Passive Dissolution and Localized Corrosion of Alloy 22 High-Level Waste Container Weldments
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D. S. Dunn, G. A. Cragnolino, and N. Sridhar Center for Nuclear Waste Regulatory Analyses, Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78238-5166, ddunn(aswri.org. ABSTRACT Localized corrosion of high level nuclear waste containers is considered an important factor that will have a strong influence on the overall performance of the proposed repository at Yucca Mountain, NV. The present candidate container material, Alloy 22 [UNS N06022 (57Ni-22Cr-13.5Mo-3W-3Fe)], is highly resistant to localized corrosion. Assessing the performance of the HLW containers also requires an evaluation of localized corrosion resistance and determination of the passive corrosion rate of Alloy 22 weldments. The localized corrosion resistance of welded Alloy 22 specimens was evaluated by measuring the repassivation potential in chloride containing solutions whereas potentiostatic tests were used to determine the passive corrosion rate of the welded material. The results for the welded material are compared to those for the base metal. INTRODUCTION A key factor in the overall performance of the proposed Yucca Mountain (YM) repository for the disposal of high level nuclear waste is the ability of the waste packages to contain the radioactive waste for an extended period [I]. Waste package designs have gradually shifted to include thick walled containers fabricated from more corrosion resistant alloys. Recently, several enhanced design alternatives (EDA) were proposed to replace the Department of Energy (DOE) viability assessment (VA) waste package design[2]. The EDA II waste package specifications include a 5-cm thick type 316 nuclear grade (NG) stainless steel (SS) (69Fe-l7Cr-12Ni) inner container surrounded by a 2-cm thick Alloy 22 (57Ni-22Cr-13.5Mo-3W-3Fe) outer container. Although additional design options in the EDA II, including a dripshield encompassing the waste packages and the use of backfill, may extend the lifetime of the waste package, this design is primarily dependent on the corrosion resistance of the outer Alloy 22 barrier. Previous investigations have shown that Alloy 22 in the mill annealed condition is resistant to localized corrosion in the range of chloride containing environments expected at the YM site [3,4]. In addition, the uniform corrosion rate of mill annealed Alloy 22 was shown to be sufficiently low to promote long waste package lifetimes in the absence of localized corrosion [5]. Welding operations during closure of the WP, followed by prolonged exposures to elevated temperatures in the emplacement drifts, may affect the phase stability of corrosion-resistant Alloy 22. In this case, generation of short- and long-range ordered structures or formation of brittle intermetallic phases [i.e., topologically close-packed (TCP) phases] may accelerate uniform and localized corrosion processes and even affect mechanical properties [6]. Although excellent performance has been demonstrated for the mill annealed material, variations in the corrosion resistance of the container material as a consequence
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