Numerical Simulation Of Migration And Decay Mechanism Of Low-and Medium Level Nuclear Waste Disposal In Abandoned Mines

With the development of economy,more and more nuclear energy is used as a clean energy in recent years,and its proportion in the energy consumption structure is getting higher and higher.Compared with traditional thermal power stations,nuclear power has many advantages,but it also brings many environmental problems,such as the storage of a large number of medium-and low-level radioactive solid wastes during the operation of power stations.The accumulation of more and more solid wastes is not conducive to the long-term operation of power stations.Therefore,it is an engineering idea with practical engineering significance to transform the abandoned mine into a medium-and low-level radioactive waste disposal repository.In this paper,we propose a conceptual disposal reservoir transformed from a abandoned coal mine.This research mainly aimed at investigating the permeability evolution process of the surrounding protective layer under multi-physical couplings during the storage of radioactive solid waste.The evolutions of permeability in backfilled bentonite during thermal-fluid-solid coupling process,the surrounding rock barrier protection are investigated through the conceptual model of disposal repository,and the accuracy for an equivalent unsaturated dual-permeability in surrounding rock was investigated in detail.The following research contents are included:(1)A conceptual model of medium-and low-level radioactive solid waste disposal repository was put forward through the transformation of abandoned coal mines.The displacements of rock mass on the bottom of the roadway before and after the excavation for radioactive waste storage caverns were analyzed by the elastoplastic deformation theory.The comparison of horizontal and vertical displacements of rock mass shows that the vertical displacement is negligible.An abandoned mine can be transformed into a low radioactive waste repository if the hydrological condition is feasible.(2)The multi-physical coupling equation and numerical model for a heat – fluid-solid coupling was established for the bentonite isolation layer.The evolution mechanism of soil permeability in the process of nuclear waste storage was investigated.The relationship between permeability and porosity established reveals the influence of the exothermic process of waste decay on the evolution of permeability.In addition,the influence of different initial saturation on the protection performance of bentonite is revealed.The higher the initial saturation is,the higher the accumulated pollutant concentration at the protection boundary of bentonite layer will be,and the faster the pollutant concentration diffusion will be.The accuracy of the model is verified by some physical field problems.Numerical results show that the temperature has an effect on the concentration diffusion in porous media,and the higher the temperature,the faster the concentration diffusion.(3)A dual permeability model is proposed to describe the dual seepage process in fractures and matrix in the surrounding rock for the disposal reservoir in the operation process.This study proposes to set parameters in the water-retention curve,the initial state of the equivalent crack area in the dual permeability model.The accuracy of this dual permeability model was investigated by comparing the boundary flow between the crack and the double-porosity medium in the unsaturated state.At the same time,the water conductivity coefficient of the interface between matrix and fracture in the unsaturated dual pore permeability model was compared and optimized for selection.After comparing three interface conductivity coefficients,a coefficient is identified for a better description of saturation variation in the early stage of the dual permeability model.