Patent Number: 058928073
Section: description

DETAILED DESCRIPTION OF THE INVENTION Zircaloy-4 and Zircaloy-2 are much stronger and have much better creep resistance than unalloyed commercially pure zirconium. Zirconium alloys can typically be strengthened by two mechanisms; solid solution hardening and precipitation hardening. A combination of these strengthening mechanisms is employed in many high strength zirconium alloys. The most prominent precipitation strengthener is niobium. It is among others used in the Russian developed zirconium alloys having 1% niobium, 1.2% tin, 0.4% iron, and in the zirconium 2.5%-2.8% niobium alloy used in Canada for CANDU pressure tubes. Other precipitation strengtheners are molybdenum and silicon. The strength of Zircaloy-4 and Zircaloy-2 derive mainly from addition of tin which, because of its solubility in the zirconium matrix, is a solid solution strengthener. The atomic radius of tin, 0.1584 nm, is nearly the same as that of zirconium, 0.1602 nm, and tin atoms take the place of or substitute for zirconium atoms in the crystallographic lattice of the alloy. Tin, therefore, is also called a substitutional alloying element when used in zirconium base alloys. The addition of iron and chromium to Zircaloy-4 and iron, chromium and nickel to Zircaloy-2 does not substantially affect the mechanical properties of these alloys since these elements are nearly insoluble in the zirconium alpha phase and are added in small amounts only. These alloy elements are added mainly to improve the corrosion behavior of the Zircaloys. At reactor operating temperatures and below, these transitional elements are present in the form of small intermetallic particles with the approximate compositions Zr(CrFe).sub.2 or Zr.sub.2 (NiFe). It has been determined in the present invention that the addition of certain alloying elements to zirconium produces alloys possessing improved strength and creep resistance. More particularly, the addition of bismuth making up about 1.5 to 6 weight percent and an element or mixtures of elements selected from the group consisting of molybdenum, tin and niobium, making up about 1 to 4 weight percent tin, 0.5 to 3 weight percent niobium and/or 0.5 to 1.5 weight percent molybdenum, the balance being zirconium, produces alloys which possess substantial improvement in strength and creep resistance. In accordance with the present invention, alloys for use as the inner layer of two layered cladding tube or an inner layer of a three or more layered cladding tube having high strength and improved creep behavior as well as reduced parasitic neutron absorption comprise zirconium with an addition of from 1.5 to 6 weight percent bismuth (Bi). Similar to tin, bismuth is a solid solution strengthener. The atomic radius of bismuth is 0.1700 nm compared to the atomic radius of zirconium which is 0.1602 nm. This makes bismuth a substitutional alloying element similar to tin. The added advantage of using bismuth as an alloying element is its very low thermal neutron cross section; 0.034 barns compared to zirconium with a cross section of 0.184 barns. The thermal neutron cross section of tin is 0.610 barns. Whereas the addition of tin to zirconium increases the parasitic neutron absorption of the alloy over that of pure zirconium metal, the addition of bismuth lowers the parasitic neutron absorption by the alloy compared to either zirconium metal or to Zircaloy. The following zirconium alloys with concentration levels of alloying elements have higher yield strength and creep resistance than Zircaloy and zirconium. I. Ternary Alloys a. Zirconium-Bismuth-Molybdenum alloys with 3-6 weight percent Bismuth, and Molybdenum, balance Zirconium, preferably 0.5 to 1.5 weight percent Molybdenum b. Zirconium-Bismuth-Tin alloys with 1-4 weight percent Tin and 1.5-6 weight percent Bismuth, balance Zirconium II. Quarternary Alloys a. Zirconium-Bismuth-Molybdenum-Niobium alloys with (A) 3-6 weight percent Bismuth, and Molybdenum and Niobium, balance Zirconium, preferably 0.5-1.5 weight Percent Molybdenum and 0.5-3 weight Percent Niobium; and PA1 (B) 1.5-3 weight percent Bismuth, 0.5-3.0 weight percent Niobium and 0.5-1.5 weight percent Molybdenum, balance Zirconium where the sum of Molybdenum and Niobium is greater than 1.5 weight percent b. Zirconium-Bismuth-Molybdenum-Tin alloys with 1-4 weight percent Tin, 1.5-6 weight percent Bismuth, and Molybdenum, balance Zirconium preferably 0.5-1.5 weight percent Molybdenum III. Quinary Alloys a. Zirconium-Bismuth-Molybdenum-Tin-Niobium alloys with 1-4 weight percent Tin. 1.5-6 weight percent Bismuth, and Molybdenum and Niobium, balance Zirconium preferably 0.5-1.5 weight percent Molybdenum and 0.5-3 weight percent Niobium All the above alloys could furthermore contain up to approximately 0.1 weight percent silicon for added strength and grain refinement purposes. In a preferred embodiment, the minimum silicon content should be 0.008 weight percent (80 ppm). These alloys could also contain between approximately 0.008 and 0.02 weight percent (80 and 200 ppm) of carbon for grain size control. The oxygen level in the above alloys could be adjusted to fall in the range of 0.06 and 0.18 weight percent (600-1800 ppm) and preferably in the range of 0.06 to 0.09 weight percent (600-900 ppm) in order to impart low temperature strength to the alloys. Referring to the drawings, FIG. 1 represents a nuclear fuel assembly 10 for a pressurized water reactor (PWR) comprising a lower tie plate 12, guide tubes 14, nuclear fuel rods 18 which are spaced radially and supported by spacer grids 16 spaced along the guide tubes, an instrumentation tube 28, and an upper tie plate 26 attached to the upper ends of the guide tubes. Control rods which are used to assist in controlling the fission reaction are disposed in the guide tubes during reactor operation, but are not shown. Each fuel rod 18 generally includes a metallic tubular fuel rod cladding 110 (120) within which are nuclear fuel pellets 80 composed of fissionable material and an upper end plug 22 and a lower end plug 24 which hermetically seal the nuclear fuel pellets within the metallic tubular fuel rod cladding. A helical spring member can be positioned within the fuel rod between upper end plug 22 and fuel pellet 80 to maintain the position of the fuel pellets in a stacked relationship. Referring to FIG. 2A which is a schematic representation of a cross-sectional view of a nuclear fuel rod for a PWR such as shown in FIG. 1 constructed according to the teachings of the present invention having a composite cladding 110 comprising an outer layer 111 composed of a corrosion resistant zirconium and/or zirconium alloy metal and an inner layer 114 bonded metallurgically to inner wall 113 of outer layer 111 and composed of a zirconium alloy consisting essentially of molybdenum and 3 to 6 weight percent bismuth and the balance zirconium, and preferably where the amount of molybdenum is in the range of 0.5 to 1.5 weight percent. In another embodiment, the inner layer 114 is made from another zirconium alloy consisting essentially of molybdenum, niobium, and 3 to 6 weight percent bismuth and the balance zirconium, and preferably where the amount of molybdenum is in the range of 0.5 to 1.5 weight percent and the amount of niobium is in the range of 0.5 to 3 weight percent. In another embodiment, inner layer 114 is composed of a zirconium alloy consisting essentially of 1.5 to 6 weight percent bismuth and 1 to 4 weight percent tin, the balance zirconium. In another embodiment, inner layer 114 is composed of a zirconium alloy consisting essentially of molybdenum and 1.5 to 6 weight percent bismuth and 1 to 4 weight percent tin, the balance zirconium. In another embodiment, inner layer 114 is composed of a zirconium alloy consisting essentially of molybdenum, niobium, and 1.5 to 6 weight percent bismuth and 1 to 4 weight percent tin, the balance zirconium, and preferably where the amount of molybdenum ranges from 0.5 to 1.5 weight percent and the amount of niobium ranges from 0.5 to 3 weight percent. In another embodiment, inner layer is composed of a zirconium alloy consisting essentially of 1.5 to 3 weight percent bismuth, 0.5 to 3 weight percent niobium, 0.5 to 1.5 weight percent molybdenum, the balance zirconium, where the sum of the amounts of niobium and molybdenum is greater than 1.5 weight percent. Referring to FIG. 2B which is a schematic representation of a cross-sectional view of another nuclear fuel rod for a PWR such as shown in FIG. 1 constructed according to the teachings of the present invention having a composite cladding 120 comprising an outer layer 121 composed of a corrosion resistant zirconium and/or zirconium alloy metal, an inner layer 124 composed a high strength zirconium alloy and an innermost layer 127 or liner bonded metallurgically on the inside surface 126 of the inner layer 124. In accordance with the present invention, inner layer 124 of composite cladding 120 is composed of a high strength zirconium alloy consisting essentially of molybdenum and 3 to 6 weight percent bismuth and the balance zirconium, and preferable where the amount of molybdenum is in the range of 0.5 to 1.5 weight percent. In another embodiment, the inner layer 124 is made from another zirconium alloy consisting essentially of molybdenum, niobium, and 3 to 6 weight percent bismuth and the balance zirconium, and preferably where the amount of molybdenum is in the range of 0.5 to 1.5 weight percent and the amount of niobium is in the range of 0.5 to 3 weight percent. In another embodiment, inner layer 124 is composed of a zirconium alloy consisting essentially of 1.5 to 6 weight percent bismuth and 1 to 4 weight percent tin, the balance zirconium. In another embodiment, inner layer 124 is composed of a zirconium alloy consisting essentially of molybdenum and 1.5 to 6 weight percent bismuth and 1 to 4 weight percent tin, the balance zirconium. In another embodiment, inner layer 124 is composed of a zirconium alloy consisting essentially of molybdenum, niobium, and 1.5 to 6 weight percent bismuth and 1 to 4 weight percent tin, the balance zirconium, and preferably where the amount of molybdenum ranges from 0.5 to 1.5 weight percent and the amount of niobium ranges from 0.5 to 3 weight percent. In another embodiment, inner layer 124 is composed of a zirconium alloy consisting essentially of 1.5 to 3 weight percent bismuth, 0.5 to 3 weight percent niobium, 0.5 to 1.5 weight percent molybdenum, the balance zirconium, where the sum of the amounts of niobium and molybdenum is greater than 1.5 weight percent. To provide additional protection against pellet cladding interactive (PCI) induced failures, innermost layer 127 can be zirconium or a zirconium alloy, or another metal and preferably is made of pure or sponge zirconium or a dilute zirconium iron alloy of about 0.4 weight percent iron. While the foregoing description and drawings represent the preferred embodiments of the present invention, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the true spirit and scope of the present invention.