Probabilistic Analysis of Radionuclide Transport for Near-Surface Disposal Facilities in Spatially Varying Soils

Abstract

The long-term safety of radioactive waste disposal facilities is ensured by developing performance assessment models. The general requirement of these models is to assess the risk caused by failure of disposal systems that leads to radionuclide release into the geosphere and their migration toward the near-field biosphere. The radiation dose and risk due to disposal practice are the endpoints of assessment of the model. A two-dimensional radionuclide transport model with a decaying source is modeled numerically to compute the radiological impact caused by radionuclide, iodine-129 (I-129) in the biosphere. The intrinsic part of the performance assessment model requires consideration of uncertainties and variabilities in the system that are indicated by the inherent variability (heterogeneity in the geological medium), measurement, and modeling uncertainties (the geohydrological, geochemical properties of the radionuclides being released and transported). In this study, the spatial variability in the geological medium is addressed by treating the hydraulic conductivity of the medium as a random field. The probability of the radiation dose exceeding the design threshold (i.e., the probability of failure) is computed using subset simulation method. The influence of autocorrelation length and coefficient of variation of hydraulic conductivity on the probability of failure and the transport behavior of radionuclide are also studied. Thus, the results showed that a probabilistic framework that accounts for spatial variability in the geological medium is necessary for performance assessment of radioactive waste disposal facilities.