In order to ensure that a repository for the geological disposal of HLW is isolated from the human environment, underground excavations, including pits and tunnels, must be properly sealed. Effective sealing requires that these excavations are backfilled, and that the Excavation Damage or Disturbed Zone (EDZ), which includes preferential flowpaths, must be intersected by sealing plugs. Methods for constructing a full-scale sealing plug and their influence on plug performance were studied and confirmed by the Tunnel Sealing Experiment (TSX). This experiment was carried out by an international partnership of the Japan Nuclear Cycle Development Institute (JNC) and Atomic Energy of Canada Limited (AECL). However certain specific roles of the sealing plugs at the scale of the whole repository were not studied. There remain issues to be clarified, notably the effectiveness of sealing plugs in a geological environment with heterogeneous characteristics and the resulting influences of the heterogeneities in performance assessment.
  Focusing on a geological environment with spatially heterogeneous characteristics, the authors have developed a method for designing the sealing plugs, based on groundwater simulations. The design method consists of: (1) defining sub-regions that are able to contain and/or retard radionuclides sufficiently to ensure the long-term safety of the surrounding disposal tunnels in the repository region, (2) assigning the sealing plugs to the role of repairing the damaged sub-regions that are caused by excavation, and (3) adoption of a variable disposal tunnel layout to reduce the number of sealing plugs that must be constructed, by using information obtained while excavating the disposal tunnels.

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  To establish sealing performance suitable for geological environmental conditions of Japan, it is necessary to obtain basic data related to the sealing functions through laboratory and in-situ experiments. These data are to be used for practical design of the sealing materials and to establish the method for analysis of the sealing performance.
  Japan Atomic Energy Agency (JAEA) had joined the international project, the Tunnel Sealing Experiment (TSX), to demonstrate the sealing performance of full scale plugs in-situ, and to develop analytical method of the sealing performance of the plugs at Underground Research Laboratory of the Atomic Energy of Canada Limited in Canada.
  This paper presents the experimental data on the sealing performance, as well as the analytical studies for clay plug performance based on the tracer test results. The results of the experiments and the analytical studies indicate that the low permeability of the clay plug was well performed. It was also shown that the boundary between the clay plug and rock could be a dominant mass transport pathway.

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  The current concept for the functioning of high-level radioactive waste repositories assumes that redox conditions will remain reducing even after repository closure. Oxidizing conditions may, however, develop in and around the waste cavern due to inorganic and/or organic redox reaction during the construction and the operational phase. Within the redox reaction, several kinds of oxides and hydroxides are inevitably formed and believed to have a strong influence on elemental migration and retardation through adsorption and/or co-precipitation in the near-filed (NF) environment, where their formation is certainly influenced by microbial activity. Such oxidized zones can be formed at the interface between the engineered barrier system and surrounding host rock and will affect the efficiency of the safety barrier after closure of the repository.
  The influence of the interaction between microbes, water and rock around waste caverns is therefore important in understanding long-term chemical stability, elemental migration, and hence the safety of proposed sites for deep geological isolation of radioactive waste. However, the complex reaction among groundwater, rock/minerals including synthetic materials and microbes under deep geological environments is not well understood and is thus not sufficiently taken into account in present safety case development. Here, we therefore try to describe the role of microbes in the repository system in our orogenic geological regime and deep geological environment for both cases of sedimentary and crystalline rocks. This kind of discussion can also be used for the planning of Underground Research laboratory's (URL) study relevant to microbial influence in NF environment for the development of realistic long-term safety of radioactive waste repositories in Japan.

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