Microbial Effects on Nuclear Waste Packaging Materials

Microorganisms may enhance corrosion of components of planned engineered barriers within the proposed nuclear waste repository at Yucca Mountain (YM). Corrosion could occur either directly, through processes collectively known as Microbiologically Influenced Corrosion (MIC), or indirectly, by adversely affecting the composition of water or brines that come into direct contact with engineered barrier surfaces. Microorganisms of potential concern (bacteria, archea, and fungi) include both those indigenous to Yucca Mountain and those that infiltrate during repository construction and after waste emplacement. Specific aims of the experimental program to evaluate the potential of microorganisms to affect damage to engineered barrier materials include the following: Indirect Effects--(1) Determine the limiting factors to microbial growth and activity presently in the YM environment. (2) Assess these limiting factors to aid in determining the conditions and time during repository evolution when MIC might become operant. (3) Evaluate present bacterial densities, the composition of the YM microbial community, and determining bacterial densities if limiting factors are overcome. During a major portion of the regulatory period, environmental conditions that are presently extant become reestablished. Therefore, these studies ascertain whether biomass is sufficient to cause MIC during this period and provide a baseline for determining the types of bacterial activities that may be expected. (4) Assess biogenic environmental effects, including pH, alterations to nitrate concentration in groundwater, the generation of organic acids, and metal dissolution. These factors have been shown to be those most relevant to corrosion of engineered barriers. Direct Effects--(1) Characterize and quantify microbiological effects on candidate containment materials. These studies were carried out in a number of different approaches, using whole YM microbiological communities, a subset of YM bacteria, and select reference organisms. Studies were carried out to determine morphological alterations to materials surfaces and using electrochemical methods to help quantify effects and modes of MIC, and to provide additional alternative means of evaluating MIC effects. They were carried out only under conservative conditions (low temperature, saturated conditions); thus, resulting conclusions may be considered an upper bound of potential biological effects on tested materials