Process for producing a guide tube of a nuclear reactor fuel assembly, mandrel for forming the guide tube, and the guide tube obtained

A pilgrim rolling mill assembly for producing a guide tube having a substantially constant outside diameter and comprising at least one main part having a first wall thickness and, on a fraction of the length of said guide tube, a reinforced part which has a second wall thickness exceeding said first wall thickness, the assembly comprising:

• a stepped mandrel having successive adjacent parts along the length thereof, including
• a) a screw threaded end for connecting the mandrel to mandrel holding and actuating rod means;
• b) a first symmetrical part of revolution having a cross-sectional diameter which diminishes along the axial direction of said mandrel from a first value less than the inside diameter of said blank to a value equal to the inside diameter of said at least one main part of a finished guide tube;
• c) a second symmetrical part of revolution having a cross-sectional diameter which diminishes in the axial direction of said mandrel from a value equal to said inside diameter of said at least one main part of said guide tube to a second value equal to the inside diameter of said reinforced part of said guide tube;
• d) each of said first and second symmetrical parts of revolution of said mandrel further having a mandrel section having a symmetrical surface of revolution and a meridian of parabolic shape and a part for calibrating said at least one main part and said reinforced part of said guide tube, respectively;
• and
• cooperating die means mounted on respective shafts for
• e) effecting, in a first stage, a rolling of a first section of said blank on the first part of said mandrel with a reduction of the outside diameter of said blank to said outside diameter of said guide tube and a reduction of the wall thickness of said blank to one of said first and second wall thicknesses;
• f) effecting, in a second stage, a rolling of a second section of said blank on the second part of said mandrel spaced from said first part in the axial direction of said mandrel, with a reduction of said outside diameter of said blank to said outside diameter of said guide tube and a reduction of said wall thickness of said blank to the other of said first and second wall thicknesses;
• said mandrel holding and actuating rod means for displacing said mandrel through said blank in an axial direction of said blank between said first and said second stages.