Methodologies for Predicting the Performance of Ni-Cr-Mo Alloys Proposed for High Level Nuclear Waste Containers
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INTRODUCTION A key factor in the overall performance of the proposed Yucca Mountain 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 [ 1]. One of the uncertainties in evaluating the repository performance is the ability to predict the long term performance of the waste package container materials in a range of possible repository environments. Over the history of the U.S. repository program, the choice of container materials has progressively shifted to more corrosion resistant alloys in order to increase the lifetime of the waste packages. Single, thin-wall Alloy 825 containers were ruled out in favor of a waste package design that features a 2-cm thick Alloy 825 corrosion resistant inner barrier inside a 10-cm thick A516 steel outer barrier designed to corrode at a slow and predictable rate in the repository environment. Improvements of this double barrier design included Alloy 625 and subsequently Alloy C-22 as inner barrier materials. In an effort to increase the lifetime of the containers beyond 10,000 years, an alternate waste package design has been proposed recently that features an Alloy C-22 outer barrier and a titanium alloy inner barrier [2]. For either of the current waste package designs, the performance of Alloy C-22 is critical to the overall containment strategy. As a result, an appropriate methodology for predicting the long-term performance of Alloy C-22 in expected repository environments must be developed and verified. Previous investigations by the authors have shown that the long term initiation of localized corrosion for Alloy 825 and type 316L stainless steel (SS) in simulated repository environments could be predicted by using the repassivation potential for crevice corrosion [3,4]. The repassivation potential or similar concepts have also been considered for predicting long term corrosion behavior in other HLW disposal programs [5,6]. The main objective of this investigation is to determine values of the repassivation potential for crevice corrosion (E~c,,.J as a means to predict environmental conditions where HLW containers constructed of Alloy C-22 are
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susceptible to localized corrosion. Although Alloys 825 and 625 are no longer candidate container materials, assessment of these materials is included as a comparison to Alloy C-22 and to show the historical evolution of candidate waste package materials to alloys with increased corrosion resistance. In addition, the passive current density, which determines the lifetime of container materials in the absence of localized corrosion [7], was evaluated for Alloy C-22 under various environmental conditions.
EXPERIMENTAL PROCEDURES All tests were conducted using specimens of Alloys C-22, 625, and 825. The compositions of these materials are shown in Table 1. Repassivation potentials were determined on specimens with artificially formed crevices using cyclic potentiodyn
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