[en] The authors' study concerns the effect of organic acids produced by microorganisms in a deep repository. In a first step, they study the conditions leading to cellulose degradation and organic acids production by cellulotytic microorganism: fermentative conditions lead to an increase of the organic acids production. Secondly, the complexation properties of uranium by these acids are shown. Finally, the effect of the organic acids on the degradation of cement, used as an engineered barrier or as an embedding matrix for nuclear waste is shown. The leaching of calcium is correlated to the pH of the solution and to organic acid production. Biodegradation of cement is expressed in terms of equivalent leach layer thickness in calcium

[en] A thorough understanding of the energy sources used by microbial systems in the deep terrestrial subsurface is essential since the extreme conditions for life in deep biospheres may serve as a model for possible life in a nuclear waste repository. In this respect, H₂ is known as one of the most energetic substrates for deep terrestrial subsurface environments. This hydrogen is produced from abiotic and biotic processes but its concentration in natural systems is usually maintained at very low levels due to hydrogen-consuming bacteria. A significant amount of H₂ gas will be produced within deep nuclear waste repositories, essentially from the corrosion of metallic components. This will consequently improve the conditions for microbial activity in this specific environment. This paper discusses different study cases with experimental results to illustrate the fact that microorganisms are able to use hydrogen for redox processes (reduction of O₂, NO^3-, Fe III) in several waste disposal conditions. Consequences of microbial activity include: alteration of groundwater chemistry and shift in geochemical equilibria, gas production or consumption, bio-corrosion, and potential modifications of confinement properties. In order to quantify the impact of hydrogen bacteria, the next step will be to determine the kinetic rate of the reactions in realistic conditions. (authors)

[en] The effect of many parameters need to be studied to characterize the long term behavior of nuclear waste in a deep repository. These parameters concern the chemical effects, radiolytic effects, mechanical properties, water composition, and microbiological activity. To evaluate microbial activity in such an environment, work was focused on an inventory of key nutrients (C, H, 0, N, P, S) and energy sources required for bacterial growth. The production of hydrogen in the nuclear waste environment leads to the growth of hydrogen oxidizing bacteria, which modify the gas production balance. A deep repository containing bituminized waste drums implies several sources of hydrogen: - water radiolysis; -corrosion of metal containers; - radiolysis of the embedding matrix (bitumen). Two deep geological disposal conditions leading to H₂ production in a bituminized nuclear waste environment were simulated in the present study: - H₂ production by iron corrosion under anaerobic conditions was simulated by adding 10% of H₂ in the atmosphere; - H₂ production by radiolysis of bitumen matrix was approached by subjecting this material to external gamma irradiation with a dose rate near real conditions (6 Gy/h). The presence of dissolved H₂ in water allows the growth of hydrogen oxidizing bacteria leading to: - CO₂ and N₂ production; - H₂ consumption; - lower NO₃^- concentration caused by reduction to nitrogen. In the first case, hydrogen consumption is limited by the NO₃^- release rate from the bitumen matrix. In the second case, however, under gamma radiation at a low dose rate, hydrogen production is weak, and the hydrogen is completely consumed by microorganisms. Knowledge about these hydrogen oxidizing bacteria is just beginning to emerge. Heterotrophic denitrifying bacteria adapt well to hydrogen metabolism (autotrophic metabolism) by oxidizing H₂ instead of hydrocarbons. (authors)

[en] This study focuses on the bituminous waste, a mixture of bitumen and insoluble salts, mainly of nitrate and sulphate carrying low and intermediate level radioactivity. The re-saturation of the disposal cells after closure will likely lead to the development of microbial activity. One of the most critical aspects limiting bacterial activity is the high alkalinity prevailing in the barrier concrete system and at the concrete-clay rock interface (pH = 11-13.5). The objective of this work is to evaluate the possible activity of nitrate reducing bacteria under these alkaline conditions, building on knowledge for alkaliphilic bacteria gained from similar conditions. Alkaliphilic bacteria develop optimally at pH values above 9, often between 10 and 12, in some cases above 12. They are likely to be predominant under alkaline conditions, but they will be replaced by neutrophilic bacteria as soon as local zones of near-neutral pH develop. In the case of a waste cell this could be at the interface to the clay rock or in the vicinity of organic matter releasing bituminous waste. Two sets of experiments have been carried out, bringing nitrate in contact with a solution containing organic matter (acetate) and a culture of alkaliphilic microbes, one focuses on the influence of nitrate concentrations, the other on the impact of changes in pH (10.0 and 12.4). The results indicate a low microbial activity at pH 12.4 whereas a significant nitrate reduction is obtained at pH 10.0. Furthermore, nitrite is the main product of nitrate reduction; traces of ammonium have also been measured. (authors)