[en] France is now investigating a number of deep geological formations (clay, granite, salt and schist) for disposal of long half-life radioactive waste, notably vitrified high-level waste (HLW) packages which would be stacked in vertical boreholes extending downward from a network of parallel handling drifts. The design of such a geological repository allows for construction and operating constraints, as well as the temperature rise in the vitrified waste packages and in the host rock. Repository temperatures are calculated using a finite-element code in conjunction with a superposition algorithm. The geometric parameters of the repository are analyzed to optimize the dimensions from a thermal standpoint, notably the borehole pitch in each drift, and the distance between adjacent drifts. Numerical examples are included for a repository in a clay environment, characterized by its relatively low thermal conductivity. The thermal consequences of vertical spacing between stacked waste packages are also discussed. 5 refs., 8 figs