Patent Number: 060144181
Section: description

The Figures the following reference numbers: 1 fuel rod 2 cladding tube 3 UO.sub.2 fuel pellet 4 end plug 5 spring 6 bead 7 heat affected zone 8 equiaxed structure 9 acicular structure 10 grain boundary DETAILED DESCRIPTION OF THE INVENTION The present invention may provide (1) A fuel rod for a light water reactor, comprising a cladding tube which comprises a zirconium alloy containing Nb and Fe; uranium oxide fuel pellets packed in said cladding tube; and end plugs comprising a zirconium alloy and closing both ends of said cladding tube, said cladding tube being sealed with said end plugs by TIG welding, wherein grain boundaries in each heat affected zone of said cladding tube adjacent to a bead formed by TIG welding of said cladding tube with said end plug have structural compositions including 4 to 30% by weight of Nb, and 0.9 to 20% by weight of Fe. The present invention may also provide (2) a method for manufacturing a fuel rod for a light water reactor, comprising: packing uranium oxide fuel pellets into a cladding tube which comprises a zirconium alloy containing Nb and Fe; closing both ends of said cladding tube with end plugs comprising a zirconium alloy; and sealing by TIG welding said cladding tube together with said end plugs, wherein: each heat affected zone of said cladding tube which is adjacent to a bead formed by TIG welding said cladding tube with said end plug is cooled at a rate of 70.degree. C./sec. to 5.degree. C./sec. According to the present invention, the zirconium alloy containing Nb and Fe for the cladding tube to be used in the fuel rod for a light water reactor has a composition including 0.6 to 2.0% by weight of Nb, 0.5 to 1.5% by weight of Sn, 0.05 to 0.3% by weight of Fe, and the balance being Zr and incidental impurities. Preferably, the zirconium alloy has a composition including 0.8 to 1.2% by weight of Nb, 0.8 to 1.1% by weight of Sn, 0.08 to 0.12% by weight of Fe, and the balance being Zr and incidental impurities. Conventional Zircaloy-2 (JIS H4751ZrNT802D) or Zircaloy-4 (JIS H475 1ZrNT804D) is used for the end plugs which close both ends of the cladding tube comprising the zirconium alloy containing Nb and Fe since the end plugs do not greatly affect the life span of the fuel rod for a light water reactor, even if they are corroded. A fuel rod for a light water reactor of the present invention comprises a cladding tube which comprises a zirconium alloy containing Nb and Fe; uranium oxide fuel pellets packed in the cladding tube; and end plugs comprising a zirconium alloy and closing both ends of said cladding tube, where the cladding tube is sealed with the end plugs by TIG welding. Grain boundaries in each heat affected zone of the cladding tube which is adjacent to a bead formed by TIG welding have compositions including 4 to 30% by weight of Nb and 0.9 to 20% by weight of Fe. In some cases, where Cr is contained in the alloy as an incidental impurity, Cr can also segregate at grain boundaries and can be detected. In the fuel rod for a light water reactor according to the present invention, for example, the following methods of cooling while controlling the cooling rate at 70.degree. C./sec. to 5.degree. C./sec. may be employed: (i) TIG welding is carried out while both areas adjacent to each portion to be welded are covered with a heat insulating material, and following sufficient cooling after completion of the welding, the heat insulating material is removed; or (ii) after welding, the cladding tube and end plugs are subjected to induction heating or heating by direct heating with electricity. When the heat affected zones due to welding are cooled at a cooling rate of greater than 70.degree. C./sec., the concentrations of Nb and Fe at the grain boundaries fall below 4% by weight and 0.9% by weight, respectively, and sufficient corrosion resistance cannot be achieved. On the other hand, when the heat affected zone due to welding is cooled at a cooling rate of less than 5 .degree. C./sec., further improvement in corrosion resistance cannot be achieved, since the concentrations of Nb and Fe at the grain boundaries in the heat affected zone formed by the welding do not exceed 30% by weight and 20% by weight, respectively, even with a much slower cooling rate. On the contrary, such a slow cooling rate causes the strength of the fuel rod to deteriorate. EXAMPLES Example 1 Zirconium alloy cladding samples were prepared which had dimensions of 10 mm in diameter and 0.6 mm in thickness, having a composition including 1.0% by weight of Nb, 1.0% by weight of Sn, 0. 1% by weight of Fe, and the balance being Zr and incidental impurities. Each zirconium alloy cladding sample was TIG welded under the conditions described below at its ends with zirconium alloy end plugs which had a composition including 1.5% by weight of Sn, 0.2% by weight of Fe, 0. 1% by weight of Cr, and the balance being Zr and incidental impurities. The cooling rate at the heat affected zones of each cladding sample was controlled by a method shown in Table 1. As a result, Samples 1 to 6 according to the present invention, Comparative Samples 1 and 2, and Conventional Samples 1 and 2 were manufactured, in which the concentrations of Nb and Fe at grain boundaries in heat affected zones were as shown in Table 1, respectively. TIG Welding Conditions: Current: 30 A PA1 Voltage: 15 V PA1 Welding rate: 500 mm/min. PA1 Cooling gas: 25 liter/min. He PA1 Voltage: 20 V PA1 Current: 100 mA PA1 Temperature: -40.degree. C. PA1 Solution: 5% perchloric acid-methanol Samples 1 to 6 according to the present invention, Comparative Samples 1 and 2, and Conventional Samples 1 and 2, which were zirconium alloy cladding samples having heat affected zones, were subjected to chemical milling in a nitric-hydrofluoric acid solution [HNO.sub.3 :HF:H.sub.2 O=45:5:50 (% by volume)] to a thickness 100 .mu.m, and were cut into disks having a diameter of 3 mm. Subsequently, the disks were subject to electrolytic milling under the conditions described below to prepare foil samples for examination by Transmission Electron Microscopy. Electrolytic Milling Conditions: The above-obtained foil samples for examination by Transmission Electron Microscopy from Samples 1 to 6 according to the present invention, Comparative Samples 1 and 2, and Conventional Samples 1 and 2 were examined with an accelerating voltage of 200 kV and a magnification of 50,000, and no precipitates of intermetallic compounds were found. Additionally, contents of Nb and Fe at the grain boundaries were measured by Energy Dispersive X-ray Analysis, with the results shown in Table 1. Samples 1 to 6 according to the present invention, Comparative Samples 1 and 2, and Conventional Samples 1 and 2 were placed in an autoclave, and subject to autoclave tests in purified water having a high temperature of 360.degree. C. for 120 days in order to examine color change in the heat affected zones of the zirconium alloy cladding samples. The results are shown in Table 1. TABLE 1 __________________________________________________________________________ Nb and Fe Concentrations at Appearance after Autoclave Grain Boundaries Test Cooling Rate (% by weight) (in 360.degree. C. Pure Water for Sample Type & No. (.degree. C./sec.) Controlling Method of Cooling Rate Nb Fe 120 Days) Remarks __________________________________________________________________________ Samples of the 1 68 Removal of the Chilling Block 4.3 0.95 Black -- Present Invention 2 45 Covering with a Heat-Insulating material 5.2 2.7 Black -- 3 30 Direct Heat with Electricity 6.3 4.8 Black -- 4 22 Induction Heating of the Cladding Tube 11 7.3 Black -- 5 15 Induction Heating of the Cladding Tube and 15e 10 Black -- End Plug Portions 6 6.0 Induction Heating of the Cladding Tube and 28e 25 Black -- End Plug Portion Comparative 1 3.8 Induction Heating of the Cladding Tube and 27e 23 Black Insufficient Samples End Plug Portions Strength 2 1.5 Induction Heating of the Cladding Tube and 25e 25 Black Insufficient End Plug Portions Strength Conventional 1 100 Natural Cooling 3.1 0.50 White (Peeled) -- Samples 2 80 Decreasing the Flow of Cooling Gas 3.3 0.65 White -- __________________________________________________________________________ As shown in Table 1, each heat affected zone of Conventional Samples 1 and 2, which were allowed to cool or were cooled with a cooling rate close to the natural cooling, turned white and had inferior corrosion resistance. On the other hand, each heat affected zone of Samples 1 to 6 according to the present invention and Comparative Samples 1 and 2, which were cooled while controlling the cooling rate at 70 to 5.degree. C./sec., and in which the concentrations of Nb and Fe at the grain boundaries in their structure were 4 to 30% by weight and 0.9 to 20% by weight, respectively, had a black color and had satisfactory corrosion resistance. Comparative Samples 1 and 2 with heat affected zones which cooled at a cooling rate of less than 5.degree. C./sec., were undesirably softened and had insufficient strength. As described above, the fuel rod for a light water reactor according to the present invention, which has improved corrosion resistance as compared to conventional rods, allows for highly efficient and highly reliable operation, and therefore, greatly contribute to the development of the atomic industry. Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein. The priority document of the present application, Japanese Patent Application No. 08-211281, filed on Aug. 9, 1996, is hereby incorporated by reference.