Effect of Thiosulfate on the SCC Behavior of Carbon Steel Welds Exposed to Concrete Pore Water Under Anoxic Conditions
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MRS Advances © 2018 Materials Research Society DOI: 10.1557/adv.2018.285
Effect of Thiosulfate on the SCC Behavior of Carbon Steel Welds Exposed to Concrete Pore Water Under Anoxic Conditions B. Kursten 1, S. Caes 1 and R. Gaggiano 2 1
SCK•CEN, The Belgian Nuclear Research Centre, R&D Waste Packages Unit, Boeretang 200, 2400 Mol, Belgium
2
ONDRAF/NIRAS, The Belgian Agency for Radioactive Waste and Enriched Fissile Materials, Avenue des Arts 14, 1210 Brussels, Belgium
ABSTRACT
The Supercontainer (SC) is the reference concept for the post-conditioning of vitrified highlevel nuclear waste and spent fuel in Belgium. It comprises a prefabricated concrete buffer that completely surrounds a carbon steel overpack. Welding is being considered as a final closure technique of the carbon steel overpack in order to ensure its water tightness. Welding is known to induce residual stresses near the weld zone, which may lead to an increased susceptibility to stress corrosion cracking (SCC). In this study, slow strain rate tests were conducted to study the SCC behavior of plain and welded P355 QL2 grade carbon steel exposed to an artificial concrete pore water solution that is representative of the SC concrete buffer environment. The tests were performed at 140°C, a constant strain rate of 5 × 10-7 s-1 and at open circuit potential under anoxic conditions. The effect of thiosulfate on the SCC behavior was investigated up to levels of 600 mg/L S2O32-.
INTRODUCTION In Belgium, the preferred long-term management option for vitrified high-level radioactive waste (VHLW) and spent fuel (SF), that is being pursued by the Belgian radioactive waste management organization (ONDRAF/NIRAS), is final underground disposal in a geologically stable and poorly indurated clay formation [1]. The Supercontainer concept is currently being considered as the reference design for the final disposal of VHLW and SF. In this concept, a watertight cylindrical carbon steel container, the so-called overpack, which contains either VHLW canisters or SF assemblies, is inserted into a cylindrical prefabricated thick concrete block, named the buffer. In the currently preferred design option, the concrete block will be fitted into an outer stainless steel container, termed the envelope [2]. Low carbon steel (mild steel) has been chosen for the overpack because it is a material for which a broad experience and knowledge already exists, it shows excellent machinability and weldability properties, and its corrosion behavior, in a concrete
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environment, in particular, is well understood and favorable for meeting the requirement for overpack longevity. The function of the carbon steel overpack is to provide total containment of the radionuclides during at least the thermal phase by preventing contact between the wasteform and
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