Derivation of an approximate analytical solution for understanding the response characteristics of the EBS
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Derivation of an approximate analytical solution for understanding the response characteristics of the EBS T. Ohi1, T. Chiba2, T. Nakagawa2, T. Takase3, T. Nakazawa3, Y. Akagi3 and K. Idemitsu4 1
Nuclear Waste Management Organization of Japan (NUMO), 1-23, Shiba 4-chome, Minato-ku, Tokyo, 108-0014 Japan 2 JGC Corporation, 2-3-1 Minatomirai, Nishi-ku, Yokohama-shi, Kanagawa, 220-6001 Japan 3 Mitsubishi Materials Corporation, 1-297 Kitabukuro-cho, Omiya-ku, Saitama-shi, Saitama, 330-8508 Japan 4 Kyushu University, 744 Motooka, Nishi-ku, Fukuoka-shi, Fukuoka, 819-0395 Japan ABSTRACT To perform a safety assessment for the geological disposal of radioactive waste, it is important to understand the response characteristics of the disposal system. In this study, approximate analytical solutions for steady-state nuclide releases from the engineered barrier system (EBS) of a repository were derived for an orthogonal one-dimensional diffusion model. In these approximate analytical solutions, inventory depletion, decay during migration and the influence of groundwater flow in the excavation damaged zone (EDZ) were considered. These solutions were simplified by the Taylor theorem in order to clearly represent the response characteristics of the EBS. The validity of these solutions was shown by comparison with numerical solutions. The response characteristics of the EBS are useful for identifying target values for important parameters that would have the effect of improving the robustness of system safety. The robustness of the geological disposal system and the reliability of the safety assessment can thus potentially be improved using the approximate analytical solutions. INTRODUCTION In order to improve the robustness of the geological disposal system and the reliability of the safety assessment, it is important to understand the response characteristics of the system and to assess the robustness of the system. Up till now, the importance of understanding such characteristics has been highlighted [1, 2]. The migration behavior of nuclides is described by mass transport equations based on advection and diffusion (dispersion) and, to date, a large number of analytical solutions for various conditions have been derived [3, 4, 5, 6]. The analytical solution is a useful tool because the response characteristics of the system can be quantitatively and qualitatively understood by a simple equation. However, it is not always easy – or even possible - to derive analytical solutions for all relevant conditions of interest. Therefore, in most recent safety assessments [7, 8, 9, 10], numerical solutions have been used for the assessment. In this case, the response characteristics of the system result from sensitivity analysis based on a large number of calculations, which can be managed easily for most conditions using modern computers. However, it is not always easy to understand the response characteristics quantitatively and qualitatively because there is no simple equation
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describing the system characteristics. Thus, there ar
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