Development of hydrogen oxidizing bacteria using hydrogen from radiolysis or metal corrosion

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)