The microbiology of subsurface, salt-based nuclear waste repositories: Toward a realistic prediction of the microbial effects on repository performance


The microbiology of subsurface, salt-based nuclear waste repositories: Toward a realistic prediction of the microbial effects on repository performance

Swanson, J.; Cherkouk, A.; Bader, M.; Reed, D.

Only two countries in the world—Germany and the United States—currently host nuclear waste facilities in subterranean salt formations, although the concept is gaining traction in other countries, as well. The evaluation of such sites for their feasibility as waste repositories is extensive and multidisciplinary. Microbiology has been one of the weaker areas of investigation, and as a result, there is still uncertainty surrounding the possible effects of microbial activity on salt-based repository performance.
The success of a nuclear waste repository is measured as its ability to prevent the release of radionuclides into the surrounding environment or to limit that release to levels deemed acceptable by the appropriate regulatory agencies and public. Microorganisms are predicted to have diverse effects on nuclear waste repository performance, all of which involve their impact on radionuclide migration. These effects are due to any activities that may affect radionuclide speciation, solubility, or mobility, including 1) complexation with carbonate or organic ligands generated from the breakdown of organic waste; 2) complexation with microbially-generated ligands; 3) creation of a reducing environment through the consumption of oxygen or generation of hydrogen; 4) alteration of pH; 5) redox reactions; and 6) bioassociation that could lead to biocolloid transport.
Because the biogeochemistry of other deep geologic (e.g., granite, clay) settings differs significantly from subterranean salt, it is not possible to extrapolate microbial activity from one site type to the other. However, because of a lack of data, this is precisely what has been done in most safety case scenarios in salt. Thus, performance models assume the worst-case scenario: that the organisms present in rock salt will thrive on the organics present in the radioactive waste, leading to the generation of complexing agents that enhance radionuclide solubility, and that they will take up significant amounts of radionuclides and transport them away from the repository. The goal of current research being conducted by Los Alamos National Laboratory for the Waste Isolation Pilot Plant (WIPP) and by the Helmholtz-Zentrum Dresden-Rossendorf for the German repository concept, is to provide a more realistic view of the potential effects of microorganisms on salt-based nuclear waste repositories.
To reach this goal, both laboratories have been performing coordinated, culture-dependent and independent studies on halite and briny groundwater samples from the Salado Formation (US) and the Zechstein Formation (Germany). Isolates (e.g., Halobacterium sp., putatively noricense,
and others) undergo further investigation into their ability to degrade specific organic waste components (e.g., citrate, cellulose) under repository-relevant conditions and into their potential interactions with waste radionuclides (e.g., uptake, toxicity, and transformation). Results thus far suggest: 1) that the activity of repository-indigenous organisms will be constrained by the projected conditions (brine composition, anoxic atmosphere) and also by the lack of suitable organic substrates but that organisms located in the far-field (overlying briny groundwaters) will not; 2) that some organisms alter brine composition in ways that may affect radionuclide solubility; 3) that the radionuclides present in the waste are inhibitory, but not completely lethal, at their soluble concentrations in repository brine; and 4) that bioassociation of radionuclides appears to differ with oxidation state, organism, and brine composition.
An overview of our current knowledge regarding the microbial impact on salt-based nuclear waste repository performance will be presented and will emphasize the much-needed collaboration between those doing basic halophile research and those applying it in non-routine settings.

  • Lecture (Conference)
    Halophiles 2016, 22.-27.05.2016, San Juan, Puerto Rico

Permalink: https://www.hzdr.de/publications/Publ-23573
Publ.-Id: 23573