Assessment of heat and radiation impacts on natural organic matter composition in bentonite for used nuclear fuel disposal

Abstract

Bentonite clay (Wyoming, USA) is proposed as a buffer layer for the engineered barrier system for the deep geologic repository to store Canada's used nuclear fuels in the long-term. The presence of natural organic matter (NOM) in the mined bentonite may support microbial activity which may promote the corrosion of used fuel containers. The NOM found in Wyoming-type bentonite is diagenetically altered, however, there is limited knowledge about how NOM chemistry may change with heat and radiation when exposed to the proposed deep geologic repository conditions. To investigate this further, molecular-level techniques were used to analyze NOM compositional differences in bentonite with different heat (30–105 °C for 3 days, and 90 °C for 14, 28 and 42 days) and gamma-radiation exposure (100 kGy for 3 days). Elemental analysis results indicated that all bentonite samples exhibited low total organic carbon concentrations (0.15–0.27%) and 1.6–3.3% of these were in the form of extractable dissolved organic carbon. Solution-state 1H nuclear magnetic resonance (NMR) and solid-state 13C NMR analyses revealed that chemical signatures of dissolved organic matter and overall NOM composition did not vary between samples. Slightly increased concentrations of short-chain (C9-C19) n-alkanoic acids were observed with heat (14, 28 and 42 days at 90 °C), which is likely due to the oxidation of long-chain (≥C20) n-alkanes or n-alkanols. However, the broader assessment of NOM composition via principal component analysis of forty-four targeted NOM compounds indicated no significant differences after heat or radiation exposure. Collectively, the molecular-level analyses indicate that heat and radiation only imparted nanogram-level differences in some targeted compound concentrations, but the overall NOM chemistry was not altered. These results expand the understanding of NOM chemistry in bentonite clay and suggest that under the conditions tested, NOM composition in bentonite was largely insensitive to heat and radiation exposure. Further work is warranted to investigate longer exposure durations and the overall stability of NOM in a deep geologic repository setting.