JPH0454491A - Nuclear fuel element - Google Patents

Abstract

PURPOSE:To prevent stress corrosion cracking by constituting the fuel cladding pipe of an inside surface layer consisting of a water corrosion resistant zirconium alloy layer having a specific thickness, an intermediate layer consisting of a pure zirconium layer and an outside surface layer consisting of a water corrosion resistant zirconium alloy layer. CONSTITUTION:The inside surface layer 2 of the cladding pipe is constitute of zircaloy-2 having the excellent water corrosion resistance in the range form the inside surface up to 5 to 10mum. The pure zirconium is used for the intermediate layer 3 next to this layer. The outside surface layer 4 consists of the zircaloy-2 having 0.78mm thickness. Nuclear fuel pellets 1 are housed via a gap 5 into the cladding pipe. since the zirconium is used for the intermediate layer of the nuclear fuel element in such a manner, the stress corrosion cracking arises hardly and further the part of 5 to 10mum inside surface of the cladding pipe which is liable to be hardened by the influence of the nuclear fuel is substd. with the zircaloy-2 having the higher water corrosion resistance than the water corrosion resistance of the zirconium and, therefore, the abrupt oxidation on the inside surface of the pipe in the event of the failure of the cladding pipe is prevented as compared to the conventional zirconium lined pipe.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a nuclear fuel element with excellent stress corrosion cracking resistance.

[Prior Art] A nuclear fuel element accommodates a plurality of nuclear fuel pellets in a stacked manner within a fuel cladding tube, has a plenum part for gas storage at the upper end and a plenum spring for stably supporting the nuclear fuel pellets, and has openings at both ends. The upper and lower end plugs are sealed and welded together.

Nuclear fuel pellets inside the fuel cladding emit radioactive fission products when burned, but in order to prevent these radioactive fission products from getting mixed into the coolant outside the cladding, the fuel cladding has a leak prevention function. It has been demanded. According to operational experience to date, when the power output increases rapidly at a stage where the fuel burnup is high, a chemical reaction occurs between the cladding and corrosive fission products, and thermal expansion of the nuclear fuel pellets causes the cladding to It was found that thermal stress is applied to the fuel cladding, and the synergistic effect of the two can cause stress corrosion cracking in the fuel cladding.

In order to prevent such stress corrosion cracking of the fuel cladding tube, the inner peripheral surface of the fuel cladding tube is coated with a thickness of, for example, 80 to 1
A so-called zirconium alloy layered tube with a 00 μm pure zirconium lychee layer as a barrier was published in Japanese Patent Application Laid-Open No. 55-164.
This is proposed in Japanese Patent No. 396, and the pure zirconium lychee layer prevents contact between the cladding tube and corrosive fission products, and also alleviates local stress generated in the cladding tube to prevent stress corrosion cracking of the cladding tube. It is expected to be effective.

Furthermore, Japanese Patent Application Laid-Open No. 1987-69793 discloses a fuel cladding tube in which a first coating is made of a Cr alloy on the inner surface of the cladding tube and a second coating is provided on the inner surface of the cladding tube to form a triple structure. Furthermore, AI, Cu, Nb, and Ni are added to the inner surface of the fuel cladding tube.
JP-A-51-71498 proposes a fuel cladding tube having a triple structure in which a buried metal barrier such as , Fe, etc. is provided and a zirconium alloy or stainless steel is arranged on the inner surface.

[Problem to be solved by the invention] However, since the conventional zirconium liner tube has an active pure zirconium layer exposed on its inner surface, it reacts with gas in the space inside the fuel element and temporarily loses its stress corrosion cracking prevention function. There is a possibility that this will decline.

In addition, the conventional triple-layered cladding may impair neutron economy due to the materials constituting each layer, which may ultimately reduce the performance of the nuclear reactor.

The present invention has been made in view of the above circumstances, and is designed to prevent stress corrosion cracking from occurring in the cladding tube when stress is applied to the cladding tube due to interaction with fuel in corrosive fission product gas, and to prevent the cladding tube from being damaged. The purpose of the present invention is to provide a highly reliable nuclear fuel element that can prevent rapid oxidation of the inner surface of the tube.

[Means for Solving the Problems] That is, the present invention provides a nuclear fuel element in which nuclear fuel pellets are housed inside a fuel cladding tube, in which the fuel cladding tube is
Three-layer structure consisting of an inner surface layer made of a zirconium alloy layer with excellent water corrosion resistance and a thickness of 5 to 10 mm, an intermediate layer made of a pure zirconium layer, and an outer surface layer made of a zirconium alloy layer with excellent water corrosion resistance. It is characterized by consisting of.

[Operation] The present inventor measured the hardness distribution in the vicinity of the inner surface of various nuclear fuel cladding tubes used to high burnup, and as a result obtained new knowledge as shown in FIG. 2.

That is, in Fig. 2, 20 is the hardness distribution of the Zircaloy-2 material when the influence of the fuel side is not included (that is, only the influence of neutron irradiation), and 21 is the actual hardness distribution when the influence of the fuel side is included. It shows the hardness distribution. As shown here,
The hardness increases on the surface side from a position of about 10 μm due to the influence of the fuel side. On the other hand, in the case of a zirconium liner tube in which the inner surface of the cladding tube is lined with zirconium, the hardness distribution is 22 and 23. In other words, the hardness distribution when the influence of the fuel side is not included is 22, while the hardness distribution when the influence of the fuel side is included is 23, and the hardness distribution when the influence of the fuel side is included is 22. It begins to rise at a position of about 10 μm from the inner surface, and becomes almost equal to the saturation value of Zircaloy-2 material at a position of about 5 μM from the inner surface.

That is, (a) even if the inner surface of the fuel cladding tube is made of a soft material such as zirconium, it hardens within a range of about 10 μω from the inner surface due to the influence of the fuel side. (b) Particularly at a position approximately 511 IIl from the inner surface, the hardness should be equivalent to that of Zircaloy-2 material, which was only affected by irradiation.

I understand. Therefore, in the range from 5 to 10 am from the inner surface, there is no significant difference in using a particularly soft material, and it was found that it is more effective to use Zircaloy-2, which has excellent water corrosion resistance.

The present invention is based on the above knowledge, and by constructing the cladding tube from 5 to 10 μm from the inner surface with Zircaloy-2, which has excellent water corrosion resistance, and using pure zirconium for the next intermediate layer, It is possible to improve the shortcomings of conventional zirconium liner tubes and provide highly safe nuclear fuel elements.

[Example] Embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a cross-sectional view of a nuclear fuel element that is an embodiment of the present invention. In the figure, 1 is a nuclear fuel pellet, 2 is an inner surface layer of the cladding tube, 3 is an intermediate layer of the cladding tube, 4 is an outer surface layer of the cladding tube, 5
is the gap. The inner surface layer 2 of the cladding tube has a thickness of 0.01
The intermediate layer 3 is made of Zircaloy-2 with a thickness of 0.0 mm.
Made of 7mm zirconium, the outer surface layer is 0.7mm thick.
Made of 8mm Zircaloy-2. Nuclear fuel pellets 1 are accommodated within the cladding tube through a gap 5.

Next, a method for manufacturing this nuclear fuel element will be explained.

Zirconium contains impurities that meet ASTM B353, including 1.50% tin, 0.14% iron, and 0.10% chromium.
, 0.05% nickel, and 0.12% oxygen were added to obtain a Zircaloy-2 ingot. This ingot was formed into a cylindrical shape. Next, add about 600 ppm of oxygen,
After machining the core of a zirconium ingot containing about 600 ppm of iron and chromium in total, a cylinder made of Zircaloy-2 material was inserted so as to fit tightly into the opening, and after rolling, the inner surface was made of Zircaloy-2 material. 2, and a double tube whose outer surface was made of zirconium was obtained. The outer diameter of this double tube was cut, and it was inserted so as to be in close contact with the inner surface of the thick-walled Zircaloy 2 tube, and the outer diameter of the double tube was 12.52.
The cladding tube was finished with a wall thickness of 0.86 mm.

It should be noted that the above manufacturing method is only an example and does not limit the present invention, of course. For example, in the above embodiments, the inner surface layer was formed by rolling, but the cladding tube of the present invention can also be obtained by adding an alloying element to the inner surface of a conventional zirconium lychee tube 5 to 10 μm thick.

[Effects of the Invention] As explained above, the nuclear fuel element of the present invention uses zirconium in the intermediate layer, so stress corrosion cracking is difficult to occur, and the inner surface of the cladding tube 5 to 5, which is easy to harden under the influence of nuclear fuel. Since the 10 μm portion was replaced with Zircaloy-2 material, which has better water corrosion resistance than zirconium, rapid oxidation of the inner surface of the tube when the cladding tube breaks can be prevented compared to conventional zirconium litchi tubes.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of a nuclear fuel element showing an embodiment of the present invention;
FIG. 2 is a diagram showing the hardness distribution near the inner surface of the cladding tube. 1...Fuel pellets 2...Inner surface layer of cladding tube 3...Intermediate layer of cladding tube 4...Outer surface layer of cladding tube 5...Gap 20...No influence from the fuel side Zircaloy when not there-2
Material hardness distribution 21... Hardness distribution of Zircaloy 2 material when fuel side influence is included 22... Zirconium hardness distribution when fuel side effect is not included 23... Fuel side Hardness distribution of zirconium when affected Figure 1 Agent: Patent attorney (8733) Yoshiaki Inomata (and 1 other person)

Claims (1)

[Claims] (1) In a nuclear fuel element formed by storing nuclear fuel pellets inside a fuel cladding tube, the fuel cladding tube has a thickness of 5 to 10 μm.
It has a three-layer structure consisting of an inner surface layer made of a zirconium alloy layer with excellent water corrosion resistance, an intermediate layer made of a pure zirconium layer, and an outer surface layer made of a zirconium alloy layer with excellent water corrosion resistance. A nuclear fuel element characterized by: