Impact of temperature on corrosion processes and porous material microstructural evolution: carbon steel/cementitious materials interfaces experimental characterization
Start in : As soon as possible
Workplace : Fontenay-aux-Roses, France
Duration : 12 months
This post-doctoral project, proposed and conducted by IRSN, is part of the research programs dedicated to the long-term safety assessment of HL-ILW radioactive waste deep geological disposal facility. Specifically, this study is concerning the impact of temperature on corrosion processes that can take place at the interface between carbon steel and cementitious materials. This interface is a key issue in the concept of disposal cell for vitrified high-level waste (HLW) in argillaceous sedimentary formations for the Belgium, Dutch and French national programs. The first two programs rely upon supercontainers containing HLW encased in cylindrical CEM I concrete buffer material. During the thermal phase and much longer beyond, pH has to be kept at high values in order to keep the carbon steel overpack passivated, to limit corrosion and ultimately radionuclide release. In the French concept, the annular gap between a carbon steel liner and the host rock is filled with a cement-bentonite grout that should impose corrosion-limiting conditions. This grout is characterized by a moderated alkalinity (pH ~ 11) that should be neutralized after the thermal phase to prevent the dissolution of the nuclear glass matrix and the associated radionuclide release. The understanding of the evolution of carbon steel/cementitious materials interfaces during the thermal phase, as well as the impact of possible heterogeneities (e.g. voids at the interface) on this evolution, are important scientific challenges.
The work proposed in this post-doctoral fellowship consists mainly in working on the preparation and the characterization of solid carbon steel/cementitious materials interfaces samples collected from the BACUCE in situ experiments that have been launched in the framework of the “European joint programme for radioactive waste management” (EURAD project/WP ACED). These experiments are conducted in the IRSN Tournemire Underground Research Laboratory (URL). Practically, each experiment is installed in a separate sub-horizontal borehole and reproduces an interface between a cementitious material and a carbon steel tube in anoxic condition. A heater system, adjusted for a temperature of 80°C, is emplaced in the carbon steel tube to simulate the thermal period at the interface. In each experiment, voids are created artificially at the interface between steel and cementitious materials to represent possible heterogeneities that could occur in a disposal facility. In the first two experiments the cementitious material consists of a cement-bentonite grout. In the third experiment the cementitious material is a CEM I paste. Cement-bentonite grout or CEM I material synthetic solutions are injected in the created heterogeneities and after several months of interaction, solutions are sampled and the carbon steel tubes are retrieved with their surrounding cement materials in order to characterise the physicochemical evolution of the materials at the interface.
An important part of the proposed work will be related to the subsampling of the interface and to their microstructural characterization. Morphology and chemical composition of the corrosion products formed at the carbon steel/cementitious materials interfaces will be investigated using (i) Scanning Electron Microscopy (SEM) coupled to Energy Dispersive Spectroscopy (EDS), (ii) X-Ray Diffraction analyses (XRD) and (iii) optical imaging and micro-Raman (µRaman) spectroscopy will be used to identify the main reactive pathways associated to phase precipitations/dissolutions at the cement/clay interface. In parallel, microstructural evolution of the carbon steel/cementitious materials interfaces will be assessed by X-ray tomography (µ-XCT), especially to identify the homogeneity of the steel corrosion layer and to characterize the iron diffusion front in the cementitious material. Finally, the post-doctoral researcher will participate to both the interpretation of the obtained experimental data and the valorisation of the associated results.
The researcher will be located in the LUTECE IRSN laboratory in Fontenay aux Roses (92). Apart from the Raman characterizations, all tools are available in the IRSN laboratory. Two or three missions are possible in the Tournemire URL laboratory (12) to sample the on-going experiments.
PhD in geochemistry – metallurgy
Experience in the field of corroded steel material experimental characterisation and porous material microstructural evolution characterisation. Experience of user for XRD, SEM-EDS, Raman is desirable.