Disruptive Effects on a HLW Repository Due to Uplift-Erosion in the Distant Future
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Disruptive Effects on a HLW Repository Due to Uplift-Erosion in the Distant Future Kaname Miyahara1, Manabu Inagaki1, Makoto Kawamura1, Takanori Ebina2 and Ian G. McKinley3 1
Geological Isolation Research and Development Directorate, Japan Atomic Energy Agency (JAEA), 4-33 Muramatsu, Tokai, Ibaraki, 319-1194 Japan 2 NESI 4-33 Muramatsu, Tokai, Ibaraki, 319-1194 Japan 3 McKinley Consulting Tafernstrasse 11, 5405 Baden/Datwil, Switzerland ABSTRACT In Japan, uplift/erosion scenarios must be analysed even if they occur far in the future, as no assessment cut-off times have yet been defined. For this purpose, an argumentation method is developed to allow sensible scenarios to be constructed. The consequences of erosion of the repository may be better estimated in terms of radionuclide fluxes and these compared with those of naturally occurring radionuclides. This paper discusses procedures to derive relevant conceptual models and resultant analyses in a credible manner, which illustrates the effectiveness and robustness of the HLW disposal system. INTRODUCTION Uplift and erosion cannot be precluded in Japan, therefore these processes need to be treated as a variant of normal “groundwater” scenarios considered in repository post-closure safety assessment. Obviously, uplift and erosion will cause major disruption of the engineered and natural barriers when the repository nears, and is eventually exposed at, the ground surface. However, erosion of the repository should be excluded for very long times by careful siting and design (especially selection of emplacement depth). In Japan, uplift/erosion scenarios must be analysed in a quantitative manner, even if they occur far in the future, as no assessment cut-off times have yet been defined. For this purpose, an argumentation method is being developed to allow sensible scenarios to be constructed - as opposed to the unreasonably pessimistic cases previously considered, which may be summarised as “the repository is exposed at the ground surface at a fixed time after repository closure”. When the repository eventually nears the surface, doses could be estimated by assuming, for example, that all the radionuclides released by erosion enter river water. Under the unknown conditions of the distant future, however, the application of the Reference Biosphere approach for estimating doses is difficult to justify. The consequences of erosion of the repository may thus be better quantified in terms of radionuclide fluxes, with these compared with those of naturally occurring radionuclides. The question is then whether it is possible to develop an argumentation method to construct sensible scenarios for uplift and erosion, and to define alternative yardsticks that may be appropriate for such a generic model. The objectives of this paper are as follows: to illustrate development of a method to construct reasonable uplift/erosion scenarios, based on system understanding formalised within conceptual models. to illustrate the impact of disruptive uplift/erosion occurring far in the fut
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