Understanding results of a groundwater transport model for a surface radioactive waste disposal through sensitivity assessment

Detailed analysis of the radiological impact on a sandy aquifer is performed for the near surface repository for low- and intermediate-level short-lived waste, developed by ONDRAF/NIRAS at Dessel, Belgium. This analysis relies on simulated radionuclide concentrations by MT3DMS in a hypothetical well located within the predicted plume. The transport model advection term was based on a steady-state flow model and the present climate. Model assumptions were carefully examined and alternative choices analyzed to assess the model conservativeness. The impact of groundwater flow model assumptions on the predicted concentrations was tested using alternative models (transient and alternative climate model). The impact of transport model assumptions on the results was assessed for the spatial discretization, negligence of the dispersion term, well conceptualization, source term variability, and amount of recharge through the repository. The results of the sensitivity analysis improve understanding of the system behavior and related uncertainties and are summarized in the present paper.