JPS6238388A - Composite coated tube for nuclear fuel - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a fuel element constituting a nuclear fuel assembly, that is, a fuel rod, and particularly to a cladding tube of the fuel rod.

[Prior Art] Fuel assemblies used in nuclear power plant reactors generally consist of columnar sintered bodies (called pellets) of uranium oxide that are covered with a zirconium alloy cladding tube, with both ends of the cladding tube covered with a zirconium alloy cladding tube. It consists of a rod-shaped fuel element, that is, a fuel rod, which is sealed with an end plug.

During operation of a nuclear power plant, the outer surface of the fuel rod is in contact with high-temperature, high-pressure cooling water, and the inside of the fuel rod is isolated from the cooling water by a zirconium alloy cladding tube. Since fuel rods are used under conditions where the pressure of cooling water is higher than the internal pressure of the fuel rod, the outer diameter of the fuel rod decreases due to creep of the cladding material. Further, as the combustion of the fuel progresses, fission products such as iodine accumulate within the pellet, and the outer diameter of the pellet increases due to volumetric expansion of the pellet called swelling. Therefore, the radial gap that existed between the outer surface of the pellet and the inner surface of the cladding tube at the beginning of operation decreases as combustion progresses, and the outer surface of the pellet and the inner surface of the cladding tube come into contact.

If the power of the nuclear reactor is suddenly increased under such conditions, the outside diameter of the pellet will further increase due to the increase in temperature of the pellet, and a large stress will be applied to the cladding tube.

Furthermore, due to the increase in pellet temperature, fission product gas such as iodine accumulated in the pellet is released, creating a corrosive atmosphere inside the fuel rod.

It is known that if excessive stress is applied to a zirconium alloy cladding tube in an atmosphere of corrosive gases such as iodine, a phenomenon called stress corrosion cracking, which causes the cladding tube to break, may occur. It is conceivable that fuel damage may occur due to stress corrosion cracking of the zirconium alloy cladding when the output suddenly increases.

In order to prevent fuel damage due to stress corrosion cracking of the fuel cladding tube, various improvements have been made to the fuel. For example, the inner surface of the zirconium alloy cladding tube is coated with lO Composite cladding tubes (hereinafter simply referred to as cladding tubes) have been developed that are metallurgically lined with substantially pure zirconium metal to a thickness of about 1.5%.

[Problems to be Solved by the Invention] However, in such a composite cladding tube, the pure zirconium metal lining the inner surface has inferior corrosion resistance in an oxidizing atmosphere compared to a zirconium alloy, so the inner and outer surfaces of the cladding tube are If a defective state occurs and cooling water enters the cladding tube, the amount of corrosion reaction on the pure zirconium metal inner surface will be greater than in a zirconium alloy cladding tube without a lining.

When zirconium metal corrodes, zirconium oxide is formed, and the generated hydrogen is absorbed into the base material of the cladding made of a zirconium alloy and precipitated as hydride.

If a large amount of hydride is deposited on the cladding, the mechanical properties of the cladding will deteriorate and the ability of the fuel rod to maintain its shape may be impaired, as is well known to those skilled in the art. The amount of hydrogen gas generated depends on the thickness of the zirconium metal lining, which controls the amount of hydrogen generated, that is, the volume of the zirconium metal that easily causes corrosion reactions.

In such conventional composite cladding, the thickness of the pure zirconium metal lining accounts for about 10% of the total wall thickness of the cladding,
If all of these parts form an oxide film due to corrosion, a large amount of hydrogen will be absorbed into the cladding tube base material, causing a problem of deterioration of the mechanical integrity of the cladding tube.

Therefore, without reducing the resistance to stress corrosion cracking, which is an improvement effect of composite cladding using pure zirconium metal, the amount of hydrogen absorbed into the cladding base material due to corrosion of pure zirconium metal on the inner surface of the cladding can be reduced. It has been desired to develop a composite cladding tube that suppresses the oxidation, and an object of the present invention is to provide such a composite cladding tube.

[Means for Solving the Problems] For this purpose, the present invention provides an outer tube formed using a zirconium alloy as a base material, and an inner tube made of pure zirconium metal whose inner surface is metallurgically lined. In the composite cladding tube for nuclear fuel, the inner tube has a radial wall thickness of
It is characterized in that the thickness is set to 3 to 5% of the radial wall thickness of the entire composite cladding tube.

[Function] Since the thickness of the inner tube made of pure zirconium metal, which easily corrodes, is reduced, the hydrogen generated by the corrosion of pure zirconium metal on the inner surface of the composite cladding tube is transferred to the base material of the composite cladding tube, i.e. The amount absorbed into the outer tube is suppressed. For example, comparing composite cladding tubes with various wall thickness ratios under the same corrosive atmosphere conditions and with the same chemical compositions of the zirconium alloy and pure zirconium metal used as the outer and inner tube materials of the composite cladding tube. Then, the wall thickness of the inner tube is 1
/2, the amount of hydrogen absorbed into the outer tube of the composite cladding due to the corrosion of pure zirconium metal on the inner surface of the inner tube is approximately 1.
/2. Therefore, even if a defect that penetrates the thickness of the composite cladding tube occurs, the amount of corrosion reaction with the cooling water that has entered the interior and the amount of hydrogen gas generated can be suppressed.

On the other hand, if the thickness of the pure zirconium metal lining is made too thin, the improvement effect on stress corrosion cracking of the composite cladding may not be as expected, but the wall thickness of the inner tube is set at 3 to 5%. It was confirmed that this possibility could be eliminated by doing so.

[Embodiments] Next, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings, in which the same reference numerals indicate openings or corresponding parts.

In FIG. 1, a composite cladding 1 according to the invention is shown with an outer tube 2 formed from a zirconium alloy and an outer tube 2 made of nearly pure zirconium metal and metallurgically bonded to the outer tube 2 in a known manner. It consists of an inner tube 3. Zirconium alloy is ASTM (American Society for Testing and Materials) B
-353, UNS number R60802 or R60901 is preferred, and pure zirconium metal has INS number R, defined by ASTM B-353.
60802 nuclear reactor grade is best.

As can be understood from FIG. 1A showing the area IA surrounded by the chain line in FIG. 1, if the radial wall thickness of the entire composite cladding tube 1 is Wt and the radial wall thickness of the inner tube 3 is Wzr,
The Wt ratio is set between 0.03 and 0.05 according to the present invention. This ratio is approximately 0.1 in the conventional case shown in FIGS. 2 and 2A.

When a defect that penetrates the thickness of the composite cladding tube I occurs due to some factor in a fuel rod using the composite cladding tube I during operation of a nuclear power plant.

In this case, cooling water infiltrates into the fuel rod through this defective portion, and the following reaction occurs on the inner surface of the inner tube 3 of the composite cladding tube 1.

Zr+2H7O→ZrO2+2H2↑ The hydrogen generated by the above reaction is transferred to the outer tube 2 of the composite cladding tube 1.
The amount of hydrogen absorbed depends on the amount of hydrogen generated. That is, by reducing the volume (which can be considered as the thickness) of the inner tube 3 made of zirconium metal, the amount of hydrogen generated decreases, and the amount of hydrogen absorbed by the composite cladding tube 1 is suppressed.

In the present invention, based on this knowledge, the wall thickness Wzr of the inner tube 3 made of zirconium metal is reduced by 173 to 1/3 compared to the conventional one.
By reducing the thickness to 2, the hydrogen absorption amount of the composite cladding tube 1 is reduced at a similar rate, and the deterioration of the mechanical properties of the composite cladding tube due to hydride precipitation is reduced. On the other hand, by reducing the thickness of the inner tube 3, there is a possibility that the stress corrosion cracking resistance of the composite cladding tube l will deteriorate. As mentioned above, the amount was set at 3 to 5% of the total cladding tube.

Figure 3 is a diagram showing the results of a comparative test of stress corrosion cracking resistance between a conventional composite cladding tube and that of the present invention. ), and the vertical axis is the amount of strain that the cladding undergoes until it breaks by applying strain in the circumferential direction with the inner surface of the cladding tube in an iodine atmosphere. It is expressed as a ratio to the amount of failure strain (thickness: 10%). The failure strain of the composite cladding tube 1 of the present invention with an inner tube wall thickness of 3 to 5% is substantially the same as that of the conventional one; It can be seen that similar stress corrosion cracking resistance characteristics can be ensured.

[Effects of the Invention] As described above, the wall thickness of the inner tube made of pure zirconium metal of the composite cladding tube is adjusted to 3 to 5% of the total wall thickness of the cladding tube according to the present invention.
%, compared to conventional composite cladding which has an inner tube with a zirconium metal layer that accounts for approximately 10% of the total cladding wall thickness, the cladding hydrogen absorption in the event of a defect penetrating the cladding wall thickness is reduced. The amount can be suppressed to about 173 to 1/2 without substantially impairing stress corrosion cracking resistance properties.

Therefore, it is possible to not only prevent fuel damage when the output of a nuclear power plant suddenly increases, but also to ensure the ability to maintain the shape of the fuel rod even if a defect that penetrates the wall thickness of the cladding tube occurs due to some reason. It becomes possible.

[Brief explanation of the drawing]

Fig. 1 is a cross-sectional view of a composite cladding tube according to the present invention, Fig. 1A is an enlarged sectional view of the area surrounded by a chain line IA in Fig. 1, and Fig. 2 corresponds to Fig. 1 of a conventional composite cladding tube. 2A is an enlarged sectional view of the area surrounded by the chain line 2A, and FIG. 3 is a diagram comparing the stress corrosion cracking resistance of a conventional composite cladding tube and that of the present invention. . ■...Composite cladding tube 2...Outer tube 3...Inner tube 11t...Full wall thickness of cladding tube Wzr...
Inner tube wall thickness Applicant: Mitsubishi Heavy Industries, Ltd. Same as above Mitsubishi Atomic Crab Industry Co., Ltd. Agent: Teru Sogado Figure 1 Figure 2 Figure 1A Figure 2A Wz, Wzr-=o
, o3~0.05 -:0.1tWt Figure 3

Claims (1)

[Claims] In a composite cladding tube for nuclear fuel consisting of an outer tube formed using a zirconium alloy as a base material and an inner tube made of pure zirconium metal metallurgically lined on the inner surface of the outer tube, the diameter of the inner tube is A composite cladding tube for nuclear fuel, wherein the thickness in the direction is set to 3 to 5% of the thickness in the radial direction of the entire composite cladding tube.