Corrosion Performance Projection of Yucca Mountain Waste Packages
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Corrosion Performance Projection of Yucca Mountain Waste Packages Alberto A. Sagüésa Department of Civil and Environmental Engineering University of South Florida Tampa, FL 33620, U.S.A. ABSTRACT 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 ~20-mm-thick outer shell of Alloy 22 as the main corrosion-resistant barrier. The operating conditions may include an initial high-temperature (>96 ºC) pulse that will last approximately several hundred to a thousand years. Ti-alloy shields are 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.
INTRODUCTION A unique feature of nuclear waste disposal is that the long life of some of the radionuclides requires anticipating the behavior of a repository over many thousands of years, which is beyond any previous technical experience. Performance-analysis calculations of the proposed Yucca Mountain repository (PYMR) indicate that widespread corrosion penetration of the waste package (WP) shell during the first 10,000 years of operation could seriously compromise repository performance [1]. In the present PYMR concept, the corrosion-resistance burden of the WP rests mainly on a ~20-mm-thick outer shell made of the Ni-Cr-Mo-W superalloy Alloy 22 (for conservatism, the corrosion resistance of other metallic WP components is ignored in current durability projections). The projected corrosion behavior of Alloy 22 under PYMR conditions is discussed below.
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Dr. Sagüés is a member of the United States Nuclear Waste Technical Review Board (NWTRB). Any statements and opinions presented in this paper are those of Dr. Sagüés and do not necessarily represent any position of the NWTRB.
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SERVICE ENVIRONMENT A detailed description of the PYMR is available in the Yucca Mountain Project (YMP) literature [2], and only relevant highlights will be mentioned here. The WPs are to emplaced in horizontal tunnels bored through tuffaceous rock above the water table. S
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