JPS59151085A - 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 relates to a nuclear fuel element, particularly to a nuclear fuel element having an improved structure for preventing stress corrosion cracking of a zirconium alloy cladding tube or a cladding tube having a zirconium liner pipe metal. be.

[Prior art]

Normally, a nuclear fuel element has a plurality of nuclear fuel pellets 2 stacked and housed in a cladding tube 1, as shown in the cross-sectional view of FIG. 1, and both ends of the cladding tube 10 are sealed with end plugs 3a and 3b. It has become. The nuclear fuel pellet 2 is formed by molding and sintering fissile oxide fuel powder into a cylindrical pellet having a length to diameter ratio of about 1, for example. still,
Reference numeral 4 in FIG. 1 denotes a spring that serves both the function of forming a gas reservoir brenum 5 within the cladding tube 1 and the function of stably supporting the nuclear fuel pellet 2.

Incidentally, the cladding tube 1 in the nuclear fuel element configured as described above has the function of preventing contact and chemical reaction between the nuclear fuel pellet 2 and the coolant, and the function of preventing radioactive fission products released from the fuel from entering the coolant. A function is required to completely prevent the contamination of the product. 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 total safety. This is an unavoidable situation.

The cladding tube ♂ of nuclear fuel elements used in water-cooled nuclear reactors is generally made of zirconium and its alloys. Zirconium and its alloys have a small neutron absorption cross section, are strong and ductile at temperatures below about 400C, and have stable properties that do not react with heat or water vapor used as a coolant. However, according to operating experience to date, even with cladding made of zirconium and its alloys, neutron irradiation is not possible. -j-
Ru.

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 pellets 2 to the outer surface of the cladding tube 1,
The total gap formed between the inner surface of the nuclear fuel pellet 2 and the nuclear fuel pellet 2 must be set to several tens of microns or less. on the other hand. During operation, the nuclear fuel pellet 2 generates heat, which causes the nuclear fuel pellet 2 itself to crack due to thermal stress, resulting in discrepancies in the fracture surfaces and volumetric expansion caused by the accumulation of fission products within the nuclear fuel pellet 2 as it burns. Then, as shown in FIG. 2, the cladding tube 1 is subjected to the stress of being expanded by the nuclear fuel pellets 2. Although the average value of the strain that the cladding tube 1 receives in the circumferential direction is not so large, the strain is locally concentrated on the wall near the crack 6 that has occurred in the nuclear fuel pellet 2, 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 pellets 2 as a result of nuclear fission, and these corrosive gases are distributed especially in the free space within the cladding tube 1, such as the cracks 6. Corrosive gas tends to collect near the portion of the cladding tube 1 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, oxygen present in the bath, alloy composition, heat treatment, degree of processing, etc., and the mechanism by which it occurs is not clear.

[Purpose of the invention]

The present invention has been made in view of the above situation, and the entire purpose of the present invention is to provide a nuclear fuel element in which the p1 zirconium cladding tube or 1'' is zirconium-like and has a low probability of failure.

[Summary of the invention]

In the nuclear fuel element of the present invention, nuclear fuel pellets are housed in a zirconium alloy cladding tube or a cladding tube having a zirconium liner tube, and the openings at both ends of the cladding tube are completely sealed. All elements are intercalated to form a thermodynamically stable iodide. In order to achieve all of the above objects, the present inventors have discovered all means for preventing stress corrosion cracking based on the stress corrosion cracking mechanism, and it is believed that the stress corrosion cracking mechanism is caused by the following process.

In the imaginary boundary where the crystal grains on the inner surface of the cladding tube are completely oriented in a specific direction, the reaction between the grain boundary precipitates and the fission product iodine (■2) is active. This becomes a so-called active bus, and when strain concentration occurs near the temple, the bottom oxide film is destroyed, and this destroyed portion becomes a focal point, generating a crack nucleus. It is presumed that at the tip of this initial crack, the interfacial energy decreases due to surface adsorption of workpiece 2, and the crack propagates. Therefore, if the reaction rate between the grain boundary precipitates and iodine can be completely reduced or stopped by some means, the initial crack nucleus will not be generated and stress corrosion cracking will not occur.

The Gibbs formation free energy change sound when each element in the cladding reacts with 1 mole of iodine at 350C, the horizontal axis shows the alloying elements and impurity elements in the cladding, and the vertical axis shows the change in the formation free energy of iodide. It is shown in Figure 3.

In Figure 3, iodine and 7. It easily reacts with r, but A
Mg shown at position and Ca shown at position B are 7. It reacts even more easily with iodine than r, and for this reason, the iodine generated from the nuclear fuel beret through nuclear fission will react with the zirconium that makes up the entire cladding tube, as well as with the calcium and magnesium inserted into the cladding tube. As a result, a nuclear fuel element can be obtained in which all iodine acting on the cladding tube has been removed.

[Embodiments of the invention]

Hereinafter, the nuclear fuel element of the present invention will be explained using one embodiment, using the same symbols and symbols for the same parts as the conventional one, omitting the explanation of the structure of the same parts, and referring to FIG. In Figure 4, the inner diameter is approximately 10.8++
Zircaloy-2 cladding tube 1 with an outer diameter of about 12.5 rrnn and a length of about 4 m is welded to an end plug 3b', and uranium dioxide nuclear fuel pellets with an outer diameter of about 10.6 mm and a height of about 10 mm are inside. Insert 2. Then, a magnesium disk 7 with a thickness of about 1 tu and about 108 is formed by punching from a high-purity magnesium plate with a thickness of about 1 porcelain, and inserted between the upper and lower nuclear fuel pellets 2, respectively.

In this way, we investigated the properties of nuclear fuel elements contained within the cladding tube, which form an iodide that is thermodynamically more stable than zirconium iodide. The method is to place 1Mi of magnesium in a bong shape on the inner surface of the cladding tube, insert a nuclear fuel pellet inside the cladding tube, and fill the hollow part of the nuclear fuel pellet with a cylindrical pure aluminum rod.
During the W period, the aluminum rod was compressed in the entire longitudinal direction in an atmosphere with a phosphorus a degree of 3 m g71c c and a cladding tube temperature of 350C, and a hollow nuclear fuel pellet was fully valved to apply stress in the circumferential direction to the cladding tube. Then, the distribution of cracks on the inner surface of the cladding tube obtained at this time was determined. On the other hand, for comparison, a similar experiment was conducted in which a conventional cladding tube was prepared without inserting a magnesium ribbon ligature. As a result, as shown in Figure 5, where the horizontal axis represents the length of cracks and the vertical axis represents the frequency of crack occurrence, the curve C of the conventional fuel rod element is significantly lower than the curve of the cladding tube containing magnesium. A high incidence of siffrac was observed.

As is clear from the results shown in FIG. 5, in the nuclear fuel element of this example, the frequency of cracking in the stress corrosion cracking test was low. It is clear that it is working effectively.

In this way, in the nuclear fuel element of this example, an element that forms all iodides, which is thermodynamically more stable than zirconium, is inserted into the cladding tube, so that the zirconium-based cladding tube is not subjected to stress in corrosive gas. The probability of corrosion cracking occurring is significantly lower, reducing the probability of damage and improving reliability. It can be significantly improved.

In the above example, the case of a cladding tube made of Silca 9E2 was described, but it can also be applied to a cladding tube formed of other zirconium alloy systems such as Zircaloy-4, and a cladding tube with a zirconium liner. are the same. Furthermore, although we have described the case where magnesium is used as the element inserted into the cladding tube, the same effects can be obtained even with zirconium or other elements that easily react with iodine, such as calcium, sodium, and potassium.

〔Effect of the invention〕

As described above, the nuclear fuel element of the present invention significantly reduces the probability of stress corrosion cracking and reduces the probability of failure when stress is applied to a zirconium alloy cladding tube or a cladding tube having a zirconium liner tube in corrosive gas. This has the effect of significantly improving reliability.

[Brief explanation of the drawing]

Figure 1 is a longitudinal sectional view of the main part of a conventional nuclear fuel element, Figure 2 is an explanatory diagram of problems that tend to occur in the nuclear fuel element of Figure 1, and Figure 3 is a diagram showing the reactions between various elements inside the cladding material and iodine. Gender illustration,
FIG. 4 is a sectional view of a main part of an embodiment of the nuclear fuel element of the present invention, and FIG. 5 is an explanatory diagram comparing the frequency of crack occurrence between the nuclear fuel element of FIG. 4 and a conventional nuclear fuel element. 1... Cladding tube, 2... Nuclear fuel belen), 3a, 3b
...Figure 1 Figure 3 Figure 4-Figure S

Claims (1)

[Claims] 1. A nuclear fuel element in which nuclear fuel pellets are housed in a zirconium alloy cladding tube or a cladding tube having a zirconium liner tube, and both openings of the cladding tube are sealed, wherein heat is A nuclear fuel element characterized by the insertion of an element that forms an iodide that is mechanically more stable than zirconium. 2. The nuclear fuel element according to claim 1, wherein the element is formed from magnesium. 3. The nuclear fuel element according to claim 1, wherein said element is formed from calcium.