Titanium alloy degradation modes are reviewed in relation to their performance in the repository environments. General corrosion, localized corrosion, stress corrosion cracking, hydrogen induced cracking, microbially influenced corrosion, and radiation-assisted corrosion of Ti alloys are considered. With respect to the Ti Grade 7 drip shields used in the repository at Yucca Mountain, general corrosion, hydrogen induced cracking, and radiation-assisted corrosion will not lead to failure within the 10,000 year regulatory period; stress corrosion cracking (in the absence of disruptive events) is of no consequence to barrier performance; and localized corrosion and microbially influenced corrosion are not expected to occur. To facilitate the discussion, Ti Grades 2, 5, 7, 9, 11, 12, 16, 17, 18, and 24 are included in this review.
The Nuclear Waste Policy Act of 1982 as amended in 1987 designated Yucca Mountain in Nevada as the potential site to be characterized for high-level nuclear waste (HLW) disposal [1]. Long-term containment of the waste and subsequent slow release of radionuclides from the Engineered Barrier System (EBS) into the geosphere will rely on a system of natural and engineered barriers including a robust waste containment design. The waste package (WP) design during the Viability Assessment (VA) phase of the Yucca Mountain Project [2] used an all-metallic, dual-barrier waste container utilizing a thick outer corrosion-allowance metal barrier (A516 carbon steel) over a thinner inner container made of a suitable corrosion-resistant alloy [2]. During the Site Recommendation (SR) phase of the Project, the container design was modified to the current configuration with a highly corrosion resistant Ni-based Alloy 22 cylindrical barrier surrounding a 316 stainless steel inner structural container [3]. The waste package was to be covered by a self-supported mailbox-shaped drip shield (DS) composed predominantly of Ti Grade 7 with Ti Grade 24 structural support members [3]. The Ti drip shield provided defense in depth, because the WP and DS do not have common failure modes, lending a further margin of safety to repository design. The current design concept is schematically shown, with illustration of the mailbox-shaped drip shield, in Figure 1 [4.
The minimum target lifetime for containment of HLW without exceeding a regulatory specified individual dose rate at the site boundary is 10,000 years [4]. Over the years, numerous studies have been performed to evaluate the susceptibility to stress corrosion cracking, general, localized, galvanic and microbially influenced corrosion for Alloy 22 [5, 6, 7, 8, 9, 10, 11, 12] and Ti Grade 7 [6, 7, 13, 14, 15]. The purpose of this work is to review the corrosion performance of Ti Grade 7 and other relevant titanium alloys for the current WP design under the repository environmental conditions. The review will concentrate on potential corrosion processes possible in aqueous environments at Yucca Mountain. A brief review of the background of titanium alloys, hydrogen absorption and the properties of passive film on titanium alloys will be given as the basis of the discussion. Next, the key corrosion processes that could occur will be addressed individually. Finally, the expected corrosion performance of these alloys under the specific environmental conditions anticipated at Yucca Mountain will be considered.
