JPH11202084A - Void detector - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a void detector with high sensitivity capable of detecting void in sodium even if pipe diameter further increases. SOLUTION: By providing with a pair of excitation coils 4a and 4b on a perpendicular place of the direction sodium 2 is flowing in the position 180 degree facing direction around a pipe 1, and a pair of or a plurality pairs of detection coils 5 are placed around the pipe 1 so as not to overlap on the excitation coils 4a and 4b putting the perpendicular plane in between. A signal processor has a function comparing the signals of a pair of the detection coils 5a and 5b.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a void detector in liquid metal, and more particularly to a sodium void detector which is effective for detecting bubbles mixed in liquid metal sodium used in a fast breeder reactor.

[0002]

2. Description of the Related Art As a coolant for a primary system of a fast breeder reactor (hereinafter abbreviated as "FBR"), liquid metal sodium (N
a) is used to circulate through the core. A chemically inert argon gas is used as a cover gas for the free surface of Na. When this argon gas is entrained in Na and circulated through the reactor core, various operational obstacles may be caused.

[0003] Examples thereof include a change in reactivity of a core core, generation of thermal stress, reduction of heat removal performance, reduction of heat exchange efficiency, activation of argon gas, and increase of measurement noise. Therefore, it must be designed and operated so that gas entrainment does not occur in the primary system of the FBR.

For this purpose, a void detector for detecting bubbles in Na is required. However, since Na is opaque, bubbles in Na cannot be detected by image analysis or visual observation.

[0005] Further, conventionally, in a point electrode probe type void detector which uses a difference in electric conductivity between gas and liquid used in a water flow test or the like, measurement can be performed only at a local position where a probe is arranged. For this reason, the eddy current type void detector is currently used for detecting bubbles in Na.

A conventional void detector is described in the literature (D. Stapleton,
CEGB Report 1973: D. Stapleton: Central Electricit
y Generating Board, Research Department, Berkeley
Nuclear Laboratories.). As shown in FIG. 15, this void detector has a configuration in which one air-core excitation coil 4 is provided on the outer periphery of a pipe 1 through which Na2 flows, and two detection coils 5a and 5b are arranged at opposing positions. I have. When an alternating current is applied to the exciting coil 4, the exciting coil 4
, A magnetic flux 7 is formed which travels toward the detection coils 5a and 5b.

At this time, an eddy current 30 is generated in the conductive fluid Na 2 so as to surround the magnetic flux 7. Since the direction of the magnetic flux generated by the eddy current 30 is opposite to the direction of the magnetic flux generated from the excitation coil 4, the magnetic flux density reaching the detection coils 5 a and 5 b from the excitation coil 4 is weakened. When the void 3 passes through the pipe 1, the eddy current 30 is interrupted, and the magnetic flux 7 passing through the detection coil 5b changes. This change is detected from the difference between the output voltages of the two detection coils 5a and 5b. It will be.

[0008]

In the eddy current type void detector of the prior art described above, when the pipe diameter is enlarged, the magnetic flux reaching the detection coil is attenuated, and the detection sensitivity is reduced.
Therefore, the present invention has been made in view of the problems of the prior art, and an object of the present invention is to provide a highly sensitive void detector capable of detecting voids in Na even when the pipe diameter is further increased. It is in.

[0009]

Means for Solving the Problems In order to achieve a high-sensitivity type void detector, (1) the magnetic flux density in a region where a void exists in the Na pipe is increased.

(2) The magnetic flux generated by the exciting coil is guided to a region in the Na pipe where a void exists.

(3) A magnetic detector is arranged in a region where a change in magnetic flux due to the presence or absence of a void is large.

(4) Electromagnetic noise from outside is cut off.

Is an effective means. It is also effective to install a void detector inside the pipe.

Therefore, the means of the present invention is to provide a pair of exciting coils as an exciting device at a position 180 ° opposite to each other on the outer periphery of the pipe and orthogonal to the direction in which the conductive fluid flows. One or more pairs of detection coils, Hall elements, and semiconductor magnetoresistive elements are provided on the outer circumference of the pipe so as not to overlap with the excitation coil. I made it possible. Further, the pair of excitation coils was provided with a core made of one ferromagnetic material, and both ends of the core were arranged so as to sandwich the outer periphery of the pipe.

According to another means, a C-shaped core made of a single ferromagnetic material whose end located at a position facing 180 ° sandwiches the outer periphery of the pipe is provided on the outer surface of the pipe and on a plane orthogonal to the direction in which the conductive fluid flows. An excitation coil is provided so as to surround the core at an arbitrary position of the core, and one or more pairs of magnetic detection devices are provided so as to sandwich an orthogonal plane on the outer periphery of the pipe and not to be sandwiched between the magnetic circuit and the pipe, The signal processing device can be achieved by having a function of comparing signals of a pair of magnetic detection devices.

In the above-mentioned two means, the shape and dimensions of the core satisfy the following conditions.

D <L ₁ and L ₂ <D (μ ₂ / μ ₁ ) where L _{1 is} the distance between the inner surfaces parallel to the line connecting both ends of the C-shaped core, and D is the distance between both ends of the core. , L ₂ : average magnetic path length of the core, μ ₁ : magnetic permeability of the conductive fluid, μ ₂ : magnetic permeability of the core.

In the means installed inside the pipe, a pair of exciting coils are provided at 180 ° opposing positions parallel to the direction in which the conductive fluid flows or the direction in which bubbles in the conductive fluid move, and the conductive fluid is supplied. A pair of magnetism detecting devices are provided so as to sandwich the exciting coil in the flowing direction or the direction in which bubbles in the conductive fluid move, so as not to overlap the exciting coil, and to be penetrated by the magnetic flux generated from the exciting coil. Alternatively, a plurality of sets are provided, and the signal processing device can be achieved by having a function of comparing signals of a pair of magnetic detection devices.

The exciting coil is a solenoid type exciting coil having a ferromagnetic core. The shape and dimensions of the core were set to satisfy the following conditions.

D <L <D (μ ₂ / μ ₁ ) where L: average magnetic path length of the core, D: distance between a pair of exciting coils, μ ₁ : magnetic permeability of conductive fluid, μ ₂ : core Is the magnetic permeability.

Further, in each of the above means, the surroundings of the excitation device and the magnetic detection device are covered with a magnetic material.

That is, according to the above-mentioned constitutional requirements of the present invention, the outer periphery of the pipe and the surface perpendicular to the direction in which the conductive fluid flows are:
By providing a pair of excitation coils at 180 ° facing positions,
A magnetic flux that penetrates the pipe can be generated.
In addition, one or more pairs of magnetic detection devices are provided so as to sandwich the orthogonal plane provided with the excitation coil in a portion where the magnetic flux changes greatly due to the presence or absence of the void and does not overlap the excitation coil on the outer periphery of the pipe. A void can be detected by having a function of comparing signals from the detection device.

Further, when the pair of exciting coils has a core made of one ferromagnetic material, and the both ends of the core are arranged so as to sandwich the outer periphery of the pipe at 180 ° opposing positions, the magnetic flux is effectively guided into the pipe. Become. According to another means, when the above-described core is provided, the exciting coil can be arranged other than on the outer periphery of the pipe. In these means, the shape and dimensions of the core, D <L ₁ and _{L 2 <D (μ 2 /} μ
The effect is greatest under the condition ₁ ).

The same effect can be obtained with the above-mentioned structure also for the means installed inside the pipe. In this means, the shape and dimensions of the core are D <L <D (μ ₂ / μ ₁ )
The effect is the greatest in the case of

In each of the above means, an exciter,
By covering the portion provided with the magnetic detection device with a magnetic material, external electromagnetic noise can be cut off.

[0026]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 shows a sectional view of a first embodiment of the present invention. This example uses Na
Exciting coils 4a and 4b of a solenoid type are arranged at 180 ° opposing positions on the outer circumference of the pipe 1 for flowing the pipe 2, and a ferromagnetic material core 6 is provided therein to constitute an exciting device.

The end of the core 6 is disposed so as to sandwich the pipe 1 at a position facing 180 °. A pair of detection coils 5a and 5b are arranged on the outer circumference of the pipe 1 along the flow direction of Na2.
FIG. 2 shows the structure viewed from the side of the pipe 1, and the arrow 9 indicates the flow direction of Na. The detection coil 5a is disposed on the upstream side of the core 6 and the detection coil 5b is disposed on the downstream side of the core 1 on the outer periphery of the pipe 1. An air core or an iron core can be applied to the detection coils 5a and 5b.

The void detector 16 according to the first embodiment includes:
The excitation device includes the pair of excitation coils 4a and 4b described above, the magnetic detection device also includes the pair of detection coils 5a and 5b, and a signal processing device (not illustrated).

Since Na flows in the pipe at a temperature of 200 to 600 ° C., the component elements adjacent to the pipe 1 need to have a heat-resistant structure. In particular, the excitation coils 4a, 4b and the detection coils 5a, 5b use a ceramic bobbin made of a heat-resistant and insulated wire such as glass fiber or alumina fiber, and the lead wire from the coil is equivalent to the coil wire or an MI cable (metal sheath wire). ) Should be used. As the ferromagnetic material for the core 6, a laminated steel sheet in which a thin silicon steel sheet is treated with an oxide insulating film having excellent heat resistance is suitable.

The operation of the present invention for detecting voids in Na will be described with reference to the first embodiment of the present invention configured as described above.

In FIGS. 1 and 2, the exciting coils 4a,
4b from several 100 to several 1 from an external constant current type AC power supply
When a sine wave of 000 Hz is supplied at a constant current, the number of turns (T) of the coil and the supply current (A) are applied to the exciting coils 4a and 4b.
A magnetic flux proportional to the product (A · T) is generated. Since the core 6 disposed at the center of the exciting coils 4a and 4b is a ferromagnetic material, the generated magnetic flux is guided into the core 6 and concentrated between the pipes to generate the magnetic flux. The magnetic flux in the ferromagnetic core 6 is
Since the eddy current is prevented from leaking out and the generation of the eddy current in the laminated core 6 can be prevented, the eddy current is supplied to the pipe 1 without the eddy current loss in the core 6.

Since the magnetic flux density inside the pipe is attenuated as its width increases, it is necessary to supply a strong magnetic field reaching the Na region in the pipe with a large diameter. Since the SUS pipe 1 and Na2 are non-magnetic materials, the magnetic flux generated from both ends of the core partially leaks to the outer periphery of the pipe 1. An eddy current is generated in the pipe 1 in a direction to cancel the magnetic flux.

Here, if the void 3 exists in the conductive Na, the void 3 has no conductivity, so that no eddy current is generated in that portion, the eddy current distribution changes, and the magnetic flux distribution changes. Also change. Therefore, the magnetic flux 8 on the outer circumference of the pipe in FIG. 1 also changes, and by measuring the change in the magnetic flux on the outer circumference of the pipe, Na
The presence or absence of the voids 3 present therein can be detected. Its magnetic flux 8
Is detected as induced electromotive force by the detection coils 5a and 5b arranged on the outer periphery of the pipe 1.

When air-core type coils are used for the detection coils 5a and 5b, a minute change in the leakage magnetic flux density can be detected without disturbing the magnetic field to be detected. In addition, since the magnetic flux near the iron core is focused and detected by using the iron core type detection coil, high sensitivity detection can be performed if the void generation position exists near the detection coil.

The detection coil for detecting the induced electromotive force is Na
The S / N ratio is easily affected by an external magnetic field other than the void signal or an induced voltage (noise), and the S / N ratio is deteriorated. This void detector 16
Is a change in eddy current due to electromagnetically induced electromotive force due to Na flow in the void detection region.

The electromagnetically induced electromotive force due to the flow of Na is generated in a direction orthogonal to the magnetic field and the flow direction according to Fleming's right-hand rule, and its magnitude is proportional to the magnetic flux density and the flow rate of Na. Thus, the higher the Na flow rate, the greater the effect. A current flows through Na due to the generated electromagnetic induction electromotive force, and the magnetic flux 8 described above
Affects as well as eddy current effects.

Therefore, a plurality of detection coils 5 are arranged around the Na pipe 1, and the presence or absence of a void is detected by comparing respective detection signals. Ideally, a pair of detection coils 5a are provided upstream and downstream with respect to the excitation magnetic field.
5b is provided to reduce the difference between the induced electromotive forces of the two detection coils, thereby reducing the influence of Na flow fluctuations and external electromagnetic noise. When a drift such as a secondary flow accompanies the Na pipe, the best point can be found by comparing the induced electromotive forces of a plurality of pairs of detection coils 5 as shown in FIG.

FIG. 4 is a diagram showing a cross section of a second embodiment of the present invention. This is a case where the exciting coils 4a and 4b do not have the core 6. When a compact void detector 16 is required, such a configuration also functions as the void detector 16.

FIG. 5 is a sectional view of a third embodiment of the present invention. The configuration is the same as that of the first embodiment except that the mounting position of the exciting coil 4 is different. Since the magnetic resistance of the core 6 is small, the exciting coil 4 may be located outside the outer periphery of the pipe if it is arranged so as to surround the core 6.

In the first and third embodiments, the shape and dimensions of the core 6 preferably satisfy the following conditions.

D <L ₁ and L ₂ <D (μ ₂ / μ ₁ ) where L _{1 is} the distance between the inner surfaces parallel to the line connecting both ends of the C-shaped core, and D is both ends of the core 6. Distance between L ₂ :
Average magnetic path length of core 6, μ ₁ : magnetic permeability of conductive fluid, μ
₂ : Permeability of the core 6.

This will be described in detail with reference to FIG. When the core 6 is smaller than the distance between both ends of the core 6, the magnetic flux emitted from the end of the core 6 does not penetrate the pipe, and the closest core 6
Because it is guided by itself, the distance between the inner surfaces parallel to the line extending between both ends of the core 6 (in this embodiment, a rectangle) is smaller than the distance between both ends of the core 6 (equal to the pipe diameter in this embodiment). (Equal to the length of the short side of the core 6). Also,
When the magnetic resistance along the magnetic path of the core 6 is larger than the magnetic resistance in the pipe 1, the magnetic flux generated by the exciting coil 4 passes outside the pipe without being effectively collected in the pipe. It is preferable that the magnetic resistance inside the pipe, that is, the conductive fluid is larger than the magnetic resistance of the conductive fluid.

FIG. 7 shows a modification in which the core 6 is structured to cover the pipe 1 in order to apply a symmetric excitation magnetic field to the pipe 1. In this modification, the magnetic flux is axially symmetric, and it is possible to estimate the diameter of the void and the passing position of the void from the signal of the detection coil 5.

FIG. 8 shows an application example in which the void detector 16 is entirely covered with a magnetic material 10 to add a magnetic shielding function to reduce the mixing of electromagnetic noise, thereby improving the S / N ratio. As the magnetic material 10, iron, nickel, or an alloy thereof can be used.

FIG. 9 is a view of the structure of FIG. 8 as viewed from the side of the pipe. The lead wire 11 from the coil is equivalent to the coil wire material or an MI cable (metal sheath electric wire) and the outlet 12
From the outside. The outlet 12 is made of the same material as the magnetic material 10 for shielding, and a shielding effect can be obtained by making the opening deeper than the inner diameter.

FIG. 10 is a sectional view showing a fourth embodiment of the present invention. The present invention is a void detector 16 used by being installed inside a Na pipe or a large tank. A pair of excitation coils 4a, 4a, 4a, 4b are positioned at a position 180 ° opposite to the direction 9 in which Na flows or the direction in which bubbles move in Na.
b so as to sandwich the exciting coils 4a and 4b and not to overlap the exciting coils 4a and 4b.
a and 5b are provided. These coils are stored in metal or ceramic protective containers 13a, 13b, 14a, and 14b, respectively, so as not to directly contact Na. The exciting coils 4a and 4b are solenoid type exciting coils provided with cores 6a and 6b made of a ferromagnetic material.

When an alternating current is applied to the excitation coils 4a and 4b, a magnetic flux 7 penetrating the detection coils 5a and 5b is generated. Since an eddy current is generated around the magnetic flux 7, when the void 3 passes, a change in magnetic flux due to the passage of the void appears in the detection coils 5a and 5b as a change in induced electromotive force. FIG.
FIG. 4 is a layout view when viewed from the direction in which a flows. The protective containers 13a, 13b, 14a, and 14c are mutually supporting members 1
It is fixed at 5. Here, the core 6 shown in FIG.
The dimensions of a and 6b preferably satisfy the following conditions.

D <L <D (μ ₂ / μ ₁ ) where L: average magnetic path length of the core, D: distance between a pair of exciting coils, μ ₁ : magnetic permeability of conductive fluid, μ ₂ : core Is the magnetic permeability. When the core is small, the magnetic flux emitted from the end of the core is not guided to the end of the adjacent core but is guided to another end of itself, so that the distance between the pair of exciting coils (this embodiment In this case, it is better that the average magnetic path length of the core (equal to the total length of the core in this embodiment) is longer than the distance between adjacent cores). Further, when the magnetic resistance along the magnetic path of the core is larger than the magnetic resistance in the pipe, the magnetic flux generated in the exciting coil does not effectively concentrate in the pipe, and therefore, the magnetic resistance of the cores 6a and 6b is smaller than the magnetic resistance of the cores 6a and 6b. A pair of exciting coils 4a, 4b
It is preferable that the magnetic resistance of the conductive fluid between them is large.

FIG. 12 shows a structural example when the fourth embodiment is installed inside a pipe. Void detector 1 in single tube 17
6 is fixed by the support member 15. The single pipe 17
Weld to the piping that needs void measurement. In this example, in order to reduce mixing of an external magnetic field and an induced voltage (noise) component, the entire outer periphery of the single tube 17 to which the void detector 16 is attached is covered with the magnetic material 10 to add a magnetic shielding function. The magnetic material 10 can be arranged inside the pipe. In order to prevent leakage of Na, the lead wire 11 is taken out of the single tube 17 through the inside of the support member 15 and taken out of the magnetic material 10 using the hermetic seal 18.

FIG. 13 shows an example in which a Hall element or a semiconductor magnetoresistive element 19 is used in place of the detection coil.
Since the Hall element or the semiconductor magnetoresistive element 19 is smaller than the coil, the void detector 16 can be made more compact.

FIG. 14 shows that the void detector 20 of the present invention is
The system configuration incorporated in the BR plant is shown. The reactor core 24 in the reactor vessel 26 and the intermediate heat exchanger 22 are connected by the primary system piping 21, and the primary system main circulation pump 25 circulates the primary system Na. The void detector 20 is provided in a part of the primary piping section 21 (for example, the inlet piping section of the intermediate heat exchanger 22), and when a Na void is detected, it is transmitted to the control rod driving device 27 by the core control system 23. , A safety system that controls the output of the reactor core 24 in a direction to suppress it.

[0052]

The void detector of the present invention can supply a magnetic flux to the void passage region powerfully and effectively, and can cope with a large-diameter Na pipe. In addition, the amount of magnetic flux that spreads outside the piping is small, so that noise is reduced.

By detecting with a plurality of detection coils installed around the Na pipe or inside the Na pipe, it is possible to correct noise components such as Na flow, electromagnetic noise, etc., and to detect a small diameter Na void with high sensitivity. it can.

Further, by providing a magnetic material covering the void detector, the S / N ratio can be improved.

[Brief description of the drawings]

FIG. 1 is a sectional view of a pipe on which a void detector according to a first embodiment of the present invention is mounted.

FIG. 2 is a side view of the piping of FIG.

FIG. 3 is a cross-sectional view of the detection coil according to the first embodiment of the present invention, in which an arrangement position of the detection coil is modified.

FIG. 4 is a sectional view of a pipe on which a void detector according to a second embodiment of the present invention is mounted.

FIG. 5 is a sectional view of a pipe showing a third embodiment of the present invention.

FIG. 6 is a cross-sectional view illustrating core dimensions of the first and third embodiments of the present invention.

FIG. 7 is a sectional view showing an example in which the core shape of the first and third embodiments of the present invention is modified.

FIG. 8 is a sectional view showing an application example in which a magnetic shield structure is applied to the first embodiment of the present invention.

FIG. 9 is a side view showing an application example in which a magnetic shield structure is applied to the first embodiment of the present invention.

FIG. 10 is a sectional view of a void detector showing a fourth embodiment of the present invention.

FIG. 11 is a structural diagram of a void detector showing a fourth embodiment of the present invention.

FIG. 12 is a diagram showing an application example in which a magnetic shield structure is applied to the fourth embodiment of the present invention.

FIG. 13 is a diagram showing an application example using a Hall element or a semiconductor magnetoresistive element in the first embodiment of the present invention.

FIG. 14 is a diagram showing an example of a safety system in which the void detector of the present invention is incorporated in an FBR plant.

FIG. 15 is a diagram illustrating the operation of a conventional void detector.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Piping 2 ... Na 3 ... Void 4 ... Exciting coil 5 ... Detection coil 6 ... Core 7, 8 ... Magnetic flux 9 ... Direction of Na flow 10 ... Magnetic material 11 ... Lead wire 12 ... Outlet 13, 14 ... Protective container 15 ... Supporting member 16 ... Void detector 17 ... Single tube 18 ... Hermatic seal 19 ... Hall element or semiconductor magnetoresistive element 20 ... Void detector 21 ... Primary system piping 22 ... Intermediate heat exchanger 23 ... Core control system 24 ... Core 25 ... Primary system main circulation pump 26 ... Reactor vessel 27 ... Control rod drive unit 30 ... Eddy current

Claims (12)

[Claims] An exciting device for forming an eddy current in a conductive fluid flowing in a pipe, a magnetic detecting device for detecting a magnetic flux generated by the exciting device, and a signal processing device for processing an output signal of the magnetic detecting device In the void detector for detecting the presence or absence of bubbles in the conductive fluid, the exciting device is located at a position 180 ° opposite to the outer periphery of the pipe, and in a plane orthogonal to the direction in which the conductive fluid flows. A pair of excitation coils disposed, wherein the magnetic detection device is provided with one or more pairs of detection coils disposed on the outer periphery of the pipe so as to sandwich the orthogonal plane and not to overlap with the excitation coil. Wherein the signal processing device detects the presence or absence of the bubble by comparing detection signals generated by each of the pair of detection coils. Detector. 2. The pair of excitation coils according to claim 1, wherein the pair of excitation coils includes a core made of one ferromagnetic material, and both ends of the core are located just so as to sandwich the outer periphery of the pipe. Void detector. 3. An exciting device for forming an eddy current in a conductive fluid flowing in a pipe, a magnetic detecting device for detecting a magnetic flux generated by the exciting device, and a signal processing device for processing an output signal of the magnetic detecting device. A void detector for detecting the presence or absence of bubbles in the conductive fluid, comprising: an end located at a position facing 180 ° in an outer periphery of the pipe and in a plane orthogonal to a direction in which the conductive fluid flows. A C-shaped core made of a single ferromagnetic material provided so as to sandwich the outer periphery of the pipe, and an exciting coil disposed so as to surround the core at an arbitrary position on the core. A pair or a plurality of pairs of detection coils arranged on the outer periphery of the pipe so as to sandwich the orthogonal plane and not to overlap with the excitation coil, and the signal processing device includes the pair of detection coils. A void detector for detecting the presence or absence of the air bubble by comparing detection signals emitted from the respective coils. 4. The void detector according to claim 2, wherein the shape and dimensions of the core satisfy the following conditions. D <L ₁ and L ₂ <D (μ ₂ / μ ₁ ) where L _{1 is} the distance between the inner side surfaces parallel to a line extending between both ends of the C-shaped core, and D is the distance between both ends of the core. distance,
L ₂ : average magnetic path length of the core, μ ₁ : magnetic permeability of the conductive fluid, μ ₂ : magnetic permeability of the core. 5. The void detector according to claim 1, wherein a portion provided with the excitation device and the magnetic detection device is covered with a magnetic material. 6. An exciting device for forming an eddy current in a conductive fluid flowing in a pipe, a magnetic detecting device for detecting a magnetic flux generated by the exciting device, and a signal processing device for processing an output signal of the magnetic detecting device. A void detector for detecting the presence or absence of air bubbles in the conductive fluid, wherein the excitation device is disposed at a position facing 180 ° in the conductive fluid and parallel to a direction in which the conductive fluid flows. A pair of excitation coils provided, wherein the magnetic detection device comprises:
One or more pairs of a pair of detection coils disposed in the conductive fluid are provided so as to sandwich the excitation coil and not to overlap with the excitation coil, and to be penetrated by the magnetic flux generated from the excitation coil. Wherein the signal processing device detects the presence or absence of the bubble by comparing detection signals generated by each of the pair of detection coils. 7. The void detector according to claim 6, wherein the exciting coil is a solenoid type exciting coil having a ferromagnetic core. 8. The void detector according to claim 7, wherein the dimension of the core satisfies the following condition. D <L <D (μ ₂ / μ ₁ ) where L: average magnetic path length of the core, D: distance between the pair of exciting coils, μ ₁ : magnetic permeability of the conductive fluid, μ
₂ : Permeability of the core. 9. The void detector according to claim 6, wherein a portion of the pipe provided with the excitation device and the magnetic detection device is covered with a magnetic material. 10. The void detector according to claim 1, wherein the magnetic detector is a solenoid coil. 11. The device according to claim 1, wherein the magnetic detection device is a Hall element or a semiconductor magnetoresistive element.
10. A void detector according to any one of claims 9 to 9. 12. The void detector is a fast breeder reactor.
The void detector according to any one of claims 1 to 11, wherein the void detector is arranged in a secondary cooling system pipe.