The Use of Repassivation Potential in Predicting The Performance of High-Level Nuclear Waste Container Materials
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THE USE OF REPASSIVATION POTENTIAL IN PREDICTING THE PERFORMANCE OF HIGH-LEVEL NUCLEAR WASTE CONTAINER MATERIALS
Narasi Sridhar, Darrell Dunn, and Gustavo Cragnolino Center for Nuclear Waste Regulatory Analyses (CNWRA) 6220 Culebra Road, San Antonio, Texas, U.S.A. ABSTRACT Localized corrosion in aqueous environments forms an important bounding condition for the performance assessment of high-level waste (HLW) container materials. A predictive methodology using repassivation potential is examined in this paper. It is shown, based on longterm (continuing for over 11 months) testing of alloy 825, that repassivation potential of deep pits or crevices is a conservative and robust parameter for the prediction of localized corrosion. In contrast, initiation potentials measured by short-term tests are non-conservative and highly sensitive to several surface and environmental factors. Corrosion data from various field tests and plant equipment performance are analyzed in terms of the applicability of repassivation potential. The applicability of repassivation potential for predicting the occurrence of stress corrosion cracking (SCC) and intergranular corrosion in chloride containing environments is also examined.
INTRODUCTION One of the requirements for the performance of the Engineered Barrier Systems (EBS), as prescribed in the U.S. Code of Federal Regulations 10 CFR 60.113, is that the waste packages shall be designed to contain the radionuclides '§ubstantially completely" for a period ranging from 300 to 1,000 years. Two key technical uncertainties in the demonstration of compliance of the EBS with the regulatory requirement are: (i) lack of understanding of the effects of environmental factors on waste package performance, and (ii) difficulty in extrapolation of short-term laboratory and prototype data to long-term performance. The environmental conditions surrounding the waste packages are determined by the complex interactions of temperature distribution, rock fracture distribution, vadose zone water composition, evaporative effects, man-made materials, and microbiological actions. Although inclusion of all these factors in a near-field environment model is a considerable challenge, it is clear, even from simplistic considerations, that the environment contacting the waste packages will vary widely in its chemistry [1]. Because of the difficulty involved in including detailed models of environmental changes and their effects, a simple scheme for incorporating the effects of these environments on corrosion of container materials must be considered for performance assessment calculations [2]. Such a scheme for corrosion resistant alloys is illustrated in Figure 1. As shown in Figure 1, the initial period after emplacement of the waste packages in the proposed repository horizon at Yucca Mountain is expected to be dry, at least for most of the waste packages. The waste package temperatures will depend upon the emplacement thermal loading strategy as well as the age of the fuel, but is expected to be as high as 280 'C
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