JPH0528797B2 - - Google Patents

Description

[Detailed description of the invention]

FIELD OF APPLICATION OF THE INVENTION The present invention relates to nuclear fuel elements. [Background of the Invention] Regarding the elements of the cladding material of nuclear fuel elements,
The ASTM standard ASTMB353-77a is known. Generally, in a nuclear fuel element, a plurality of nuclear fuel pellets are stacked and stored in a cladding tube, and both openings of the cladding tube are sealed with end plugs. Nuclear fuel pellets are formed by forming and sintering fissile oxide fuel powder into cylindrical pellets having, for example, a length to diameter ratio of about 1. In addition, the cladding tube of a nuclear fuel element configured in this manner has the function of preventing the coolant from coming into contact with the nuclear fuel pellets and preventing chemical reactions from occurring, and the function of preventing radioactive fission products released from the fuel. A function is required to prevent coolant from entering the coolant. Therefore, in the event that the cladding tube does not satisfy these functions, that is, the cladding tube is damaged, the radioactivity level in the cooling system plant will increase, and reactor operation must be stopped to ensure safety. It will stop happening. On the other hand, cladding tubes of nuclear fuel elements used in water-cooled nuclear reactors are generally made of zirconium and alloys thereof. Zirconium and its alloys have a small neutron absorption cross section and approximately
It is strong and ductile at temperatures below 400°C, and has the property of not reacting with water vapor, which is used as a coolant. However, according to operational experience to date, even cladding tubes made of zirconium and its alloys are susceptible to brittleness due to a decrease in material strength due to neutron irradiation and corrosion due to chemical reactions with nuclear fission products. Cracks have occurred.
It is thought that such an undesirable phenomenon occurs as follows. That is, in order to efficiently transfer the heat generated by the nuclear fuel pellet to the outer surface of the cladding tube, it is necessary to set the gap formed between the inner surface of the cladding tube and the nuclear fuel pellet to several tens of microns or less. on the other hand,
During operation, nuclear fuel pellets generate heat, so the pellets themselves crack due to thermal stress, resulting in discrepancies in the fracture surfaces, and furthermore, fission products accumulate within the nuclear fuel pellets as they burn, resulting in volumetric expansion. The tube is expanded and stressed by the nuclear fuel pellets. Although the average value of the strain to which the cladding tube is subjected in the circumferential direction is not very large, the strain is not only locally concentrated on the wall near the crack that occurs in the nuclear fuel pellet, but this strain reaches more than the yield stress. Further, as nuclear fission occurs, corrosive gases such as iodine and iodine compounds, cesium and cesium compounds are generated from nuclear fuel pellets, and these corrosive gases collect in free spaces within the cladding tube, such as cracks. In particular, corrosive gas tends to collect near parts of the cladding where strain is particularly concentrated. Generally, when stress (particularly greater than yield stress) is applied in a corrosive gas atmosphere, the ductility of the material decreases and a brittle fracture phenomenon called stress corrosion cracking occurs. Stress corrosion cracking is also affected by temperature, stress, concentration of corrosive gas, dissolved oxygen, alloy composition, heat treatment, degree of processing, etc., and the mechanism by which it occurs is not unique. The concept of lining cladding pipes to prevent these undesirable fractures is well known;
U.S. Pat. No. 3,502,549, U.S. Pat. No. 3,625,821, JP-A-51-69792, JP-A-51-69795, JP-A-51-69796, and JP-A-Sho. 51-
In Publication No. 71497, Mo as a liner material,
W, Nb, Ni, Fe, Mg, Cu, pure Zr, Al, Ni−
Cr alloys, aluminized coatings, silicided coatings, etc. are shown. However, some liner materials used as barrier materials have drawbacks such as a large neutron absorption cross section, which reduces the economic efficiency of the reactor. Additionally, the use of liner materials not only increases the manufacturing process of the cladding, but also presents inherent technical problems and economic disadvantages. [Object of the invention] The present invention has been made in view of the above points,
The object of the present invention is to provide a nuclear fuel element that has increased stress corrosion cracking resistance and improved reliability and economic efficiency. [Summary of the Invention] That is, the present invention provides a nuclear fuel element in which nuclear fuel pellets are sealed inside a cladding tube, in which the cladding tube has a carbon concentration of 80 ppm or less and an oxygen concentration of 80 ppm or less.
It is characterized by being formed from a zirconium alloy of 600 ppm or less by weight, which makes stress corrosion cracking of the cladding less likely to occur. The inventor investigated how stress corrosion cracking resistance could be improved. Among the elements constituting the zirconium alloy, the elements that increase the strength at high temperatures (approximately 350°C) are mainly elements such as oxygen, which are dissolved in zirconium. In addition, carbon is a major element that is dispersed within the crystal grains and increases the strength of the cladding. Conventionally, carbon is contained at about 140 ppm by weight, but it is industrially possible to further reduce this concentration. In addition, the conventional oxygen concentration was approximately 1100 to ensure the initial strength of the cladding tube.
However, in the present invention, in order to keep the high temperature strength of the cladding tube low, limiting the concentration of oxygen as well as carbon will prevent stress corrosion cracking caused by the interaction between the fuel and the cladding tube. We focused our attention on the fact that an excellent cladding tube could be obtained. [Embodiments of the Invention] Table 1 shows the chemical compositions of the zirconium alloys used in the embodiments of the present invention, excluding zirconium.
A and B are examples. First, the chemical composition standard (ASTM, B-353,
The following amount of alloying elements satisfying Grade, RA-1) are added to zirconium. i.e. 1.20-1.70% by weight of tin, 0.07-0.20% by weight of iron, chromium
0.05 to 0.15% by weight, 0.03 to 0.08% by weight of nickel, and 0.18 to 0.38% by weight of iron + chromium + nickel. Next, according to the conventional standards mentioned above, carbon weighs 270
ppm or less, but in Examples A and B, the carbon concentration was 50 and 80 ppm by weight, respectively, the oxygen concentration was 400 and 600 ppm by weight, respectively, and the other impurity elements were zirconium ingots that satisfied the above specifications. . Conventional examples C and D have carbon concentrations of 140 and 170 ppm by weight, respectively, and oxygen concentrations of 1100 and 1300 ppm by weight, respectively, satisfying the conventional carbon concentration of about 140 ppm by weight and oxygen concentration of about 1100 to 1300 ppm by weight, as described above. It was made into an ingot.

【table】

〔Effect of the invention〕

As mentioned above, the present invention has increased stress corrosion cracking resistance, improved reliability and economical efficiency,
It is possible to obtain a nuclear fuel element with increased stress corrosion cracking resistance and improved reliability and economic efficiency.

[Brief explanation of the drawing]

FIG. 1 is a process diagram for manufacturing a cladding tube according to one embodiment of the nuclear fuel element of the present invention, and FIG. 2 is a diagram showing the elongation at break in the circumferential direction of the cladding tube manufactured according to the same embodiment.

Claims (1)

[Scope of Claims] 1. A nuclear fuel element in which nuclear fuel pellets are sealed inside a cladding tube, wherein the cladding tube has a carbon concentration
A nuclear fuel element characterized in that it is formed from a zirconium alloy with an oxygen concentration of 80 ppm or less and an oxygen concentration of 600 ppm or less. 2. The nuclear fuel element of claim 1, wherein the zirconium alloy is formed from Zircaloy.