Mechanical Properties of Oxides Formed by Anaerobic Corrosion of Steel
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Mechanical Properties of Oxides Formed by Anaerobic Corrosion of Steel N.R. Smart1, A.E. Bond1, J.A.A. Crossley1, P.C. Lovegrove2 and L. Werme3 1 AEA Technology plc, Culham Science Centre, Abingdon, Oxfordshire, OX14 3ED, United Kingdom, Email: [email protected] 2 British Gyroplanes Ltd, Wallingford, Oxfordshire, OX10 OJA, United Kingdom 3 SKB, Box 5864, SE-10240, Stockholm, Sweden. ABSTRACT In Sweden, it is proposed that spent nuclear fuel should be encapsulated in sealed cylindrical canisters for disposal in a geologic repository. The canisters would consist of a thick ferrous inner container and a copper overpack. If mechanical failure of the copper overpack occurred, allowing water to enter, there would be a build up of ferrous corrosion product, which could induce stresses in the outer copper canister. This paper describes an apparatus, the ‘stress cell’, which was designed to measure the expansion caused by the anaerobic corrosion of steel under compressive loads. The apparatus consisted of a stack of steel and copper discs, which were immersed in simulated anoxic groundwater. A system of levers amplified the change in height of the stack, and the displacement was measured using sensitive transducers. Three cells were set up; two contained alternate mild steel and copper discs, and the third, a control cell, consisted of alternate stainless steel and copper discs. A slight contraction of the control cell was observed but no expansion was measured in the mild steel - copper cells. In parallel, coupons of mild steel and cast iron were corroded in anoxic, artificial groundwater at 50ºC and 80ºC for several months. The coupons were examined by atomic force microscopy (AFM) to determine the mechanical properties and the structure of the corrosion product films, and X-ray photoelectron spectroscopy (XPS) to identify the chemical composition of the film. INTRODUCTION The proposed design for a final repository for spent fuel and other long-lived residues in Sweden is based on the multi-barrier principle. The waste will be encapsulated in sealed cylindrical canisters, which will then be placed in vertical storage holes drilled in a series of caverns excavated from the granite bedrock at a depth of about 500 m and surrounded by compacted bentonite clay. The canister design is based on a thick cast iron inner container, designed to provide mechanical strength and to keep individual fuel elements at a safe distance from one another, thereby minimizing the risk of criticality. The container is fitted inside an inherently corrosion resistant copper overpack that is designed to provide containment over the long timescales required for safety reasons. As part of the safety case for the repository, one of the scenarios being addressed by SKB involves the early mechanical failure of the outer copper overpack, allowing water to enter the outer container and corrode the inner one. One consequence of this failure would be the long-term build up of corrosion product, which could induce stresses in the spent fuel canister.
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