JPH0510415B2 - - Google Patents

Description

[Detailed description of the invention]

[Field of Application of the Invention] The present invention relates to a novel highly corrosion-resistant zirconium-based alloy, and particularly to a fuel assembly member for BWR that has excellent nodule corrosion resistance. BACKGROUND OF THE INVENTION Zirconium-based alloys are used in structural members of fuel assemblies in nuclear power plants because of their excellent corrosion resistance and very small thermal neutron absorption cross sections. Since these members are used in a furnace for a long time, their corrosion resistance is especially important. Zircaloy-2 is a typical zirconium-based alloy.
(Zr contains approximately 1.5% Sn, approximately 0.1% Fe, approximately 0.1% Cr, approximately 0.1% Ni)
0.05% alloy) and “Zircaloy-4”
(an alloy in which approximately 1.5% Sn, approximately 0.2% Fe, and approximately 0.1% Cr is added to Zr) is known. In addition, Scatarch alloy (zirconium with Nb0.4~1.2%, Sn0.09% or less,
An alloy containing 0.04% Fe, 0.01-0.5% Cr, and approximately 0.3% Mo has been reported. Zircaloy-2 and Zircaloy-4 mentioned above are fully performing their functions under the current operating conditions of nuclear reactors, but in the future, due to longer operating periods in order to improve the economic efficiency of nuclear plants, even harsher operating conditions will be required. Since corrosion resistance is unstable due to the addition of In the structural members of BWR fuel assemblies made of zirconium-based alloys, speckled grayish-white corrosion products called nodular corrosion are generated on the surface. If this corrosion product grows significantly after being used in a nuclear reactor for a long period of time, there is a risk that it will peel off. There is a concern that the decrease in the wall thickness of the member due to this peeling will result in a decrease in the mechanical strength of the fuel structural member. Figure 3 is a schematic diagram of a BWR fuel assembly. First, a large number of fuel rods (roughly divided into fuel pellets 1, fuel cladding tubes 2 covering the pellets, and end plugs 3) and spacers 4 that hold them at a predetermined distance from each other are attached to the fuel rods. It consists of square tube channel boxes 5 extending along the longitudinal direction of the fuel rods on the outside, and these have an important role in terms of fuel efficiency and plant safety. Channel boxes are manufactured by forming plates into rectangular tubes and butt-welding the joints. The spacer is similarly assembled by welding. As shown in Figure 5, the fuel cladding tube is in contact with the fuel pellets on the inside and with the reactor water on the outside, and is a strength member as well as a barrier between corrosive gases (such as iodine) generated during combustion and the reactor water. Used in corrosive environments. It is particularly important to prevent nodule corrosion on the outer surface that comes into contact with reactor water. Fuel spacers secure a number of fuel rods at several locations along the length of the fuel assembly.
This prevents lateral vibration and longitudinal bending of the fuel rods. Figure 6 is a plan view of the spacer, Figure 7 is a plan view of the spacer.
The figure shows a side view of the spacer, in which the cladding tube 2 is fixed by a spacer grit 6 and a lantern-shaped leaf spring 7. For this reason, the spacer is used under stress from the fuel rods. Since this member comes into contact with reactor water, there is a risk of nodule corrosion. The fuel channel box is located outside the fuel rods assembled with the fuel spacer, and is used with the fuel rods fixed by the upper tie plate 8 and the lower tie plate 9. FIG. 8 shows an enlarged view of the fuel channel box, which has the shape of a rectangular cylinder made by welding 10 two-part processed plates. The member forcibly guides high-temperature water and steam generated in the fuel rods during operation to the top, and the square tube is used for a long time under stress that spreads outward. Since this member also comes into contact with reactor water, it is necessary that it has excellent nodule corrosion resistance. Furthermore, for parts, hydrogen is generated during the oxidation reaction of zirconium-based alloys (Zr + 2H ₂ O → ZrO ₂ +
2H ₂ ), and there is a concern that this hydrogen accumulates in the material, impairing the strength of the material and having an adverse effect on nodular corrosion resistance. The Zr-2.5%Nb alloy is used in pressure pipes at a Canadian plant. Mechanical properties of this material, improved irradiation growth Zr−2.5~4.0%Sn−0.5~1.5%Mo
There is a −0.5 to 1.5Nb alloy (Japanese Patent Application Laid-Open No. 134304/1983).
Also Scatarch alloy 4Zr-0.25~1.5%Nb-0.025
~0.2%Sn−0.02~1.0%(Cr+Mo), or Zr−
0.45~1.2%Nb−0.04~0.1%Sn−0.25~0.6%(Cr
+Mo) -0.7 to 0.7 to 1.8% (Nb+Cr+Mo) (Japanese Patent Application Laid-open No. 148213-1983). Also, Nb in Zircaloy-4
63 (1977)). However, with the exception of Zr-2.5%Nb alloy, most of these materials have no experience in use, and are said to be insufficient to replace Zircaloy alloys. [Object of the Invention] An object of the present invention is to provide a zirconium alloy with excellent nodular corrosion resistance. [Summary of the invention] The present invention contains 1 to 2% tin, 0.05 to 0.3% iron,
Contains 0.05-0.2% chromium, 0.1-0.3% niobium, 0.05-0.3% vanadium and 0.05-0.5% molybdenum,
This is a highly corrosion-resistant zirconium-based alloy characterized by the remainder being zirconium. The present inventors investigated ways to improve nodular corrosion resistance without significantly changing the chemical composition of Zircaloy, and as a result, discovered the following fact. An ingot made of a Zircaloy alloy with trace amounts of Mo, V, and Nb added was processed into a plate material through the steps of forging, hot plastic working, cold plastic working, and annealing, and then a corrosion test was conducted. As a result, it was found that addition of Mo, V and Nb significantly improved nodule corrosion resistance. This is known as β or β+, which is conventionally known as heat treatment for high corrosion resistance of Zircaloy.
It was found that manufacturing is easier and the corrosion resistance is dramatically improved compared to the method of quenching from the temperature of the α phase. Sn is 1 to 2% because it has the effect of suppressing the negative effect of reducing corrosion resistance due to nitrogen in zirconium-based alloys.
It is good to add it. If it is less than 1%, the effect is small, and if it is added more than 2%, there is no problem with corrosion resistance, but it is not good for mechanical properties. Addition of Fe is effective in improving the corrosion resistance and mechanical properties of zirconium-based alloys. Therefore Fe is 0.05
Contains ~0.3%. If Fe is less than 0.05%, the effect will be small, and if it is added more than 0.3%, it will have a negative effect on the core properties and the ductility of the member. Addition of a small amount of Mo is effective in improving nodular corrosion resistance. In addition to the addition of Mo, a small amount of Nb and V are added in combination, resulting in stable corrosion resistance with a small amount added. Mo is 0.05% or more, Nb is 0.1% or more, V
is effective when added at 0.05% or more. The reason for setting the upper limit for the amount of these elements added is due to the metallographic structure.
This is because a large amount of intermetallic compounds or metal second phases precipitate, and the elements that contribute to corrosion resistance are saturated in the matrix of the alloy, which are effective for nodular corrosion resistance. On the other hand, these precipitations are basically undesirable because they reduce the workability and ductility of the member. Moreover, addition of a large amount at the same time is not preferable in terms of nuclear properties. In other words, these elements have significantly larger thermal neutron absorption cross sections than Zr, so
This is because the neutron economy decreases. Addition of Cr is effective in improving the corrosion resistance of zirconium-based alloys. However, if Cr is added in an amount of 0.05% or less, the effect is small, and if it is added in an amount of 0.2% or more, a more significant effect cannot be expected. The remainder consists of zirconium, unavoidable impurities, and oxygen, which has a large effect on strength. In the chemical composition range of the present invention, there is no significant change in plastic workability or weldability compared to conventional materials, and in terms of corrosion resistance, fuel cladding tubes manufactured by normal manufacturing processes,
It has been found that the nodular corrosion resistance of fuel spacers and fuel channel boxes can be significantly improved. The zirconium-based alloy of the present invention exhibits particularly remarkable effects when subjected to processing in which cold working and annealing are repeated at least three times after hot working. In both hot working and annealing after cold working, intermetallic compounds due to added elements in the zirconium alloy precipitate, and the excessive precipitation reduces corrosion resistance. However, the alloy of the present invention has remarkable corrosion resistance because the precipitation is suppressed. Hot working temperature is 650~750℃ and annealing temperature is 550℃
Preferably it is carried out at ~700°C. In particular, the final annealing
Preferably, the temperature is 500-550°C. Furthermore, the zirconium-based alloy of the present invention has a remarkable effect particularly on butt welds. As mentioned above, precipitates are formed in the weld heat affected zone and the corrosion resistance is reduced, so this is effective in such cases. [Embodiments of the Invention] Example 1 Industrial pure Zr was used as the material, predetermined alloying elements were added, and an ingot was produced by arc melting. Table 1 shows its chemical composition. Each ingot was processed into a tube material by carrying out the material manufacturing process shown in FIG. In addition, the samples were divided into two types of hot working temperatures during the manufacturing process so that it was possible to confirm the presence or absence of the influence of the working temperature. The cold working rate is 70% in area reduction rate. No. 1 to 4, 6.9 to 13 are comparative materials, No. 5, 7,
8 is the material of the present invention. To evaluate corrosion resistance, a long pipe material was cut to a length of about 50 mm and a corrosion test piece was taken.
A corrosion test was conducted for 20 hours in 150Kg/cm ² steam. Second
The test results are shown in the table. The corrosion characteristics of the comparison material are No.
The corrosion weight increase was 450 mg/dm ² in the A-processed material of No. 1, and more than 1000 mg/dm ² in the B-processed material, and an extremely large amount of nodular corrosion was observed in the external observation of the test piece. Note that the characteristic of processed material A is that the corrosion weight increase is lower than that of processed material B, but there is not much difference in terms of nodular corrosion susceptibility.

【table】

【table】

〔Effect of the invention〕

According to the present invention, a zirconium alloy with extremely excellent nodular corrosion resistance can be obtained, and therefore highly reliable fuel cladding tubes, fuel spacers, and fuel channel boxes using the same can be expected.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the relationship between corrosion weight increase and Mo, Nb, Mo+Nb addition amount, Figure 2 is a diagram showing the relationship between tensile strength and elongation and Nb or Mo+Nb addition amount, and Figure 3 is a fuel assembly. Figure 4 is a manufacturing process diagram of each member of the fuel assembly, Figure 5 is an exploded view of the fuel rod,
FIG. 6 is a plan view of the fuel spacer, FIG. 7 is a side view of the fuel spacer, FIG. 8 is a perspective view of the fuel channel box, and FIG. 9 is a manufacturing process diagram of the tube material. 1...Fuel pellets, 2...Fuel cladding tube, 3...
...End plug, 4...Fuel spacer, 5...Fuel channel box, 6...Spacer brit, 7...
Lantern template spring, 8... Upper tie plate, 9
...Lower tie plate, 10...Welding part.

Claims (1)

[Claims] 1 By weight: 1-2% tin, 0.05-0.3% iron, 0.05-0.2% chromium, 0.1-0.3% niobium, vanadium
A highly corrosion-resistant zirconium-based alloy containing 0.05-0.3% molybdenum and 0.05-0.5% molybdenum, with the balance being zirconium. 2. The highly corrosion-resistant zirconium-based alloy according to claim 1, wherein the alloy is subjected to cold plastic working and annealing after the working, and has precipitates. 3. The highly corrosion-resistant zirconium-based alloy according to claim 1 or 2, wherein the alloy is joined by welding and has a desired shape.