Uncertainty quantification of radionuclide migration in fractured granite

Abstract

Deep geological disposal is a widely accepted approach for safe management and long-term disposal of high-level radioactive waste (HLW). However, high uncertainty associated with subsurface properties of fractured rocks is a significant obstacle to practical safety assessment of HLW disposal. In this study, we develop an integrated statistical framework for uncertainty quantification of radionuclide migration related to the geological disposal of HLW. We employ a response surface methodology integrated with Monte Carlo simulations of radionuclide migration in fractured granite to perform global sensitivity and statistical analysis by coupling the uncertainty quantification tool, PSUADE, and radionuclide migration simulator, FRACPIPE. FRACPIPE is a semi-analytical simulator that models solute transport in fractures and matrix slabs with an arbitrary-length decay chain and an arbitrary time-varying influent concentration history by considering a variety of transport mechanisms. The statistical risk metrics include nuclide breakthrough (BT) time, total release dose, single nuclide release dose, and single nuclide flux. The global sensitivity analysis identifies fracture aperture, matrix diffusion coefficient, hydraulic gradient, and dispersivity as the most sensitive parameters. Considering the uncertainty ranges for independent variables (e.g., dispersivity, hydraulic gradient, fracture spacing, fracture aperture, sorption distribution coefficient, matrix diffusion, and matrix porosity), we apply post-processing results of Monte Carlo simulations to conduct statistical analysis of the risk metrics. Under this studied condition, the average BT time for radionuclides at 1000 m is between 85 and 331 thousand years. This study provides valuable insights into the impact of input parameter (independent variables) uncertainty and sensitivity on radionuclide migration behavior in an HLW repository.