Disruptive Effects on a HLW Repository ue to Uplift-Erosion in the Distant Future

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.

Consequence Analysis of Fluvial Erosion Scenarios for an HLW Repository

“…We assume the average concentration of radionuclides in the volume of the repository as in the previous study, 2) where all radionuclides are assumed to be retained in the near field, treated as simplistically averaged over a thickness of 3 m as the vicinity of the EBS spread over the repository footprint of 2 Â 2 km 2 for 40,000 vitrified waste packages. 6) This is rather unlikely as the consequences of prior dispersion, especially when the repository enters oxidising/higher-flow zones near the surface, would probably dilute radionuclide concentrations through a larger volume of rock.…”

“…In a previous study, 2) a simple linear uplift process was combined with a more detailed assessment of fluvial erosion, which was identified as the key erosion process in Japan. This indicated the robustness of the reference HLW disposal system: the consequences of erosion of the repository being small when compared with the yardstick provided by natural radionuclide fluxes.…”

Key natural analogue input required to build a safety case for direct disposal of spent nuclear fuel in Japan

“…Although there is certainly a degree of measurement uncertainty associated with the error values quoted (both in the corrosion depth and the time over which corrosion occurred), it is also reasonable to assume that such ranges also represent true variability-as would be expected in terms of both the composition of the metal involved and the environment in which corrosion occurred. For the case of steel OPs or emplacement casks, compositional variation would be relatively small, but the variation in the chemical environment may be significant in a repository which could spread over an area of several km 2 and which will vary with time due to processes like uplift/erosion and sea level change (Miyahara et al 2009). …”

“…It is important to note, however, that the lifetimes for &50 % of the cases would be [1 Ma, which is about the expected timescale of exposure of a repository in Japan due to uplift and erosion (JNC 2000;Miyahara et al 2009). It would be anticipated that the corrosion rate would increase significantly as erosion brings a repository closer to the surface, but it might be worth considering if better performance-at least in terms of the IRF-might be obtained with a thinner OP that would ensure a spread of failures while the repository is still well below the surface (e.g.…”