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Report No.
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Model development to evaluate evolution of redox conditions in the near field

Sasamoto, Hiroshi   ; Yui, Mikazu; Chiba, Tamotsu*; Miki, Takahito*; Inagaki, Manabu*

Deep underground is thought to be a potential place for high level radioactive waste repository. It is believed that the chemical condition of deep groundwater is generally anoxic and reducing. However, during construction and operation phase of repository, oxygen will diffuse some distance into the surrounding rock mass, and diffused oxygen may remain in the surrounding rock mass even after repository closure. In such a case, the transitional redox condition around the drift is not preferable in view point of safety assessment for HLM disposal. Hence, it is very important to evaluate evolution of redox conditions in the near field. This report describes the status of model development to evaluate evolution of redox conditions in the near field. We use the commercial solver to equate the mathematical equations which mean evolution of redox condition in the near field. The target area modeled in this report are near field rock mass and engineered barrier (buffer). In case of near field rock mass, we consider the following two geological media : (1) porous media for sedimentary rock, (2)fractured media for crystalline rock. In case of the engineered barrier, we the regard buffer as porous media. We simulate the behavior of dissolved oxygen and Fe$$^{2+}$$ in groundwater during evolution of redox condition in the near field rock mass and the buffer. In case of the porous media, we consider diffusion of chemical species as dominant transport mechanism. On the other hand, in case of the fractured media, we consider diffusion of chemical species in rock matrix and advection of that (only dissolved oxygen considered in this model) in fracture as transport mechanism. We also use the rate raw of iron oxidation reaction and dissolution of Fe-bearing minerals in this model besides.

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