Patent Number: 052456455
Section: summary

The invention relates to a structural part for a nuclear reactor fuel assembly, in particular a cladding or casing tube for a nuclear fuel-filled fuel rod or a spacer for such fuel rods, and to a method for producing the structural part. German Published, Non-Prosecuted Application DE-OS 34 28 954 discloses a cladding or casing tube made of a zirconium alloy for a nuclear reactor fuel rod that can be filled with nuclear fuel. The zirconium alloy may be Zircaloy-2, containing from 1.2 to 1.7% by weight tin, 0.07 to 0.2% by weight iron, 0.05 to 0.15% by weight chromium, 0.03 to 0.08% by weight nickel, 0.07 to 0.15% by weight oxygen, and zirconium for the remainder. The geometric mean value of the grain diameter of the zirconium alloy is less than or equal to 3 .mu.m. In particular, the geometric mean value is from 2.5 to 2 .mu.m. Such a cladding tube is supposed to possess great resistance to stress corrosion cracking. Stress corrosion cracking is a corrosion mechanism on the inside of the cladding or casing tube in the nuclear reactor, for which the expansion of the of the cladding or casing tube resulting from the swelling of the nuclear fuel filling it and from nuclear fission products such as iodine liberated from the nuclear fuel are responsible. Stress corrosion cracking plays a particular role in nuclear ractor fuel rods that are used in boiling water reactors. There, abrupt changes in power of the nuclear reactor, in particular, can cause breaching of the cladding or casing tube walls of the nuclear reactor fuel rods from stress corrosion cracking. The grain diameter in a zirconium alloy can be determined by A.S.T.M. (American Society for Testing Materials) designation E 112-61. The geometric mean value of n diameters is defined as X.sup.G =(d.sub.1 .d.sub.2 . . . d.sub.i .d.sub.n).sup.1/n, where d.sub.i is the i.sup.th diameter. It is accordingly an object of the invention to provide a structural part for a nuclear reactor fuel assembly and a method for producing this structural part, which overcome the hereinafore-mentioned disadvantages of the heretofore-known methods and devices of this general type, and which include a zirconium alloy that has a high corrosion resistance not only to the nuclear fuel or nuclear fission products but also to the coolant which is water in a nuclear reactor, even at relatively high prevailing temperatures, for instance in a pressurized water reactor, which is higher than in a boiling water reactor. With the foregoing and other objects in view there is provided, in accordance with the invention, a structural part being formed of a cladding or casing tube of a nuclear fuel-filled fuel rod or a spacer for a fuel rod for a nuclear reactor fuel assembly, comprising: a) a zirconium alloy material having at least one alloy ingredient selected from the group consisting of oxygen and silicon, a tin alloy ingredient, at least one alloy ingredient selected from the group consisting of iron, chromium and nickel, and a remainder of zirconium and unavoidable contaminants; b) the zirconium alloy material having a content of the oxygen in a range of substantially from 700 to 2000 ppm, a content of the silicon of substantially up to 150 ppm, a content of the iron in a range of substantially from 0.07 to 0.5% by weight, a content of the chromium in a range of substantially from 0.05 to 0.35% by weight, a content of the nickel of substantially up to 0.1% by weight, and a content of the tin in a range of substantially from 0.8 to 1.7% by weight; c) the alloy ingredients selected from the group consisting of iron, chromium and nickel being precipitated out of a matrix of the zirconium alloy as secondary phases, having a diameter with a geometric mean value in a range of substantially from 0.1 to 0.3 .mu.m; and d) the degree of recrystallization of the zirconium alloy being less than or equal to 10% and a sample of the zirconium alloy, after a recrystallization annealing with a degree of recrystallization of 97.+-.2%, having a grain size with a geometric mean value less than or substantially equal to 3 .mu.m. In accordance with another feature of the invention, the content of iron in accordance with characteristic (b) is in a range of substantially from 0.07 to 0.3% by weight, and the content of chromium is in a range of substantially from 0.05 to 0.15% by weight, in the zirconium alloy. The diameter of secondary phases, that is independent crystallites of alloy components precipitated out of the zirconium alloy, can be determined either with high accuracy by using a transmission electron microscope, or with an accuracy which is not as high by using a scanning electron microscope. The geometric mean value of these diameters is defined in correspondence with the definition of the geometric diameter of particle or grain diameters. The degree of recrystallization is defined as the percentage of recrystallized crystal matrix in the zirconium alloy. The relatively low content of tin in the zirconium alloy in accordance with characteristic (b) mentioned above and the relatively high geometric mean value of the diameter of the secondary phases precipitated out of the matrix of the zirconium alloy in accordance with characteristic (c), in particular, bring about the increased corrosion resistance with respect to water. In accordance with a further feature of the invention, the zirconium alloy has a texture with a Kearns parameter f.sub.r wherein 0.6.ltoreq.f.sub.r .ltoreq.1 and preferably 0.6.ltoreq.f.sub.r .ltoreq.0.8. An even further increased corrosion resistance to both nuclear fuel or nuclear fission products and to water at increased temperatures can be attained in this way. A body of a zirconium alloy has a texture, if the hexagonal crystallites of this body have a 3-dimensional ordered alignment (for instance attainable by mechanical deformation), as compared with a purely random alignment (for instance virtually attainable by .beta.-quenching). One measure for the alignment of the crystallites which form right angles with the surface of the body being formed of the zirconium alloy and thus for the texture, is the Kearns parameter f.sub.r, which can be calculated in accordance with "Metallurgical Transactions A", Volume 10A, April 1979, pages 483 through 487. The necessary measurements are carried out in a goniometer with the aid of directional X-radiation. In accordance with an added feature of the invention, the content of tin in the zirconium alloy is in a range of substantially from 0.9 to 1.1 % by weight. In accordance with an additional feature of the invention, the contents of the alloy ingredients iron and chromium in the zirconium alloy are in a ratio of substantially 2:1, and/or the contents of the alloy ingredients iron and chromium have a sum of substantially 0.4 to 0.6% by weight. In this way, the structural parts being formed of the zirconium alloy can be given an optimal corrosion resistance to water at elevated temperatures. In accordance with yet another feature of the invention, the contents of the alloy ingredients iron and chromium have a sum of substantially 0.4 % by weight. In accordance with yet a further feature of the invention, in accordance with characteristic (b), the content of oxygen is in a range of substantially from 1000 to 1800 ppm, the content of silicon is in a range of substantially from 80 to 120 ppm, the content of iron is in a range of substantially from 0.35 to 0.45% by weight, the content of chromium is in a range of substantially from 0.2 to 0.3% by weight, and the content of tin is in a range of substantially from 1 to 1.2% by weight. In accordance with yet an added feature of the invention, the zirconium alloy is Zircaloy-2 or Zircaloy-4. With the objects of the invention in view, there is also provided a method for producing a structural part, which comprises: a) annealing a starting body of a zirconium alloy at a temperature in the .beta. range below the melting temperature to dissolve precipitated-out alloy ingredients, then quenching the starting body at a quenching rate of at least 30 K/s at a surface of the starting body, at a temperature transition through the .alpha.+.beta. range; b) then annealing the starting body at a first temperature in the .alpha. range until formation of precipitates of the alloy ingredients having a precipitate diameter with a geometric mean value in a range of substantially from 0.1 to 0.3 .mu.m; c) hot-forging the starting body into a forged part at a second temperature in the .alpha. range below the first temperature; d) then hot-rolling or hot-extruding the forged part at a temperature in the .alpha. range below the first temperature; and e) then cold-rolling the hot-rolled forged part in at least two rolling steps having recrystallization annealing carried out between two rolling steps with a degree of recrystallization in a range of substantially from 95% to 99% at an annealing temperature in the .alpha. range, while cold-pilgering the hot-extruded forged part in at least two pilgering steps, with a recrystallization annealing carried out between two pilgering steps with a degree of recrystallization in a range of substantially from 95% to 99% at an annealing temperature in the .alpha. range. With the objects of the invention in view, there is additionally provided a method for producing a structural part, which comprises: a) annealing a starting body of a zirconium alloy at a temperature in the .beta. range below the melting temperature to dissolve precipitated-out alloy ingredients, then quenching the starting body at a quenching rate of at least 30 K/s at a surface of the starting body, at a temperature transition through the .alpha.+.beta. range; b) hot-forging the starting body into a forged part at a first temperature in the .alpha. range; c) then heating the forged part to a second temperature in the .alpha. range above the first temperature, until formation of precipitations of the alloy ingredients having a precipitation diameter with a the geometric mean value in a range of substantially from 0.1 to 0.3 .mu.m; d) then hot-rolling or hot-extruding the forged part at a temperature in the .alpha. range below the second temperature; and e) then cold-rolling the hot-rolled forged part in at least two rolling steps having recrystallization annealing carried out between two rolling steps with a degree of recrystallization in a range of substantially from 95% to 99% at an annealing temperature in the .alpha. range, while the hot-extruded forged part is cold-pilgered in at least two pilgering steps, with a recrystallization annealing carried out between two pilgering steps with a degree of recrystallization in a range of substantially from 95% to 99%, at an annealing temperature in the .alpha. range In accordance with another mode of the invention, there is provided a method which comprises performing a final pilgering step and pilgering steps preceding the final pilgering step in the at least two pilgering steps, selecting a logarithmic cold work of at least 1.0 in the pilgering steps preceding the final pilgering step, and selecting a logarithmic cold work of at least 1.6 in the final pilgering step. In accordance with a further mode of the invention, there is provided a method which comprises performing a final pilgering step and pilgering steps preceding the final pilgering step in the at least two pilgering steps, selecting a quotient of a logarithmic wall thickness variation to a logarithmic diameter variation of at least 1 in the pilgering steps preceding the final pilgering step, and selecting a quotient of a logarithmic wall thickness variation to a logarithmic diameter variation of at least 5 in the final pilgering step. In accordance with a concomitant mode of the invention, there is provided a method which comprises performing a final rolling step and a final pilgering step, performing a final annealing following the final rolling step or the final pilgering step, and performing the final stress relief annealing with a degree of recrystallization of a maximum of 10%. The logarithmic wall thickness variation .epsilon..sub.S is the natural logarithm of the quotient S.sub.o /S of the wall thickness of a tube before (S.sub.o) and after (S) a pilgering step. The logarithmic diameter variation .epsilon..sub.D is the natural logarithm of the quotient D.sub.o /D of the mean diameter (between the inside and outside diameter) of this tube before (D.sub.o) and after (D) the same pilgering step. The logarithmic cold work value .phi. of the tube effected by this pilgering step is defined as .phi.=.epsilon..sub.S +.epsilon..sub.D, and a cold-deformation C.sub.W associated with the pilgering step is C.sub.W =100 (1-exp-.phi.) in percent. The .alpha. range of a zirconium alloy is the temperature range in which the crystal of the zirconium alloy has a hexagonal structure. The .beta. range is the temperature range in which the crystal of the zirconium alloy has a cubically body-centered structure. The (.alpha.+.beta.) range is the temperature transition range in which both of these crystal structures are present in the zirconium alloy. Other features which are considered as characteristic for the invention are set forth in the appended claims. Although the invention is illustrated and described herein as embodied in the manufacture of two cladding tubes for a nuclear reactor fuel assembly, for instance for a UO.sub.2 -filled fuel rod in a fuel assembly, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.