發展數值模擬工具探討膠體加速放射性核種衰變鏈多成員核種遷移與放射性廢棄物地質處置安全評估應用

The disposal in a deep crystalline rock formation is considered for the long term isolation of high level waste (HLW) from biosphere due to its geological stability and low hydraulic conductivity and high capacity for sorbing radionuclides. The most likely pathway for radionuclides coming back into the biosphere is the release of the waste and subsequent being transported by groundwater flow through the fracture present in rocks surrounding the HLW repository. Radionuclide migration through a fracture-matrix system is thus an important issue for HLW safety assessment carried out during site selection, site characterization, operation and closure. Presence of colloids in the fracture groundwater is derived from degradation of near field materials or natural colloids transported into the repository by groundwater flow. Considerable laboratory and field studies have observed that colloids increase the migration of radionuclides in groundwater owing to their potential for transporting sorbed radionuclides. This project is thus designed to develop a novel numerical model for predicting the coupled transport of the colloid and multiple members of a radionuclide decay chain in a fracture-matrix system. The transport processes for the colloid include the advection-dispersion in fracture groundwater and sorption onto the fracture surface. The transport of multiple members of the decay chain consist of advection-dispersion, radioactive decay/ingrowth in facture groundwater and sorption directly onto the fracture surface, onto the mobile colloids in the fracture groundwater, onto the immobile colloids on the fracture face as well as the diffusion into groundwater of the host rock, radioactive decay/ingrowth and sorption onto solids of the host rock. A hybrid analytical-numerical technique is applied to solve the a system that couples partial differential equations, ordinary differential equations and algebraic equations. The newly developed numerical model will be subsequently applied to understand how various processes or mechanisms affect colloid-facilitated transport of multiple members of radionuclide decay chain in a fractured-matrix system. Ultimately, the developed numerical model is applied to carry out a safety assessment of a HLW deep geological disposal considering the presence of colloids.