Patent Number: 050230484
Section: description

DESCRIPTION OF PREFERRED EMBODIMENTS In order that the invention may be more celarly understood, there will now be described by way of example several embodiments of a fuel rod according to the invention intended for a fuel assembly in a pressurized water nuclear reactor. In all cases, the sheath of the fuel rod according to the invention is produced by the extrusion and then rolling in a step-by-step rolling mill of a blank constituted by a tubular core of zirconium alloy on which is mounted an outer tube having a composition corresponding to the composition of the surface or outer layer defined hereinabove. In its final state, the sheath of the fuel rod according to the invention has the following dimensional characteristics: outside diameter 9.63 mm.+-.0.04 mm, minimum total thickness of the wall of the sheath 0.605 mm, minimum thickness of the surface or outer layer 0.060 mm. EXAMPLES 1, 2 AND 3 The inner tubular layer is made from a zirconium alloy of conventional type and currently employed in the case of the manufacture of sheaths of fuel rods for assemblies intended for pressurized water nuclear reactors. Such a zirconium-base alloy, designated by the name Zircaloy 4, includes 1.2 to 1.7% tin, 0.18 to 0.24% iron, 0.07 to 0.13% chromium, 0.0080 to 0.00200% carbon, 0.0050 to 0.012% silicon and 0.0900 to 0.1600% oxygen, the indicated percentages being percentages by weight, the total of the percentages by weight of the iron and chromium components being between 0.28 and 0.37%. The balance is constituted by the zirconium apart from inevitable impurities in very low proportions. The following table indicates the compositions (in percentages by weight) of three surface layers respectively corresponding to the Examples 1, 2 and 3 of the invention, these layers having, relative to one another, certain differences in composition concerning the addition elements introduced or the percentages by weight of these elements. __________________________________________________________________________ Sn Fe Cr O Nb V Zr __________________________________________________________________________ EX. 1 0.35/0.65 0.22/0.28 -- 0.09/0.16 0.35/0.65 -- BALANCE EX. 2 0.35/0.65 0.35/0.45 -- 0.09/0.16 0.35/0.65 -- BALANCE EX. 3 0.35/0.65 0.55/0.65 -- 0.09/0.16 -- 0.25/0.35 BALANCE __________________________________________________________________________ The compositions of the surface layer of the fuel rods according to the invention are characterized by the presence of tin in a significant proportion of 0.35 to 0.65%, by the presence of iron in a proportion which may vary but which is always between 0.20 and 0.65%, by the absence of chromium which may only be present as residual impurities in a very small amount, and by the presence of niobium or vanadium in significant and well-determined proportions. In all cases, the simultaneous presence of tin and an element such as niobium or vanadium permits obtaining both very satisfactory mechanical characteristics and in particular high hardness and corrosion resistance characteristics comparable to those of zirconium alloys including vanadium and devoid of tin. The vanadium and niobium permit reducing the surface absorption of hydrogen by the sheath and therefore improve the corrosion resistance in the environment of the reactor. After its forming and heat treatment, the duplex sheath of the fuel rod according to the invention has on the whole a homogeneous crystalline structure in the form of a recrystallized phase. EXAMPLES 4, 5 AND 6 In these examples of a fuel rod according to the invention, the inner tubular layer is constituted by a zirconium-base alloy including substantially 1% niobium, to the exclusion of any other metal alloy element in a significant quantity. This inner layer has in all cases the following composition by weight: niobium 0.8 to 1.2%, oxygen 0.09 to 0.16%, the balance being constituted by the zirconium apart from inevitable impurities in very small amounts. The sheaths of the fuel rods according to Examples 4, 5 and 6 differ from one another by the composition of their surface layer. In the case of Example 4, the surface layer has the composition mentioned hereinbefore in Example 1. Likewise, the surface layer of the sheaths of the rods according to Examples 5 and 6 have the compositions mentioned hereinbefore in Examples 2 and 3, respectively. In the final state, the sheath has an entirely recrystallized structure. Tests carried out to ascertain the corrosion resistance at 400.degree. C. have shown that the composite sheaths of the rods according to the invention have characteristics which are distinctly improved over those of sheaths of Zircaloy 4. Moreover, the resistance to creep at 400.degree. C. of the sheaths of fuel rods comprising an inner layer of zirconium-niobium alloy is very much higher than the resistance to creep of homogeneous or composite sheaths of Zircaloy 4. Furthermore, according to a particular advantage of the invention, the alloy Zr-Nb markedly reduces the risk of corrosion under stress due to the interaction between the pellet and the sheath, since this alloy has a lower relative ductility loss than that of alloys of the prior art. In all cases, the hardness of the surface layer at low or high temperature is very much higher than the hardness of the corresponding surface layer of composite sheaths according to the prior art. The scope of the invention is not intended to be limited to the described examples. Thus, it is possible to employ an inner layer constituted by a zirconium alloy such as Zircaloy 2 including 1.2 to 1.7% tin, 0.07 to 0.2% iron, 0.05 to 0.15% chromium, 0.03 to 0.08% nickel and 0.07 to 0.15% oxygen, the sum of the percentages by weight of iron, chromium and nickel being between 0.18 and 0.38%. It is also possible, as concerns the surface or outer layer, to select more precise composition limits within the limits given hereinbefore. The fuel rods according to the invention may be employed both in the case of fuel assemblies for pressurized water nuclear reactors and in the case of fuel assemblies for boiling water nuclear reactors.