Abstract: A self-propelled carriage for inspecting the primary vessel of a fast reactor is capable of displacement within the interspace between the primary vessel and the containment vessel in order to inspect and test any predetermined zone of the primary vessel, the carriage being associated with a drive mechanism and applied against the oppositely-facing wall of the containment vessel. The carriage is suspended from a composite cable actuated by a handling apparatus for introducing the carriage into the interspace and withdrawing it therefrom. The composite cable supplies electric power as well as the different fluids required for positioning and operation of the inspection devices which are mounted on the carriage.