Simulation And Uncertainty Analysis Of Multi-component Radionuclide Transport In Natural Barriers

Deep geological disposal is a widely accepted method for the safety management and long-term disposal of high-level radioactive waste(HLW).The natural barriers serve as the last line of defense to isolate HLW from the biosphere,which is crucial for the safety of long-term disposal of HLW.When the engineered barrier fails,the natural barriers can effectively block the further transport of radionuclide into the biosphere.In order to ensure the safe operation of HLW geological disposal,it is particularly important to accurately analyze the transport characteristics and patterns of radionuclides in natural barriers,and predict the radionuclide transport process and spatiotemporal distribution.This study aims to meet the practical needs of safety assessment for high-level radioactive waste geological disposal.By combining the advantages of parameter upscaling in predicting the variability of parameters in complex heterogeneous systems,multicomponent radionuclide transport simulation and uncertainty analysis were carried out.Specifically,mathematical models and semi-analytical/numerical models for radionuclide transport in "natural barriers"(including equivalent porous media and fractured media),as well as spatial variability of transport parameters and algorithms for quantifying parameter uncertainty were developed.Based on model construction and theoretical research,an applied study in a site in northwest China was conducted to analyze the spatiotemporal distribution characteristics of radionuclide transport,identify the main influencing factors of transport mechanisms,and quantify the sensitivity and uncertainty of transport parameters.The main achievements and insights of this study are as follows:1.This paper studied the transport mechanism of multicomponent radionuclides in natural barriers,including convection,dispersion,molecular diffusion,matrix diffusion,sorption,and decay.A high-efficiency radionuclide transport simulation model was constructed,and radionuclide transport algorithm and program were developed.2.This article studied the spatial variability of transport parameters and established upscaling models for fracture aperture,solute displacement variance,dispersivity,and effective retardation factor.The effectiveness of the upscaling models was validated using the random walk particle tracking model and solute transport experiment to accurately predict transport parameters at different scales.The upscaling model indicated that the effective fracture aperture is the geometric mean of the fracture aperture,while the solute displacement variance and dispersivity increase with the increase in the variance and integral scale of the log fracture aperture.The effective retardation factor converged to the arithmetic mean of the retardation factor with an increase in time.The upscaling model and geological statistical parameters of finite laboratory cores were used to predict the dispersivity of transport parameters at the site.3.The developed radionuclide transport model and upscaling model were applied to simulate radionuclide transport in the study area to analyze the spatiotemporal distribution patterns of radionuclide and their influencing factors.The analysis results showed that an increase in fracture aperture and dispersivity would reduce breakthrough time,while an increase in matrix diffusion coefficient and retardation factor would increase breakthrough time.Changes in fracture aperture and retardation factor would affect the peak value of the breakthrough curve.4.Combining the on-site radionuclide transport model,a Monte Carlo uncertainty assessment algorithm based on response surface analysis was constructed to quantify the uncertainty of parameters in the on-site radio nuclide transport model.Global sensitivity analysis was performed to analyze the sensitivity of parameters,and the results showed that breakthrough time,individual nuclear species release dose,and total release dose were most sensitive to fracture aperture and hydraulic gradient.Using response surface analysis,the impact of transport parameters on radionuclide transport results was quantified,revealing the range and trends of the impact of transport parameters on the results.The research results of the paper expanded the theoretical foundation and technical application of predicting the multicomponent radionuclide transport in natural barriers,providing new ideas and methods for the safety assessment of deep geological disposal of HLW in China.