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Bioréactivité des nitrates en contexte de stockage de déchets radioactifs profond

Abstract : France has the project to dispose long-lived intermediate-level radioactive waste (ILW-LL) in a deep geological repository. The radioactive waste is loaded with nitrate salts and after thousands of years, the release of nitrates could promote the mobility of radionuclides out of the waste. However, in the presence of organic matter or hydrogen, bacterial activity can theoretically reduce nitrate to smaller nitrogen species through denitrification. The first objective of this thesis is to evaluate the ability of bacteria to adapt to the environment near the waste, i.e. in the absence of oxygen, at alkaline pH between 9 and 13, in the presence of high nitrate concentrations, in the presence of an organic (acetate) or mineral (hydrogen) electrons donor and in the presence of solid cement and solid steel. The second objective is to quantify nitrate reduction kinetics under the conditions described above. Bacterial reduction of nitrates was observed up to pH 11 and 400 mM of nitrates in the presence of acetate and up to pH 10.5 and 150 mM of nitrates in the presence of hydrogen. In the presence of hydrogen, nitrate reduction kinetics were generally slower and bacteria were more sensitive to alkaline pH than heterotrophic bacteria using acetate. This is a consequence of the low solubility of hydrogen, the mineral carbon assimilation expense for bacteria and the alkalinisation during denitrification in the presence of hydrogen. In the presence of acetate the pH equilibrate toward 10, due to CO2 generation when acetate is oxidized. Finally, while the presence of solid cement did not have a significant impact on bacterial activity, solid steel stimulated the reduction of nitrates.
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Pierre Albina. Bioréactivité des nitrates en contexte de stockage de déchets radioactifs profond. Matériaux. Université Paul Sabatier - Toulouse III, 2019. Français. ⟨NNT : 2019TOU30286⟩. ⟨tel-03104059⟩

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