Abstract: The invention relates to an underwater measuring device of the hardness of a vertical wall of the partitioning of the core of a pressurized water nuclear reactor. The device comprises a hollow pole for the manipulation from the edge of the pool of the reactor of the pole in vertical position. A body fast to the pole at its lower end contains a first chamber in which is mounted a movable piston bearing support means intended to come into contact with walls at right angles and a second chamber in which is movably mounted a second piston bearing a hardness measuring head designed to form an impression on the wall opposite one of the walls. Hydraulic fluid supply means for the chambers and conductors pass through the hollow pole. The conductors are connected to a measuring station. The invention is applicable particularly to determining the influence of irradiation on the walls of the partitioning of a nuclear reactor, after a certain period of operation.