JPS6313125B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention is based on pure zirconium (hereinafter referred to as zirconium) used to construct the core of a nuclear reactor.
The present invention relates to a method for measuring the wall thickness of a composite material made of a zirconium alloy and a zirconium alloy, particularly a composite material in which the inner surface of a tubular cladding material (hereinafter referred to as cladding tube) made of a zirconium alloy that encloses nuclear fuel is lined with a metal barrier of a zirconium layer. Nuclear fuel elements of power reactors currently designed, manufactured, or operated currently enclose nuclear fuel material inside a cladding tube that is corrosion-resistant, non-reactive, and has good thermal conductivity. Nuclear fuel assemblies are assembled by assembling such nuclear fuel elements in a lattice pattern at regular intervals, and by combining an appropriate number of these nuclear fuel assemblies, nuclear fission chain reaction assemblies and reactor cores capable of self-sustaining nuclear fission reactions are formed. The core is contained within a reactor vessel through which coolant passes. The main purpose of using a cladding tube to contain nuclear fuel in the core of such a nuclear reactor is, first, to prevent chemical reactions between the nuclear fuel and the coolant or between the nuclear fuel and the moderator; The purpose is to prevent radioactive fission products, which are partially gaseous, from escaping from the fuel into the coolant or moderator. The main cladding tubes commonly used are stainless steel and zirconium alloys. When manufacturing and operating nuclear fuel elements using certain metals or alloys as cladding tubes, various problems arise due to mechanical or chemical reactions that occur in these cladding tubes under certain conditions. It's on. Zirconium and its alloys are excellent nuclear fuel cladding under steady-state conditions. The reason for this is that zirconium and its alloys have a small neutron absorption cross section, and furthermore, at temperatures below about 400°C, zirconium and its alloys cannot be used in the presence of pure water or steam, commonly used as reactor coolants and moderators. This is because it has excellent strength, elongation, is extremely stable, and is non-reactive. However, it has become clear that the behavior of nuclear fuel elements is such that the interaction between the nuclear fuel, the cladding, and the fission products produced during the fission reaction causes the cladding to become brittle and eventually crack. It has been determined that this undesirable behavior is further exacerbated by local mechanical stresses due to differential thermal expansion between the fuel and the cladding. During the operation of a nuclear reactor, fission products are released from the nuclear fuel due to the fission reaction, and when they are present on the surface of the cladding tube and certain fission products such as iodine and cadmium are present,
Stress corrosion cracking occurs in cladding tubes due to the effects of local stress and strain. As a measure to prevent such troubles, attempts have been made to provide a metal barrier between the cladding tube and the nuclear fuel sealed therein. As this metal barrier, zirconium of moderate purity is used, and a composite cladding tube made of a zirconium alloy and lined with zirconium alloy is considered to be the most promising.
In this case, the thickness of the zirconium layer barrier is approximately 5 to 30% of the thickness of the composite cladding. Compared to zirconium alloys, zirconium remains soft during irradiation, reducing local strain within the nuclear fuel element and protecting the cladding from stress corrosion cracking or liquid metal embrittlement, as well as reducing neutron capture penalties.
It is also superior in that it does not cause heat transfer penalties or material incompatibility problems. Such a composite cladding tube is usually manufactured through the steps shown in FIG. That is, zirconium and zirconium alloy briquettes are melted separately to produce ingots, hollow billets of zirconium alloy with a large diameter and zirconium with a small diameter are produced, and then the zirconium alloy hollow billets are made into ingots. A hollow billet of zirconium is inserted inside and integrated by explosion welding or diffusion bonding to form a composite billet. Next, this composite billet is made into an extruded composite tube by a conventional hot extrusion method, and the extruded composite tube is processed into a continuous tube to form a composite cladding tube of desired size. In the composite cladding tube manufactured in this manner, the zirconium alloy layer portion serving as the base material and the zirconium layer portion serving as the metal barrier are metallically bonded at the mutual interface. The most important thing when using the composite cladding obtained in this way in the central part of a nuclear reactor is that the thickness of the zirconium layer that serves as a barrier is manufactured accurately so that it is within the required dimensional range. The zirconium layer and the zirconium alloy layer serving as the base material must be completely metallically bonded over the entire boundary surface between them. This is why it is necessary to inspect the thickness of the zirconium layer in each part of the composite cladding tube.
However, because the zirconium layer and zirconium alloy layer are completely metallically bonded and their materials are extremely similar, it has been difficult to accurately measure the thickness of the zirconium layer non-destructively. It was considered impossible. For example, standard Zircaloy-2, which complies with ASTM standard B353 grade R60802, whose main component is 98% zirconium.
(Sn1.47%, Fe0.123%, Cr0.087%, Ni0.056%)
base material layer and substantially pure zirconium 99.9%
(Sn0.0021%, Fe0.034%, Cr0.014%, Ni0.001
In the case of a composite cladding consisting of a barrier layer of
Since the physical properties such as thermal conductivity, magnetic permeability, and acoustic impedance are very similar, the thickness of the barrier layer cannot be determined by any inspection method such as ultrasonic inspection, electromagnetic inspection, or radiographic inspection. It was impossible to measure. Referring to Figure 2, the case where the thickness of a barrier layer is measured using a conventional water immersion ultrasonic inspection device is as follows.
When ultrasonic waves are emitted from the probe 6, the cathode ray tube 8
, a transmitted echo T appears first, and when the ultrasonic wave reaches the outer surface of the composite cladding tube 1, an outer surface echo S appears, but the ultrasonic wave is propagated within the thickness of the tube and the zirconium alloy base material Layer 2 and zirconium barrier layer 4
Even if it reaches the boundary surface 3 of the barrier layer, the acoustic impedance between the two layers does not change, so it passes through as it is, and no echo appears on the cathode ray tube 8, and the internal reflection echoes B and B' that hit the inner surface of the barrier layer and are reflected. only appears. Although it is possible to measure the total wall thickness of the composite cladding tube 1 based on the propagation time between the surface echo S and the internal reflection echo B, since the ultrasonic waves pass through the boundary surface 3, it is difficult to measure the thickness of the barrier layer 4. There was no way I could measure the thickness. In the figure, 5 is a water tank and 7 is an ultrasonic inspection device. The above-mentioned composite cladding tube has attracted attention for its good mechanical properties, and research and development is progressing worldwide as a nuclear fuel cladding tube that can be used in the operation of boiling water type and load-following type nuclear reactors. It has not yet been put into practical use, and the reason for this is that there are issues with quality assurance of the barrier layer, and in particular, it has not been possible to accurately measure the wall thickness of the barrier layer, which is a major reason for the delay in practical application. It was becoming. The present invention aims to solve the above-mentioned problems by heat-treating a composite material in which a substantially pure zirconium layer and a zirconium alloy layer are metallically bonded at a temperature higher than the recrystallization temperature. At the interface between the pure zirconium layer and the zirconium alloy layer, crystals with a clear grain size difference are generated between the two layers, and the composite material after heat treatment is subjected to ultrasonic waves (a) with a frequency in the 10 MHz range and with a frequency of 100 to 500 MHz. Ultrasonic waves (d) are applied from the zirconium alloy layer side at the same time or in succession, and the total thickness value of the composite material is determined based on the echoes reflected by the ultrasonic waves (a). The gist is to obtain the thickness value of the zirconium alloy layer based on the reflected echo, and to detect the thickness value of the pure zirconium layer based on the difference between the total thickness value of the composite material and the thickness value of the zirconium alloy layer. It is something. Here, a composite material is defined as a material in which a pure zirconium layer and a zirconium alloy layer are metallurgically bonded, regardless of its shape, whether it is a pipe, a plate, a strip, or a container. It may be something like. Embodiments of the present invention will be described below using a composite material as an example of a composite cladding tube in which nuclear fuel is sealed.
Table 1 shows the chemical compositions of the zirconium alloy (Zircaloy) that will be the base material layer of the composite cladding tube and the pure zirconium (pure Zr) that will be the barrier layer.

[Table] A composite cladding tube having a zircaloy layer and a zirconium layer having the chemical composition shown in Table 1 is manufactured through the tube manufacturing process explained with reference to Fig. 1 above, and consists of a zirconium alloy layer as a base material and its inner surface. The zirconium layer, which is a barrier lined on the side, is completely metallically bonded at the mutual interface, and when such a composite cladding tube is placed in a vacuum annealing furnace, the zirconium layer is 0.5 at a temperature higher than the recrystallization temperature. When heat treatment is performed for an appropriate time of ~3 hours, a clear difference in the degree of grain growth occurs between the zircaloy layer and the zirconium layer. This phenomenon is due to the behavior of the elements contained in both layers, and generally speaking, the more types of alloying elements there are, or the higher the content of those elements, the finer the grains become. What affects the size
Sn. The difference in grain size between the two layers is due to the fact that the two layers are completely metallically bonded.
The mutual boundary surfaces are clearly defined, as shown in the micrograph of FIG. 3. The crystal grain sizes of the zircaloy layer and the zirconium layer were as shown in Table 2 when the heat treatment temperature was varied. However, the overall degree of processing from the raw pipe stage of a composite cladding tube to the finished pipe by performing several tube manufacturing processes and intervening an annealing process between each tube manufacturing process is determined by the area reduction rate. 86%, heat treatment time is 2Hr
And so. Particle size display was based on ASTM standards.

[Table] Next, regarding wall thickness measurement by ultrasonic testing, it is generally known that there is a close relationship between the crystal grain size of the metal structure and the degree of scattering of ultrasonic waves. That is, the larger the crystal grains, the greater the degree of scattering of ultrasonic waves, and therefore the more difficult it is for ultrasonic waves to pass through the material to be inspected. The degree of scattering of ultrasonic waves is also closely related to the frequency of the ultrasonic waves themselves; the higher the frequency, the greater the degree of scattering. It is known that the rate of children returning to their offspring is also high. We conducted an experiment to see how this general phenomenon appears in composite cladding tubes. In this experimental example, the frequency of sample material No. 2 shown in Table 2 was
When ultrasonic waves in the 10 MHz range were applied, it was confirmed that the ultrasonic waves passed through the Zircaloy layer and the pure zirconium layer. Therefore, it was found that by using ultrasonic waves in this frequency range, it is possible to measure the total wall thickness of a composite cladding tube.
However, in order to detect the thickness of a pure zirconium layer, even though the zircaloy in the outer layer passes through, it does not pass through the zirconium layer and is reflected at the mutual interface and returns to the probe. Must explore ultrasonic frequencies. Therefore, ultrahigh-frequency ultrasonic waves with various frequencies are incident on the outer surface of the composite cladding tube, that is, from the zirconium layer side, and they are reflected at the interface between the zircaloy layer and the zirconium layer and returned to the probe. I checked the rate of return. The results were as shown in FIG. i.e. frequency 50M
The Hz signal hardly reflected from the boundary surface and did not come back, but the 100 MHz signal had a reflectance of 40
%, and the one at 200MHz exceeds 80%,
It is recognized that it reaches almost 100% at 220MHz. If the rate of ultrasonic waves returning to the probe is 40% or more, it is possible to measure the thickness of the Zircaloy layer by the reflected echo, so the frequency is
Uses ultrasound of 100MHz or higher. However, if the frequency is too high, the S/N ratio deteriorates, so in order to obtain a detectable S/N ratio (2 or more), it is necessary to suppress the frequency to 500 MHz or less. in short,
Sufficient reflected echo for measurement and S/ that does not interfere with detection
To obtain the N ratio, the thickness of the Zircaloy layer is measured with the frequency set in the range of 100 to 500 MHz. It goes without saying that if the total thickness of the composite cladding tube and the thickness of the zircaloy layer among them can be detected as described above, the thickness of the zirconium layer can be detected from the difference between the two thicknesses. Of course, this is applicable not only to composite cladding tubes, but also to composite templates and other composite materials, in view of the technical purpose described above. FIG. 5 schematically shows an ultrasonic wall thickness measuring device used to carry out the above wall thickness measuring method. In the same figure, a probe 6, a composite cladding tube 1, a base material layer (zircaloy layer or zirconium alloy layer) 2, a boundary surface 3, a barrier layer (zirconium layer or pure zirconium layer) 4,
The water tank 5 is similar to that in FIG. Reference numeral 11 denotes an ultrasonic inspection device that uses ultrasonic waves (a) in the frequency range of 10 MHz, and a cathode ray tube 12 is connected to this. On the other hand, 13 is an ultrasonic wave with a frequency range of 100 to 500 MHz.
This is an ultrasonic inspection device using (b), to which a cathode ray tube 14 is connected. 15 is an operation for inputting a signal corresponding to the total thickness value obtained by the cathode ray tubes 12 and 14 and a signal corresponding to the thickness value of the base material layer, and calculating the difference between the two, that is, the thickness value of the barrier layer. The signal value thus obtained is output to the display 16 to display the thickness value of the barrier layer 4. Emitted from ultrasonic inspection devices 11 and 13
The ultrasonic waves (a) and (b) in the 10 MHz range and 100 to 500 MHz range are made perpendicular to the surface of the composite cladding tube 1, and are incident in the thickness direction of the tube from the same location. This may be done simultaneously or one after the other. The ultrasonic inspection apparatuses 11 and 13 are equipped with filters that can handle only the respective frequencies. The reflected waves of the ultrasonic waves (a) appear on the cathode ray tube 12 as a transmitted echo T, an external echo S caused by reflection from the outer surface of the composite cladding tube 1, and an inner echo B caused by reflection from the inner surface of the barrier layer 4. Further, t ₀ corresponds to the total wall thickness of the composite cladding tube 1 . The reflected wave of the ultrasonic wave (b) appears on the cathode ray tube 14 as a transmitted echo T and an external echo S as in the case of the cathode ray tube 12.
A reflected echo F from the boundary surface 3 appears without passing through the boundary surface 3. And _t1 corresponds to the thickness of the base material layer 2. The signal at t ₀ and the signal at t ₁ are sent to the arithmetic unit 15.
The thickness value of the barrier layer 4 is obtained by calculating (t ₀ −t ₁ ), which is displayed on the display 16. As detailed above, according to the wall thickness measurement method of the present invention, a composite cladding tube with a base layer made of zirconium alloy and a barrier layer of pure zirconium metallically bonded to the inner surface of the base layer made of zirconium alloy, which was previously considered impossible. It is now possible to measure not only the total wall thickness of the reactor, but also the wall thickness of the base material layer and barrier layer, which will greatly contribute to the design of materials used in the core of the reactor, and will help prevent stress corrosion cracking and liquid metal The quality assurance level of composite cladding tubes that can withstand embrittlement can be greatly improved.

[Brief explanation of the drawing]

Figure 1 is a block diagram showing the manufacturing process of the composite cladding tube, which is the material to be measured. Figure 2 is a diagram showing how the wall thickness of the tube is measured using conventional water immersion ultrasonic testing. A micrograph clearly showing the difference in crystal grain size between the recrystallized zirconium alloy layer and the zirconium layer, Figure 4 is a diagram showing the relationship between the ultrasonic frequency used and the reflected echo, and Figure 5 is a diagram showing the present invention. FIG. 2 is a diagram schematically showing an ultrasonic inspection device used to implement the wall thickness measurement method of FIG. DESCRIPTION OF SYMBOLS 1... Composite cladding tube, 2... Base material layer, 3... Boundary surface, 4... Barrier layer, 5... Water tank, 6... Probe, 7... Ultrasonic inspection device, 8... cathode ray tube,
11... 10MHz range ultrasonic inspection device, 12... Braun tube, 13... 100-500MHz range ultrasonic inspection device, 14... Braun tube, 15... Arithmetic unit, 16
……display.

Claims (1)

[Claims] 1 By heat-treating a composite material in which a substantially pure zirconium layer and a zirconium alloy layer are metallically bonded at a temperature higher than the recrystallization temperature, a difference in crystal grain size is created between the pure zirconium layer and the zirconium alloy layer. After tightening, the composite material after the heat treatment is subjected to ultrasonic waves (a) in the frequency range of 10MHz and frequencies of 100 to 500MHz.
The ultrasonic waves (b) are incident simultaneously or one after another from the zirconium alloy layer side, and the total thickness value of the composite material is determined based on the echoes reflected by the ultrasonic waves (a),
The wall thickness value of the zirconium alloy layer is determined based on the echo reflected by the ultrasonic wave (b), and the wall thickness value of the pure zirconium layer is detected from the difference between the total wall thickness value of the composite material and the wall thickness value of the zirconium alloy layer. A method for measuring the wall thickness of a zirconium composite material.