Corrosion performance projection of Yucca Mountain waste packages

The proposed high-level nuclear Waste repository at Yucca Mountain relies heavily on the corrosion resistance of waste packages (WP) emplaced in tunnels bored through tuffaceous rock, for adequate performance during the anticipated 10,000 years regulatory period. Present WP design uses a similar to20-mm-thick outer shell of Alloy 22 as the main corrosion-resist ant barrier. The operating conditions may include in initial high-temperature (>96 degreesC) pulse that will last approximately several hundred to a thousand years. Ti-alloy shields ire envisioned to prevent water from directly dripping on the WP. However, recent findings suggest that deliquescent salts and other contaminants on the WP surfaces may cause liquid water to form there, even at high temperatures. Current performance projections predict that during the anticipated regulatory period localized corrosion modes will be unlikely and that the Alloy 22 barrier will degrade primarily by very slow uniform dissolution, essentially under passive surface conditions. A review is presented of the assumptions and experimental findings leading to those projections, as well as a discussion of findings of a recent Workshop on the challenges involved in extrapolating limited information on corrosion behavior over an extremely long service period that extends beyond the time frame of common engineering experience. Potential mechanisms for deterioration of the passive regime that may be encountered under those circumstances are discussed.