Analogues for the Corrosion-induced Expansion of Iron in HLW containers
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$QDORJXHVIRUWKH&RUURVLRQLQGXFHG([SDQVLRQRI,URQLQ+/:FRQWDLQHUV Nicholas R. Smart1, Rachel Adams1 and Lars Werme2 1 Serco Assurance, Culham Science Centre, Abingdon, Oxfordshire, OX14 3ED, U.K. 2 SKB, Box 5864, SE-10240, Stockholm, Sweden. $%675$&7 In Sweden, spent nuclear fuel will be encapsulated in sealed cylindrical canisters, consisting of a cast iron insert and a copper outer container. The canisters will be placed in a deep geologic repository and surrounded by bentonite. If a breach of the outer copper container were to occur the cast iron insert would undergo anaerobic corrosion, forming a magnetite film whose volume would be greater than that of the base metal. In principle there is a possibility that accumulation of iron corrosion product could cause expansion of the copper canister. Anaerobic corrosion rates are very slow, so in the work described in this paper reference was made to analogous archaeological materials that had been corroding for long periods in natural anoxic aqueous environments. This paper describes a number of archaeological artefacts containing iron and copper corroding in constrained geometries in anoxic natural waters. No evidence has been obtained which would suggest that severe damage is likely to occur to waste canisters as a result of expansive corrosion of cast iron under repository conditions. ,1752'8&7,21 In Sweden, spent nuclear fuel will be encapsulated in sealed cylindrical canisters for disposal in a geologic repository. The canisters will consist of a thick cast iron inner container and a copper overpack. After emplacement in the repository they will be surrounded by bentonite clay to limit the release of radioactivity, if for any reason there is a leakage from the canister. If failure of the copper overpack occurred, allowing water to enter, the cast iron insert would corrode anaerobically, liberating hydrogen and forming a corrosion product, which is likely to be predominantly magnetite. This corrosion product would have a greater volume than the base metal and so the possibility of it causing deformation and expansion of the outer copper canister has been studied both experimentally [1,2] and through modelling. In view of the very slow anaerobic corrosion rates of cast iron and carbon steel [3] and the long timescales involved in the geological disposal of radioactive waste containers, natural analogues for expansion caused by anaerobic corrosion have been sought. The objective was to identify natural analogues for iron, or combinations of iron and copper alloys, which were corroding anaerobically in constrained geometries over long periods of time, with the intention that if such items could be found further detailed analysis would be undertaken where possible. Although it is recognised that it is impossible to find natural analogues exposed to exactly the same conditions as in a geological repository, particularly in terms of temperature, radiation flux and microbial activity, natural analogues have the potential to provide valuable support for prediction
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