Method and apparatus for inspecting the geometry of a guide tube

A method for monitoring a degree of straightness of a guide tube of a nuclear fuel assembly having a zone of smaller diameter in a terminal portion thereof, comprising the steps of:

• inserting, into said guide tube, from an end opposite to said terminal portion, an apparatus comprising:
• stick-shaped means having a distal portion which presents in succession, starting from a distal end of the stick-shaped means,
• a guiding swelling having a predetermined diameter selected to enable said swelling to pass through the zone of smaller diameter with an easy fit, and
• a first projection and a second projection at different distances from said swelling, with at least the first projection carrying at least two sets of diametrically opposite strain sensors provided with conductors linking them to a proximal end of the stick-shaped means;
• and
• a sleeve having an outer diameter selected for a sliding fit in a running portion of the guide tube above said zone of smaller diameter and an inner diameter selected for sliding fit over the stick-shaped means;
• said sleeve and said stick-shaped means having mutually cooperating abutment means for limiting displacement of the sleeve towards said distal portion to a position where the sleeve surrounds the stick-shaped means, until said sleeve comes into abutment against an end of said zone of smaller diameter,
• further moving the guide tube relative to the stick-shaped means to make the projections scan said zone of smaller diameter,
• and deriving an amount of deformation and an orientation of deformation at each axial level of said narrower zone from signals delivered by the sensors and from measurement of displacement of the stick-shaped means along the guide tube.