Disposal of High-Level Nuclear Waste in Deep Horizontal Drillholes

Abstract

Spent nuclear fuel and high-level radioactive waste can be disposed in deep horizontal drillholes in sedimentary, metamorphic or igneous rocks. Horizontal drillhole disposal has safety, operational and economic benefits: the repository is deep in the brine-saturated zone far below aquifers in a reducing environment of formations that can be shown to have been isolated from the surface for exceedingly long times; its depth provides safety against inadvertent intrusion, earthquakes and near-surface perturbations; it can be placed close to the reactors and interim storage facilities, minimizing transportation; disposal costs per ton of waste can be kept substantially lower than for mined repositories by its smaller size, reduced infrastructure needs and staged implementation; and, if desired, the waste could be retrieved using “fishing” technology. In the proposed disposal concept, corrosion-resistant canisters containing unmodified fuel assemblies from commercial reactors would be placed end-to-end in up to 50 cm diameter horizontal drillholes, a configuration that reduces mechanical stresses and keeps the temperatures below the boiling point of the brine. Other high-level wastes, such as capsules containing 137Cs and 90Sr, can be disposed in small-diameter horizontal drillholes. We provide an overview of this novel disposal concept and its technology, discuss some of its safety aspects and compare it to mined repositories and the deep vertical borehole disposal concept.

Post-Closure Safety Analysis of Nuclear Waste Disposal in Deep Vertical Boreholes

“…Boreholes may be drilled along a line at a given interval, on a lattice, or other suitable patterns. This flexibility can be further increased by making use of directional drilling technology, which enables drilling multiple wells from a single well pad, using slanted or horizontal boreholes, with the possibility to create long disposal sections, potentially in multiple directions or vertically stacked [17,46].…”

“…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].…”

“…These differences are related to all aspects of repository design and operation, including pre-and post-closure safety as well as socio-economic considerations [15,21]. Some of these similarities and differences are described in [10,11,15,17,[22][23][24] and will be further discussed below.…”

The State of the Science and Technology in Deep Borehole Disposal of Nuclear Waste

“…Two safety cases are discussed: one for horizontal [8] and one for vertical [9] boreholes. The horizontal borehole concept uses relatively small diameter holes that can be drilled with off-the-shelf equipment; the holes are cased and disposal sections backfilled [8]. This concept is deemed suitable for spent nuclear fuel and capsules with highly concentrated Cs/Sr from defence applications.…”

“…Waste types, including waste types suitable for deep borehole disposal and the circumstances under which a national waste management programme might wish to consider deep borehole disposal [1], and the status of deep boreholes as a potential option for disposal of high-level waste in Germany [2]; • Engineered barrier performances, including corrosion performances of engineered barrier systems [3] and potential cementing systems suitable for deep borehole disposal [4]; • Performance assessments, including the thermal evolution around heat-generating waste for a wide range of material properties and disposal configurations [5], geochemical analyses of deep brines focussed on fluid-rock interactions [6] and post-closure performance assessment calculations for the deep borehole disposal of Cs/Sr capsules [7] and • Safety cases, including deep horizontal drill-holes in sedimentary, metamorphic or igneous rocks [8] and safety case aspects for deep vertical boreholes [9].…”