The Influence of Environmental Conditions and Passive Film Properties on the MIC of Engineered Barriers in the Yucca Mou
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The Influence of Environmental Conditions and Passive Film Properties on the MIC of Engineered Barriers in the Yucca Mountain Repository A.C. Lloyd,1 R.J. Schuler,1 J.J. Noël,1 and D.W. Shoesmith1 1 Department of Chemistry, University of Western Ontario, London, Ontario, N6A 5B7, Canada F. King2 2 Integrity Corrosion Consulting, Calgary, Alberta, T3B 3K8, Canada [email protected] ABSTRACT A combination of gamma radiation fields, the absence of moisture, and the high temperatures on the drip shield (DS) and waste package (WP) should combine to suspend microbial activity on the DS/WP surfaces for many tens of thousands of years. This lack of microbial activity, coupled with the corrosion resistance of the titanium Grade7 (Ti-7 drip shield) and the Alloy-22 (waste package) materials make microbially induced corrosion (MIC) of these engineered barrier materials extremely unlikely. INTRODUCTION Nuclear waste repositories are harsh environments for micro-organisms [1-3]. Microbial activity tends to be limited by one or more of the following factors: (i) the presence of a radiation field, (ii) the availability of water, (iii) the elevated temperature, (iv) the availability of nutrients and electron acceptors, and (v) the prevailing redox conditions. Any one of these factors can limit the spatial and temporal distribution of microbial activity in the repository. In addition, different materials exhibit different inherent resistances to attack by micro-organisms [4]. Here, the expected evolution of environmental conditions at the Yucca Mountain repository is used to predict the impact of microbial activity on the DS and WP. In addition, evidence is presented to suggest that the inherent stability of the passive film on Alloy 22 will prevent MIC even once microbial colonization of the WP becomes possible. EXPECTED EVOLUTION OF MICROBIAL ACTIVITY AT YUCCA MOUNTAIN It is clear from various microbial studies that a range of microbes are naturally present or can be expected to be introduced during repository construction. Relatively large numbers of microbes representing a variety of species can be cultured from samples of tuff from Yucca Mountain [5,6]. However, it is not the population of microbes that is important for MIC or for the alteration of the environment, but their activity. If microbes are not active they cannot produce the aggressive metabolic by-products that lead to MIC or potentially crevice-forming EPS, regardless of the total number of microbes present. A number of factors affect the activity of microbes. Irradiation has been shown to either kill or to render microbes dormant [7-10]. Desiccation is well known to inhibit microbial activity [11]. Elevated temperatures will also restrict the extent of microbial activity to cooler locations within the repository, away from the WP and DS in the drifts and tunnels. The prevailing aerated conditions within Yucca Mountain will restrict anaerobic microbial activity to microenvironments formed under biofilms on rock surfaces in regions of the repository in whic
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