Three-dimensional Model Tests And Discrete Element Simulations For The Effects Of Water Flow And Heat Transfer On The Temperature And Stress Of Sparsely Irregularly Fractured Rocks

Deep underground disposal is a feasible scheme for permanent disposal of high radioactive waste. In order to evaluate the safety of the high level radioactive waste repository, it is necessary to study the influence of the water flow and heat transfer on the temperature and stress. The main contents and results are as follows:(1) A meter-scale physical modelling test is conducted to study the effects of heat source temperature and water flow velocity on the temperature and stress of sparsely irregularly fractured rocks. The granite rocks are taken from the Beishan area in Gansu province, which is being investigated as a potential site for hosting the first high-nuclear waste repository in China. The sparsely irregularly fractured rock model of 750mm×300mm×1000mm (widthX thickness×height) is made. The test results reveal that: oblique fracture water flow mainly from the side inlet to outlet obliquely,and Vertical fracture water flow mainly from the top inlet to the bottom outlet vertically, with weak interchange flows at the intersections of the oblique and vertical fractures; Due to the gap between the heat source and rock, the natural convection occurs in the water of gap after the heater source heated,so showing the high temperature characteristics of upper rock; heat conduction and oblique fracture water flow control the temperature distributions in the model, while the vertical fracture water flow hinders heat conduction to the far side of the heat source; heat transfer paths are mainly from left to right in the upper rock layer and from top to bottom in the middle rock layer and the low rock layer, due to the locality of heat source location,the irregularities of heat conduction and water flow and heat transfer; the temperature gradients are smaller and thus contraction of rock produces tensile stress in the upper and lower rock layers, whereas the temperature gradients are larger and expansion of rock produces compressive stress in the middle rock layer; the higher the heat source temperature, the lower the fracture water flow velocity, the higher the rock temperature, the greater the rock stress, and the system requires longer time to reach steady state; the sands filled in the fractures enhance the heat transfer, with both the temperature and the thermal stress being larger than those cases with unfilled fractured rocks; there is no phase change of the water in the fractures,and the temperature and water flow velocity variations have little effect on the water pressure.(2) The 3D discrete element model of water flow and heat transfer effect on temperature and stress in the sparsely irregular fractured rock mass model is established, the temperature and stress of fractured rock mass under different schemes are compared, and the influence of fracture aperture and boundary conditions on the temperature of fractured rock mass is analyzed. discrete element simulation Indicate,From the transient state to steady state, the temperature gradient of the rock decreases gradually, and the discontinuity of the rock isotherme on both sides of the fracture increases gradually,and the influence of oblique fracture water flow on the heat transfer path of middle layer rock is more and more obvious, heat transfer paths are mainly from bottom to top in the upper rock layer and from left to right in the middle rock layer and the low rock layer; the model boundary gap effect on the temperature and heat transfer path of rock is more obvious; the upper, middle and lower layers of rock are subjected to thermal pressure stress, the stress increment increasing with the increases of temperature gradients in the oblique planar direction, and the major principal stress direction being approximately orthogonal to the intersections of the oblique fracture planes and the vertical fracture planes; since the current version of the 3DEC software does not consider water density change with temperature, the coupling of heat conduction with vertical fracture water flow occurs earlier than with the oblique fractures in the discrete element simulations, in which rock matrix heat conduction and oblique fracture water flow and vertical fracture water flow adjacent to the heat source play dominant roles on the temperature distribution; the lower the heat source temperature, the higher the fracture water flow velocity, the larger the fracture aperture,the lower the rock temperature, and the smaller the rock stress, and the system requires shorter time to reach steady state; the heat conduction of lower velocity fracture water flow is dominant, the convection heat transfer of high velocity fracture water flow is dominant, the junction of fractures exist local heat convection.(3) Considering the thermal power of the HLW glass solidified body after cooling 30a and fracture Water flow and heat transfer effect on near-field of high level radioactive waste repository,and the influence of the Water flow and heat transfer on the near field temperature of the repository is analyzed by 3DEC software. the farther fracture water flow distance canister, the lower fracture nearby rocks temperature,the weaker the coupling of rock heat transfer and fracture water flow,which on the isotherm of rock central horizontal section is not obvious and on the isotherm of rock central vertical section is obvious; due to the endothermic effect of fracture water flow,the temperatures of the bentonite at the canister surfaces obtained from using the fractured rock model are lower than those from the intact rock model, and the time to reach steady state for the former is shorter; fracture water flow close to canister, the temperatures of the bentonite at the canister surfaces in the upstream are significantly lower than the downstream region of the fractured rock model,while fracture water flow distance canister, the temperatures of the bentonite at the canister surfaces in the upstream are not significantly more obvious than the downstream region of the fractured rock model; under given conditions, the maximum temperature of the canisters surface bentonite for the fractured rock model is about 75% of that for the intact rock model, and when the fracture water velocity is increased from 0.2 mm/s to 0.5 mm/s,the peak temperature of the canisters surface bentonite is reduced by about 4%, and when the fracture aperture is increased from 0.5 mm to 0.7 mm, the peak temperature of the canisters surface bentonite is rised by about 3%.