Long-term in-situ experiment simulating disposal of low-level nuclear waste

An in situ Gas Generation Experiment was initiated at Olkiluoto, Finland, to simulate gas generation from low-level maintenance waste under underground repository conditions. Although the primary focus was on gas generation, the 26-year experiment also provided an opportunity to investigate carbon steel corrosion and concrete degradation upon its termination in 2023. Early heterogeneous conditions enabled the formation of microbial niches and the initiation of gas generation, despite the influence of initially high-pH cementitious water. Approximately 7% of the theoretical gas generation potential was realized over the 26-year duration of the experiment, and at the time of termination, gas production was continuing at a stabilized rate. Microbiological analyses confirmed the presence of microbial groups essential for the complete biodegradation of waste, including bacteria capable of decomposing and fermenting organic matter, as well as acetogens and methanogens. Methane and carbon dioxide were the main gases produced, with CO2 precipitating as CaCO3. Simulated disturbances, including sulphate addition and an increase in pH, had no long-term impact on gas generation. At termination, the exterior paint on the steel drums remained in surprisingly good condition, although it had become brittle and exhibited blistering. The drum interiors showed both uniform and pitting corrosion. The concrete retained its strength within the design limits, with carbonation attributed to pre-experiment storage conditions.