Method for using a thimble of a pressurized water nuclear reactor and device for adjusting the axial position of the thimble

In a nuclear reactor comprising a vessel, a core comprised of fuel assemblies and lower internals of the reactor disposed in the vessel, a measurement room outside the vessel, at least one guidance tube joining the measurement room to the vessel, at least one vertical channel passing through the lower internals, beneath the core in line with a vertical tube guide of a fuel assembly of the core, and at least one thimble ensuring guidance of a probe, the method comprising the steps of

• (a) introducing the thimble in a guidance duct comprising the guidance tube, the vertical channel and the vertical tube guide of the assembly, so that a closed end of the thimble penetrates inside the tube guide as far as a predetermined level, corresponding to a first position of the thimble, in the guidance duct;
• (b) measuring neutron flux inside the assembly when the reactor is operating, through the probe guided into the thimble, over a first period of use;
• (c) displacing the thimble along the axis of the guidance duct from said first position to a second position, from the measurement room;
• and
• (d) measuring neutron flux inside the assembly in the reactor, over a second period of use, the position of at least one preferential wear zone of the thimble being different with respect to at least one zone of the guidance tube during said first and second periods of use.