Risk-based Assessment of Salt Domes as Disposal Sites for Nuclear Waste: Uncertainty of Groundwater Age in the Salt Dome Problem

Long-term safety has to be assured for the disposal of high-level nuclear waste to prevent contamination of the biosphere. A numerical framework for the probabilistic assessment of hazardous events regarding nuclear waste disposal in salt domes is developed here. The goal is to numerically simulate transport times and mass fluxes of radionuclides into the biosphere, and to couple this with a probabilistic framework for reliability assessment. This model can deal with uncertainties such as the impact of external events (major climate changes, human activities, earthquakes) on subsurface structure, material properties and boundary conditions. To quantify the impact of external events, a numerical model of the far field of a salt dome disposal site is generated. It includes the simulation of density-driven (thermohaline) flow, heat transport, transport of dissolved salt and a radionuclide in discretely-fractured porous media using the FE-code HEATFLOW-SMOKER Version V3/TC2.

As a first step in quantifying the effects of uncertain parameters in the context of nuclear waste disposal in salt domes, the salt dome problem (HYDROCOIN level 1 case 5) is further investigated in terms of groundwater age. Groundwater age is one of the exclusion criteria in the site selection process for nuclear waste deposits and therefore of great importance. Groundwater age is here calculated as a transport problem using steady-state flow velocities. This problem is density-driven due to transport of salt into the model domain and the steady-state solution is highly dependent on dispersion.  A sensitivity analysis is carried out to quantify the effect of uncertain dispersion on flow and salt distribution and resulting groundwater age. Preliminary simulation results demonstrate that both salt distribution and groundwater age in steady-state are significantly affected by longitudinal and transversal dispersivity. Resulting flow velocities are higher with increased dispersion and therefore lead to a decreased maximum groundwater age in the model domain.