Abstract
In a geological disposal system for high-level radioactive waste (HLW), maintenance of a reducing geological environment is considered to be important for long-term safety. However, formation of a redox front in the near-field of a HLW repository can be accompanied by oxidation of artificial barriers and basement rocks during the operational phase. In order to provide an analogue for secondary elemental migration associated with the formation of such a redox front, we examined granitic rock with a redox front formed along a fracture.
Geochemical analyses revealed that Fe concentrations in the oxidized zone are higher than those of the parent rock, and that secondary Fe accumulation occurred. Secondary accumulation of Mn was also identified in the oxidized zone, which has about 5 times the Fe concentration of the parent rock. These results show that these elements were firstly carried by reduced groundwater, and then oxidized and precipitated after oxygen diffusion into the matrix of the host rock.
It is also found that heavy metals such as Pb, Ba and Rb are concentrated in the oxidized zone, and that the concentrations of Pb and Ba are also increased by about 2 times compared with those of the parent rock. The concentration of these heavy metals can be considered to occur due to adsorption on Mn precipitates, and this suggests that Mn precipitates are capable of retaining heavy metals within the oxidized zone. Additionally, it has also been revealed that REE and U have accumulated in parts of the redox front, and are considered to be adsorbed onto Fe precipitates.
These results of an investigation into a redox front, show that secondary migration and accumulation of heavy metals was accompanied by redox front migration. Coupled phenomenon like these may be analogous to processes that might occur in the near-field of a HLW repository.
