Heat released by the radioactive decay of nuclear wastes in an underground repository causes a long-term thermal disturbance in the rock mass. The nature of this disturbance for a planar repository 3000 m in diameter at a depth of 500 m below surface is investigated. Loaded initially with a power density of 10 W/m2 of spent fuel assemblies 10 years after discharge from a reactor, the maximum increase in temperature of the repository in granite is about 50 ¿C and the epicentral thermal gradient about 70 ¿C/km. Different waste forms and periods before buriel have significant effects on the thermal disturbance. The effects of temperature changes in the groundwater flow are evaluated with simple models of a vertical fracture connected to a horizontal fracture in the rock mass. The buoyancy groundwater flow through the vertical fracture is a function of both the vertical and the horizontal fracture transmissivities, as well as the changes in density and viscosity of groundwater caused by the temperature changes. Finite hydraulic recharge from the surrounding rock mass affects the thermohydrologic disturbance.