The thimble (10) is introduced into a guidance duct consisting of a guidance tube (5) joining a measurement room (3) to the vessel (1) of the nuclear reactor, a vertical channel passing through the lower internals (14) of the reactor beneath the core (12) and a tube guide (17) of an assembly of the core (12). The closed end of the thimble penetrates into the tube guide of the fuel assembly as far as a defined level. The position of the thimble (10) is modified along the axis of the guidance duct, between two periods of use of the thimble in the nuclear reactor, so as to modify the position of the heavy wear zones of the wall of the thimble (10) with respect to the guidance duct. Preferably, the axial position of the thimble (10) is adjusted by using an extension of the thimble fixed to a seal (8) of the guidance duct and into which the end of the thimble is screwed.
