JPS6165187A - 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 invention relates to improvements in nuclear fuel elements.

[Background of the invention]

Generally, a nuclear fuel element has a plurality of nuclear fuel pellets 8 stacked and stored in a tube to be formed 7, as shown in FIG.
Openings at both ends of the cladding tube 7 are end plugs 9a.

9b. The nuclear fuel pellet 8 is made of fissile oxide fuel powder, for example, with a length to diameter ratio of about 1.
It is molded and sintered into cylindrical pellets. Furthermore, the first
Reference numeral 10 in the figure denotes a spring that has the function of forming a gas reservoir brenum 11 within the cladding tube 7 and the function of supporting the nuclear fuel pellets 8 in a stable state. In addition, in the nuclear fuel element configured as described above, the cladding tube 7 has a function of preventing the coolant from coming into contact with the nuclear fuel pellets 8 and preventing a chemical reaction from occurring, and a function of preventing the coolant from coming into contact with the nuclear fuel pellets 8 and preventing a chemical reaction from occurring between the cladding tube and the nuclear fuel pellets 8. A function is required to prevent radioactive fission products from entering the coolant.

Therefore, the cladding tube 7 that does not satisfy such functions, that is,
In the event that the cladding tube 7 is damaged, the radioactivity level in the cooling system plant increases, resulting in a situation where the reactor operation must be stopped to ensure safety.

On the other hand, the cladding tubes of nuclear fuel elements used in water-cooled nuclear reactors are generally made of zirconia and its alloys. Zirconium and its alloys have a small neutron absorption cross section, are strong and ductile at temperatures below about 400 C, and have characteristics that do not react with water vapor used as a coolant.

However, according to operational experience to date, even if the cladding tube 7 is made of zirconium and its alloys, interactions such as a decrease in material strength due to neutron irradiation and corrosion due to chemical reactions with fission products may occur. Brittle cracking has occurred due to Such an undesirable phenomenon is thought to occur as follows. That is, in order to efficiently transfer the heat generated by the nuclear fuel pellet 8 to the outer surface of the cladding tube 7, the gap formed between the inner surface of the cladding tube 7 and the nuclear fuel pellet 8 is set to several tens of microns or less. There is a need. On the other hand, during operation, the nuclear fuel pellet 8 generates heat, so the pellet itself cracks due to thermal stress.
Due to the misalignment of the fracture surfaces and the volumetric expansion caused by the accumulation of fission products within the nuclear fuel pellet 8 during combustion, the cladding tube 7 is pushed and expanded by the nuclear fuel pellet 8 as shown in FIG. subject to stress. flannel jacket 7
Although the average value of strain in the circumferential direction is not very large,
Strain is locally concentrated on the wall of the nuclear fuel pellet (8) near the crank 6, 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 the nuclear fuel pellet 8 due to nuclear fission, and this corrosive gas is released into the free space within the cladding tube 7, that is, the crank 6, etc. get together. In particular, corrosive gas tends to collect near the portion of the cladding tube 7 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 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 such undesirable destruction, the concept of lining the cladding tube is well known and is disclosed in U.S. Pat. Nos. 3,502,549 and 36
Specification No. 25821, JP-A No. 51-69792.
-69795, 51-69796 and 51-7
In Publication No. 1497, the liner material is melon Mo, W
, Nb, Cr, Ni, Fe, Mg, Cu
, pure zr.

A/, Ni-Cr alloy, aluminized coating, silicided coating, etc. are shown.

However, some of the liner materials used as barrier materials mentioned in the above prior art have drawbacks such as a large neutron absorption cross section that reduces the economic efficiency of the reactor. Further, the higher the purity of the liner material, the better the SCC (stress corrosion cracking resistance) performance, so it was necessary to use a liner material of high purity. For example, zirconium used as a liner material is mainly obtained by the Kroll method in which zirconium tetrachloride is reduced with magnesium. This zirconium is called sponge zirconium and is widely used. If even higher purity is required, the above-mentioned sponge zirconium is once reacted with iodine, the zirconium iodide is made red-hot, and it is decomposed by touching it with a tungsten wire, and microcrystals are precipitated around the red-hot wire to crystallize it. Obtain Verzirconium. In this case, not only does the number of manufacturing steps for the liner material increase, but there are also inherent technical problems and economic disadvantages.

Thus, in some of the proposals cited above, the material used as the wall may be a material that is incompatible with nuclear fuel pellets or a material that is incompatible with cladding; It cannot be said that a fundamental solution to the local chemical-mechanical interaction between the nuclear fuel and the cladding tube, which has been a problem recently, has been reached.

[Object of the Invention] The present invention was made in view of the above-mentioned circumstances, and its purpose is to provide a nuclear fuel element that can significantly increase stress corrosion cracking resistance and improve economic efficiency and reliability. be.

[Summary of the invention]

In the nuclear fuel element of the present invention, nuclear fuel pellets are exposed to light jσ(,
The openings at both ends of the extraction tube are sealed through end plugs, and a liner layer made of a zirconium material is formed on the inner surface of the cladding tube, and the cladding tube is made of Zircaloy-2 or Zircaloy-4 material. The liner layer is composed of
It is made of a material in which the carbon content in the zirconium is 50% by weight or less, and at least a part of the inside of the powder receiving tube is lined. The present inventors have discovered that a liner layer in which carbon I's content is restricted to a low level among impurities in zirconium is excellent against stress cracking due to fuel interaction, and The surface is lined with this low carbon zirconium.

The following is the chemical composition standard for reactor grade zirconium (A8T
M・B-352-79・Grade, R60001)
shows.

Aluminum 75! P or less, horine o, s IF or less, cadmium 0.5- or less, carbon 270I! II+ or less, cobalt 20 or less, chromium 200p or less, copper 50P
Hereinafter, iron 1500 or less, Hafuniwam 1001] 1u or less, Magnesium 20 or less, manganese 50PF or less,
Molybdenum 50 or less, nickel 70Pl or less, silicon 1
20IF1 or less, tin 50p or less, tungsten/100p or less, furan 3.5p or less, niobium 100p or less, titanium 50p or less, hydrogen 25p or less, nitrogen 804 or less.

Among the impurity elements mentioned above, zirconium forming the liner layer of the nuclear fuel element of the present invention has a lower allowable amount of 270p for carbon atoms. When carbon is included in zirconium, the solid volume within the zirconium interstitial space is extremely small, so it forms a carbide with zirconium (Z
form rC).

Then, it segregates at the zirconium grain boundaries. Until now, little attention has been paid to the influence of grain boundary segregated carbides on stress corrosion cracking. However, according to the experimental results of the present inventors, even a small amount of carbon can induce grain boundary embrittlement in a stress corrosion cracking environment, causing microcracks to occur in the zirconium liner layer and preventing the original purpose of the liner from forming. It was found that the effect of preventing stress corrosion cracking was impaired.

That is, it has been found that the carbon content has a greater influence on the stress corrosion cracking susceptibility of zirconium than the oxygen content, which is the main impurity in zirconium, and the degree of oxidation. From the above, the lower the carbon content in the zirconium liner layer, the more preferably 50 ni or less.

[Embodiments of the invention]

Hereinafter, the nuclear fuel element of the present invention will be explained with reference to FIGS. 3 to 5, with the same parts as the conventional ones being designated by the same numbers and explanations of the structures of the same parts being omitted. Fig. 3 is an explanatory diagram of the relationship between carbon content 1i and cracks, where the carbon content is taken to explain the manufacturing process of zirconium liner 1I, and the cumulative length of the crank within 0.02 mm2 is plotted on the vertical axis. Figure 5 shows the embodiment shown in Figure 4 and the conventional one.
It is an explanatory diagram of impurity content (review) in zirconium of j. First, in order to use it as a zirconium liner material, from sponge zirconium and hycrystalline verzirconium, we selected a 2-1 lot with a carbon content of 50 weight or less. is 5
We selected zirconium lots of 3<2+l that exceeded 0. The carbon content and other impurity amounts of these lots used are shown in FIG. Example 1 shown in Figure 5
．． Five zirconium liner materials of Example 2 and Conventional Examples 3 to 5 were used to prepare zirconium liner-clad tubes by the process shown in FIG.

In Figure 3, pure zirconium raw material is added to step 11.
The ingot is melted in step 12 and forged and formed into a hollow billet in step 12. This hollow billet is inserted into a separately formed hollow billet of a zirconium alloy such as Zircaloy-2, and the two combined hollow billets are hot tube extruded in step 13 so that the inside of the zirconium liner cladding tube is a layer of pure zirconium. The outside is the raw tube with a zirconium alloy layer. This raw tube undergoes cold rolling in step 14 and annealing in step 15 a predetermined number of times, and then becomes a zirconium liner-clad tube in step 16. Five zirconium liner clad tubes were manufactured in this way, and the zirconium raw material used in this example was as described above. A sea urchin carbonaceous material and a zirconium sponge raw material with an IF or less were selected and manufactured. Note that contamination of carbon during melting and other manufacturing steps was avoided.

And reduce the carbon content to less than 50pp? IJ limited Example 1.2 and Conventional Examples 3 to 5 were prepared at an iodine concentration of 1 mg
/ cm", in an atmosphere with a cladding tube temperature of 350C, apply internal pressure to the cladding tube with high-purity argon gas, and apply 10
% of circumferential plastic strain was generated.

At this time, in order to quantify the ease with which stress corrosion cracking occurs in the liner layer of each cladding tube, the cumulative length of cranks occurring within a predetermined area of the inner surface of the cladding tube was determined. The results are shown in FIG.

As is clear in FIG. 4, as the carbon content in zirconium decreases, the cracks that occur also decrease. moreover,
When the carbon content exceeds sop, the crank length increases rapidly, but in Example 1゜2, cracks occur significantly less than in Conventional Examples 3 to 5, and stress corrosion cracking of the cladding tube can be prevented. .

Also, by simply limiting the carbon content in zirconium,
Since the stress corrosion cracking resistance can be improved, the liner material is not limited to high-purity crystal versilconicum, and sponge zirconium can also be used, making it possible to improve economic efficiency.

As described above, in the nuclear fuel element of this example, by reducing the carbon content of the zirconium liner of the zirconium alloy cladding tube to 50p or less, the stress corrosion cracking resistance can be significantly increased, and the economic efficiency and reliability can be improved. . In addition, although the above example describes the case where a zirconicum liner with a carbon content of 501- or less is used over the entire surface, the same effect can be obtained even if it is used only in the axially central portion of the test tube where corrosion is particularly severe. have

〔Effect of the invention〕

As described above, the nuclear fuel element of the present invention has the effect of significantly increasing stress corrosion cracking resistance and improving economic efficiency and reliability.

[Brief explanation of drawings]

Fig. 41 is a vertical cross-sectional view of a normal nuclear fuel element, Fig. 2 is an explanatory diagram of problems that are likely to occur in the nuclear fuel element of Fig. 1, and Fig. 3 is an explanation of the cladding manufacturing process of the embodiment of the nuclear fuel element of the present invention. Figure 4 is an explanatory diagram of the relationship between the carbon content and stress corrosion cracking of the zirconium liner manufactured by the process shown in Figure 3,
FIG. 5 is an explanatory diagram of the impurity content in zirconium of the sample shown in FIG. 4. 7... Cladding tube, 8... Nuclear fuel pellets, 9a, 9b
...End plug.

Claims (1)

[Claims] 1. A cladding tube made of a zirconium alloy material is filled with nuclear fuel pellets, the openings at both ends of the cladding tube are sealed via end plugs, and a liner layer made of a zirconium material is formed on the inner surface of the cladding tube. In the cladding tube, the cladding tube is made of a Zircaloy-2 or Zircaloy-4 material, the liner layer is made of a material in which the carbon content in the zirconium is 50 ppm by weight or less, and the cladding A nuclear fuel element characterized in that at least a portion of the inside of the pipe is lined.