A Quantitative Assessment of Microbiological Contributions to Corrosion of Candidate Nuclear Waste Package Materials
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ABSTRACT The U.S. Department of Energy is contributing to the design of a potential nuclear waste repository at Yucca Mountain, Nevada. A system to predict the contribution of Yucca Mountain (YM) bacteria to overall corrosion rates of candidate waste package (WP) materials was designed and implemented. DC linear polarization resistance techniques were applied to candidate material coupons that had been inoculated with a mixture of YM-derived bacteria with potentially corrosive activities, or left sterile. Inoculated bacteria caused a 5- to 6-fold increase in corrosion rate of carbon steel C 1020 (to approximately 7-8ýim/yr), and an almost 100-fold increase in corrosion rate of Alloy 400 (to approximately ltm/yr) was observed due to microbiological activities. Microbiologically Influenced Corrosion (MIC) rates on more resistant materials (CRMs: Alloy 625, Type 304 Stainless Steel, and Alloy C22) were on the order of hundredths of micrometers per year (gtm/yr). Bulk chemical and surfacial endpoint analyses of spent media and coupon surfaces showed preferential dissolution of nickel from Alloy 400 coupons and depletion of chromium from CRMs after incubation with YM bacteria. Scanning electron microscopy also showed greater damage to the Alloy 400 surface than that indicated by electrochemical detection methods.
INTRODUCTION The U.S. Department of Energy is engaged in a suitability study for a potential geological repository at Yucca Mountain, Nevada, for the containment and storage of commercially generated spent nuclear fuel and high-level nuclear waste. There is growing recognition of the role that microorganisms could play in this repository, through MIC of waste packages and other repository components. Bacterial isolates from YM geologic material were shown in earlier studies to possess biochemical activities associated with MIC. Various microbial isolates demonstrated abilities to oxidize iron, produce sulfide, generate acids, and produce
exopolysaccharides or "slime" layers on metal surfaces, thus establishing some potential for MIC in the potential repository environment [I]. A system was implemented to predict and quantify the contribution of YM bacteria to the overall corrosion of candidate WP materials by testing candidate WP materials under accelerated conditions employing sterile vs. non-sterile conditions. Using this approach, results from tests employing sterile conditions were compared to those conducted under non-sterile conditions, thus elucidating bacterial contributions to corrosion of specific materials. Coupled with well-accepted electrochemical polarization techniques, overall corrosion rates were 1175 Mat. Res. Soc. Symp. Proc. Vol. 556 © 1999 Materials Research Society
determined in the presence of characterized, YM MIC-causing microbes, or in their absence. Subsequent endpoint chemical analyses of spent aqueous media from these tests and surfacial analysis of exposed coupons further contributed to determining the mode and extent of MIC by YM bacteria. EXPERIMENT
Electrochemical polarizat
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