Radionuclide Uptake and Transport on Microbes in Potential Repository Drifts at Yucca Mountain, Nevada
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Radionuclide Uptake and Transport on Microbes in Potential Repository Drifts at Yucca Mountain, Nevada Darren M. Jolley Engineered Barrier System Department Duke Engineering and Services 1180 Town Center Drive Las Vegas, Nevada 89134 ABSTRACT Radionuclide adsorption onto microbes, microbial retention in the engineered barrier system (EBS), and their potential release from the EBS as microbial colloids have been investigated. The microbial source term for these calculations was derived using MING V 1.0 software code [1]. Multiple model calculations from MING representing variations in possible microbial communities in the EBS were abstracted into two equations representing one meter segments of potential repository drift containing either commercial spent nuclear fuel (CSNF) or defense high level waste (HLW) packages. These two equations (Equations 1 and 2) represent the average cumulative microbial biomass generated in the EBS at any given time. A distribution for uranium uptake onto microbes (162.88 ± 133.05 mg U/gm dry cell) was applied to the microbial source term. The distribution was derived from the data set in Suzuki and Banfield [2] representing 45 different species of bacteria and fungus, covering uranium uptake at optimum pH values of 1 to 7. The mass of uranium sorbed onto the biomass was either sequestered in the EBS or transported as a microbial colloid based on a regression of data from Jewett et al. [3] representing microbial sorption onto air-water interfaces in unsaturated column experiments. Over one million years, it is estimated that EBS microbes may adsorb from 77 to 2302 kg of uranium [2302 kg U > 100% of the uranium available in a one meter segment of a CSNF waste package] per meter of waste package depending on the saturation of the invert and type of waste package. Over the same time, microbial colloids may transport from 8 to 1250 kg of adsorbed uranium per meter of waste package from the EBS. INTRODUCTION The objective of this paper is to present a scoping calculation of the mobilization/sequestration of uranium by microbial action. The calculation attempts to bound the adsorption of radionuclides to potential microbial communities and predict the mass of uranium that could be transported as a type of colloid through the proposed Engineered Barrier System (EBS) at Yucca Mountain, Nevada (Figure 1). The transport of microbe-associated radionuclides through the EBS is primarily a function of: the size of microbial community that could be present in the EBS, the metabolism independent adsorption of dissolved radionuclides onto the cell walls, and the subsequent mobility of microbes through the degraded waste package materials and invert (Figure 1). Regardless of the mobility of individual microbes, they will sorb metals including radionuclides onto or within their cell walls [2]. Once an individual microbe has sorbed the available radionuclides within, or onto, its cell walls, the radionuclides become subject to the transport regime imposed on the microbe by the EBS and by the ability of the
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