Abstract: The distance between substantially parallel metal tubes forming a flat sheet and the thicknesses of oxide layers covering the tubes are simultaneously measured. The tubes may particularly be sheaths of a nuclear fuel assembly.

Abstract: The positively acting mechanism comprises a jack (2) with a rod (10) connected, at one of its ends, to a member whose actuation or retention it ensures, and, at its other end, to a thrust piece (16), on which a spring (18) comes to bear, in order to actuate or retain the member. An axle (25) parallel to the rod (10) of the jack is rotatably mounted in a housing (14) fixed relative to the body (3) of the jack (2) and has a threaded part (26) in engagement with a nut (34) forming an engagement fork for the thrust piece (16) of the rod (10). The axle (25) has an end part (30) engaging with a tool allowing the axle (25) to be set in rotation remotely. It is thus possible, in the event of failure of the jack, to rotate the axle (25) and displace the rod (10) of the jack (2) in the direction of release of the mechanism.