Patent Number: 051907216
Section: description

DETAILED DESCRIPTION OF THE INVENTION The alloys of this invention can be melted by methods similar to conventional methods for alloying zirconium. The alloying is accomplished preferably by arc melting a zirconium billet having the desired amount of the alloying metals encased in a hollow portion of the billet. The alloys of this invention can contain the conventional impurities found in sponge zirconium and zirconium alloys in conformance with ASTM B 349, 1991 Annual Book of ASTM Standards, Vol. 12.01, pp. 1-2, incorporated herein by reference. The melt is cast as an alloy billet, which can be finished by conventional zirconium alloy processing methods to produce the final shapes. Referring to FIG. 1, a nuclear fuel material 16 forming the central core of a fuel element 14 is surrounded by a cladding container 17, which is a composite cladding. The composite cladding container 17 encloses the core 16 so as to leave a gap 23 between the core and the cladding container. The composite cladding container 17 is comprised of a conventional zirconium alloy tube 21, for example formed from Zircaloy-2 or Zircaloy-4. The alloy tube 21 has bonded on the inside surface thereof a barrier layer 22 so that the barrier layer 22 forms a shield between the alloy tube 21 and the nuclear fuel material 16 facing the barrier 22. The barrier layer 22 forms about 1 to about 30 percent of the thickness of the cladding and is characterized by low neutron absorption. The barrier 22 protects the zirconium alloy tube portion of the cladding from contact and reaction with gases and fission products from the nuclear fuel, and minimizes the localized stress and strain on the cladding from the expansion of the nuclear fuel. The improved corrosion resistance and good ductility, as compared to sponge zirconium, of the alloys of this invention make the alloys especially suitable for use as the barrier layer 22 in the nuclear fuel cladding. In the following examples, sheet samples of sponge zirconium and high-purity zirconium were compared to alloys of sponge zirconium and high-purity zirconium comprised of tin, bismuth, or bismuth and niobium. Conventionally processed plates of the sponge zirconium, high-purity zirconium, or zirconium alloys about 1.3 centimeters thick were obtained. The composition of the plates is shown in the Tables below, where the term "sponge" means the moderate-purity zirconium. Some of the plates were cold reduced to 0.762 millimeter thick sheet, and annealed at 576.degree. C. for 2 hours in vacuum. Some plates were given the same processing, but were beta quenched prior to the cold reduction and annealing. The beta quenching was performed by heating the plates to about 1000.degree. C. for 1 minute, and rapidly cooling the plate by quenching in water. Tests for both uniform corrosion resistance and nodular corrosion resistance were conducted on samples about 0.762 by 11.2 by 22.5 millimeters cut from the sheet. Resistance to uniform corrosion was measured by exposing samples to steam at 400.degree. C., and 1500 psig for 7 days, conditions which can produce the uniform corrosion in an accelerated test that is found on cladding in the core of a nuclear reactor. Resistance to nodular corrosion was measured by exposing samples to steam at 1500 psig, and 410.degree. C. for 4 hours, followed by 520.degree. C. for 16 hours, conditions which can produce the nodular corrosion that is sometimes found on cladding in the core of a nuclear reactor. Samples were also tested under conditions that are believed to promote stress corrosion cracking in the fuel cladding. Tension test samples were milled from the sheet, and tested in a conventional stress corrosion cracking test, see L. F. Coffin, "Localized Ductility Method for Evaluating Zircaloy-2 Cladding," ASTM STP 681, 1977, p. 72, and D. S. Tomalin, "Localized Ductility of Irradiated Zircaloy-2 Cladding in Air and Iodine Environments," ASTM STP 633, 1976, pp. 557-572. The test samples were heated to 280.degree. C. and exposed to an atmosphere of argon or iodine while being strained in tension. The tension test was performed as a slow strain rate test, straining at about 1 mil per minute to failure. In addition, the alloys of this invention were tested for hydriding resistance, another factor that is believed to promote stress corrosion cracking in the cladding. Samples were heated to 365.degree. C. in a hydrogen atmosphere comprised of 3 percent water for 1 to 4 weeks, and the hydrogen absorbed in the sample was measured. The hydrogen absorption was measured by the conventional Leco fusion technique. EXAMPLE 1 The Knoop Hardness Number (KHN) was determined for samples of the annealed sheet on the longitudinal surface using a 300 gram load in conformance with the Knoop Hardness Test Method, ASTM E 384, 1991 Annual Book of ASTM Standards, Vol. 03.01. The hardness test results are shown in FIG. 2 where hardness is plotted on the ordinate, and the sample composition is shown on the abscissa. EXAMPLE 2 Table I below lists the composition of the sheet samples of alloys of this invention, and a sponge zirconium sheet sample. The weight gain from uniform corrosion testing is shown for each sample. TABLE I ______________________________________ Weight Gain From Uniform Corrosion Test Weight Gain Sn Bi Nb Zr (mg/dm.sup.2) Comment ______________________________________ Sponge 966 Spalling Oxide 0.5 Bal. 1251 Spalling Oxide 0.4 0.1 Bal. 1115 0.4 0.15 Bal. 944 Edge Corrosion 0.4 0.2 Bal. 885 Edge Corrosion ______________________________________ EXAMPLE 3 Table II below lists the composition of the sheet samples of alloys of this invention, and a sponge zirconium sheet sample. The weight gain from nodular corrosion testing is shown for each sample. TABLE II ______________________________________ Weight Gain From Nodular Corrosion Test Weight Gain Sn Bi Nb Zr (mg/dm.sup.2) Comment ______________________________________ Sponge 6952 Nodules 0.25 Bal. 5120 Nodules 0.50 Bal. 5223 Nodules 0.75 Bal. 5455 Nodules 1.0 Bal. 6320 Nodules 0.5 Bal. 2141 Few Nodules 0.4 0.1 Bal. 1608 Few Nodules 0.4 0.15 Bal. 952 Few Nodules 0.4 0.2 Bal. 968 Black Oxide *BQ 0.4 0.1 Bal. 1595 Few Nodules *BQ 0.4 0.15 Bal. 684 Black Oxide *BQ 0.4 0.2 Bal. 862 Black Oxide ______________________________________ *BQ Beta Quenched plates EXAMPLE 4 Table III below lists the composition of the sheet samples of alloys of this invention, high-purity zirconium samples, and sponge zirconium sheet samples. The tensile properties under conditions that simulate stress corrosion cracking in nuclear fuel cladding, and hydriding resistance for each sample is shown. The results from a hydriding test on a sample of Zircaloy-2 is also shown for comparison. TABLE III __________________________________________________________________________ Tensile and Hydriding Properties Uniform Hydrogen (ppm) Additions Yield Str. Elongation Exposed Zirconium Sn Bi Nb (ksi) (percent) 1 wk 4 wk __________________________________________________________________________ Sponge Argon 26.1 13.1 15 25 Sponge Iodine 21.6 11.8 21 32 Sponge Iodine 22.3 11.8 11 24 Sponge 0.5 Argon 25.9 6.1 22 95 Sponge 0.5 Iodine 23.7 6.3 Sponge 0.5 Iodine 28.3 6.1 High-Purity Argon 18.4 20.2 26 37 High-Purity 0.1 Argon 16.1 15.9 15 30 High-Purity 0.5 Argon 18.01 20.7 16 35 High-Purity 1.0 Argon 24.1 15.9 28 44 Sponge 0.4 0.1 Argon 25.4 12.1 8 14 Sponge 0.4 0.15 12 14 Sponge 0.4 0.2 Argon 28.7 5.0 9 10 *BQ Sponge 0.4 0.1 Argon 31.9 10 *BQ Sponge 0.4 0.15 Argon 39.1 10 *BQ Sponge 0.4 0.2 Argon 27.4 8.6 Sponge 0.25 Argon 24.7 6.6 26 39 Sponge 0.5 Argon 26.4 8.1 18 44 Sponge 0.75 Argon 22.6 6.5 19 24 Sponge 1.0 Argon 32.2 7.8 21 25 ZR-2 Tube 44 51 __________________________________________________________________________ *BQ Beta Quenched plates By making reference to FIG. 2 it can be seen that sponge zirconium has a relatively low hardness as compared to the conventional cladding material Zircaloy-2. It is believed the much lower hardness of sponge zirconium is one reason why it provides improved resistance to stress corrosion cracking as the barrier layer in Zircaloy-2 cladding. FIG. 2 also shows the zirconium alloys of this invention comprised of bismuth and niobium have a low hardness that is comparable to the relatively low hardness of sponge zirconium. Therefore it is believed the alloys of this invention will improve resistance to stress corrosion cracking in a manner comparable to sponge zirconium. The corrosion testing results in Tables I and II above, show that the alloys of this invention containing bismuth and niobium have improved corrosion resistance over sponge zirconium, and alloys comprised of tin and zirconium. The samples of sponge zirconium, or tin-zirconium alloys exhibited heavy nodular corrosion and weight gains of over 5000 mg/dm.sup.2, as compared to the few nodules and less than 2000 mg/dm.sup.2 weight gain found on the samples of alloys of this invention after the nodular corrosion test. From Table III above, it can be seen that the alloys of this invention have ductility, as measured by uniform elongation, that is comparable to sponge zirconium. It is believed that high ductility is another important property for improving resistance to stress corrosion cracking in the cladding. Sponge zirconium samples comprised of 0.5 percent bismuth had a uniform elongation about half the uniform elongation of the sponge zirconium samples. Surprisingly, alloys of this invention comprised of bismuth and niobium had a uniform elongation that was comparable to the uniform elongation of sponge zirconium. Both beta quenched and non-beta quenched samples of alloys of this invention had ductility comparable to sponge zirconium. The comparable corrosion resistance and ductility of beta quenched as compared to non-beta quenched samples, shows the relative insensitivity to heat treatment of the alloys of this invention. Alloys of this invention also have a higher hydriding resistance as compared to sponge zirconium comprised of bismuth, high-purity zirconium comprised of bismuth, sponge zirconium comprised of tin, and Zircaloy-2 tubing. Since hydriding resistance has been shown to be related to stress corrosion cracking, the alloys of this invention should have improved resistance to stress corrosion cracking.