According to the French concept for deep geological disposal, high level radioactive waste will be emplaced in carbon-steel overpacks in a host rock consisting in a Callovo-Oxfordian clayey formation located in the Eastern Paris Basin. These overpacks have to ensure containment for several hundred years. During the first years after closure, they will be in unsaturated and aerated conditions and after a transition period aerated/deaerated the system would be in anaerobiosis. The study presented here is a part of a collaboration between LAPA and the French Institute of Radioprotection and Nuclear Safety (IRSN) in Fontenay-aux-Roses in France. IRSN has been conducting researches in its Tournemire Underground Research Laboratory (URL), a railway tunnel which crosses a Toarcian argillaceous formation. This geological layer is particularly interesting for its physical and chemical properties close to those of the Callovo-Oxfordian formation.
An original corrosion set-up was carried out: steel coupons were placed in drilled argillite with flowing of Tournemire pore water either in absence or in presence of a bacterial strains mix, at various temperatures, in oxic or anoxic conditions, and during one, four or nine months. The aerated conditions were chosen to understand what could take place at the beginning of the storage of the nuclear waste before the anoxic phasis. The mix of bacterial strains is representative of those reported in studies of the deep clayey biodiversity: Sulphate-Reducing Bacteria (SRB), Sulfur-Oxidizing Bacteria (SOB), Iron-Reducing Bacteria (IRB), biofilm forming strains. After the corrosion experiment, the steel coupons (surface and polished transverse sections) were characterized by complementary techniques: µRaman spectroscopy and XRD, so as to identify the crystalline structure of the corrosion products and FESEM at low voltage (5 kV) to evidence bacteria and biofilms and their association to the corrosion products. The bacterial diversity on the surface of steel coupons was also characterized by DNA Preparation and sequencing at IPREM (Equipe Environnement et Microbiologie). The corrosion rate was also determined for all the coupons.
The aim of this study is to determine the bacterial impacts on steel corrosion in conditions prevailing in a repository in the presence of a complex biodiversity with the following questions:
Let us see an illustration concerning the system corroded in aerobiosis at 25°C. Without added bacteria, FeIII-oxyhydroxides as lepidocrocite (