Patent Number: 052456455
Section: description

The invention and its advantages will be described in further detail below with respect to the production of two embodiments of a cladding or casing tube for a fuel rod filled with nuclear fuel such as UO.sub.2, for a nuclear reactor fuel assembly. Referring first to FIG. 1 in particular, there is seen a diagram which shows the relationship between an annealing temperature .phi..sub.A and an annealing time t.sub.A and a resultant geometric mean value X.sub.G.sup.A of the diameter of deposits precipitated out of alloy components after the annealing of a first zirconium alloy Zircaloy-4 (1.2 to 1.7 % by weight tin, 0.18 to 0.24% by weight iron, 0.07 to 0.13% by weight chromium, 0.10 to 0.16% by weight oxygen, up to 120 ppm silicon, with the remainder zirconium and unavoidable contaminants; and the sum in % by weight of iron and chromium: 0.28 to 0.37% by weight) and a second zirconium alloy with the alloy ingredients being 1 to 1.2 % by weight tin, 0.35 to 0.45% by weight iron, 0.2 to 0.3% by weight chromium, 0.1 to 0.18% by weight oxygen, and 80 to 120 ppm silicon, with the remainder zirconium and unavoidable contaminants. In the diagram of FIG. 1, the equation below applies to the geometric mean value X.sub.A.sup.G of the diameters of the precipitates of alloy ingredients i.e. components or alloying elements, in the applicable zirconium alloy: ##EQU1## In the diagram of FIG. 1, the upper scale in the abscissa applies to the first zirconium alloy (Zircaloy-4), and the lower abscissa scale applies to the second zirconium alloy. The above equation was obtained empirically for the two zirconium alloys by determining the geometric mean values of the diameters of precipitated of alloy ingredients in test bodies, in each case involving these two zirconium alloys. These test bodies were first heated to a temperature of 1150.degree. C. and then quenched with water. After this .beta.-quenching, the majority of the alloy ingredients, iron and chrome, have precipitated out in finely-dispersed form. A geometric mean value of the diameters of the precipitates in the test bodies was found to be X.sub.G.sup.A min=20 nm. The various test bodies were then exposed to various annealing temperatures for variously long annealing times. After they had cooled down, the geometric mean values of the diameters of the precipitates were determined for each test body. This geometric mean value became higher, as the annealing temperature became higher or the annealing time for the applicable test body became longer. However, this geometric mean value is limited at the top by the relatively small quantity of the iron and chrome alloy ingredients in the applicable test bodies. This limitation is taken into account for the first zirconium alloy by a constant .DELTA.X.sub.A.sup.G =1000 nm, and for the second zirconium alloy by a constant .DELTA.X.sub.A.sup.G =1260 nm in the above-given equation. In this equation, the following symbols have the following meanings: k.sub.1 =0.47.times.10.sup.-7 per hour, n=0.57; the activation temperature Q/R=18,240K (R=general gas constant) and, T=.phi..sub.A +273 K. The diagram in FIG. 2 shows the relationship between a logarithmic variation .epsilon..sub.D in diameter, a logarithmic variation .epsilon..sub.S in wall thickness, a logarithmic cold work .phi., and a quotient q=.epsilon..sub.S /.epsilon..sub.D when a tube is produced by pilgering. The following equations apply: ##EQU2## The diagram of FIG. 3 shows the relationship between cold-deformation C.sub.W, an annealing temperature .phi..sub.R and an annealing time t.sub.R, and a resultant degree of recrystallization R.sub.x after the annealing of the Zircaloy-4 tube cold-deformed by pilgering. The degree of recrystallization R.sub.x is the proportion in percentage of the crystal structure recrystallized after annealing. The empirically obtained equations below apply to the degree of recrystallization R.sub.x : ##EQU3## where the annealing temperature A=t.sub.R .multidot.exp-Q/RT, with the constant k.sub.2 =8.3.times.10.sup.18 min.sup.-1, and the activation temperature Q/R =40,000K (R=general gas constant), and T=.phi..sub.R +273K (see "Zirconium in the Nuclear Industry"; A.S.T.M. Special Technical Publication 824; 1984; pages 106 through 122). A first solid cylindrical blank or tubular body of Zircaloy-4 has the alloy composition 1.2% by weight tin, 0.24% by weight iron, 0.12% by weight chromium, 0.15% by weight oxygen, 0.01% by weight silicon, and the remainder zirconium with technically unavoidable contaminants, and a second solid cylindrical blank or tubular body has the alloy composition 1.1% by weight tin, 0.4% by weight iron, 0.25% by weight chromium, 0.14% by weight oxygen, 100 ppm silicon and the remainder zirconium with technically unavoidable contaminants. In the case of both of these zirconium alloys, the .alpha. range extends to approximately 810.degree. C., the (.alpha.+.beta.) mixed range extends from approximately 810.degree. to 940.degree. C., and the B range begins at approximately 940.degree. C. The diameter of the blank or tubular body is 600 mm. Both blanks or tubular bodies are heated to a temperature in the .beta. range of 1150.degree. C. At this temperature, each of the two solid cylindrical blanks or tubular bodies is forged into a solid cylindrical starting body having a diameter of 350 mm. Next, the two starting bodies are again heated to a temperature in the .beta. range of 1150.degree. C., until such time as precipitated alloy components have dissolved. Then each starting body is quenched in water at a quenching rate of 35 K/s at the surface of the starting body during temperature passage through the (.alpha.+.beta.) range, in other words through the temperature range from 940.degree. to 810.degree. C. Finally, both starting bodies are left to cool to a temperature of approximately 100.degree. C. on the body surface. Then both starting bodies are annealed at a temperature in the .alpha. range, to form precipitates of the iron and chromium alloy components as secondary phases. The annealing temperature selected is 750.degree. C. for both starting bodies. As can be seen from the diagram of FIG. 1, the associated annealing time for forming the desired precipitates is 5 hours. In the alloy of the first starting body, precipitates of the alloy components iron and chromium form as secondary phases having a geometric mean value of 0.2 .mu.m, and in the alloy of the second starting body they form with a geometric mean value of 0.252 .mu.m. Next, the two starting bodies are hot-forged to a diameter of 150 mm at a temperature of 700.degree. C., which is in the .alpha. range. This hot-forging at 700.degree. C. can also be performed before the development of the precipitates of the alloy components in the starting body. After cooling to room temperature, a continuous hole which is 50 mm in diameter is drilled in the axial direction into each of the two cylindrical forged parts produced, forming a hollow cylinder. The hollow cylinders are reheated to a temperature in the .alpha. range of 700.degree. C. and hot-extruded with a cylinder press. The applicable hollow cylinder is pressed by a cylindrical die with an internal mandrel, producing a tube with an unchanged inside diameter of 50 mm and an outside diameter of 80 mm. Both tubes obtained by extrusion are then placed in a pilgering machine and pilgered in four pilgering steps to a finished tube having an inside diameter of 9.30 mm, an outside diameter of 10.75 mm, and thus a wall thickness of 0.72 mm. A pilgering machine is described in U.S. Pat. No. 4,233,834. In the diagram of FIG. 2, a pilgering path I associated with these four pilgering steps is shown with pilgering steps a from 0 to A, b from A to B, c from B to C, and d from C to D. A logarithmic cold work per pilgering step, for the first three pilgering steps a, b, c, is approximately .phi..sub.a =.phi..sub.b =.phi..sub.c =1.1 each time, which is equivalent to a cold-deformation C.sub.Wa and C.sub.Wb and C.sub.Wc of approximately 67% per pilgering step a or b or c. In the last pilgering step d, the logarithmic cold work .phi..sub.d =1.65, which is equivalent to a cold-deformation C.sub.Wd of 81% as a result of this pilgering step d. The ratio q.sub.a =q.sub.b =q.sub.c for the first three pilgering steps a, b and c, is approximately 1.1 each, and for the fourth and last pilgering step d, q.sub.d =approximately 5.5. Between each two pilgering steps, recrystallization annealing is carried out virtually without secondary recrystallization. The degree of recrystallization R.sub.x is 99%, for example. In the case of this degree of recrystallization, the result is good deformability of the tube for the ensuing pilgering step. Through the use of the diagram of FIG. 3, the annealing parameters A represented by the straight line II can be found for a cold-deformation C.sub.W of 67% and a degree of recrystallization R.sub.x at 99%. The intersections of the straight line II with the annealing isotherms define the annealing temperature .phi..sub.R and the associated annealing time t.sub.R, which are necessary to avoid secondary recrystallization in the recrystallization annealings between the pilgering steps. For instance, an annealing temperature of 590.degree. C. and an annealing time of 100 minutes, in a Zircaloy-4 tube having a cold-deformation of 67%, lead to a degree of recrystallization of 99%. The diagram of FIG. 3 is practically equally applicable to a tube made from the zirconium alloy of the second blank or tubular body. After the last pilgering step, both of the tubes are stress relieved in a final annealing. The result is a degree of recrystallization of at most 10%. In the diagram corresponding to FIG. 3, the maximum annealing parameter A for a cold-deformation of 81% and a degree of recrystallization R.sub.x of a maximum of 10% can be seen. The intersections of the straight line IV with the annealing isotherms define the annealing temperature .phi..sub.R and the associated annealing time t.sub.R for a suitable final annealing. For instance, an annealing temperature of 500.degree. C. and an associated annealing time of 400 minutes leads to the degree of recrystallization R.sub.x =10%. The two finished tubes thus obtained have a texture having the Kearns parameter of 0.63, which is determined substantially by the selection of the ratio q.sub.d for the fourth and last pilgering step d. Moreover, the geometric mean value of the diameter of the alloy components iron and chrome precipitated out of the matrix of the first zirconium alloy as second phases is practically unchanged at 0.2 .mu.m, and in the second zirconium alloy is also practically unchanged at 0.252 .mu.m. A material test sample is taken from each of the finished tubes. Both material samples are annealed for two minutes at a temperature of 660.degree. C. The zirconium alloy of the two test samples is found to have a degree of recrystallization of 99% and the geometric mean value of the grain size of the matrix is 2.8 .mu.m for both zirconium alloys. Both finished tubes are used as nuclear fuel-filled cladding or casing tubes of fuel rods and have such high corrosion resistance that they obtain a service life of 4 years in a pressurized water reactor without becoming damaged, while a service life of only 3 years is obtainable with typical tubes of Zircaloy-4.