JPS61205892A - Nuclear fuel element - 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 Application of the Invention] The present invention particularly relates to a nuclear fuel element having an improved structure for preventing stress corrosion cracking of a cladding tube.

[Background of the invention]

Regarding the elements of the cladding material of nuclear fuel elements, AS'r
The M standard, ASTM B553-71, 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 one cladding tube are sealed with end plugs. Nuclear fuel pellets are made by molding and sintering fissile oxide fuel powder into cylindrical pellets with a length to diameter ratio of about 1, for example. In addition, the cladding tube in the nuclear fuel element configured as described above has the function of preventing 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 substances from entering the coolant. Therefore, if the cladding tube does not satisfy these functions, that is, if the cladding tube is damaged, the radioactivity level in the cooling system plant will increase, and the reactor operation will have to be stopped to ensure safety. This is an unavoidable situation.

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, are strong and ductile at temperatures below about 400C, and have the characteristics of not reacting with heat or water vapor used as a coolant. According to the operational experience of zirconium and zirconium alloys, brittle cracking occurs due to interactions such as a decrease in material strength due to neutron irradiation and corrosion due to chemical reactions with fission products. It has occurred. Such an undesirable phenomenon is thought to occur as follows. In other words, it is necessary to set it below nuclear fuel. On the other hand, when driving,
As the nuclear fuel pellets generate heat, the pellets themselves crack due to thermal stress, resulting in misalignment of the fracture surfaces, and furthermore, the volume expansion caused by the accumulation of fission products within the nuclear fuel pellets during combustion causes the cladding tube to break into the nuclear fuel pellets. ) is expanded and subjected to stress by K.

Although the average value of the strain that the cladding receives in the circumferential direction is not very large, strain is locally concentrated on the wall near the cracks that occur in the nuclear fuel pellet, and this strain reaches more than the yield stress.

Furthermore, corrosive gases such as iodine and iodine compounds, cesium and cesium compounds are generated from nuclear fuel pellets as a result of nuclear fission, 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 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 influenced 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. In order to prevent these undesirable destructions, the concept of lining the cladding tube as a conventional example is well known and is disclosed in a) U.S. Pat. Nos. 3,502,549 and 36
Specification No. 25821, JP-A-51-69792, JP-A No. 51-69792
No. 1-69795, No. 51-69796 and No. 51-
In Publication No. 71497, as a liner material, M o
+ W * N b m N l , F e +
Mg + Cu* pure zr, ht, Nt-cr gold alloy aluminized coating, silicided coating, etc. are shown.

However, some of the liner materials used as barrier materials mentioned in the above-mentioned prior art have drawbacks such as a large neutron absorption cross section that 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.

[Purpose of the invention]

The present invention was made in view of the above situation, and an object of the present invention is to provide a nuclear fuel element that can significantly increase stress corrosion cracking resistance and improve reliability and economic efficiency.

[Summary of the invention]

In the nuclear fuel element of the present invention, the cladding tube is filled with nuclear fuel pellets, the openings at both ends of the cladding tube are sealed via end plugs, and the material of the cladding tube has a carbon content of 60% by weight or less. Some are made of zirconium alloy. The present inventor discovered that among the impurities of zirconium alloy,
It has been discovered that a zirconium alloy with a low carbon content, which has not received particular attention in the past, is excellent against stress corrosion cracking caused by the interaction between the fuel and the cladding.

[Embodiments of the invention]

FIG. 1 shows an embodiment of the nuclear fuel element of the present invention.

This will be explained with reference to FIG. FIG. 1 is an explanatory diagram of the chemical composition excluding zirconium, and FIG. 2 is a flowchart of the manufacturing process. First, the chemical composition standard for reactor number Zircaloy-2 (A8TM
B-353Grade RA-1) Add the following amounts of alloying elements satisfying f into zirconium. tin 1.20
~1.70% by weight, iron 0.07~α20% by weight, chromium 0.05~0.15% by weight, nickel α03~O,OS
% by weight, iron+chromium-nickel 0.18-0.38% by weight. Next, according to the above-mentioned conventional standards, the carbon content is 270% by weight or less, but we selected a zirconium ingot that had a carbon content of 50 and 60F and other impurities that met the above-mentioned standards. . For comparison, 20 zirconium ingots having a carbon content exceeding 60% by weight were selected. Using the Zircaloy-2 raw material whose composition was adjusted in this way, a Zircaloy-2 cladding tube was manufactured according to the flowchart shown in FIG.

Incidentally, the contamination of carbon during the manufacturing process was avoided as much as possible.

In order to investigate the properties of the manufactured low carbon Zircaloy-2 cladding tube, hollow nuclear fuel pellets are inserted into the cladding tube. Then, a cylindrical pure aluminum rod was inserted into the hollow part of this nuclear fuel pellet, and the iodine concentration was 3 wq/c1! In an atmosphere with a cladding tube temperature of 350 C, the aluminum rod was compressed in the longitudinal direction, and a circumferential stress of 90°C was applied to the cladding tube through the hollow nuclear fuel pellet. Then, the elongation at break that occurred in the cladding tube obtained at this time was determined. The characteristics shown in the elongations at break a and b of this example in FIG. 3, where the horizontal axis represents the carbon content and the vertical axis represents the elongation at break in the circumferential direction of the cladding tube, were obtained. On the other hand, for comparison, the results of a similar experiment using a conventional cladding tube are shown in Figure 3.
However, it is recognized that the elongation at break is lower than that of the former. That is, by limiting the amount of carbon, which is an impurity in Zircaloy-2, to less than 60% by weight, it is possible to reduce the Ω stress of the cladding when stress is applied to the cladding due to interaction with fuel in a corrosive gas. Corrosion cracking becomes less likely to occur and thick elongation is allowed.

As described above, the nuclear fuel element of this example is formed from a zirconium alloy with a carbon content of 60 weight P or less, so that the elongation at break in the circumferential direction is significantly lower than that of conventional cladding tubes, and the stress resistance is reduced. It can significantly increase corrosion cracking performance and improve economy and reliability.

〔Effect of the invention〕

As described above, the nuclear fuel element of the present invention has the effect of significantly increasing resistance to single force corrosion cracking, as well as improving K, economy, and reliability.

[Brief explanation of the drawing]

Claims (1)

[Claims] 1. A fuel cladding tube is filled with nuclear fuel pellets and the openings at both ends of the cladding tube are sealed via end plugs, and the material of the cladding tube is a zirconium alloy with a carbon content of 60 μm or less by weight. A nuclear fuel element characterized in that it is formed from.