Corrosion Considerations Related to Carbon Steel Radioactive Waste Packages Exposed to Cementitious Materials
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Corrosion Considerations Related to Carbon Steel Radioactive Waste Packages Exposed to Cementitious Materials Bruno Kursten1, Frank Druyts1, Nick R. Smart2, Digby D. Macdonald3 and Robert Gens4 1
SCKyCEN, The Belgian Nuclear Research Centre, R&D Waste Packages Unit, Boeretang 200, 2400 Mol, Belgium 2 Amec Foster Wheeler, Clean Energy, Building 150, Harwell Oxford, Didcot, Oxfordshire, OX11 0QB, U.K. 3 University of California at Berkeley, Department of Materials, Science and Engineering, Berkeley, CA 94720, U.S.A. 4 ONDRAF/NIRAS, The Belgian Agency for Radioactive Waste and Enriched Fissile Materials, Avenue des Arts 14, 1210 Brussels, Belgium ABSTRACT The Supercontainer is the reference concept for the post-conditioning of vitrified high-level radioactive waste and spent fuel in Belgium. It consists of a prefabricated concrete buffer that completely surrounds a carbon steel overpack. In this highly alkaline environment (pH ~ 13.6) and under normal conditions (i.e. without the ingress of aggressive species), the carbon steel overpack will be protected by a passive oxide film, which is believed to result in very low uniform corrosion rates. This paper gives an overview of the status of the uniform corrosion, pitting corrosion and stress corrosion cracking behaviour of carbon steel expected during the waste disposal period. INTRODUCTION The Supercontainer (SC) is currently being studied as the reference design for the final disposal of vitrified high-level radioactive waste (VHLW) and spent fuel (SF) in Belgium. It is essentially a massive cylindrical prefabricated concrete block, named the buffer, into which a watertight cylindrical carbon steel container, the so-called overpack, holding either VHLW canisters or SF assemblies, will be inserted. In the presently preferred design option, the conrete block will be fitted into an outer stainless steel container, termed the envelope [1]. A Portland cement-based (PC) concrete has been chosen for the buffer because it will provide a highly alkaline chemical environment, in which the external surface of the overpack will be passivated and it is expected only to be prone to slow uniform corrosion (passive dissolution) [2-4]. Considering the long timescales in waste disposal, determining accurate and reliable estimates of the uniform corrosion rate under anaerobic conditions becomes very important. On the other hand, metals that are protected by a passive oxide film are known to be susceptible to localised breakdown if the proper environmental conditions arise [5,6]. Therefore, it is crucial to demonstrate that no other forms of corrosion other than uniform corrosion can take place. Considering the environmental conditions to which the overpack is expected to be exposed during the disposal period, pitting corrosion and stress corrosion cracking are considered to be potentially the most detrimental.
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