JPH0540186A - Nuclear fuel element - Google Patents

Abstract

PURPOSE:To obtain a nuclear fuel element capable of preventing stress corrosion cracking of nuclear fuel clad and hydrogen embrittlement even in the case of clad rupture. CONSTITUTION:Between fuel pellets 1 in a fuel clad 4, a mixture of burnable poison (Gd, etc.) and a good hydrogen absorption material (Zr, etc.) which is coated with material 3 with good hydrogen permeability and combined in one, is provided in a fuel element. Thus, during a reactor power escalation, the local stress concentration in a clad is mitigated. Furthermore, even in the case of clad rupture, the hydrogen embrittlement can be prevented.

Description

Detailed Description of the Invention

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a nuclear fuel element used in a nuclear reactor, and more particularly to a nuclear fuel element having an improved cladding tube for improved reliability.

[0001]

2. Description of the Related Art A nuclear fuel element has a fuel cladding tube in which a plurality of nuclear fuel pellets are stacked and housed, and a plenum portion for gas storage and a plenum spring for stably supporting the nuclear fuel pellet are provided at an upper end portion, and both end opening portions are provided. It has a structure in which the upper and lower end plugs are hermetically welded. In the above configuration,
In order to prevent the radioactive fission products emitted from the nuclear fuel pellets from mixing into the coolant around the outer circumference of the cladding, the cladding tube is required to have a function to prevent the radioactive fission products from leaking to the outside. However, according to the operation experience to date, if the output suddenly rises when the burnup of the fuel becomes high, a chemical reaction occurs between the cladding and corrosive fission products, and the nuclear fuel pellets become hot. It has been found that stress is applied to the cladding tube by so-called ridge deformation in which both end surface portions of the pellet are deformed in a localized manner due to expansion, and the stress corrosion cracking may occur in the cladding tube due to the overlapping action of the both.

For the purpose of preventing the stress corrosion cracking of the cladding tube as described above, a so-called zirconium, for example, having a pure zirconium liner layer having a thickness of 80 to 100 μm stretched as a barrier on the inner peripheral surface of the cladding tube is used. A liner tube has been proposed as JP-A-55-164396, and the zirconium liner tube prevents contact between the cladding tube and corrosive fission products and relaxes local stress generated in the cladding tube. The effect of preventing stress corrosion cracking of the cladding tube is expected.

As disclosed in Japanese Utility Model Laid-Open No. 55-55796, the output of the end surface of the fuel pellet is made smaller than that of the pellet abdomen, and the ridge deformation caused by the thermal stress of the fuel pellet is made small so that the cladding tube Nuclear fuel elements have been proposed that have weakened interactions. Further, there is a proposal to insert a plastic interposition material in the fuel pellet for the purpose of reducing the above-mentioned ridge deformation.
It is proposed in Japanese Patent No.

[0004]

However, in the nuclear fuel element manufactured by the conventional technique, it is disadvantageous in terms of manufacturing cost to manufacture a nuclear fuel element which gives a distribution to the heat output of the fuel pellets. Further, when the cladding tube is damaged for some reason and the cooling water enters the cladding tube, the fuel is immediately oxidized and hydrogen is generated. There is a concern that this hydrogen will diffuse into the coated pipe material made of a zirconium alloy, causing hydride formation and hydrogen embrittlement.

The present invention has been made in view of the above circumstances, and local stress concentration called a ridge is less likely to occur in a nuclear fuel cladding tube, so that stress corrosion cracking does not occur in the cladding tube, and at the same time, the cladding tube is caused for some reason. Even if the
An object is to provide a highly reliable nuclear fuel element capable of preventing hydrogen embrittlement of a cladding tube.

[0006]

The structure of a nuclear fuel element according to the present invention for solving the above-mentioned problems is such that a plurality of nuclear fuel pellets are stacked and housed inside a fuel cladding tube made of a zirconium alloy. In a nuclear fuel element in which both end plugs are sealed, a combustible poison and a substance having a good hydrogen absorbing property are covered with a film having a good hydrogen permeable property so as to be integrated and arranged between the nuclear fuel pellets. .

[0007]

In the nuclear fuel element, it was known that gadolinium (Gd) is effective for limiting the heat output in the vicinity of the end face portion of the fuel pellet, but there was a technical problem in the arrangement method.

The present inventors considered the above-mentioned Gd arrangement method to be integrally molded together with the hydrogen absorbing means, and also examined the rationalization in manufacturing. That is, when the cladding tube is damaged for some reason, the high temperature fuel pellets are oxidized by the infiltrated cooling water, and as a result, hydrogen is generated, and the hydrogen concentration in the cladding tube increases with water vapor.

On the other hand, Gd or gadolinia (Gd oxide) is mixed with a substance (Zr, etc.) having an excellent hydrogen absorbing property which has been studied in advance, and the mixture is covered with a film having good hydrogen permeability. Prepare the substance you prepared. By disposing the mixture in the vicinity of the fuel pellets in the fuel cladding tube, the generated hydrogen is absorbed, and the influence of hydrogen, such as embrittlement, is eliminated.

On the other hand, according to this method, in the initial stage of combustion, the fuel pellets tend to have few cracks and a large thermal expansion deformation tends to occur at the pellet end face portion. The heat output of the part is suppressed. As combustion progresses, the effect of combustible poisons (Gd, etc.) disappears, but many cracks occur in the fuel pellets at that time, deformation due to thermal expansion becomes small, and there is no need to suppress heat output. .

[0011]

Embodiments of the present invention will be described below with reference to FIGS. 1, 2 and 3.
Will be explained. FIG. 1 is a partial vertical sectional view of a nuclear fuel element of the present invention. In FIG. 1, 1 is a nuclear fuel pellet, 2 is a mixture of the present invention, 3 is a hydrogen permeable coating,
4 is a fuel cladding tube.

That is, in the configuration of FIG. 1, a mixture 2 of a burnable poison (Gd or a compound thereof) and a substance having a good hydrogen absorbing property (zirconium or a compound) is charged with hydrogen in at least a part of the middle of the nuclear fuel pellet 1. It is provided in the coating tube 4 by being coated with a coating having excellent permeability (a substance containing palladium, lanthanum or cerium as a main component) 3 and integrally molded.

The present invention has the effect of suppressing the thermal expansion of the end surface of the fuel pellet when the reactor power is increased. Further, it is effective in preventing hydrogen embrittlement even when the fuel cladding tube is damaged.

FIG. 2 is a schematic cross-sectional view of the single-piece molded mixture. In FIG. 2, a hydrogen-absorbing metal (Zr or its compound) is used as a base and a burnable poison (Gd or its compound) is mixed with this. FIG. 3 shows an even mixture of hydrogen absorbing metal and fine particles of a burnable poison. It goes without saying that in any case, the same effect as that of the embodiment shown in FIG. 1 is exhibited.

Regarding the coating for coating the mixture,
Any material having excellent hydrogen permeability may be used. For example, substances containing palladium, lanthanum and cerium as main components have been verified as hydrogen permeable substances.

[0016]

According to the present invention, it is possible to suppress the thermal expansion of the end surface portion of the fuel pellet in the cladding tube when the power output of the reactor is increased, and to relieve the local stress concentration in the cladding tube. further,
Even if the nuclear fuel element is damaged, hydrogen embrittlement of the cladding tube can be prevented, so that the reliability of the nuclear fuel element can be improved.

[Brief description of drawings]

FIG. 1 is a partial longitudinal sectional view of a nuclear fuel element of the present invention.

FIG. 2 is a schematic diagram of a substance in which burnable poison particles are mixed with a hydrogen absorbing metal.

FIG. 3 is a schematic diagram of a substance in which a hydrogen-absorbing metal and burnable poison particles are uniformly mixed.

[Explanation of symbols]

 1 Nuclear Fuel Pellet 2 Mixture of Hydrogen Absorbing Metal and Burnable Poison Particles 3 Hydrogen Permeable Coating 4 Cladding Tube 5 Mixture A 6 Mixture B

Claims (2)

[Claims] 1. A nuclear fuel element in which a plurality of nuclear fuel pellets are stacked and housed inside a fuel cladding tube made of a zirconium alloy, and upper and lower end plugs are hermetically sealed, a combustible poison and a substance having a good hydrogen absorbing property. A nuclear fuel element, characterized in that it is covered with a film having good hydrogen permeability and integrated, and is disposed between the nuclear fuel pellets. 2. The nuclear fuel element according to claim 1, wherein the main component of the burnable poison is gadolinium or its compound, the main component of the hydrogen absorbing material is zirconium or its compound, and the coating material thereof is palladium, A nuclear fuel element characterized by mainly containing lanthanum or cerium.