Gas Generation From Organic Waste Over a Seven-Year Period: Implication for the Management of Low- and Intermediate-Level Radioactive Waste

Abstract of the technical paper presented at:
Microbes in Nuclear Waste Disposal (MINWD) Lausanne, Switzerland
May 7–9, 2018
Prepared by: Richard Goulet
Canadian Nuclear Safety Commission

Abstract:

Deep geological repositories have been adopted in many countries for the permanent disposal of low- and intermediate-level radioactive waste (LILW). Two key areas of attention in assessing long residence times of LILW in geologic facilities are: how gas pressure affects re-saturation with meteoric water of underground cavities and how these two forces affect the transport of aqueous radionuclides by groundwater and gaseous radionuclide migration through rock fractures or shaft seals; and how microbial processes affect both the waste and the speciation and transport of radionuclides present in the waste. Over long time scales, in situ anaerobic biodegradation of the non-radiological components of the waste is expected to produce:

• Gas and volatile compounds that could result in pressure build-up reducing the re-saturation rate of an underground cavity and delaying migration of soluble radionuclides
• Acids that can affect the initial integrity of both the host bedrock and the shaft seals that isolate and contain the radioactive waste.

Hydrogen, carbon dioxide, methane and other volatile compounds are the gases expected to be generated by the biodegradable organic components of the waste.

We monitored the gas pressure evolution, headspace gas composition, and microbiology of candidate organic waste spanning a seven-year period. The gas pressure evolution and changes in gas composition are interpreted according to the fungal, bacterial and archaeal composition of the candidate waste and in terms of the functional genes for methane and acetate formation, which are both processes that consume hydrogen and carbon dioxide, thus reducing pressure build-up in an underground cavity. In our experiment, methane formation appeared low while acetate formation was prevalent. In such a scenario, acetogens could produce acidity and locally impact barriers, a factor which may need to be considered in the design of shaft seals. Long-term safety cases of repositories typically have sufficient margins of safety to account for the presence of acetogens. However, including acetogenesis in safety assessments of repositories will reduce uncertainty.

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