The biogeochemistry of gas generation from low-level nuclear waste: Modelling after 18 years study under in situ conditions

Gas generation from low- and intermediate-level nuclear waste (LILW) is an important process related to the safety of near surface and geological nuclear waste repositories. Generation of a gas phase has the potential to transport gaseous radionuclides such as 14C to the biosphere. Furthermore, pressurisation and gas phase formation can affect the engineered barrier system and water flow, which can affect the transport of radionuclides and other soluble contaminants in water. Microbial gas generation is coupled with a wider range of biogeochemical processes that are also relevant to the safety of LILW repositories through effects on pH, Eh and organic complexation. The large scale Gas Generation Experiment (GGE), located at the VLJ repository, Olkiluoto, Finland has been in operation for 18 years to study processes of gas generation from cellulose-containing low-level waste (LLW) from the Olkiluoto power plants. Here we provide an update of the experiment data set that records, over a period of 18 years, the neutralisation of the initial pH 10-11 conditions in the water filled regions of the experiment that were buffered by a concrete container, of relatively small mass, by acidity generated by degradation of cellulose LLW materials. The neutralisation of pH below pH 9 coincides with a doubling of the generation rate of a CH4-rich gas to around 1 m3 per year. Aqueous sulfide is also present at low levels (<3 × 10−6 M) at this time, consistent with equilibrium with mackinawite (FeS). Dissolved organic carbon attained a peak concentration of 8 mM during the early stages of the experiment, prior to the higher gas generation rate, but after 9 years declined. After 18 years the Eh of the tank water measured by a Pt electrode is consistent with that calculated for the S(-2)/S(6) and methanogenesis redox couples suggesting equilibration of redox processes. Carbonate concentration has increased steadily to around 15 mM. After 7 years CO2 gas and aqueous and solid carbonates are also close to equilibrium. The experimental data has been interpreted with the aid of a biogeochemical model that represents the coupled processes of organic waste degradation, pH buffering and microbial gas generation. The model simulates that the majority of the gas is generated by H2 and organic consuming methanogenic processes in the waste drums. The increase in CH4 generation is simulated to be the result of methanogenic processes that consume soluble cellulose degradation products that diffuse into the tank water from the waste drums. It is expected that under the steady state conditions now established, resulting from gas/liquid/solid equilibrium and diffusion processes, that gas generation should continue until the cellulose and steel materials are exhausted. The experimental data and biogeochemical model can be used to predict the rate of CH4-rich gas generation from the VLJ and other repositories with higher proportions of cementitious materials. In addition the study has shed further light on the role of microbial processes in affecting pH buffering of cementitious conditioned LILW.