Thermal Hydrology Modeling of Deep Borehole Disposal of High-Level Radioactive Waste

Disposal of high-level radioactive waste, including spent nuclear fuel, in deep (3 to 5 km) boreholes is a potential option for safely isolating these wastes. Existing drilling technology permits reliable and cost-effective construction of such deep boreholes. Conditions favorable for deep borehole disposal in crystalline basement rocks, including low permeability, high salinity, and geochemically reducing conditions, exist at depth in many locations. Coupled thermal-hydrologic processes induced by heat from the radioactive waste may impact fluid flow and the associated migration of radionuclides. Numerical simulations of thermal hydrology in the deep borehole disposal system were carried out with waste emplaced between depths of 3 km and 5 km. The geometry of the system consisted of a disturbed zone of higher permeability within a radius of 1m from the borehole, and low permeability rock beyond the 1m radius. The simulations considered borehole spacing of 100m and 200m, and number of boreholes of 1, 9 and 25. The base case was taken to be 9 boreholes with 200m borehole spacing. Simulations were conducted for disposal of spent nuclear fuel assemblies and for the higher heat output of vitrified waste from the reprocessing of fuel. Physical, thermal, and hydrologic properties representative of granite host rock at a depth of 4 km were used in the models. The simulations studied temperature and fluid flux in the vicinity of the boreholes. The results show that for all runs single phase liquid conditions persist throughout the model area due to the large hydrostatic pressures present at the specified depths. Simulated base case temperatures for fuel assemblies and vitrified waste showed peak temperature increases of about 30 °C and 180 °C, respectively. Temperatures near the boreholes peak within about 10 years of waste emplacement. Results show minimal thermal perturbations at depths above the top of the waste, for both types of radioactive waste. Axial temperature profiles are dominated by conduction, as convection is constrained by the low permeability and porosity of the host rock. Simulations with borehole spacing of 100m, and number of boreholes of 1 and 25 gave similar temperature results as the base case. For the base case, vertical flux profiles showed similar trends as the temperature profiles, peaking within about 10 years of waste emplacement due to the thermal expansion of water, followed by much lower flow rates at later times. The magnitude of peak vertical specific discharge varies along the length of the emplaced waste. Simulated peak upward vertical specific discharge values at 4000m depth (center of waste) were 3.6 mm/year and 57.0 mm/year for fuel assemblies and vitrified waste, respectively, with fluxes of less than 1 mm/year beyond 100 years. Just as with temperature profiles, vertical upwards fluxes diminish above the depth of the top of the waste. Axial migration of fluid is constrained by the low permeability of the host rock. Simulations with borehole spacing of 100m, and number of boreholes of 1 and 25 gave similar flux results as the base case. Future simulations will model the effect of salinity on thermal hydrology of the deep borehole disposal system, as well as sensitivity studies on model geometry and rock properties.