[en] For assessing the long term safety of a thorium-based fuel stored in a deep geological repository, we have considered the possible release of the radionuclides generated in the fuel and their transport from the disposal to some outlets of the geosphere over periods of time as long as 10⁶ years. The simulation code MELODIE, tested in the case of an uranium-based fuel (EVEREST program), has been used to model this transport, with the same scenario and assumptions. The contribution of the actinides, activation products and fission products to the total effective dose has been analysed separately. The behavior of each long lived radionuclide under consideration is explained on the basis of its bonds with the ThO₂ matrix, on its solubility and on the capacity of the geosphere to retain it. In the case of the transuranium elements, a comparison with the results previously obtained on UO₂ is presented. The influence of the burn up on the initial inventory and, consequently, on the radiological impact is also discussed. (authors)