Fundamental understanding of technetium interactions in the environment of nuclear waste repositories

Technetium (Tc) is an element of concern for the environment and living organisms. In particular, 99Tc a beta emitter with a long half-life (t1/2 = 2∙105 years) is the most abundant isotope of Tc. Besides its generation by medical applications, it is a fission product in nuclear reactors during energy production and must be considered in the safety assessment of nuclear waste repositories. Tc is known to be very mobile in water fluxes under aerobic condition, when mainly TcVII is present. TcVII barely interacts with minerals or microorganisms that could restrict its migration. Thus, the strategy to immobilize TcVII employs the reduction of TcVII to TcIV, since the main species of TcIV is a low soluble oxide (TcIVO2), limiting Tc migration through aquifers. The change in redox state is triggered by reducing agents, such as Fe2+, sulfide or by microbiological redox cascades. Thus, it is necessary to evaluate the interaction of Tc with different materials present in engineered or natural environments also under more complex experimental conditions. We use a broad variety of advanced techniques along a value chain that starts with determination of thermodynamic data (i.e. complex formation constants, solubility constants of minerals, redox potentials and Tc distribution coefficients) it is followed by structural verification by micro-spectroscopy approaches up to establishment in thermodynamic databases. Our particular interest is the in situ monitoring of molecular sorption and redox processes at biogeochemical interfaces and in solutions. As a result, a deep chemical understanding of environmental behavior of technetium will allow the development of sound risk assessments and remediation strategies. This is only feasible by a multidisciplinary approach, including chemistry, geosciences and microbiology.
Some of the work presented here is part of the BMBF funded young investigator group TecRad (Ref 02NUK072, further details can be found at https://www.hzdr.de/db/Cms?pNid=1375).