The lower end plug is welded to the cladding tube, and the rod is loaded with the pellets, an insulation pellet and the spring. The upper end plug is welded on, the tube is evacuated, pressurised with helium and welded shut. The helium pressure depends on the reactor type and is 3 to 10 bar (STP) for BWR and 10 to 20 bar for PWR fuel rods. These pressures double during warming up to the operating temperature and are chosen to be high enough to provide some support against fast cladding collapse under the outside system pressure (70 bar BWR, 150 bar

PWR, but slow cladding creep-down still occurs, see Chapter 14) and to maintain a good heat conductance across the pellet-cladding gap even when the helium fill gas is diluted with low conductivity fission gas (see Chapter 14). The amount of fill gas is low enough to leave room for released fission gas without exceeding rod pressure safety limits (see Section 9.6.1).

The whole assembly and control process is highly automated. The girth weld of the end plugs is inspected by various (automated) methods, e. g., helium leak testing, computerised optical scanning of the weld surface and determination of permissible weld bulge that does not interfere with the spacer grid, X-ray fluorescence spectrography, and ultrasonic inspection.