PURPOSE: To form nuclear fuel coating tubes having the superior strength, corrosion resistance, and heat conductivity by extruding a composite billet, whose pure zirconium inner cylinder is inserted into the zirconium alloy outer cylinder, at a temp. in the range of 500W750°C.

CONSTITUTION: Under a vacuum, air is removed from a gap 3 of a composite billet 6 whose pure zirconium hollow billet 2 is inserted into a zirconium alloy hollow billet 1. After both ends of the billets 1 and 2 are weld beaded 4, the billet 6 is hot extruded at a temp. in the range of 500W750°C. The adhesion of the billets 1 and 2 increases because a temp. of about 500°C is the recrystallization starting temp. of zirconium alloys, so that the heat conductivity of the billet 6 is improved. Wall deviation ratio causing a short strength is restrained to ≤10% at a temp. of ≤750°C. Therefore, stress corrosion cracks caused by a wall deviation of the zirconium billet is restrained and nuclear fuel coating tubes having the superior strength, corrosion resistance, and heat conductivity are formed.