JPS61165685A - Method of evaluating stress corrosion crack sensibility of fuel coated tube - 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 method for evaluating stress corrosion cracking susceptibility of zirconium-based alloy fuel cladding [Background of the Invention] Zirconium-based alloys are known for their excellent corrosion resistance. Because of its extremely small neutron absorption cross section, it is used in fuel cladding tubes for atomic couplants. Fuel cladding tubes are used for long periods in nuclear reactors, but sintering due to combustion of the fuel pellets causes stress to expand on the inner surface of the cladding tubes, and stress is generated by the release of corrosive gases (mainly iodine) from the fuel pellets. Corrosion cracking occurs.

Such stress corrosion cracking is a major cause of fuel rod failure and must be adequately addressed from the standpoint of reactor safety and reliability.

From the above-mentioned viewpoints, methods for preventing stress corrosion cracking have been studied, and methods for determining the corrosion resistance of materials outside the reactor have become important issues.

In other words, Zircaloy-Z (
Approximately L518n, 0.1*1i'e, 0.1% Or to Zr
and 0.051Ni-added alloy), Zircaloy-4
(ZrK approx. L5%Sn, oytspe and 0.1%Or
A method of preventing stress corrosion cracking by providing a liner section on the inner surface of the cladding tube of 7. 147680, 1982
195185). In addition, as a material evaluation method, there is also a method of determining the soundness of the material by observing the surface structure [JP-A-59-208541] On the other hand, as a measure to prevent nodular corrosion on the outer surface of the cladding tube in contact with reactor water in a nuclear reactor, 4
There is a method of β-quenching during the production of 1 bond f. However, the stress corrosion cracking susceptibility in this case is not necessarily clear.

As described above, attempts are being made to prevent stress corrosion cracking of zirconium alloys by treating the inner surface of the cladding tube and improving the structure of the tube itself. These #evaluations are carried out inside the reactor or determined by tests outside the reactor.

Although demonstration tests inside a nuclear reactor are a more reliable evaluation method, it is not possible to set test specimens easily because it is not possible to set severe test conditions and it is necessary to take sufficient safety precautions against fuel damage. There is a restriction that it cannot be placed inside an atom.

On the other hand, characteristics evaluation outside the reactor involves placing the inside of the cladding tube in a corrosive state, and then (1) increasing the internal pressure and determining from the relationship between stress and time when a crack penetrates; (2) inside the cladding tube. There are two main types of methods: bulging the surface core and determining the amount of strain and the presence or absence of crack penetration. However, the previous method (1
) is not sufficient as a simulation test because actual furnace cracking is caused by the swelling of fuel PV, but it is planned based on the internal pressure load. The previous method (the former method applies stress by bulging the core, which makes it easy to control strain and allows stress concentration to be applied, etc., is effective as a mock test, but it is mainly judged by the presence or absence of through cracking, and cracks are It is difficult to evaluate the occurrence and progress of the disease.

However, although it is possible to measure cracks by cutting a cross section of the cladding tube during the test, this requires a huge amount of test material and labor.

Next, we will discuss the problems of the conventional method and its effectiveness in detecting the occurrence and progression of cracks and evaluating stress corrosion cracking susceptibility. In the conventional method, the determination of a through crack differs depending on the amount of deformation, but it is determined as a through crack, and these are ranked lower. On the other hand, all cladding tubes determined to have no through cracking are classified as having the same stress corrosion cracking susceptibility and are used for convenience. However, although some of them have resulted in through cracking,
Some materials have a tendency to crack quickly, which can cause unexpected problems in actual equipment.

On the other hand, if a plan for the occurrence and propagation of cracks can be made, depending on the extent of the cracks, it will be possible to decide whether to apply the method to high-temperature fuel cladding tubes scheduled for future operation, current cladding tubes, or equipment whose operating conditions are not severe.

Each cladding tube can be applied to suit the equipment usage conditions. This will dramatically improve the safety of the equipment and has the potential to supply cladding tubes at low cost.

For this reason, there has been a strong desire for a simple evaluation method that can reliably determine the stress corrosion cracking susceptibility of cladding outside the reactor. From the above-mentioned viewpoints, methods for preventing this stress corrosion cracking have been studied, and a method for determining the corrosion resistance of materials in detail outside the nuclear reactor is strongly desired.

[Purpose of the invention]

The purpose of the present invention is to provide a method for determining stress corrosion cracking susceptibility in a nuclear reactor environment by an evaluation test outside the reactor. It was discovered that the stress corrosion cracking susceptibility can be determined from the crack depth, which can be detected from the change in potential difference.

The details of the present invention will be described below.

As shown in FIG. 1, the present invention is based on the S
In the method for determining CC sensitivity, the electrode configuration is such that multiple electrodes are provided in the circumferential direction on the outer surface of the cladding tube, two electrodes adjacent to each other are used as measurement terminals, and two electrodes adjacent to the outer surface are used as measurement terminals. Use constant current terminal. The potential difference is determined by passing a predetermined current between the constant current terminals, measuring the potential difference between the measuring terminals, estimating the crack depth on the inner surface of the cladding tube from the change, and determining the SCC susceptibility of the cladding tube from the magnitude of the value. This is a method of determining. Note that the determination of the entire circumference of the cladding tube can be achieved by moving one block in nine circumferential directions using a contact switching method and repeating the above-mentioned potential difference measurement.

More stable measurements can be made by spot welding the conducting wire directly to the cladding tube than shown in FIG. 2, but electrodes with reduced contact as shown in FIGS. 4 and 5 can also be used. In this contact type electrode, each electrode is fixed with an insulator, and a spring allows it to contact the cladding tube evenly, and the electrode can be attached and detached using a lock method (lock part 13 in Fig. 4).
It also has a structure that allows it to be moved arbitrarily.

The reason why the detection operation time is limited in the present invention is as follows. The progress of corrosion largely depends on the rate of electrochemical reactions. Therefore, in the present invention, it is desirable to carry out the above-mentioned potential difference measurement in a range where the load current does not affect the corrosion itself, and the detection time range is 5x or less in one area and 100'HJ or less in the entire circumference area. did. If the detection time is longer than that, there will be variations in the judgment of stress corrosion cracking susceptibility. [Embodiment of the invention] Example 1 The Zircaloy-2 fuel cladding used for convenience was 1°41mm Sn,
0.151Fe, 0.12% Cr, 0.06% and balance Z
It was manufactured using an ingot having a chemical composition of r.

The cladding tube used was manufactured by conventional manufacturing methods. Figure 6 is an example of confirming the characteristics of the method of the present invention, and shows the relationship between the slit depth (simulating stress corrosion cracking depth) and the potential difference between the measurement terminals ((B)-(C) in the figure). According to this, an increase in the slit depth significantly increases the potential difference, and this relationship suggests that the present invention can quantitatively detect the occurrence and progression of stress corrosion cracking. do.

On the other hand, the above material was subjected to a stress cracking test in an iodine environment using the method of the present invention using a core tube expansion method. (Test conditions are 350C, a
(1mg/crn2), loading speed L7 x 10
-' g/sea) As a result, as shown in FIG. 7, no through cracking occurred even when the load was applied until the 1 diameter strain (E) reached 10 inches or more. However, it was estimated from the relationship shown in FIG. 6 that micro-cracks of about 0.45 W were generated on the inner surface of the cladding tube due to the change in potential difference. Further, in the subsequent destructive inspection of the test specimen, a crack with a depth of 0.4 was was confirmed on the inner surface of the tube, which was in relatively good agreement with the estimated value by the method of the present invention.

As described above, it has been found that the method of the present invention can successively measure the occurrence and depth of stress corrosion cracking on the inner surface of a tube.

Example 2 The material was Zircaloy-2, and four types of fuel cladding tubes of different manufacture were used. The stress corrosion cracking test was carried out using the method of the present invention and the conventional method using a core tube expansion method at 350C, iodine (1 mg/z"), and a loading rate of 1.7 x lO-4.

As a result, as shown in FIG. 8, in the present invention, differences in stress corrosion cracking susceptibility between lots were determined in detail based on crack depth, etc. On the other hand, in the conventional method, the evaluation is based on the presence or absence of penetrating cracks, so the judgment is rough. As described above, the present invention makes it possible to determine the stress corrosion cracking susceptibility between lots even by non-penetrating cracking, and allows for more detailed material selection than conventional methods. Especially when applying new materials, design margins can be appropriately evaluated.

〔Effect of the invention〕

According to the present invention, the stress corrosion cracking resistance of a Zircaloy fuel cladding tube for a nuclear reactor can be determined more accurately. Therefore, this effect can prevent fuel damage and provide a low-cost fuel cladding tube.

[Brief explanation of drawings]

Fig. 1 is a schematic diagram of potential difference measurement during the implementation of the stress corrosion cracking susceptibility evaluation method of fuel cladding of the present invention, Fig. 2 is an explanatory diagram of the setting positions of the electrodes in Fig. 1, and Fig. 3 is the Figure 4 is a schematic diagram of stress corrosion cracking in the axial cross section of the fuel cladding tube, Figure 4 is a schematic diagram of the contact plate electrode in Figure 1, Figure 5 is a detailed diagram of the contact type electrode in Figure 1, Figure 6 is A) is a detailed diagram of the slit position of the electrode in Figure 1, (B) is an explanatory diagram of the relationship between the potential difference and the slit depth in the measurement area of (A), and Figure 7 (A) is a detailed diagram of the slit position of the electrode in Figure 6. (o) is an explanatory diagram of the stress cracking test results in (a), (c) is the cross-sectional origin of the crack from the inner surface between C and 0 of the test specimen in (a), and Figure 8 is an explanatory diagram of the corrosion cracking site. FIG. 2 is a comparison diagram of stress corrosion cracking susceptibility evaluation according to the present invention and a conventional method. DESCRIPTION OF SYMBOLS 1... Fuel cladding tube, 2... Electrode, 3... Lead wire, 4... V-switch, 5... Constant current power supply, 6...
...Amplifier, 7.Switching controller, 8.Internal pressure, 9.
...Zhou Xinli.

Claims (1)

[Claims] 1. In a method for determining the stress corrosion cracking susceptibility of a zirconium-based alloy fuel cladding tube, a plurality of electrodes are provided in the circumferential direction on the outer surface of the tube, and any two nearest adjacent electrodes are used as constant terminals, and A constant DC current is passed between the outer electrodes, and the depth of the inner crack in that area is detected from the change in potential difference between the measurement terminals.Then, the depth of the inner crack is detected by moving to the next area and repeating the crack detection. A method for evaluating stress corrosion cracking susceptibility of fuel cladding tubes, which is characterized by comparing maximum crack depths.