JPH06308104A - Ultrasonic probe - Google Patents

Abstract

PURPOSE:To prevent the deterioration of a piezoelectric element by providing a protecting member from liquid Na at the side of a transmitting/detecting surface of the piezoelectric element which has the thickness of not larger than a specific ratio of the basic resonance wavelength of the element. CONSTITUTION:A stainless plate (protecting member) 4 is set at the front face of a silver electrode 2a at the side of a transmitting/detecting surface of a piezoelectric body 1. Since a nickel thin film is formed in the stainless plate by vapor deposition, sputtering or the like manner, the wettability of the stainless plate 4 to Na is improved. Corrosion by Na is hence prevented. The thickness of the stainless plate 4 is not larger than about 10% the wavelength determined by the basic resonant frequency of the body 1, for instance, 8.6%. A packing member 3 formed of porous ceramic with approximately 10Mrayl acoustic impedance is provided at the front face of a silver electrode 2b. The member 3 supports the piezoelectric body 1, absorbing the ultrasonic waves radiated from the side of the electrode 2a and transmitting only the ultrasonic waves from the electrode 2b to a transmitting medium. In this manner, when the thickness of the protecting member 4 against Na is set to be approximately not larger than 10% the basic resonance wavelength, a good detecting wave having a short wave train length is obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ultrasonic probe, and more particularly to an ultrasonic probe used in high temperature liquid sodium in a fast breeder reactor.

[0002]

2. Description of the Related Art In recent years, a nuclear reactor called a fast breeder reactor (FBR) is expected as a future nuclear reactor. The fast breeder reactor uses a nuclear reaction that occurs when fast neutrons collide with Pu, which is a fission nucleus, and is characterized in that more fuel is produced than consumed fuel.

A transfer material for transmitting heat generated by nuclear fission,
That is, as the coolant used in the fast breeder reactor, liquid sodium is used, in which fast neutrons are difficult to slow down and the thermal conductivity is 100 times that of water. The liquid sodium is heated by the core in the reactor vessel, then guided to a cooling system provided outside the reactor vessel, returned to the reactor vessel again, and circulated. Liquid sodium is opaque and reacts violently with water and air, so be careful when handling it.

By the way, in a nuclear reactor, in order to ensure safety, it is necessary to constantly monitor the core structure and foreign substances in the reactor. However, visual inspection is not possible due to the opacity of liquid sodium. Therefore, an ultrasonic flaw detector that can non-destructively inspect the internal state is used for monitoring the core structure and the like. This is because ultrasonic waves are easily propagated in a liquid and pulsed ultrasonic waves are transmitted to the inside of the reactor, and the property of reflecting from an interface with different acoustic impedance (product of density and sound velocity) is used. The reflected wave is detected and the result is displayed as an image.

The ultrasonic flaw detector comprises an ultrasonic probe for transmitting and receiving ultrasonic waves, and an apparatus main body for displaying an image by subjecting the detection result of the ultrasonic probe to predetermined signal processing.

The ultrasonic probe uses, as an electroacoustic transducer, a piezoelectric element having a structure in which a piezoelectric body is held between two electrodes. For the piezoelectric body, lead zirconate titanate-based or titanate-based ceramics, single crystals such as lithium niobate or lithium tantalate, polymers such as polyvinylidene fluoride, or composite materials of these are used according to the purpose. Has been done. That is, the temperature of liquid sodium is FB
About 200 ° C when R is stopped, about 500 ° C when operating
Therefore, considering the Curie point of the piezoelectric body, ceramics or single crystals are mainly effective in the former case, and single crystals are effective in the latter case.

The types of ultrasonic probe include a mechanical probe that mechanically scans one transducer to form an image, and a plurality of arrayed transducers that are electronically delayed to focus an ultrasonic beam. There is an array probe that performs deflection. The basic structure is the same for both, and a matching layer for acoustic matching with the propagation medium is provided on the ultrasonic wave transmitting / receiving surface side, while the piezoelectric element support and back surface are provided on the rear surface side of the ultrasonic wave transmitting / receiving surface side. And a backing member that serves to absorb ultrasonic waves emitted to the side.

The thickness of the matching layer is approximately 1/4 of the wavelength determined by the fundamental resonance frequency, and the material thereof has an acoustic impedance intermediate between the piezoelectric body and the propagation medium (for example, the square root of the product of the two). In some cases, a multi-layered structure is used for widening the band and increasing the sensitivity. Also,
In the case of a mechanical probe, a concave transducer is often used to focus the ultrasonic beam, while in the case of an array probe, the ultrasonic beam is focused in the direction perpendicular to the array direction of the transducers. Often acoustic lenses are used.

Further, since corrosiveness and wettability become problems in liquid sodium, the surface of the ultrasonic probe must be made of metal such as stainless steel or nickel, or ceramic such as alumina or silicon nitride. Generally, a stainless steel plate is used for corrosion resistance, and a metal thin film such as a nickel thin film is vapor-deposited and coated on the surface of the stainless steel plate for wettability.

However, the acoustic impedance of the protective member made of metal or ceramic for liquid sodium resistance and the protective member coated with a metal thin film is 45.
Before and after Mrayl, the ceramic material or the single crystal material as the piezoelectric body has a volume of about 30 to 40 Mrayl, and the liquid sodium has a volume of 2.2 Mrayl, so that the acoustic mismatch between them reduces the sensitivity of the probe. However, there was a problem that the transmission / reception waveform was disturbed.

[0011]

As described above, in the conventional ultrasonic probe used in the liquid sodium which is the coolant of the FBR, the protective member made of metal or ceramic is used to prevent the corrosion by the liquid sodium. In addition, it was necessary to coat it with a metal thin film such as a nickel thin film for the purpose of wetting resistance.

However, there are problems that the sensitivity of the probe is lowered and the transmission / reception waveform is disturbed due to acoustic mismatch between the protective member, the piezoelectric body, and the liquid sodium.

The present invention has been made in consideration of the above circumstances. An object of the present invention is to provide an ultrasonic probe which is resistant to deterioration even when used in a sodium liquid and has good performance. Especially.

[0014]

In order to achieve the above object, an ultrasonic probe of the present invention (claim 1) has a piezoelectric element having a thickness on the wave transmitting / receiving surface side of a piezoelectric element for transmitting / receiving ultrasonic waves. It is characterized in that a protective member for liquid sodium resistance having a wavelength of 10% or less determined by the fundamental resonance frequency of the element is provided.

According to another ultrasonic probe of the present invention (Claim 2), the thickness of the wavelength determined by the fundamental resonance frequency of the piezoelectric element is approximately on the wave transmitting / receiving surface side of the piezoelectric element for transmitting / receiving ultrasonic waves. A matching layer of 2m-1) / 4 times (m is a natural number) is provided, and a thickness of the matching layer is approximately n / 2 times (n is a natural number) the wavelength determined by the fundamental resonance frequency of the piezoelectric element. ) Is provided with a protective member for liquid sodium resistance. Here, the term “n / 2 times the wavelength determined by the fundamental resonance frequency” is specifically a natural multiple of 580 μm at 5 MHz when a stainless plate is used. The protective member is preferably a metal plate such as a stainless plate or a ceramic plate.

[0016]

According to the research conducted by the present inventors, if the thickness of the protective member for liquid sodium resistance is set to 10% or less of the wavelength determined by the fundamental resonance frequency of the piezoelectric element, a good reception wave with a short wave length is obtained. It was found that (reflected wave) was obtained. Therefore,
According to the present invention (Claim 1), it is possible to prevent corrosion due to liquid sodium without causing performance deterioration due to acoustic mismatch.

Further, according to the study by the present inventors, on the transmitting / receiving surface side of the piezoelectric element, the thickness is approximately (2m-1) / 4 times (m is a natural number) of the wavelength determined by the basic resonance frequency of the piezoelectric element. In the ultrasonic probe provided with the matching layer,
If the thickness of the protective member for liquid sodium resistance provided on the front surface of the matching layer is approximately n / 2 times the wavelength determined by the fundamental resonance frequency of the piezoelectric element (n is a natural number), good reception waves with short wave lengths can be obtained. It turns out that Therefore, according to the present invention (claim 2), it is possible to prevent corrosion due to liquid sodium without causing performance deterioration due to acoustic mismatching.

[0018]

Embodiments will be described below with reference to the drawings. FIG. 1 is a diagram showing a structure of an ultrasonic probe according to an embodiment of the present invention. This is an example in which the present invention is applied to an ultrasonic probe having a single probe structure.

In the figure, 1 is 10 mm in diameter and 420 in thickness.
A μm disk-shaped piezoelectric body is shown. This piezoelectric body 1
Is made of lead titanate-based ceramics with a Curie point of 360 ° C. and a relative dielectric constant of 250, and its fundamental resonance frequency is 5 MHz.
Is. The piezoelectric body 1 is held between silver electrodes 2a and 2b having a thickness of 5 μm, and the piezoelectric body 1 and the silver electrodes 2a and 2b form a piezoelectric element.

On the front surface of the silver electrode 2a on the wave transmitting / receiving surface side of the piezoelectric body 1, there is provided a stainless plate 4 (protective member) having a nickel thin film (not shown) formed thereon. This stainless steel plate 4
Is for preventing corrosion due to sodium, and its thickness is 100 μm, which corresponds to about 8.6% of the wavelength determined by the fundamental resonance frequency of the piezoelectric element. The nickel thin film is for improving the wettability with respect to liquid sodium, and is formed by vapor deposition or sputtering. A metal thin film other than the nickel thin film may be used as long as it has good wettability to liquid sodium.

On the other hand, on the front surface of the silver electrode 2b, there is provided a backing member 3 made of porous ceramic having an acoustic impedance of 10 Mrayl. The backing member 3 supports the piezoelectric body 1 and the silver electrode 2 of the piezoelectric body 1.
The ultrasonic waves emitted from the a side are absorbed, and only the ultrasonic waves emitted from the silver electrode 2 side of the piezoelectric body 1 are transmitted to the transmission medium. The silver electrode 2b is electrically connected to the cable 6.

The silver electrode 2a and the stainless steel plate 4, and the silver electrode 2b and the backing member 3 are joined by silver solders 5a and 5b by heat treatment at about 800.degree.
Then, the piezoelectric body 1 was polarized. Further, the piezoelectric body 1, the silver electrodes 2a and 2b, the backing member 3, etc. are made of stainless steel, and are housed in a housing 7 that is electrically connected to the silver electrode 2a.

A reflected wave from a stainless steel plate placed in liquid sodium was measured by the pulse echo method using the ultrasonic probe thus constructed. FIG. 2 shows the measurement results, FIG. 2 (a) shows the reflected wave displayed in the time domain, and FIG. 2 (b) shows the reflected wave displayed in the frequency domain. From FIG. 2, it can be seen that the frequency spectrum has a single-peak characteristic with no ripple, the reflected wave (waveform a) has a straight damping characteristic, and this is reflected in the envelope (waveform b) after detection.

FIG. 3B shows a frequency spectrum when the thickness of the stainless steel plate is 1 μm, which corresponds to about 0.01% of the wavelength determined by the fundamental resonance frequency of the piezoelectric element. For comparison, FIG. 3 (a) shows the result of FIG. 2 (b) displayed on the same scale as FIG. 3 (b). 12 for 1 μm
Bimodality appears in the region of ^{5 to} 200 (× 10 ⁴ Hz), but since the level is low in that region, it can be regarded as a good reflected wave as in the case of 100 μm.

According to the study by the present inventors, in order to obtain such a good reflected wave, the thickness of the stainless steel plate 4 should be 10% or less of the wavelength determined by the fundamental resonance frequency of the piezoelectric element. I understood. If it exceeds 10%, for example, 26
% (Thickness 300 μm), as shown in FIG. 4B, the frequency spectrum was bimodal, and the waveform was also long due to overlapping of both frequency components. Comparing the envelopes after detection, it can be seen from FIG. 4A that the beam width up to −20 dB is 20% or more longer than that of this embodiment.

FIG. 5 shows the measurement results of an ultrasonic probe having a conventional matching layer. The specific structure of this ultrasonic probe is shown in FIG. The basic structure is the same as that of the ultrasonic probe of FIG. 1 except that a matching layer 8 is provided between the piezoelectric body 1 and the stainless steel plate 4. The matching layer 8 has an acoustic impedance of about 12 Mra.
The glass plate of yl is used. Also, stainless plate 4
Has a thickness of 100 μm, which corresponds to about 8.6% of the wavelength determined by the fundamental resonance frequency of the piezoelectric element, and is the same as that of the ultrasonic probe of FIG. Further, the silver electrode 2a and the matching layer 8, the silver electrode 2b and the backing member 3, and the stainless steel plate 4 and the matching layer 8 are silver brazes 5a, 5 respectively.
It is joined by b and 5c.

In the case of the ultrasonic probe provided with the matching layer 8, even if the thickness of the stainless steel plate 4 is 10% or less of the wavelength determined by the fundamental resonance frequency as in the case of this embodiment,
As shown in FIG. 5, as compared with the case of the present embodiment, the wave length is long, the frequency spectrum has a bimodal characteristic, and −20d
The beam width up to B is about twice as long.

Therefore, according to the present embodiment, the stainless steel plate 4 has high resistance to liquid sodium, and since the thickness of the stainless steel plate 4 is adjusted to the above conditions, the resolution is also high.

In such an ultrasonic probe having no matching layer, by setting the thickness of the stainless steel plate 4 to 10% or less of the wavelength determined by the fundamental resonance frequency of the piezoelectric element, a good reflected wave can be obtained as follows. Can be thought of as.

In the case of this embodiment, since the thickness of the stainless steel plate 4 is 10% or less of the wavelength determined by the basic resonance frequency of the piezoelectric element, the basic resonance frequency determined by the thickness of the piezoelectric element,
The basic resonance frequency determined by the combined thickness of the piezoelectric element and the stainless steel plate 4 is close to each other. Furthermore, since the difference in acoustic impedance between the piezoelectric element and the stainless steel plate 4 is small, the reflection at the interface between the two is as small as 5 to 10%.

For this reason, the basic resonance frequency of the piezoelectric element and the stainless steel plate 4 falls within the band (Q value) determined by the mechanical resonance sharpness of the ultrasonic probe, and the frequency spectrum has no ripple and has a single peak characteristic. Becomes

Further, in the case of the third harmonic component, the thickness of the stainless steel plate 4 is three times that in the case of the fundamental resonance, so that the resonance of only the piezoelectric body 1 and the piezoelectric body 1 and the stainless steel plate 4 are combined. The resonance does not have a single-peaked frequency spectrum but a double-peaked one. As a result, the peak value of the spectrum is lower than that of the fundamental resonance (it was confirmed to be -10 dB or less), and the effect of unnecessary vibration is reduced.

Therefore, the reflected wave has few frequency components other than the fundamental resonance, and has a short wave length, resulting in high resolution. In the present embodiment, the single probe structure has been described, but the present invention has an array probe structure as shown in FIG. 7 and a matrix probe structure in which transducers are two-dimensionally arranged. It can also be applied to.

FIG. 8 is a view showing the structure of an ultrasonic probe according to another embodiment of the present invention. In the figure, 11 has a diameter of 1
A disk-shaped piezoelectric body having a thickness of 0 mm and a thickness of 420 μm is shown. The piezoelectric body 11 is made of lead titanate-based ceramics having a Curie point of 360 ° C. and a relative dielectric constant of 250, and its basic resonance frequency is 5 MHz. Piezoelectric body 11 has a thickness of 5μ
m of the silver electrodes 12a and 12b, the piezoelectric body 1
A piezoelectric element is formed by the silver electrodes 12a and 12b.

A glass plate as a matching layer 18 is provided on the front surface of the silver electrode 12a on the wave transmitting / receiving surface side of the piezoelectric body 11. The acoustic impedance of this glass plate is 12 Mr.
In ayl, the thickness is about 300 μm, which corresponds to ¼ of the wavelength determined by the fundamental resonance frequency of the piezoelectric element.

A stainless steel plate 14 (protective member) having a nickel thin film (not shown) deposited on the front surface of the matching layer 18
Is provided. The thickness of the stainless steel plate 14 is 580 μm, which corresponds to about ½ of the wavelength determined by the fundamental resonance frequency of the piezoelectric element.

On the front surface of the silver electrode 12b, there is provided a backing member 13 made of porous ceramic having an acoustic impedance of 10 Mrayl. Also, silver electrode 1
2b is electrically connected to the cable 16.

Silver electrode 12a and matching layer 18, silver electrode 12b and backing member 13, and stainless plate 14
The matching layer 18 and the matching layer 18 are joined by silver brazing materials 15a, 15b, 15c by heat treatment at about 800 ° C., respectively. Then, the piezoelectric body 11 was polarized. Also,
Piezoelectric body 11, silver electrodes 12a and 12b, backing member 1
3, the matching layer 18 and the like are made of stainless steel, and are housed in a housing 17 that is electrically connected to the silver electrode 12a.

The reflected wave from the stainless steel plate placed in the liquid sodium at 200 ° C. was measured by the pulse echo method using the ultrasonic probe thus constructed. Figure 1
0 indicates the measurement result. It can be seen from FIG. 10 that the frequency spectrum has a single-peaked characteristic with a center frequency of about 5 MHz and has a wide band, and the reflected wave has excellent damping characteristics.

According to the research conducted by the present inventors, in order to obtain such a good reflected wave, the thickness of the stainless plate 14 is n / 2 times the wavelength determined by the fundamental resonance frequency of the piezoelectric element (n is a natural number). ) Was found to be good. Other than n / 2 times, for example, 1/12 times (wavelength 8.6 determined by fundamental resonance frequency)
%), The frequency spectrum becomes bimodal, as shown in FIG. 11B, and the low frequency component of 2.5 MHz becomes dominant instead of the fundamental resonance frequency of 5 MHz as in the present embodiment. Further, it can be seen that the level is about half that of the present embodiment. Further, it can be seen from FIG. 11A that the wave length is about 15% longer than that of the present embodiment.

Further, the thickness of the stainless plate 14 is 300 μ, which is about ¼ times the wavelength determined by the fundamental resonance frequency of the piezoelectric element.
In the case of m, as shown in FIG. 12B, it can be seen that the frequency spectrum contains almost no 5 MHz component, and the level thereof is as low as about 25% of that of the present embodiment. Further, as shown in FIG. 12A, it can be seen that the wave length is about 70% longer than that of the present embodiment.

Therefore, according to the present embodiment, the stainless steel plate 14 has high resistance to liquid sodium, and since the thickness of the stainless steel plate 14 is adjusted to the above conditions, the resolution is also high.

In the ultrasonic probe having the matching layer 18, the thickness of the stainless plate 14 is set to n / 2 times the wavelength determined by the fundamental resonance frequency of the piezoelectric element (n is a natural number), which is preferable. The reason why the reflected wave is obtained is considered as follows.

In the ultrasonic probe of this embodiment, when a drive pulse is applied to the piezoelectric body 11, a resonance phenomenon occurs,
A compression wave is generated. A coarse wave (wave having a minimum amplitude value) and a dense wave (wave having a maximum amplitude value) pass at an interval of λ / 2v seconds at an arbitrary point in the path along which the compressional wave propagates. The waves are emitted as ultrasonic waves after passing through the matching layer 18 and the stainless plate 14. Here, λ is the wavelength [m] and v is the speed of sound [m / s].

It is now assumed that a dense wave (hereinafter referred to as dense wave 1) is radiated from the front surface of the stainless steel plate 14 at a certain time. A part of the dense wave 1 is reflected at the interface between the stainless plate 14 and the sodium liquid and propagates toward the matching layer 18. This reflected wave (hereinafter referred to as the first reflected wave) is
Since the acoustic impedance of the stainless steel plate 14 is larger than that of the sodium liquid, the phase is 180 degrees inverted.

When the first reflected wave reaches the interface between the stainless plate 14 and the matching layer 18, a part of the first reflected wave is reflected at the interface (hereinafter referred to as the second reflected wave) and the rest passes. At this time, since the acoustic impedance of the stainless steel plate 14 is larger than that of the matching layer 18, the phases of the first reflected wave and the second reflected wave are different by 180 degrees, and the second reflected wave is the same as the dense wave 1. Become in phase. Also, stainless plate 1
The thickness of 4 is n of the wavelength determined by the fundamental resonance frequency of the piezoelectric element.
/ 2 (n is a natural number). As a result, the second reflected wave is transmitted from the piezoelectric body 11 to the stainless plate 14 and the matching layer 1.
The dense wave 1 propagating to the interface with 8 overlaps with the next dense wave 2 in phase, and the composite wave is radiated from the front surface of the stainless plate 14.

Therefore, since a wave containing a large number of frequency components whose phases are uniform is radiated in the vicinity of the fundamental resonance frequency determined by the band of the probe, the reflected wave becomes good. In addition,
In this embodiment, a single probe structure is explained, but the present invention is also applied to an array probe structure as shown in FIG. 9 and a matrix probe structure in which transducers are two-dimensionally arranged. it can. In the array probe shown in FIG. 9, the matching layer 18 is not separated for each element, but may be separated for each element in order to reduce crosstalk.

The thickness of the matching layer 18 is not limited to 1/4 of the wavelength determined by the fundamental resonance frequency of the piezoelectric element, and the point is that the thickness of the wavelength determined by the fundamental resonance frequency is (2 m).
It may be -1) / 4 times (m is a natural number).

The present invention is not limited to the above embodiment. For example, although silver is used as the brazing material in the above embodiments, the material is not particularly limited, and for example, copper or titanium may be used as the active material. Further, if the brazing material may also serve as the electrode, it is not necessary to form electrodes on both surfaces of the piezoelectric body in advance.

If there is no particular problem in terms of support, an air bag may be used instead of the backing member. Further, in this embodiment, the stainless steel plate is used as the protective member against the liquid sodium, but other material such as a ceramic plate may be used. In addition, various modifications can be made without departing from the scope of the present invention.

[0051]

As described in detail above, according to the present invention, it is possible to obtain an ultrasonic probe which is resistant to corrosion even when used in liquid sodium and has high resolution.

[Brief description of drawings]

FIG. 1 is a diagram showing a structure of an ultrasonic probe according to an embodiment of the present invention.

FIG. 2 is a diagram showing pulse echo characteristics of the ultrasonic probe of FIG.

3] In the ultrasonic probe of FIG. 1, the thickness of the stainless steel plate is about 0.001% of the wavelength determined by the fundamental resonance frequency.
The figure which shows the pulse echo characteristic in the case of.

FIG. 4 is a diagram showing pulse echo characteristics when the thickness of the stainless steel plate is 26% of the wavelength determined by the fundamental resonance frequency in the ultrasonic probe of FIG.

FIG. 5 is a view showing a structure of an ultrasonic probe including a conventional matching layer.

6 is a diagram showing pulse echo characteristics of the ultrasonic probe of FIG.

FIG. 7 is a perspective view showing the structure of an array probe to which the present invention is applied.

FIG. 8 is a view showing the structure of an ultrasonic probe according to another embodiment of the present invention.

FIG. 9 is a perspective view showing the structure of an array probe to which the present invention is applied.

10 is a diagram showing pulse echo characteristics of the ultrasonic probe of FIG.

11 is a diagram showing pulse echo characteristics when the thickness of the stainless steel plate is 1/12 times the wavelength determined by the fundamental resonance frequency in the ultrasonic probe of FIG.

FIG. 12 is a diagram showing pulse echo characteristics when the thickness of the stainless steel plate is 1/4 times the wavelength determined by the fundamental resonance frequency in the ultrasonic probe of FIG.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Piezoelectric body 2,2a ... Silver electrode 3 ... Backing member 4 ... Stainless plate (protective member) 5,5a ... Silver braze 6 ... Cable 7 ... Case 8 ... Matching layer 11 ... Piezoelectric body 12,12a ... Silver electrode 13 ... backing member 14 ... stainless steel plate (protective member) 15, 15a ... silver brazing 16 ... cable 17 ... housing 18 ... matching layer

Claims (2)

[Claims] 1. An ultrasonic probe used in liquid sodium, the thickness of which is 10% or less of a wavelength determined by a fundamental resonance frequency of the piezoelectric element on the wave transmitting / receiving surface side of the piezoelectric element for transmitting / receiving ultrasonic waves. 2. An ultrasonic probe, characterized in that a protective member for liquid sodium resistance is provided. 2. An ultrasonic probe used in liquid sodium, wherein a thickness of a piezoelectric element for transmitting and receiving ultrasonic waves is approximately 2 m at a wavelength determined by a fundamental resonance frequency of the piezoelectric element. -1) / 4 times (m is a natural number) matching layer is provided, and the thickness of the matching layer is approximately n / n of the wavelength determined by the fundamental resonance frequency of the piezoelectric element.
An ultrasonic probe, which is provided with a protection member for liquid sodium resistance that is twice (n is a natural number).