Model Experiment And Calculation Theory Of Water Flow And Heat Transfer In Fractured Rocks

For safe disposal of high level radioactive nuclear waste, the only currently suitable approach is underground disposal facilities. Better understandly of water flow and heat transfer in fractured rocks is needed for evaluating the safety of the nuclear waste repositories. The main contents and results of this thesis are follows:(1) Physical modelling experiments were conducted to study water flow and heat transfer in fractured rocks. Granite rocks were taken from the Beishan area in Gansu province, which is being investigated as a potential site for the high-nuclear waste repository in China, to construct a fractured rock model of height-1502.5mm*width-904mm*thickness-300mm, consists of nine rock blocks with two vertical and two horizontal fractures. The experimental results revealed, water flow and heat transfer in fractures can significantly change the temperature field of the model, for higher heat source temperatures, lower water velocities and sands filled in the fractures the range of influence of the heat source to rocks matris beyond fractures is larger, and the time for asymptotically approaching steady-state is longer; the vertical fracture adjacent to the heat source controled the temperature distribution in the model while the other fractures mainly influenced the boundary temperature and the time for approaching steady-state; there was no phase change of the water in the fractures, and the temperature variations had little effect on the water pressure.(2) Influence of thermal dynamic equilibrium assumptions for water and filled particles fractures as well as for rock face and fracture water was analyzied. The time to reach thermal equilibrium in the fractures is controlled by the ratio of convective heat transfer coefficient to particle diameter, longer time is needed for larger ratios; the temperature of water in the fractures calculated from using thermal equilibrium assumption is larger than that calculated by considering transient heat convection; and the discrepancy from using thermal equilibrium assumption is insignificant, for larger ratios of water travel distance to water flow velocity, for smaller ratios of the product of volume specific heat and thermal conductivity of rock matrix to convective heat transfer coefficient and to aperture of the fracture.(3) Finite difference equations of water flow and heat transfer in reqularly fractured rocks were developed and with the computer code written and TOUGH2code simulations of the experiments and the paramete sensitivity analyses were conducted. There was a good agreement between the experiment data, and the numerical calculations; parametric sensitivity analyses indicated that the temperature distribution was highly sensitive to the heat source temperature, fracture aperture, water flow velocity and the volume specific heat of the rock matrix, and the water temperature in the vertical fracture adjacent to the heat source was not significantly affected by the other fractures.(4) Considering the thermal output of the nuclear waste canisters as a function of time, thermal parameters of rock and water as functions of temperature, volume specific heat of the fill materials as a function of saturation, the characteristics of water flow and heat transfer in fractured rocks in the near-field of the repositories were analyzed by using the developed finite difference code. Water flow enhances heat transfer along the flow, changes the temperature field of the rock matrix, and thus increases the range of influence of the repositories, and the peak value of the canister temperature becomes lower if the water flow velocity and the aperture of the vertical fracture are increased and the distance between the vertical fracture and the canister is decreased; water flow into the repository from horizontal fracture slows the canister heating, but has little effect on the peak value of the temperature.