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Deep Borehole Disposal Research: Demonstration Site Selection Guidelines, Borehole Seals Design, and RD&D Needs

The U.S. Department of Energy has been investigating deep borehole disposal as one alternative for the disposal of spent nuclear fuel and other radioactive waste forms, along with research and development for mined repositories in salt, granite, and clay, as part of the used fuel disposition (UFD) campaign. The deep borehole disposal concept consists of drilling a borehole on the order of 5,000 m deep, emplacing waste canisters in the lower part of the borehole, and sealing the upper part of the borehole with bentonite and concrete seals. A reference design of the disposal system (Arnold et al. 2011a) includes emplacement of 400 waste canisters in the lower 2,000 m of the borehole, seals and plugs in the uncased borehole for 1,500 m above the disposal zone, and standard borehole plugging in the cased upper 1,500 m of the borehole.  Identifying a demonstration project site is one of the first steps in a demonstration project.  The site selection strategy for the demonstration project aims to maximize the probability of successfully completing the borehole, at a site with favorable geological, hydrogeological, and geochemical conditions, within budgetary and schedule constraints.

A number of geological, hydrogeological, and geophysical factors are being evaluated (depth to crystalline basement, structure / faulting, topography, stress, tectonics, heat flux).  Research on alternative seals has been directed at the main seals emplaced above the waste disposal zone and at the sealing and support matrix that surrounds the waste canisters.  Large-scale thermal hydrologic modeling has been updated to incorporate more realistic geological and hydrogeological conditions, and to provide input to an updated performance assessment (PA) model of the deep borehole disposal system.  Preliminary results of Performance Assessment studies show no (negligibly small peak mean annual dose primarily from releases of I-129) radionuclide releases to the biosphere within 1,000,000 years for the undisturbed base-case scenario.  A generic safety case study has been documented and indicates that a defensible safety case can be developed for deep borehole disposal of high-level nuclear waste, assuming that the demonstration project confirms the technical bases underlying the deep borehole disposal concept.