The influence of chemical conditions on gas generation in the disposal of low level maintenance waste (KaMu)

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Abstract

In Finland, low-level maintenance waste (LLW) originated in nuclear power plants is disposed of in the repositories situated ca.100 meters below the sea-level. A large-scale in situ Gas Generation Experiment (GGE) was established in 1997 in Olkiluoto, Finland, to simulate the gas generation from LLW under geological repository conditions. The gas generation started already during the first year of the GGE as a result of microbial degradation of waste and steel corrosion. The experiment is operated by Teollisuuden Voima Oyj (TVO) and it has been regularly analysed for chemistry, gas generation and composition of gas. VTT has been responsible for microbiological analyses of the GGE.
The aim of KaMu project was to induced disturbances to the GGE and study the influences on gas generation, gas composition, chemical parameters, and microbiology. The disturbances were induced by adding sulphate and raising the pH. The addition of sulphate simulates sulphate-rich flow of groundwater to the repository and higher pH value the influence of concrete in the repository. The results obtained in this project can be used for safety assessments and for modelling of gas generation.
After sulphate addition, quantitative PCR analysis showed a moderate increase in the number of sulphate reducers and a decrease in the number of methanogens. The diversity of methane-producing microbes stayed similar during the disturbance but certain sulphate reducing bacteria emerged to the microbial population in the tank water. Despite of these microbial changes the sulphate reduction was not enhanced significantly and no changes in gas generation or gas composition were detected. Similarly, the increase of tank water pH from 6.7 to 8.1 did not influence the rate of gas generation. However, some changes in gas composition were detected. CO2 concentration in the released gas was reduced because CO2 was partly absorbed into the more alkaline tank water. In addition, soluble iron concentration in the tank water was reduced indicating precipitation of iron.
Original languageEnglish
PublisherVTT Technical Research Centre of Finland
Number of pages23
Publication statusPublished - 25 Mar 2022
MoE publication typeD4 Published development or research report or study

Publication series

SeriesVTT Research Report
NumberVTT-R-00151-22

Keywords

  • biodegradation
  • low level radioactive waste
  • gas generation
  • methanogen
  • sulphate

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