Patent Number: 054935920
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1 thereof, there is seen a cross section of a fuel rod which is situated in a flow of water taking place axially at a temperature of, for example, 326.degree. C. and a pressure of about 160 bar. In the interior of a cladding-tube 2 there is a column of fuel pellets 1 which is formed of uranium oxide or a uranium oxide/plutonium oxide mixture, that releases increasingly aggressive gases and fission products during the reactor operation. Under the influence of the reactor radiation, the material of the cladding tube 2 undergoes structural changes which result in an increase in the length of the cladding tube, while at the same time the pressure of the coolant compresses the tube. Since the volume of the fuel increases with increasing burn-up, contacts between an inner surface 3 of the cladding tube and the hot fuel occur, with the result that finally, not only aggressive chemical conditions but also mechanical and thermal stresses occur at the inner surface. In view of these stresses, in a first exemplary embodiment of the invention, the cladding tube is manufactured as a composite body including two layers that are bound metallurgically to one another. An inner layer 4 has a thickness being about 75 to 95% of the total cladding-tube wall thickness and determining the mechanical robustness of the entire cladding tube. An alloy of the inner layer ("first alloy") is formed of sponge zirconium containing 1.5.+-.0.1% tin, 0.21.+-.0.03% iron, 0.1.+-.0.03% chromium, 0.14.+-.0.02% oxygen, 0.01.+-.0.002% silicon and less than 0.007% nickel. This alloy is therefore zircaloy 4 with a comparatively high content of tin, oxygen and silicon. Under the conditions of the pressurized-water reactor, it cannot be expected of this material that the cladding tube undergoes damage which starts from the inside and proceeds through the entire cladding tube. A thin outer layer 5 is formed of an alloy ("second alloy") which, in addition to sponge zirconium, contains 0.8.+-.0.1% tin, 0.21.+-.0.03% iron, 0.1.+-.0.03% chromium, 0.01.+-.0.002% silicon and an oxygen content of between 0.12 and 0.16%. In this connection, it is assumed that no special measures are necessary to reduce the corrosion in a lithium-containing medium. In zircaloy, the amount of tin is raised above 1.2% in view of the mechanical properties required, but it is limited to 1.7% to take account of the susceptibility to corrosion, which is increased by tin. FIG. 2 shows how the corrosion rate measured as an increase in weight in milligrams per dm.sup.2 and per day depends on the tin content in the case of Zircaloy 4 ("Zry-4") in a suitable autoclave in the presence of water (350.degree. C. at 170 bar) or steam (420.degree. C. at 105 bar). FIG. 3 shows the corresponding measured values of oxide-layer thicknesses which have formed on different workpieces during the reactor operation. In this case, an Sn gradient was maintained in a single Zry-4 melt and the material of the individual workpieces was taken from this single melt at different points. A particularly low oxidation accordingly occurs at the operating temperature of the reactor provided the tin content is kept below about 1.1%, in particular in the second alloy. However, since the inner layer is exposed to the aqueous medium only if the outer layer is destroyed, tin contents higher than this, in particular tin contents above 1.4% by weight, can be permitted in the first alloy in order to obtain advantageous mechanical cladding-tube properties. With an iron content of more than about 0.5% by weight, zirconium alloys are brittle and can virtually no longer be processed mechanically, for example, in pilger machines. The specification of the iron content in the case of Zircaloy 2 and Zircaloy 4 was based on laboratory experiments in which an oxide layer of about 2 .mu.m which protected the alloy against further oxidation and caused only a low corrosion rate (largely dependent on the iron content) formed at 400.degree. C. even after about 30 days. This transition to low corrosion rates is temperature-dependent and at 360.degree. C. it occurs, for example, only after 110 to 120 days. However, at a low iron content, a renewed growth of the oxide layer, that is to say a greater increase in weight gain, occurs as soon as the oxide layer reaches values between about 7 and 11 .mu.m after even longer test times. FIG. 4 shows corresponding measured values for a Zr1Sn0.1Cr alloy and iron contents of 0.2 or 0.4% in an autoclave at 370.degree. C. and 190 bar. Within the framework of the processable iron content, as high concentrations as possible, in particular in the range of Zry-4, should be aimed for. This also emerges from the comparison shown in FIG. 5, in which the corrosion rates in a long-term test at 370.degree. C. and 190 bar are shown for a Zry-4 spacer plate, an unforged Zry-4 plate and a pilgered Zry-4 tube by measured points 10, 11 and 12. Reference numeral 13 denotes an iron range permitted by the ASTM standard for Zry 4. Curves 15 and 16 circumscribe a range of measured values 17 which have been obtained by varying the iron content in Zry 4. If, however, the hydrogen take-up (FIG. 6) is considered in a corrosion test in which zirconium containing 1% Sn, 0.2% or 0.4% Fe and various chromium concentrations was used in an autoclave at 370.degree. C. for 410 days, it is found that an increase in the chromium content beyond 0.1% has an effect similar to an increased iron content. The limitation of the iron content in the case of Zry 4 and, in particular, in the case of Zry 2 does not therefore take sufficient account of the favorable effect of this metal on the corrosion behavior in a long-term test. For this purpose, for tin contents of more than about 1%, in particular, a total content of iron and chromium of between about 0.4 and 0.5 is advantageous but this is above the specification of Zry. In the case of duplex tubes containing less tin in the outer layer, an Fe+Cr content of at least 0.25%, and in particular at least about 0.35%, can therefore be chosen in the outer layer while the zircaloy limits are approximately maintained for the inner layer. In other words, despite a higher tin content, the chosen Fe content and the chosen (Fe+Cr) content are lower than, or at most about equal to, that of the outer layer. The specified expedient upper limits for the total content of iron and chromium of the second alloy may, for example, be 0.8% or 0.6%. The invention makes it possible to feed back waste produced without difficulty. For this purpose, for example, a cladding tube may be considered which has an external diameter of 10.7 mm and a wall thickness of 0.27 mm. The outer layer, which is formed of Zr1.1Sn0.4Fe0.25Cr, accounts for 16% of the wall thickness, that is to say about 16.5% of the material. The inner layer is formed of Zry 2 having the composition Zr1.7Sn0.16Fe0.12Cr0.03Ni. Both alloys are furthermore specified by an oxygen content of about 0.07% and a silicon content of about 0.012%. Although the high iron content of the outer layer is beneficial in relation to the hydrogen take-up of this layer with this comparatively high tin content, this alloy is difficult to process and a relatively high reject level in the manufacture of the duplex cladding tubes can be anticipated. This reject material has the overall composition of the entire cladding tube, namely Zr1.6Sn0.2Fe0.14Cr0.03Ni, which is within the range of Zry 2. As a result of adding about the same amount of freshly drawn, comparatively cheap alloying material of the overall composition Zr1.8Sn0.12Fe0.1Cr0.05Ni to the reject material being fed back, it is therefore possible to manufacture a new melt of the alloy which is necessary for the inner layer and which is shaped by forging and extrusion into a tube and forms the core of a concentric tube blank which supports on its outside a tube that is formed in a similar manner from fresh material with the composition of the second alloy. The two tubes are welded together at the ends in vacuo so that there is no gas between the two tubes. In order to obtain a metallurgical bond between the different alloys, the tube blank is extruded and then processed mechanically to the desired dimensions, for example in a pilger machine. Expediently, annealings are carried out between or after the individual mechanical processing steps. In contrast to this exemplary embodiment, a cladding tube in which Zr1Sn0.2Fe0.3Cr is used as the second alloy for manufacturing the outer layer, has an iron content which is reduced with a view to improved processability, with the higher hydrogen take-up associated therewith being partly compensated for by an increased chromium content. In accordance with FIGS. 2 and 3, the reduced tin content results in a lower growth of the oxide layer. Although FIG. 2 initially makes tin contents of less than 0.6% appear advantageous, this range is unfavorable. The oxide layer formation depends, on one hand, on the time ("transition point") at which the more severe corrosion shown in FIGS. 3 and 4 occurs in the long-term test ("post-transition corrosion rate", PTCR) and, on the other hand, on the PCTR itself. Measures which are favorable for as low a PCTR as possible may have an unfavorable effect insofar as the transition point is brought forward, that is to say the oxide thickness growth described by the PCTR occurs even earlier. A tin content of less than 0.7% may therefore even prove to be unfavorable in those cases in which the alloy is exposed to an aqueous LiOH solution, in which case, although the Li content itself may be very low, it considerably alters the corrosive effects, for example, because of the local boiling in the pores of the oxide layer already mentioned. Since, in view of FIGS. 5 and 6, the (Fe+Cr) content is advantageously kept above 0.25%, and in particular above 0.35%, the invention in any case provides a total content of iron, chromium and tin of more than 1% in the second alloy. The fuel rods can consequently also be used for outputs and temperature ranges for which dangerous corrosion damage was heretofore to be anticipated with an Li content in the cooling water. This is shown by FIG. 7 which reproduces the corrosion-induced increase in weight of the surface of a Zr 0.2 Fe 0.1 Cr workpiece under pressurized water at 170 bar and 350.degree. C. containing 70 ppm of Li and after 153 days in an autoclave as a function of the Sn content. FIG. 8 shows the measurement results of the same corrosion test for a Zr-base alloy containing 0.5% Sn as a function of the iron content. Similar relationships often also result if further alloying constituents (for example 0.5% Nb) are also added in addition. FIGS. 2 to 8 result in an advantageous outer layer of the cladding tube composed of an alloy of Zr, (0.8.+-.0.1) Sn (0.28.+-.0.04) Fe (0.17.+-.0.03) Cr. Advantageously, Zircaloy 4 with a comparatively high content of tin (of between 1.4 and 1.6) can be chosen for the inner layer. In the case of both alloys, it is advantageous to establish a defined content of oxygen and silicon, for example (0.14.+-.0.02) % O and (0.01.+-.0.002) % Si. This first alloy is not optimum on the basis of the long-term investigations submitted in this case in relation to the tin content with regard to corrosion in water (FIG. 2) and in relation to the low iron content and total content of iron and chromium in regard to corrosion and hydrogen take-up (FIGS. 4 to 6). However, according to the previous experience with pressurized-water reactors, no cladding tube defects originating from the inside and extending up to the second layer are to be expected with this alloy composition. This first layer largely determines the required mechanical properties of the cladding tube. The cladding tube is protected against a corrosive attack and hydrogenation due to the coolant (even in the case of a lithium-containing solution) by the second layer which has, for this purpose, a higher content of iron and Fe+Cr and, with a low tin content, a total content of tin, chromium and iron which is above 1.0%. Both layers of the composite tube include the same metals as alloying additives. TABLE 1 ______________________________________ Content in % by weight Sponge Zr Zry 2 Zry 4 ______________________________________ Sn &lt;0.005 1.2 . . . 1.7 1.2 . . . 1.7 Fe &lt;0.150 0.07 . . . 0.20 0.18 . . . 0.24 Cr &lt;0.020 0.05 . . . 0.15 0.07 . . . 0.13 Ni &lt;0.007 0.03 . . . 0.08 &lt;0.007 Fe & Cr & Ni: Fe & Cr: 0.18 . . . 0.38 0.28 . . . 0.37 O.sub.2 &lt;0.14 Si &lt;0.012 &lt;0.012 &lt;0.012 ______________________________________ TABLE 2 __________________________________________________________________________ Alloying constituents (% by weight) Remainder: zirconium 1st alloy 2nd alloy min. max. min. max. __________________________________________________________________________ Sn 1(1.2/1.4) 2(1.7/1.6) 0.5(0.7) 1.3(1.1/0.9) Fe 0.05(0.07/0.1/0.18) 0.25(0.24) 0.15(0.18/0.24) 0.5(0.4/0.35) Cr 0.05(0.07) 0.2(0.13) 0.05(0.07/0.13) 0.4(0.25/0.21) Fe + Cr + Sn 1.0 (1.1) (1.8/1.5) Ni .about.0 (0.08/0.007) .about.0 0.007 Si (0.005/0.007) (0.02/0.012) (0.005/0.007) (0.02/0.012) O (0.05/0.07/0.12) (0.2/0.16) (0.05/0.07/0.12) (0.2/0.16) __________________________________________________________________________