Numerical Analysis Of Coupling Between Heat Transfer And Seepage In Fracture Rock Near Field Of Nuclear Waste Repository

ABSTRACT:For analysis of theory and technical feasibility of the underground disposal of high level radioactive waste repository preselected site-Gansu Beishan area, project group use granite stones in Beishan area to make form fracture rock for experimental model, set the heat flow and water flow boundary condition, simulate the coupling effect between heat flow and water flow. As early theory analysis for the indoor model experiment, this paper summarized the previous experience and carried out the coupling between seepage and heat transfer of single fracture rock; simulated the experimental model with TOUGH2 calculating software, pre-processing software WINgridder and post-processing software MATLAB; Analyzed the parameters of coupling between seepage and heat transfer; simulated the underground disposal of high level radioactive waste preselected site-Beishan area.Conclusions of calculation suggested that:convective heat under seepage has significant effect on heat transfer. When water encountered with heat transfer in fracture, it took a lot of heat away. The temperature of the rock dropped sharply and the heat gradient reduced. The water in vertical fissure was also being heated when it flowed, and its role of heat transfer strengthened. So the heat in the lower part of model transferred more quickly, leading to the temperature curves upward, making the effect of bypassing flow. Temperature rise was accompanied by volume expansion and phase transition, volume expansion of water in fracture. When temperature of the water approaching or exceeding 100℃, some water vapored and pore water pressure increased significantly. Therefore part of rock mass stress increased, and this was not conducive for the stability of rock.Fracture aperture, heating power, the velocity of water in fracture and the time of heating were important factor in the coupling of heat transfer and seepage. In the beginning of heating time, heat transfer had not met with the fracture water seepage, the temperature curve was arc-shaped, and in the later heating time, thermal conductivity and fracture water flow coupled with each other, the temperature curve was nearly straight. The increasing of the heating power would not only lead to the rising temperature of whole rock mass, but also increased the heat gradient, raised the temperature extreme point. In the case of constant water flow, changes in fracture aperture had very big impact on the thermal conduction. The increase of fracture aperture, in fact, increased the flow cross-section and so that lower the fracture flow velocity, so that temperature of the rock would increase a little; while flow velocity is constant, change of the fracture aperture have a significant impact on the heat transfer. The increased fracture aperture significantly increased flow volume, took away more heat, reduced the temperature of rock; in case of increasing volume of it, the facture water would take away more heat, the temperature of rock would reduced. But this case would only take place along the direction of the heat transfer.The model which simulated nuclear waste disposal repository in Gansu Beishan with related materials suggests that:short term storage of nuclear waste has limited impact on the environment, especially on groundwater because of the limited releasing heat by nuclear waste; but in a long period time, the impact would be stronger with the accumulation of heat caused by occlusive environment. And the high temperature surrounding nuclear waste repository ranges in the radius of 50m. the exiting of water makes the heat transfer along with the direction of the fracture water seepage, and the exiting of little fracture water in granite rock had limited effect on temperature caused by nuclear waste repository. The surrounding environment temperature is closed related with the decay of the nuclear waste, the temperature changes similar to the change of heat released by nuclear waste decay. But the changes of temperature delay the changes of heat released by nuclear waste decay.