Author(s): S. Grousset, C. Wittebroodt, A. Dauzères and L. Urios
Paper category: Proceedings
Book title: Proceedings of the Final Conference of RILEM 253-MCI Microorganisms-Cementitious Materials Interactions Volume II
Editor(s): Alexandra Bertron and Henk Jonkers
ISBN: 978-2-35158-207-7 (Set)
ISBN: 978-2-35158-210-7 (Volume 2)
e-ISBN: 978-2-35158-206-0
Publisher: RILEM Publications SARL
Publication year: 2018
Pages: 567-568
Total Pages: 308
Language : English

Abstract: The French concept for geological disposal of nuclear waste is based on a multi-barrier system confined in a clayey host rock at about 500 m depth. In high level radioactive waste galleries, a carbon steel liner is put in contact with clay. The injection of a cement-bentonite grout [1] in the annular gap between the clay and the liner is considered to (i) counter the acidic pH transient (pH ~ 4.5) resulting from the host rock oxidation which may lead to high corrosion rates (200 μm/year) [2] and (ii) passivate metallic surfaces and prevent the O2 presence. Yet, the grout injection in an annular gap of several tens of meters in length should lead to heterogeneities of filling, resulting in voids soon filled with grout pore water solution mixed with argillite’s one. Besides, the clayey host rock [3] and the bentonite [4] are sources of microorganisms, and the pH of the grout pore water (~ 11.5) may not be high enough to prevent bacterial colonisation [5]. Therefore, bacterial activity must be considered as it can lead to higher corrosion rates through phenomena such as biofilm formation [6], pH heterogeneities [7], or H2S production by sulphate-reducing bacteria (SRB) [8] and thus impact the choice of the initial liner’s thickness. The first question that must be answered is which bacterial strains from the bentonite and/or the clayey host rock are able to grow in the storage conditions. To this end, anoxic culture tests are conducted at 25°C in several media based on a mix of a cement-bentonite suspension with a synthetic clay solution in the following ratios: 100/0 (pH = 11.5); 75/25 (pH = 11.0); 50/50 (pH = 10.2); 25/75 (pH = 9.5). The cement-bentonite suspension is obtained by mixing ultrapure water with crushed cement-bentonite paste samples (43 %w CEM III/A, 43 %w silica fume, 11 %w bentonite, 3 %w hydrotalcite, W/C = 2: 1) previously cured for one month in a sealed bag at room temperature (liquid/solid = 9 ml/g; 24h stirring). The composition of the synthetic clay solution is as close as possible to the pore water in equilibrium with the Toarcian argillite from the IRSN Tournemire experimental platform (Aveyron, France) [9, 10]. To help slow- growing bacterial strains, some media are enriched with organic acids (acetate 1 mM, lactate 1 mM, formate 1 mM, pyruvate 1 mM) and/or 3 mM (NH4)2SO4 with 0,3 mM KH2PO4 0,3 mM and/or vitamins. Finally, two samples are used as inocula: a suspension of argillite, obtained by mixing crushed argillite from Tournemire with ultrapure water (liquid/solid = 5 ml/g; 24h stirring), and a suspension of bentonite prepared by mixing commercial bentonite powder with ultrapure water (liquid/solid = 15 ml/g; 24h stirring). After a month at 25°C, bacterial growth is observed in argillite (cocci or coccobacilli, 3 – 6. 105 cell/ml) and bentonite (bacilli, 3 – 12. 105 cell/ml) suspensions in culture media composed of the 75/25 mix in which were added organic acids, (NH4)2SO4 and KH2PO4. In bentonite cultures, the production of H2S suggests that microbial sulphate-reduction is likely. Then, DNA extraction and 16S rRNA gene sequencing are carried out to determine the taxonomic diversity.
Secondly, to figure out the efficiency of the cement-bentonite grout to protect the liner against corrosion, carbon steel coupons (S235) are immersed for 2, 5 or 10 months in five different immersion media. Three of them have the same composition as in the successful culture tests, but they are prepared differently: (i) the first one is sterilized before introducing the coupons, (ii) the second one is used as it is, namely with the bacterial strains likely present in the cement-bentonite suspension, and (iii) the third one is enriched with the bacteria grown in both argillite and bentonite culture tests (final cell densities ~ 6.105 and ~ 9.105 cell/ml respectively). To compare the corrosion patterns obtained in cementitious conditions with those observed in clayey conditions, the two last immersion media are composed of the synthetic clay solution with organic acids, (NH4)2SO4 and KH2PO4, like the three cementitious solutions. These last two media are treated like the first and the third cementitious media. At each end date, the general corrosion rates are calculated from the loss of mass and the corrosion product layers are characterised by complementary techniques: μRaman spectroscopy and μ-XRD for the nature of the corrosion compounds, SEM-EDS for the elementary composition. Furthermore, the evolution of the bacterial population is studied and the formation of biofilms on the coupons is researched by microscopy using ApoTome.

Online publication : 2018
Publication type : full_text
Public price (Euros) : 0.00

>> You must be connected to view the paper. You can register for free if you are not a member