Several countries including Japan are planning to select a site for the high-level radioactive waste disposal by step by step approach. Among the site selection phases, the initial site investigation phase is considered to have greater uncertainty due to the limited amount of site investigations.
JNC held an international workshop on October 22-24,2003 to discuss the goal of modeling at this phase, the level of uncertainty and the investigation methods useful to reduce uncertainties. This paper presents a summary of discussion during the international workshop.
The goal of modeling at this phase differs between the crystalline rocks and sedimentary rocks, since the former tends to have greater degree of heterogeneities due to fractures/faults and require more boreholes and widely exposed outcrops which are generally not available, whereas the latter is more homogeneous and requires less understanding of geologic structures in various scales. The goal for modeling the crystalline rocks at this phase is to understand the site rather than to perform the rigorous safety assessment. The regional scale and the site scale are more reasonably modeled than the block scale at this phase, because the outcrops and boreholes required to model the block scale are generally limited. However, Sweden has relatively abundant outcrops and aims at the block scale, as well. On the other hand, the goal for modeling the sedimentary rocks could be safety assessment, since these rocks are relatively homogeneous and the models for all scales could be constructed with greater confidence.
Uncertainties can be divided into the expected uncertainties and the unexpected uncertainties. An example for the former is the fracture network in the block scale and an example for the latter is the compartments in the crystalline rocks. Reduction in uncertainties can be achieved not only by increased amount of investigations but also by demonstrating that the hypothesis is supported by the multiple-lines of evidence. In order to reduce the unexpected uncertainties,1) demonstrating the validity of the concept through iterations of prediction and confirmation,2) collecting baseline data as much as possible and carefully analyzing the trend of data, are also useful.
There is no single investigation method that can drastically reduce uncertainties and it is important to obtain the multiple-lines of evidence based on the various methods. However, examples of useful methods to reduce uncertainties are: flow logging and fluid logging to measure the transmissivity of the individual fractures, tracer experiments to derive flow porosity, and 3D seismic method to derive geologic structure in the sedimentary rocks.

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Studies on the groundwater flow in fractured rocks were reviewed, with a focus on breakthrough, mixing and dispersion properties of the tracer (or solute) in fractures. A few findings obtained from collective analyses of the published results are shown. Many previous papers using the tracer approach showed breakthrough curves with a rapid concentration peak followed by a tailing. Referring to the available test results, the present paper demonstrates that the peak arrival time and the recovery time (or the tailing duration) are highly correlated with R²=0.82. A comprehensive rearranging of much data on longitudinal dispersivity reported in the literatures revealed that there existed a significant positive correlation between the dispersivity and the testing scale with R²=0.76.

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Hydrogen Release Compound (HRC) is an important electron donor that is now becoming widely used in accelerating the biological reductive dechlorination of chlorinated aliphatic hydrocarbons (CAHs). HRC is a polylactate ester specially formulated for the slow time-release of lactic acid. Lactic acid is then metabolized while producing hydrogen, which can be used in the reductive dechlorination of CAHs. We investigated the effect of HRC on the generation and maintenance of reductive condition in river sand microcosms with the nonaddition or addition of the HRC-degrading bacteria. Oxidation-reduction potential (ORP) was used as an evaluation parameter for detecting the development of reductive condition following HRC addition to such microcosms. The results revealed that HRC, particularly purified HRC (polymer components of commercial HRC), excels in producing and maintaining a low ORP in the long term, which is the most desirable condition for the reductive dechlorination of CAHs.

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