Abstract
Isolation of spent nuclear fuel assemblies in deep vertical boreholes is analyzed. The main safety features of the borehole concept are related to the repository’s great depth, implying (a) long migration distances and correspondingly long travel times, allowing radionuclides to decay, (b) separation of the repository from the dynamic hydrological cycle near the land surface, (c) stable geological and hydrogeological conditions, and (d) a geochemically reducing environment. An integrated simulation model of the engineered and natural barrier systems has been developed to examine multiple scenarios of the release of radionuclides from the waste canisters, the transport through a fractured porous host rock, and the extraction of potentially contaminated drinking water from an aquifer. These generic simulations include thermal effects from both the natural geothermal gradient and the heat-generating waste, the influence of topography on regional groundwater flow, moderated by salinity stratification at depth, and the role of borehole sealing. The impact of these processes on the transport of select radionuclides is studied, which include long-lived, soluble, sorbing or highly mobile isotopes along with a decay chain of safety-relevant actinide metals. The generic analyses suggest that a deep vertical borehole repository has the potential to be a safe option for the disposal of certain waste streams, with the depth itself and the stable hydrogeological environment encountered in the emplacement zone providing inherent long-term isolation, which allows for reduced reliance on a complex engineered barrier system.
Highlights
Disposal of spent nuclear fuel and high-level radioactive waste in deep geological formations is posited a viable option to isolate radionuclides from humans and the environment for sufficiently long time periods
In addition to centralized mined repositories excavated from suitable host rocks, modular waste disposal in vertical or horizontal boreholes drilled into deep sedimentary formations or crystalline basement rocks has been proposed as complementary or alternative solutions to the long-term disposition of radioactive waste
The current study examines the disposal of canisters containing individual spent nuclear fuel assemblies from a pressurized water reactor in a 3 km deep vertical borehole drilled into crystalline basement rock at a generic disposal site
Summary
Disposal of spent nuclear fuel and high-level radioactive waste in deep geological formations is posited a viable option to isolate radionuclides from humans and the environment for sufficiently long time periods. In addition to centralized mined repositories excavated from suitable host rocks, modular waste disposal in vertical or horizontal boreholes drilled into deep sedimentary formations or crystalline basement rocks has been proposed as complementary or alternative solutions to the long-term disposition of radioactive waste. It was proposed that the modular and scalable deep borehole disposal concept provides a cost-effective alternative to mined repositories, for countries with small waste inventories [13,14] or to accommodate waste forms generated by next-generation advanced reactors [15]. As a variant of such a vertical borehole repository, directional drilling technology can be used to gradually deviate from the vertical direction of the access hole at a kick-off point above the target depth and to create a horizontal disposal section within a suitable formation, which may be a sedimentary or crystalline host rock. The deep horizontal borehole concept has been described in [17], with generic safety calculations for a repository in shale discussed in [18,19,20]
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