JPH0137463B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a nuclear fuel element, and more particularly to a nuclear fuel element that can reduce the occurrence of stress corrosion cracking in a zirconium alloy cladding tube.

As shown in FIG. 1, a nuclear fuel element usually stores a plurality of nuclear fuel pellets 2 in a stack in a cladding tube 1, and the openings at both ends of the cladding tube 1 are closed with end plugs 3a, 3b.
It is sealed. 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. In addition, numeral 4 in FIG. 1 indicates a spring which functions to form a gas reservoir plenum 5 within the cladding tube 1 and to stably support the nuclear fuel pellets 2.

By the way, in the nuclear fuel element configured as described above, the cladding tube 1 has the function of preventing contact and chemical reaction between the nuclear fuel pellets 2 and the coolant, and
A function is required to prevent radioactive fission products released from the fuel from entering the coolant.
Therefore, if the cladding tube does not satisfy these functions, or if the cladding tube is damaged, the radioactivity level in the cooling system plant will increase, and reactor operation will have to be stopped to ensure safety. It becomes a situation where it has to happen.

The cladding of nuclear fuel elements used in light water-cooled nuclear reactors is generally made of zirconium and its alloys. Zirconium and its alloys have a small neutron absorption cross section and a temperature of about 400℃.
It is strong and ductile at temperatures below, and has stable properties that do not react with water vapor used as a coolant.

However, based on my driving experience to date,
Even in cladding tubes made of zirconium and zirconium alloys, brittle cracking occurs due to interaction such as reduction in material strength due to neutron irradiation and corrosion due to chemical reaction with nuclear fission products.

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 2 to the outer surface of the cladding tube 1, the gap formed between the inner surface of the cladding tube 1 and the nuclear fuel pellet 2 must be set to about 10 microns or less. On the other hand, during operation, the nuclear fuel pellet 2 generates heat, which causes the pellet itself to crack due to thermal stress, resulting in discrepancies in the fracture surfaces, as well as volumetric expansion caused by fission products accumulating within the nuclear fuel pellet as it burns. The second cause is
As shown in the figure, the cladding tube 1 is expanded by the nuclear fuel pellets 2 and is subjected to stress. Although the average value of the strain that the cladding tube 1 receives in the circumferential direction is not very large,
Strain is locally concentrated on the wall near the crack 6 formed 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 pellet 2 as a result of nuclear fission.
This corrosive gas collects in the free space within the cladding tube 1, ie in the cracks 6 and the like. In other words, cladding tube 1
Corrosive gas tends to collect especially near areas 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 clear.

An object of the present invention is to provide a highly reliable nuclear fuel element that can significantly reduce the probability that a zirconium alloy cladding tube will undergo stress corrosion cracking when stress is applied in a corrosive gas, and has a low probability of failure. It is in.

A nuclear fuel element according to the present invention for solving the above problems is a nuclear fuel element in which nuclear fuel pellets are housed in a zirconium alloy-based cladding tube and both openings of the cladding tube are sealed, and the material constituting the cladding tube includes: The content of impurity elements that form iodide, which is thermodynamically more stable than zirconium, is
This uses Zircaloy reduced to 1 ppm or less.

First, the mechanism by which stress corrosion cracking occurs will be explained. Grain boundaries where the crystal grains on the inner surface of the cladding tube are oriented in a specific direction are active in the reaction between grain boundary precipitates and iodine (I ₂ ), which is a fission product, resulting in a so-called active path. If strain concentration occurs near this area,
The surface oxide film is destroyed, and the components of this destruction form pits and generate crack nuclei. It is presumed that at the tip of this initial crack, the interfacial energy decreases due to surface adsorption of iodine, causing the crack to propagate.

Therefore, if the reaction rate between grain boundary precipitates and iodine can be reduced or stopped by some means, the initial crack nucleus will not be generated, and stress corrosion cracking will no longer occur.

In addition to the alloying elements of iron, chromium, and nickel, the grain boundaries of Zircaloy-clad tubes contain aluminum,
Magnesium, calcium, etc. are precipitated.
Figure 3 shows the change in the Gibbs free energy of formation when each element in the cladding reacts with 1 mole of iodine at 350°C. As shown in Figure 3, iodine and zirconium react easily, but magnesium and calcium, which are thought to precipitate at grain boundaries, react with iodine even more than zirconium, as indicated by the symbol 31 in Figure 3. It turns out it's easy. For this reason, it is thought that intergranular cracks occur in the cladding tube due to iodine, and these cracks progress.

Therefore, in the present invention, the cladding tube is made of a zirconium alloy that has a lower content of impurities that are more likely to react with iodine than zirconium, that is, magnesium and calcium, thereby eliminating the main cause of intergranular cracking. I did it like that.

An embodiment of the present invention will be described in detail below.

First, sponge zirconium is obtained by the Kroll method, which is a common method for producing zirconium. This zirconium contains about 10 ppm each of magnesium and calcium, so next,
In order to reduce the content of magnesium and calcium in zirconium as much as possible, sponge zirconium is once iodized to produce zirconium iodide. This zirconium iodide is brought into contact with a red-hot tungsten wire to decompose it, and zirconium microcrystals are precipitated around the red-hot wire. This method is called the iodine method. The contents of magnesium and calcium in the crystal verzirconium thus obtained were each 1 ppm or less. Therefore, alloying elements were added to this base material to obtain Zircaloy, and using this Zircaloy, an outer diameter of 12.52 mm,
It was processed into a cladding tube with a thickness of 0.86mm, and used to construct a nuclear fuel element.

In order to investigate the characteristics of the cladding tube, which has a reduced content of impurities such as magnesium and calcium that easily react with iodine, a hollow nuclear fuel pellet was inserted into the cladding tube, and a cylindrical shape was inserted into the hollow part of the nuclear fuel pellet. filled with pure aluminum rods, and compressed in the longitudinal direction in an atmosphere with an iodine concentration of 3 mg/cc and a cladding temperature of 350°C.
Circumferential stress was applied to the cladding tube through a hollow nuclear fuel pellet. Then, the elongation at break that occurred in the cladding tube at this time was determined. As a result, the bar line 42 in Fig.
The characteristics shown are obtained. On the other hand, for comparison, a conventional cladding tube was prepared and a similar experiment was conducted, and the results are shown by the bar line 41 in FIG. This shows that the cladding tube with a reduced content of impurities that easily react with iodine has a larger elongation at break than the conventional cladding tube. Note that the configuration of the nuclear fuel element according to the present invention is the same as that shown in FIG. 1, and the description thereof will be omitted here.

According to the nuclear fuel element according to the present invention described above, as is clear from the results shown in FIG. You can reduce the cost significantly.

The present invention is not limited to cladding tubes made of Zircaloy-2, but can also be applied to cladding tubes made of other zirconium alloys such as Zircaloy-4.

As explained above, according to the present invention, the probability of stress corrosion cracking occurring in the zirconium alloy cladding tube can be significantly reduced, and a highly reliable nuclear fuel element with a low probability of breakage can be obtained. .

[Brief explanation of drawings]

The drawings are for explaining the present invention; FIG. 1 is a longitudinal cross-sectional view of a nuclear fuel element, FIG. 2 is a partially cutaway perspective view for explaining problems that tend to occur in nuclear fuel elements, and FIG. FIG. 4 is a relative diagram showing the reactivity between elements inside the cladding material and iodine, and is a graph showing a comparison of elongation at break in the circumferential direction of the cladding tube used in the present invention and a conventional cladding tube. 1... Cladding tube, 2... Nuclear fuel pellet, 3a,
3b... end plug.

Claims (1)

[Scope of Claims] 1. A nuclear fuel element in which nuclear fuel pellets are housed in a zirconium alloy cladding tube and openings at both ends of the cladding tube are sealed, in which iodine, which is thermodynamically more stable than zirconium, is used as the material constituting the cladding tube. A nuclear fuel element characterized by using Zircaloy in which the content of impurity elements that form compounds is reduced to 1 ppm or less. 2. The nuclear fuel element according to claim 1, wherein the impurity elements that form stable iodides are magnesium and calcium.