The Nuclear Research Centre in Belgium (SCK•CEN) is looking for PhD candidates. The topic is microbial processes in geological disposal of radioactive waste.
The aim of the project is to unravel the impact of microbial processes on cement in repository-like conditions and on bitumen (derivatives) at a high, cement-derived, pH. The two parts of the project, one on cement and one on bitumen, will be complementary and will overlap mostly in the experimental set-up and methodology used.
The PhD project will be part of an international project. The PhD candidate will have ample opportunity to exchange experiences and samples with European colleagues, to relate the lab scale experiments to complementary in situ experiments and to follow the first steps towards the validation of the experimental results towards modelling and performance assessment.
Worldwide, geological disposal is considered the key solution for the disposal of radioactive waste. In Belgium, Boom Clay is considered a candidate host rock for the geological disposal of both long lived intermediate level waste (ILW) and high level waste (HLW), and is in detail investigated at SCK•CEN, among others via the HADES underground laboratory. In the Belgian concept developed by NIRAS/ONDRAF, ILW disposal will consist of introducing concrete monoliths containing bitumen- or cement-conditioned waste in the host rock. For the disposal of HLW, a concrete supercontainer design, with each container containing multiple waste drums, with metal and cementitious barriers has been developed. Overall, the safety of geological disposal is based on a multi-barrier concept, i.e. a container-repository-clay engineered barrier system, and is being thoroughly studied using a multidisciplinary approach. In this respect, our current understanding of the impact of microbial metabolism on the safety of geological repositories remains tenuous, even though microorganisms may have controlling influences on waste form evolution in situ, multi-barrier integrity and ultimately radionuclide migration from the repository into the host-rock. It is therefore deemed necessary to characterise and measure the impact of microbial activity on relevant processes that might affect disposal safety. This will possibly lead to refinements of the safety case models that are currently being implemented to evaluate the long-term evolution of radioactive waste repositories.
The aim of this project is to unravel the impact of microbial processes on cement in repository-like conditions and on bitumen (derivatives) at a high, cement-derived, pH. The two parts of this project, one on cement and one on bitumen, will be complementary and will overlap mostly in the experimental set-up and methodology used.
For cementitious materials as such, the focus will mostly be on the impact of microbial processes on the integrity of plugs and seals in repository systems, for which the integrity should be ensured over a long period of time. Microbial activity, although expected to be inhibited by high pH, might affect the performance of cementitious components on the long term, mostly by either (i) the production of biogenic acids, thereby lowering pH and/or enhancing calcium leaching (a detrimental effect), (ii) the enhancement of carbonation, thereby clogging the cement pores (a desirable effect) or (iii) minor processes, like biologically induced sulphate release, triggering the production of the voluminous ettringite (also detrimental).
In this part of the project, the microbial communities indigenous to backfills or infiltrating host rock water that are expected to be in contact with cement plugs and seals will be characterised. Their metabolic processes with respect to their impact on cementitious materials, being either deterioration (pH lowering, calcium leaching) or improvement (carbonation) of its barrier function, will be monitored. In a lab scale batch set-up, cementitious reference materials will be subjected to incubation in the presence or absence of (i) backfill materials, (ii) representative host rock waters and (iii) microbial inocula. The test conditions will include a range of parameters to estimate the boundary conditions of the processes (e.g. neutral and high pH, different bicarbonate concentrations, addition of cement plasticizers as carbon source). Microbial activity, biofilm development and overall shifts in microbial communities will be monitored by state of the art enzymatic, microscopy and molecular techniques, while cement integrity will primarily be monitored by electron microscopy and chemical analysis of the liquid phase.
As for bitumen, the project will focus on the biodegradation of the products that are released by a solid bitumen simulant that has been subjected to radiation and high pH, such as acetate, formate, and oxalate. Such bitumen degradation products are interesting energy and carbon sources for microbes, and thus could provide, in combination with electron acceptors present in the repository or host rock, potentially sufficient fuel to boost unknown or unexpected microbial processes that could affect the surrounding concrete barrier, the (local) pH and/or the radionuclide (RN) release and mobility from the ILW.
Therefore, a range of batch experiments will be set up, in which an artificial or leached mix of bitumen degradation products will be subjected to microbial degradation, using as inocula microbial communities collected from the host rock or a community that is selected to thrive at a higher pH. These batch experiments will be run (i) at neutral (host rock pore water) versus higher pH (young cement water), (ii) with different concentrations of nitrate, nitrite and/or sulphate, (iii) in the presence or absence of a solid bitumen simulant and (iv) in the presence or absence of solid cement. The liquid phases will be analysed using an integrated approach of microbial, state of the art molecular, bio-informatic, chemical and microscopy analyses to assess specific microbial metabolisms on bitumen derivatives, while solid bitumen and cement samples will be assessed with state-of-the-art molecular and imaging techniques to monitor substrate colonization with respect to pore size, surface ratio and the presence of salt crystals and precipitates.
The expected outcome of both parts of the project is a detailed characterisation of the process kinetics, end-products and boundary conditions of those microbial metabolisms acting specifically on bitumen (derivatives) and cementitious repository components.
This PhD project will be part of an international project. The PhD candidate will have ample opportunity to exchange experiences and samples with European colleagues, to relate the lab scale experiments to complementary in situ experiments and to follow the first steps towards the validation of the experimental results towards modelling and performance assessment.
The minimum diploma level of the candidate needs to be: Master of sciences , Master of sciences in engineering
The candidate needs to have a background in: Bio-engineering , Chemistry , Other
If this sounds interesting please contact:
Wouters Katinka, email, +32 (0)14 33 88 09
Leys Natalie, email, +32 (0)14 33 27 26