Penalising groundwater scenario: Detailed calculation report

Penalising scenarios are typically developed to assess the performance of the disposal system when the uncertainties in the properties of the components of the system are too great to provide confidence in safety function performance. Penalising scenarios are applied within the safety analysis, to assess the impact of the residual radiotoxicity in the disposal system over timeframes of several thousands of years. This report details the conceptual model for the penalising groundwater scenario (leaching) with all major assumptions, chosen geometry, initial and boundary conditions, included processes, governing equations, material parameters and computation of effective dose rates. A base case and a number of alternative assessment cases have been described. These cases are intended to explore the sensitivity of the model response (effective dose rates) to various parameters and scenarios. This includes varying crack/matrix hydraulic conductivity ratio, crack density, spatial heterogeneity of waste distribution and initial decay times. Pressure field governs the radionuclide transport pathway. This largely depends on the equilibrium crack/matrix hydraulic conductivity ratio. To set this ratio, a phenomenological based approach is considered, which is discussed in Appendix C. A particular section is devoted to the discussion of radionuclide transport behaviour through the matrix-crack system. Results in terms of annual dose curves and maximum annual dose are presented for the base case and sensitivity cases, and compared to the 3 mSv/a dose reference value set for this type of scenarios. Indeed the results will differ for each of the cases. However, in terms of sensitivity, it is found that the initial decay time is the most sensitive parameter. This is followed by the sensitivity of spatial heterogeneity, crack density and crack/matrix hydraulic conductivity ratio. Equivalent doses to the skin and the lens of the eye are also quantified. Their compliance with the legal limits for members of the public has been demonstrated.