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This paper reviews the current understanding of hydrogen-induced cracking (HIC) of Ti Grade 7 and other relevant titanium alloys within the context of the current waste package design for the repository environmental conditions anticipated within the Yucca Mountain repository.The review concentrates on corrosion processes possible in the aqueous environments expected within this site. A brief background discussion of the relevant properties of titanium alloys, the hydrogen absorption process, and the properties of passive film on titanium alloys is presented as the basis for the subsequent discussion of model developments. The key corrosion processes that could occur are addressed individually. Subsequently, the expected corrosion performance of these alloys under the specific environmental conditions anticipated at Yucca Mountain is considered. It can be concluded that, based on the conservative modeling approaches adopted, hydrogen-induced cracking of titanium alloys will not OCCUT under nuclear waste repository conditions since there will not be sufficient hydrogen in the alloy after 10,000 years of emplacement.

The evaluation studies of the proposed repository for long-term storage of spent nuclear fuel and high-level nuclear waste at Yucca Mountain, Nevada, are underway. Fulfillment of the requirements for limiting dose to the public, which includes containment of the radioactive waste emplaced in the proposed repository and subsequent slow release of radionuclides from the Engineered Barrier System (EBS) into the geosphere, will rely on a robust waste container design, among other EBS components. Part of the evaluation process involves sensitivity studies aimed at elucidating which model parameters contribute most to the waste package and overlying drip shield degradation characteristics. The model parameters identified for this study include (1) general corrosion rate parameters and (2) stress corrosion cracking (SCC) parameters. Temperature dependence and parameter uncertainty are evaluated for the general corrosion rate model parameters while for the SCC model parameters, uncertainty treatment of stress intensity factor, crack initiation threshold, and manufacturing flaw orientations are evaluated. Based on these evaluations new uncertainty distributions are generated and recommended for future analyses. Also, early waste package failures due to improper heat treatment were added to the waste package degradation model. The results of these investigations indicate that the waste package failure profiles are governed by the manufacturing flaw orientation model parameters.

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How would you… …describe the overall signifi cance of this paper? This paper describes the general corrosion of Alloy 22 under the Yucca Mountain environment. Long-term modeling and data analysis for Alloy 22 are presented to estimate the performance of the Alloy 22 waste package outer barrier in the repository. …describe this work to a materials science and engineering professional with no experience in your technical specialty? Relatively few studies focus on longterm general corrosion behavior of highly corrosion-resistant Ni-Cr-Mo alloys such as Alloy 22. This work presents Alloy 22 general corrosion data analysis and modeling for longterm performance prediction. …describe this work to a layperson? General corrosion is an important mode of degradation of alloys. This paper summarizes the Alloy 22 general corrosion testing and modeling that has been undertaken in support of the Yucca Mountain Project. Longterm modeling and data analysis for Alloy 22 are presented. This paper presents the general corrosion model used by the Yucca Mountain Project to account for general corrosion degradation of the Alloy 22 waste package outer barrier. The general corrosion rates were determined from mass-loss measurements of specimens exposed to repository-relevant solutions at 60°C and 90°C for about fi ve years. An Alloy 22 general corrosion temperature dependence was obtained from short-term polarization resistance measurements and found to be consistent with values quoted in the literature. Uncertainty in mass-loss data and model fi tting was evaluated and incorporated into the resultant Alloy 22 general corrosion model.