JP2008122309A - Fuel cladding tube for boiling water reactor and method for manufacturing it - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fuel cladding tube for a boiling water reactor and a method for manufacturing it which make it possible to maintain its better corrosion resistance and mechanical characteristics by specifying the range of ideal cladding thicknesses for catching up with higher burnups as an oxide film formed on the surface of the fuel cladding tube to make its hydrogen absorption suppressibilities better than ever. <P>SOLUTION: In the fuel cladding tube for a boiling water reactor made of zircalloy, which is a fuel cladding tube for higher-burnup fuel assemblies whose average burnup is 45 GWd/t or higher; the oxide film whose thickness ranges between 0.05 μm and 0.2 μm inclusive is formed on the outer surface of the cladding tube. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a cladding tube of a fuel element such as a fuel rod constituting a fuel assembly for a boiling water reactor, and more specifically, formed on the outer surface of a fuel cladding tube for a high burnup fuel assembly. It relates to an oxide film.

  Conventionally, a cladding tube made of a zirconium alloy has been used for each nuclear fuel element such as a fuel rod for a boiling water reactor. Zirconium alloy is a material with excellent corrosion resistance, but the amount of corrosion tends to increase as the combustion of the fuel element proceeds in the nuclear reactor. When hydrogen absorption accompanying corrosion occurs in this zirconium alloy fuel cladding tube, hydrogen embrittlement of the fuel cladding tube proceeds.

  Therefore, by improving the corrosion resistance of the zirconium alloy fuel cladding tube, it suppresses hydrogen generated due to corrosion and at the same time suppresses the hydrogen from being absorbed into the fuel cladding tube, thereby reducing the risk of hydrogen embrittlement. It is hoped that

As a means for suppressing the hydrogen absorption of the fuel cladding tube, there is a method of forming an oxide film on the outer surface of the cladding tube. For example, the effect of suppressing hydrogen absorption by increasing the barrier effect by applying an oxide film of 0.2 μm or more to the outer surface of the cladding tube has been confirmed (see, for example, Patent Document 1). .)
JP-A 63-179286

  On the other hand, as part of the advancement of nuclear power generation, fuel burnup is being promoted in order to effectively use uranium resources and reduce the amount of spent fuel generated. The burnup of the boiling water reactor fuel assembly is currently about 45 GWd / t, but in the future, raising the burnup to a higher burnup side is under consideration.

  However, as the fuel burns up, the fuel stays in the furnace for a long period of time. As a result, it is clear that the hydrogen absorption of the fuel cladding tube increases. Therefore, as a means of suppressing hydrogen absorption in the fuel cladding, the conventional method of applying an oxide film to the surface of the cladding is an effective method. It is important to set the film thickness within a suitable range.

  However, in the conventional oxide film formation technology as described above, the hydrogen absorption test is performed at 450 ° C. for 3 hours in hydrogen gas and at 70 ° C. for 300 hours in a hydrochloric acid solution. It is only a test in a relatively short time and at a low temperature, and at present, sufficient studies have not yet been made from the viewpoint of optimizing an oxide film assuming a high burnup.

  In view of the above problems, the object of the present invention is to specify a suitable film thickness range corresponding to high burnup as an oxide film formed on the surface of the fuel cladding tube, and to improve the hydrogen absorption suppression specification than before. An object of the present invention is to provide a fuel cladding tube for a boiling water reactor capable of maintaining good corrosion resistance and mechanical properties, and a method for producing the same.

  In order to achieve the above object, a boiling water reactor fuel cladding tube according to the first aspect of the present invention is a boiling water reactor fuel cladding tube made of a zirconium alloy having a fuel assembly average burnup of 45 GWd. This is a fuel cladding tube for a high burnup fuel assembly of / t or more, and an oxide film having a film thickness of 0.05 μm or more and 0.2 μm or less is formed on the outer surface of the cladding tube.

  According to a second aspect of the present invention, there is provided a method of manufacturing a fuel clad tube for a boiling water reactor, which uses a billet formed from a Zircaloy alloy ingot to refine a tube that has undergone a cold rolling process and a vacuum annealing process. A method for producing a fuel cladding tube for a boiling water reactor, comprising a step of forming an oxide film on the outer surface of the cladding tube, wherein the oxide film forming step is an atmospheric annealing as an oxide film forming method. Any one of a method, an anodic oxidation method, and an autoclave method is used.

  In the fuel cladding tube for a boiling water reactor according to the present invention, the fuel assembly average burnup is 45 GWd / t or more by setting the film thickness of the oxide film formed on the outer surface of the cladding tube to a specific range. Even under high burn-up conditions, the hydrogen absorption suppression characteristics are improved as compared with the conventional fuel cladding tube, and there is an effect that better corrosion resistance and mechanical characteristics can be maintained.

  In the present invention, the film thickness of the oxide film formed on the outer surface of the fuel cladding tube for boiling water reactors is set to a range showing an excellent hydrogen absorption suppression effect even in a long-time corrosion treatment as described later, 0.05 μm. As described above, by specifying 0.2 μm or less, it is possible to maintain better corrosion resistance and mechanical characteristics than the conventional one corresponding to the high burnup and long life of the fuel assembly average burnup of 45 GWd / t or more. Is.

  There is a suitable thickness range for the oxide film as a hydrogen permeation barrier, and the effect is reduced if it is too thick or thinner than this range. When the thickness of the oxide film exceeds the upper limit of the range, cracks occur in the oxide film and the hydrogen permeation barrier effect is impaired. In addition, an oxide film thinner than the lower limit of the above range is insufficient to exhibit the effect as a hydrogen permeation barrier.

  However, it is difficult to determine whether or not cracks occur in the oxide film in the short-time hydrogen absorption test, and it can be clearly determined only after a long-time test. The result that the hydrogen absorption when the thickness is too thick is not obtained. That is, a suitable upper limit value of the oxide film thickness can be specified only after a long-time hydrogen absorption test.

  As a standard corrosion test method for a zirconium alloy cladding tube, JIS H 4751 specifies a 400 ° C. corrosion test. In this method, a corrosion test is performed at a temperature of 400 ° C. for 72 hours. Usually, in the 72-hour corrosion test, the oxide film has a dense black color and no film cracking is observed.

  In general, in the case of a 400 ° C. corrosion test, a corrosion transition that is considered to be due to the destruction of a dense oxide film is observed in about 1000 hours, and the amount of corrosion increases after the transition. Based on this knowledge, in the present invention, as shown in Examples described later, the corrosion time is set to 2000 hours which is sufficiently longer than the corrosion transition, and the oxide film thickness is used as a parameter, and the corrosion test is performed at 400 ° C. for 2000 hours. By measuring the amount of hydrogen absorbed later, a suitable oxide film thickness range was identified.

  As a result, when the thickness of the oxide film is in the range of 0.05 μm or more and 0.2 μm or less, the hydrogen absorption amount is found to be the smallest value, and the present invention has been achieved. In the fuel clad for a boiling water reactor of the present invention in which an oxide film that specifies this film thickness range is formed on the outer surface, for example, even under high fuel burnup conditions of about 45 GWd / t, The hydrogen absorption suppression characteristic is improved from the fuel cladding tube, and better corrosion resistance and mechanical characteristics can be maintained.

  Since the oxide film formed on the outer surface of the fuel cladding tube of the present invention is within the specific film thickness range as described above, the oxide film forming method is selected so that the film thickness can be adjusted. . For example, an atmospheric annealing method, an anodic oxidation method, and an autoclave method can be mentioned, and all of them are methods that can control the treatment time, temperature, and atmosphere to make the oxide film thickness within a desired range. is there.

  An example in which the fuel cladding tube for a boiling water reactor according to the first embodiment of the present invention is manufactured along the steps shown in the flowchart of FIG. 1 will be described below. First, in the ingot manufacturing process 1, zircaloy-2 briquettes are dissolved in multiple to obtain a zircaloy ingot. The obtained ingot is stretched into a round bar material by hot forging in the billet manufacturing process 2 and then drilled at the center to become a billet. This billet is subjected to β quenching for the purpose of improving corrosion resistance.

  Next, in the raw tube manufacturing process 3, the billet is made by combining the outer layer Zircaloy-2 tube and the inner layer zirconium tube by electron beam welding to form a billet, and then extruding the billet by high-frequency heating to produce the outer layer Zircaloy-2 tube. The inner layer zirconium pipe is metallically bonded to obtain a raw pipe.

  The raw tube is finished to a predetermined size by repeatedly performing a cold rolling process 4 and a vacuum annealing process 5 a plurality of times. Of the vacuum annealing treatments 5 after each cold rolling treatment 4, the intermediate annealing treatment other than the final annealing is mainly intended to enable the next rolling process by sufficiently softening the work hardening by rolling. In the final vacuum annealing treatment, the recrystallization annealing temperature is set in consideration of the mechanical properties of the final product.

The corrosion resistance and hydrogen absorption characteristics of a zirconium alloy generally depend largely on the thermal history based on the heat treatment performed during the processing, and the value of the cumulative heat input parameter ΣAi represented by the following formula (1) is 2 × 10. It is known that when it is in the range of ⁻¹⁹ to 2 × 10 ⁻¹⁸ , the corrosion resistance and hydrogen absorption characteristics of the zirconium alloy are the best.

ΣAi = t · exp (−Q / RT) (1)
t: Holding time at temperature T (h)
T: annealing temperature (K)
Q: Activation energy
R: Gas constant
Q / R; 40000

  The process following the cold rolling process 4 and the vacuum annealing process 5 is a refining process 6, where the bending of the pipe is corrected, the surface is finished, the dimensions are adjusted, and the like. The bending of the pipe after annealing is straightened with a roll straightener. The surface finish is performed by performing center-less mechanical polishing on the outer surface of the tube, and acid cleaning and sand blasting on the inner surface of the tube. By these treatments, fine surface scratches are removed, the entire surface is made uniform, and the dimensions are finely adjusted. Finally, degreasing and cleaning are performed to obtain a fuel cladding tube.

  In this example, the method further includes a step 7 of forming an oxide film having a thickness of 0.05 μm or more and 0.2 μm or less on the outer surface of the cladding tube obtained by the above steps. The oxide film can be adjusted to the film thickness range by controlling the temperature and time by atmospheric annealing.

  The method for forming an oxide film on the outer surface of the cladding tube is not limited to the atmospheric annealing shown in the above embodiment, and any method that can form an oxide film in a desired film thickness range can be adopted. is there. For example, a method of annealing in a mixed gas in which the amount of oxygen is adjusted, an anodic oxidation method, an autoclave method and the like can be mentioned.

  In the anodic oxidation method, an oxide film can be formed by immersing the cladding tube in an electrolyte such as sodium hydroxide or nitric acid, and applying an appropriate voltage with the cladding tube side as the anode. The thickness of the oxide film can be adjusted. In the case of the autoclave method, the film thickness of the oxide film can be adjusted by controlling the temperature, time, and atmosphere in the autoclave process.

  As a second example of the present invention, a corrosion test was performed on the fuel cladding tube obtained in Example 1 above (an oxide film formed by atmospheric annealing) as follows. Test coated tubes were prepared in which oxide films were formed with different film thicknesses from 0.01 μm to 2 μm, and each was subjected to a test.

The corrosion test uses a circulating autoclave, set the temperature to 400 ° C., the pressure to 10.3 MPa, the dissolved oxygen 200 ppb, the dissolved hydrogen 5 ppb, the electrical conductivity 2 × 10 ⁻⁶ S / cm, and the pH to 5-8. The test time was 72 hours and 2000 hours. After the corrosion test, the hydrogen absorption amount was measured for each test cladding tube, and the relationship between the thickness of the oxide film and the hydrogen absorption amount was examined. The results are shown in FIG. The cases are shown in FIG.

  As is apparent from the results of FIGS. 2 and 3, when the corrosion test time is short, the hydrogen absorption amount is suppressed as the film thickness increases, and thereafter, the dependence of the hydrogen absorption amount on the film thickness is recognized. However, when the test was performed for a long time, it was confirmed that the hydrogen absorption amount was suppressed as the film thickness increased to a certain film thickness, and the hydrogen absorption suppression effect decreased as the film thickness increased beyond a certain film thickness.

  That is, in the fuel cladding tube, the excellent hydrogen absorption suppression effect by the oxide film on the outer surface is obtained when the film thickness is within a limited range. Specifically, from the result of FIG. It became clear that it was in the range of 0.05 μm or more and 0.2 μm or less including 0.1 μm to 0.2 μm where the absorption amount was the lowest.

  As described above, in the combustion cladding tube for boiling water reactor according to the present invention, the thickness of the oxide film applied to the outer surface is set in the range of 0.05 μm or more and 0.2 μm or less, thereby increasing the fuel Good corrosion resistance and mechanical characteristics can be maintained due to the excellent hydrogen absorption suppression effect with respect to the burnup, that is, the long life of the fuel.

It is a schematic flowchart explaining the manufacturing process of the fuel cladding tube for boiling water reactors by Example 1 of this invention. It is a diagram which shows the hydrogen absorption amount (vertical axis: relative value) with respect to the film thickness (horizontal axis: micrometer) of the oxide film in the short-time corrosion test of the fuel cladding tube by Example 2 of this invention. It is a diagram which shows the hydrogen absorption amount (vertical axis | shaft: relative value) with respect to the film thickness (horizontal axis: micrometer) of the oxide film in the long-time corrosion test of the fuel cladding tube by Example 2 of this invention.

Explanation of symbols

1: Ingot manufacturing process 2: Billet manufacturing process 3: Element pipe manufacturing process 4: Cold rolling process 5: Vacuum annealing process 6: Refinement process 7: Oxide film forming process

Claims (2)

In fuel clad tubes for boiling water reactors made of zirconium alloys,
A fuel cladding tube for high-burning fuel assemblies having an average fuel assembly burnup of 45 GWd / t or more, and an oxide film having a film thickness of 0.05 μm or more and 0.2 μm or less is formed on the outer surface of the cladding tube A fuel cladding tube for a boiling water reactor characterized by being made. Fuel for boiling water reactors comprising a step of forming an oxide film on the outer surface of a cladding tube obtained by refining a raw tube that has undergone cold rolling and vacuum annealing using a billet formed from a Zircaloy alloy ingot A method for manufacturing a cladding tube, comprising:
2. The fuel clad for a boiling water reactor according to claim 1, wherein the oxide film forming step uses any one of an atmospheric annealing method, an anodizing method, and an autoclave method as an oxide film forming method. Production method.

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