Illustration of HLW Repository Performance: Using Alternative Yardsticks to Assess Modeled Radionuclide Fluxes
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Illustration of HLW Repository Performance: Using Alternative Yardsticks to Assess Modeled Radionuclide Fluxes Kaname Miyahara and Tomoko Kato Geological Isolation Research and Development Directorate, Japan Atomic Energy Agency (JAEA), 4-33 Muramatsu, Tokai, Ibaraki, 319-1194 Japan ABSTRACT Complementary indicators have been used in developing a safety case in order to avoid uncertainties in the biosphere modeling used to estimate conventional dose or risk. For example, radionuclide fluxes can be used to evaluate the effectiveness of barrier performance. However, it is difficult to define relevant yardsticks for comparison, because the fluxes of naturally occurring radionuclides due to geological processes vary considerably depending on time and location. This paper discusses the relevance of alternative yardsticks for assessing modeled radionuclide fluxes by selecting yardsticks calculated from fluxes of natural radionuclides at the groundwater discharge point from the geosphere to an aquifer; these are then compared with fluxes of repository-derived radionuclides at the same point. Such yardsticks avoid surface geological processes that may also contribute to natural fluxes, allowing comparison at a suitable, common evaluation point that avoids dependence on site-specific conditions. The effectiveness and robustness of barrier performance is demonstrated using the developed yardsticks and the sensitivity of the analysis to groundwater flux is illustrated. INTRODUCTION Complementary safety indicators, such as radionuclide concentration and flux in groundwaters, have been used in developing safety cases for deep geological disposal of radioactive waste based on multiple lines of reasoning [1-3]. These indicators can be used to avoid uncertainties in the biosphere modeling used to estimate conventional dose or risk, which inevitably arise from the assumptions made regarding future human behavior. These indicators can also demonstrate the effectiveness of barrier performance, which is useful for showing redundancy within the natural and engineered barrier system, i.e. that safety is not dependent on the performance of a single component of the disposal system. Such presentations can provide a more transparent means of communicating a safety concept, particularly to non-technical stakeholders, even though alone they are inadequate indicators of total system safety. For example, the effectiveness of barrier performance can be evaluated based simply on calculated fluxes of radionuclides to the accessible environment, if relevant yardsticks for comparison can be defined. Natural radionuclide fluxes (natural fluxes) due to geological processes (e.g. erosion and river flow) can potentially be used for this purpose [4,5]. However, these fluxes vary considerably depending on location and time. The question is, therefore, whether it is possible to define alternative yardsticks that may be more appropriate to a generic model. This paper discusses relevant yardsticks based on natural fluxes for illustrating the effectiveness an
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