Patent Number: 053735412
Section: description

DESCRIPTION OF PREFERRED EMBODIMENTS The drawing FIGURE shows a cross-section of a fuel rod with heavy fuel pellets 3 in a cladding having an inner layer 2 and an outer layer 1. Some test results showing the advantages of cladding compositions in accordance with the invention will now be given. Compositions Containing Tin, with a Low Tin Content and a High Oxygen Content Tests were first performed to determine the consequences of reducing the tin content as compared with those in a conventional Zircaloy 4 having an oxygen content of 0.12%, and in alloys for making the outer layer having a low tin content for reducing corrosion in a high temperature aqueous medium. The greatest differences relate to resistance to thermal creep under conditions representative of those found in a reactor at 400.degree. C. over a period of 240 hours and under a stress of 130 MPa. Diameter deformation was then as follows: alloy A (Zircaloy 4 having 1.5% tin and 0.12% oxygen): 1.3% PA1 alloy B (Zircaloy 4 having 1.3% tin and 0.12% oxygen): 1.5% PA1 alloy C (alloy having only 0.5% tin and 0.12% oxygen): 3.6% PA1 alloy D (alloy having 0.5% tin and 0.19 to 0.2% oxygen): 1.7% to 1.8%. PA1 alloy E: 1% niobium and 0.08% to 0.10% oxygen PA1 alloy F: 1% niobium and 0.125% oxygen. PA1 recrystallized alloy A: 0.5% to 0.6% PA1 recrystallized alloy B: 1% PA1 recrystallized alloy E: 0.60% PA1 recrystallized alloy F: 0.25% to 0.30%. PA1 under thermal creep testing at 400.degree. C., under 130 MPa for a period of 240 hours, typical diametral deformation was as follows: PA1 (a) oxygen doping of the D-type alloy constituting the outer layer of the B/D duplex tubes makes it possible PA1 (b) the high oxygen content in alloy F of the A/F duplex These results were obtained with an alloy in stress-relieved condition. It can be seen that the substantial increase in oxygen content makes it possible, with an alloy having a very low tin content, to obtain resistance to thermal creep and thus resistance under radiation almost equivalent to that of a Zircaloy 4. Creep values thus obtained are compatible with the design requirements of a fuel rod. In addition, creep tests under conditions representative of LOCA conditions show, in particular for certain temperatures (of higher end range .alpha., ranges .alpha.+.beta. and .beta.) that the high temperature creep behavior of alloy D doped with O.sub.2 is comparable to or even better than that of Zircaloy 4 alloys A and B in terms of fracture time and of ductility. Tests for measuring the yield strength under tractive force and in bursting, mostly at ambient temperature, have also shown a clear deterioration relative to standard Zircaloy 4 when the tin content is reduced to 0.5% and in the absence of an increase in the oxygen content. These tests have shown that substantially the same resiliency limit is obtained as with Zircaloy 4 containing 1.5% tin when the relaxed alloy contains 0.19% to 0.20% oxygen. These favorable results are obtained with an alloy that is stress-relieved: If on the other hand the alloy is recrystallized, then the deterioration in the ability to withstand thermal creep caused a decrease in the tin content remains, even with a high oxygen content. However, it may be preferred, in certain cases, when it is essential to obtain long term stability in a reactor and to provide the inner layer with particularly high resistance to corrosion, to recrystalize the entire cladding by a final thermal treatment. Then that treatment may be carried out at 780.degree. C..+-.25.degree. C. Composition Further Comprising Silicon An amount of silicon of up to 200 ppm may be added to improve resistance to generalized corrosion, while it has no substantial effect on the nodular corrosion (which is present in BWRs rather than PWRs). Compositions Containing Niobium, at Low Niobium Content and with Different Oxygen Contents For an outer layer containing tin and not having an appreciable niobium content, an oxygen content that is much higher than that of usual Zircaloy 2, 3 and 4 type alloys makes it possible to obtain mechanical characteristics that are close to those of Zircaloy 4, in particular when the alloy is in a relaxed state. When the outer layer has niobium as its only metal additive (apart from inevitable impurities), then the alloy in relaxed state, even when it has a high oxygen content, presents very poor resistance to thermal creep. This drawback is avoided by using an alloy that is doped with oxygen, and by simultaneously subjecting the cladding to final heat treatment for recrystallization purposes. In addition, for this ZrNb alloy, recrystallizing and adding oxygen also make it possible to considerably improve resistance to corrosion under stress in the presence of iodine, to improve the resistance limit to fatigue, to reinforce its conventional mechanical characteristics, and to return to a resistance to LOCA (loss of primary coolant accident) that is as good as that of Zircaloy 4 in the recrystallized state. Comparison Between Zircaloy 4 Alloys and Alloys Containing Niobium The same comparison as above was performed between (a) alloys A and B and (b), the following niobium alloys E and F: The alloys containing only 1% niobium, even when heavily doped with oxygen, have mechanical characteristics when stress-releaved they are too unfavorable, in particular with respect to creep, for it to be possible to envisage using them. In contrast, tests performed on alloys in the recrystallized metallurgical state have shown advantage of oxygen-doped alloy F. The following results are obtained for thermal creep at 400.degree. C. over a period of 240 h under 130 MPa: In addition, measuring yield strength has shown that the degradation in mechanical characteristics on passing from alloy A or B to alloy E is almost totally compensated with alloy F. The results of a comparison between solid cladding made of alloy A or B, i.e., Zircaloy 4, and duplex cladding having an inner layer occupying 80% of its thickness and an outer layer occupying 20% of its thickness, the inner layer being made of alloy A or B and the outer layer of alloy C, D or F, will now be given. The results obtained were as follows, for the recrystallized state: solid alloy A: 0.30-0.40% PA2 solid alloy B: 1% PA2 Duplex B/C: PA2 Duplex B/D: 1.1%-1.25% PA2 Duplex A/F: 0.75%-0.85%. PA2 During bursting tests at 400.degree. C., the R.sub.P0.2 elastic limits are as follows: PA2 to improve resistance to hot creep and to improve mechanical characteristics as compared with the B/C duplex that has little oxygen; and PA2 to bring these properties up to the same level as those obtained using solid alloy B. PA2 gives rise to increased resistance to creep and improved mechanical characteristics; and PA2 makes it possible to obtain properties that are close to or better than those of solid alloy B. solid alloy A: 215 MPa PA3 solid alloy B: 182 MPa PA3 Duplex B/C: 176 MPa PA3 Duplex B/D: 194 MPa PA3 Duplex A/F: 187 MPa. An analysis of these results shows that: In addition, selecting the recrystallized state in combination with oxygen doping for the B/D duplex and the A/F duplex also makes it possible to obtain improved corrosion resistance under stress and in the presence of iodine, to improve as regards growth under irradiation and to obtain a phase texture of the crystal lattice that is more radial. As a general rule, an amount of dopeing oxygen improves mechanical resistance and particularly yield strength. Recrystallization of the inner layer, when caused by the final thermal treatment, increases resistance to corrosion under stress by iodine from the fuel. Recrystallization of the entire cladding, when caused by the final treatment, increases overall resistance to hot creep of the cladding.