The Influence of Preoxidation on the Corrosion of Copper Nuclear Waste Canisters in Aqueous Anoxic Sulphide Solutions
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The Influence of Preoxidation on the Corrosion of Copper Nuclear Waste Canisters in Aqueous Anoxic Sulphide Solutions J. M. Smith, Z. Qin, J. C. Wren, and D. W. Shoesmith Chemistry, The University of Western Ontario, 1151 Richmond St., London, N6A5B7, Canada
ABSTRACT Scandinavian/Canadian high-level nuclear waste repository conditions are expected to evolve from initially warm and oxic to eventually cool and anoxic. During the warm, oxic period corrosion products will accumulate on the container surface. These deposits could impede the reaction of Cu with aqueous sulphide, the only reaction that could lead to the significant accumulation of additional corrosion damage under the long-term anoxic conditions. The kinetics of the reaction of Cu with aqueous sulphide solutions have been studied using electrochemical and surface analytical techniques. Corrosion potential measurements were used to follow the evolution of the surface as oxides/hydroxides were converted to sulphides in the sulphide concentration range 10-5 to 10-3 mol/L. Changes in composition were followed by insitu Raman spectroscopy. Of critical importance is whether or not a period of preoxidation of a Cu container surface can prevent subsequent reaction of the surface with remotely produced sulphide. INTRODUCTION A proposed method for the permanent disposal of Scandinavian/Canadian high-level nuclear waste is to bury it 500-1000 m deep in granitic host rock [1-3]. The waste-containing containers would be placed in vertical boreholes, surrounded by a self-sealing bentonite clay, and residual repository space backfilled with a mixture of bentonite and crushed granite. For the conditions anticipated, Cu is an ideal container material since it should be thermodynamically stable over the majority of the required containment period. The container design is discussed elsewhere [4]. The evolution of repository conditions has been well studied [5]. Initially trapped atmospheric O2 will be relatively rapidly consumed by reaction with the container surface, oxidizing minerals and microbial processes. At the time aqueous, anoxic conditions are established, models predict corrosion damage accumulation to a maximum depth of 300 µm [6] and corrosion products will be primarily Cu2O covered by various CuII salts (i.e. Cu2CO3(OH)2 [7], CuCl2.3Cu(OH)2 [8], etc). Further corrosion should be minimal since Cu is thermodynamically stable under anoxic conditions unless sulphide is present. Potential sources of sulphide include pyrite (Fe2S) and sulphate (SO42-), the latter via the action of sulphate-reducing bateria (SRB) [9]. Radioactivity and heat, leading to low water activity make the environment immediately adjacent to the Cu surface hostile to bacteria [9]. However, remotely produced sulphide is free to diffuse slowly, through the buffer material to the container surface [9]. In the presence of sulphide, copper can react with water to produce extremely stable and insoluble sulphides [10].
In this paper, results on the effects of Cu preoxidation on the su
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