE concluded that mined geologic repositories were the best option for disposal because of favorable characteristics of ready retrievability during placement, continued retrievability after disposal (though increasingly difficult), status of technology, and conformance with international agreements when compared to liquid disposal in geologic cavities with rock melt and in injection wells and solid disposal in deep boreholes, in the sub-seabed, on islands, in continental ice sheets, and in space with or without transmutation of transuranic radioisotopes.
Borehole Disposal as Alternative
Although the 1980 Generic EIS selected mined repositories for geologic disposal, the DOE noted that deep borehole disposal was worthy of further consideration. The 1980 Generic EIS projected costs for deep borehole disposal to be about three times greater than for mined, geologic repositories. However, DOE conducted an engineering analysis of deep borehole emplacement in 1983 that found the concept feasible and costs similar to that of mined repositories in the future, provided technological advances in drilling methods continued. These technologic advances have indeed occurred and DOE investigated the concept in the 1990s for the disposal of surplus plutonium. Both the United Kingdom and the Swedish high-level waste programs continue to mention deep borehole disposal as the primary feasible alternative to mined geologic repositories in periodic reviews of alternatives. However, while it is the most common feasible alternative mentioned, it has not been demonstrated.
Other Disposal Options Not Considered
The 1980 Generic EIS also suggested further consideration of sub-seabed disposal and DOE studied the feasibility of this option through 1988, along with many other countries. It remains the most studied alternative to mined, geologic disposal. The seabed disposal concept consisted of emplacement of SNF and HLW by: (1) dropping streamlined penetrators containing waste packages into the unconsolidated seabed sediments; or (2) lowering waste packages into boreholes drilled into either unconsolidated or lithified seabed sediments. Although studies showed that radionuclide doses were often lower and subject to less uncertainty than repositories on land, participating countries decided to focus their efforts on land-based repositories and terminated the sub-seabed program in the late 1980s because seabed disposal had been prohibited under the United Nations Convention on Law of the Sea and the London Convention and Protocol.
No new information has been developed since the early 1980s to suggest that other options evaluated and screened from further consideration in the past should be re-evaluated. However, should these or other disposal concepts be identified that warrant further investigation, they will be evaluated by the UFD Campaign. Specifically, the UFD Campaign recognizes that analyses to date indicate that both sub-seabed disposal and the mined repository in volcanic tuff at Yucca Mountain have the potential to provide safe long-term isolation.