Microbial influence on the immobilization of radionuclides in crystalline rock environments

In crystalline rock, the dominant transport medium for radionuclides is groundwater flowing through subsurface fractures. Since groundwater is containing microorganisms, fracture surfaces support biological growth of microbial communities. The formed subsurface microbial communities have a significant effect on the adsorption capacity of host rock formations by forming a barrier between the rock surface and the groundwater. But how do these microbial communities influence the mobilization or immobilization of radionuclides in the case of a nuclear incident? As known from experiments performed on biofilms from the underground rock characterization facility tunnel ONKALO in Finland and from the Äspö Hard Rock Laboratory (HRL) in Sweden, microbes can significantly affect subsurface biogeochemical interactions, leading to the immobilization and (bio-)adsorption of radionuclides. Under the ambient conditions in the Äspö HRL (neutral pH of the groundwater, high amount of ferrous iron in the groundwater, aerobic conditions) the uptake of radionuclides like U(VI) and Np(V) was determined to be 85% and 95%, respectively due to the abundant surface area of the bacteriogenically formed ferrihydrite.

In order to define the influence of microbial communities and their relevance in the safety assessment of a nuclear waste repository some important points have to be considered;

• We have to characterize the possible microbial diversity during changing geochemical conditions (Eh, pH, T, p, chemical composition of the groundwater) in an opened and closed deposit. Bioaccumulation experiments performed on selected microbes using different radionuclides have to emphasize at least on aerobic conditions.
• Information about the sources for electron donors and electron acceptors for microbial activity are needed.
• To estimate the relevance of microbial process, we have to know more about the kinetic of microbial growth. Therefor we have to trace the effect of microbial metabolism and growth in a geochemical system using a generalized kinetic rate law. We need to estimate biofilm growth for process description, including different scales and different types of radionuclides. What are the parameters, which are needed for modeling the kinetic of microbial growth? The aim of the modeling will be a quantification of microbial processes and the assessment of microbially mediated retention of radionuclides.