Fission fragment tracks produced on prepared surfaces of uranium dioxide were observed by replica electron microscopy. Several statistical analyses and measurements were performed on the observed tracks to be compared with theoretical studies of a fission fragment passing through a stopping medium. A temperature distribution around the fragment is evaluated by applying the thermal diffusion model to the particle energy losses. Since the temperatures attained at the surface could not explain any track formation, a shock wave is shown to be a possible cause by considering processes due to thermal pressures engendered by the very hot core surrounding the fragment path.