Corrosion Performance of Engineered Barrier System in Deep Horizontal Drillholes

Abstract

The disposal of spent nuclear fuel and other high-level radioactive waste in deep horizontal drillholes is an innovative system. Canisters of highly corrosion-resistant nickel-chromium-molybdenum (Ni-Cr-Mo) alloys are specified for the disposal of this nuclear waste. The canisters are emplaced along a steel casing in a horizontal drillhole that is one to three kilometers deep into or below a low-permeability geologic formation. The drillhole is in fully saturated rock with anoxic and reducing pore waters. A time-interval analysis method was used to track the evolution of the environment and to analyze corrosion performance of a representative engineered barrier system (EBS) configuration. In this analysis, the canisters remained perforation-free for tens of thousands of years. The amounts of hydrogen and metal oxides formed as by-products of the metal corrosion process were determined. These by-products are of interest, because both hydrogen and metal oxides can affect the chemical composition of the environment and the transport and sorption behavior of radionuclides and other species. Beneficial attributes that contribute to the extraordinarily long life of the canisters were identified. Several inherent characteristics of the horizontal drillhole disposal system reduced the complexities and uncertainties of the analysis.

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

“…In the concept of deep borehole disposal, less reliance is placed on engineered barriers, because at depths of several kilometres, the host rock is assumed to provide greater isolation and containment than for shallower geological repositories. Nevertheless, a minimum level of engineered barriers are considered for both horizontal [3] and vertical [4] boreholes. Two papers focus on the performances of engineered barriers: corrosion-resistant alloys for use in horizontal holes [3] and cementitious materials for sealing and support matrices [4].…”

“…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].…”

Disposal of High-Level Nuclear Waste in Deep Horizontal Drillholes

“…In the passive state, metals are protected by a thin, self-forming film on the surface, leading to very low general corrosion rates (on the order of 0.01 µm/y). A canister made with Alloy 625 with a wall thickness of 1 cm takes at least 50,000 years to degrade by general corrosion [38].…”

“…Calculations for carbon steel casings indicate that their lifetime in a reducing environment can reach several hundred years [38]. This is long enough to provide a reliable and relatively smooth conduit for canister retrieval, should that become necessary or worthwhile.…”

“…In Section 2.5, we briefly mentioned the role that the canisters play in protecting the waste during the initial, critical period after emplacement up to about 50,000 years [38]. After a breach of the canister (either by corrosion or mechanical destruction, for example, during earthquake-induced fault reactivation), radionuclides dissolve into the resident brine and are released to the near field either by diffusion or advective and dispersive transport.…”