above only 30 of the measurements of the internal zirconium cladding thickness could be used a first embodiment to improve the regularity of the zy2 zr interface in particular to create a difference in grain size u03b4i of at least 2 is shown in fig2 it consists in working the zircaloy 2 bar in the alpha phase after quenching at a3 to provoke substantial grain refining of the zircaloy 2 at the blank stage which is retained after assembly at c1 extruding at c2 and rolling at c3 of the composite tubular blank significantly regularizing the zy2 zr interface more precisely after heating at a2 for 1 hour at 1050 u00b0 c 1030 u00b0 c to 1070 u00b0 c a bar which had been worked by forging or rolling at a 39 1 to a diameter of about 300 mm 250 to 350 mm rather than the 177 mm 150 to 200 mm of the prior art was alpha quenched after heating for 3 to 5 hours between 750 u00b0 c and 780 u00b0 c the diameter of the bar was reduced to u03c6 177 mm by forging in the alpha phase at a4 before cutting machining and boring the billets at a5 for assembly at c1 and transformation in accordance with the prior art ______________________________________for the internal unalloyed zr cladding i1 10 for the external zy2 sleeve i2 12 i e a difference u03b4i 2 astm index numbers ______________________________________ at the same time ultrasound measurements of the thickness of the internal cladding on a series of 10 composite zy2 zr tubular blanks made.