Geogas in crystalline bedrock and its potential significance for disposal of nuclear waste

In assessments of the safety of final repositories for nuclear waste situated in crystalline basement rock it is usually postulated that the transfer of radionuclides to the biosphere can take place only through transport by water. However, in order for such an assumption to be valid, it must be verified that any geogas that is present will not affect the transport. Geogas in crystalline rock consists of species such as nitrogen, argon, helium, hydrogen, methane, carbon monoxide, carbon dioxide, hydrogen sulfide, and oxygen. The gas originates from the atmosphere, chemical reactions in the rock, te decay of radioactive elements in the rock, and degassing from the mantle of the earth. In most observed cases, geogas is dissolved in the groundwater. The transfer of geogas through the rock and to the surface takes place through flow in fractures. Firstly, dissolved geogas migrated due to the flow of the groundwater, and secondly, pockets of gas may form and eventually be released in the form of bursts. In the second case, the gas might act as a carrier for heavy elements through four different mechanisms: (1) formation of volatile compounds, (2) formation of surface active complexes, (3) flotation, and (4) formation of aerosols. When a potential site for waste disposal is being evaluated, studies of geogas should form part of such a characterization program. Favorable conditions for the formation of free gas may develop as a result of the heating of the rock by radioactive decay in the waste. It is also conceivable that methane-ice might form in the backfill of a repository in connection with a glaciation. The decomposition behavior of such methane-ice appears to be largely unknown. Positive aspects may include the possibility of utilizing geogas flow for the non-destructive monitoring of a site after closure of the repository