WO1994018540A1

WO1994018540A1 - Sensing probe

Info

Publication number: WO1994018540A1
Application number: PCT/HU1994/000003
Authority: WO
Inventors: Tamás PAJKOSSY; Gábor Nagy; Róbert SCHILLER; János MÁNDICS; Péter TILKY; Imre BUGLYÓ; Csaba Fetzer; Lajos Matus
Original assignee: Kfki Atomenergia Kutató Intézet
Priority date: 1993-02-10
Filing date: 1994-02-10
Publication date: 1994-08-18
Also published as: HUT69275A; AU6042394A; HU9300335D0

Abstract

The invention relates to a sensing probe for examining the local corrosion state of inner tube surfaces, which comprises a radiation source (6) emitting electromagnetic radiation, a radiation detecting means (7), built into the same structural unit as the radiation source, measuring the irradiation reflected from the inner surface to the tube, and an element (3) arranged between the radiation source (6) and the detecting means (7), the said element (3) being opaque for the irradiation, wherein the wavelength of the said electromagnetic radiation emitted by the radiation source (6) is chosen in the range where the intensities of the irradiation reflected from the bare metal and from the corroded surface are different. Furthermore, the invention relates to a method for examining the local corrosion state of inner tube surfaces.

Description

SENSING PROBE

The invention relates to a sensing probe for examini the local corrosion state of inner tube surfaces and to examining method performed with the aid of this the sensi probe. The invention can advantageously be used in nucle power reactors to investigate.the corrosion of tubes of he exchangers in the primary water cooling circuit.

For the safe operation of nuclear power plants t emission of radioactive materials due to corrosion structural materials is inadmissible. In the so-call pressurized water reactors the energy of water heated fission in the primary cooling circuit is utilized turbines arranged in the secondary water circuit. The he exchangers between the two water circuits are consider hazardous areas from the point of view of corrosion due the large flowing rates of cooling water and hi temperature differences.

The radioactivity appearing in the secondary wat circuit, coming from the primary circuit, is detectabl e.g. by a radiologically non-active tracer techniq (published German patent application No. 2,810,699) b these methods either cannot foresee the defects of tubes cannot reveal the reason of leakage, or they are n applicable in heat exchangers. The necessity of inspection of tubes in heat exchange is supported by the fact that leakage of radioactivi because of corrosion of pipes in heat exchangers has alrea been detected.

Up to now no method has been developed for the dire inspection of the effects of corrosion in the inner part pipes in heat exchangers of nuclear power reactors. This c be performed indirectly with the aid of specimens reference electrodes.

The first method comprises putting structural materia as specimens into the nuclear reactor for a given period a investigating the specimens after removal (French patent N 2,468,963). The drawback of this method is that th specimens are radioactive and the results of investigation are inaccurate since it is not always possible to place th specimens into such positions where the conditions are th same as for the built-in structural material.

It is possible to inspect structural material continuously with the help of. reference electrodes but thi method provides only average information about the surface without giving information on local conditions that would b important since corrosion generally takes place locally.

A number of methods are known for the inspection o mechanical state of tubes. They generally comprise a prob or pig that transmits signals to an outer processing unit.

A group of these methods is based on the measurement o (ultra)sonic waves. The probe detects either the emitte and echoed sound or the sound generated by the leaking par of a tube (German patent No. 3,131,883). These methods giv information on wall thicknesses and on possible flaws but d not give a direct characterization of surface conditions. Local metallurgical properties of tubes can be detecte bymethodsmeasuringthe changes ofmagnetic field (JP 54-156,69 and JP 84-014186) . These methods are based on the fact tha eddy current is induced by magnetic fields in metals and th changes of this electric current are characteristic of th homogeneity of the metal. They are advantageously applicabl for detecting cracks and flaws but give no information abou corrosion processes.

The characterization of pipes coated with insulatin materials can be performed by measuring insulatio resistance (published European patent application No 370,466). This method is useful for qualification o insulating coatings but not for direct observation o corrosion processes.

Other known methods are based on transmitting electri or ultrasonic waves to one end of a pipe and measuring th transmitted and reflected waves at the other end (JP 59-038,31 and JP 86-006,127). This method provides only averag information about tube state.

Optical methods are also widely used. In some cas video cameras are used for the detection of light (Briti patent No. 2,242,497) or photos are taken from the surfa of tubes (European patent No. 282,687) and the surface sta is visually evaluated. The drawback of this method is th it gives no possibility for using simple but fast a quantitative evaluation.

Surface inspection methods based on detection of lig reflected from the outer surface of tubes have already be developed applying instruments that need large room (Britis patent No. 2,249,169). For the inspection of flaws of pipes method based on fluorescence is known, which comprises fillin the tube with a special fluid marking the flaws in this way a measuring the fluorescence of the fluid in the flaws by fib optics resulting in the determination of the position surface defects (European patent No. 222,635). This metho however, does not measure the rate of corrosion and i application would be strongly disadvantageous from t viewpoint of cleanness of nuclear power reactors, since a additional foreign material would be filled into the tube of the primary cooling circuit.

Our aim was to develop a sensing probe that can measu and characterize the inner surface of pipes with sma diameters and can advantageously be used in the tubes heat exchangers of nuclear power plants.

The invention is based on the recognition that t actual corrosion state of metal surfaces can characterized by irradiating the surface to be tested wi an electromagnetic wave, which is reflected at least partl differently from the bare metal and from the corrod surface. Measuring the differences in the intensities of t reflected irradiation the local grade of corrosion can detected. Accordingly, the present invention relates to a sensi probe for examining the local corrosion state of inner tu surfaces, which comprises a radiation source (6) emitti electromagnetic radiation, a radiation detecting means (7) built into the same structural unit as the radiation source measuring the irradiation reflected from the inner tub surface, and an element (3) arranged between the radiatio source (6) and the detecting means (7) , this element (3 being opaque for the irradiation, wherein the wavelength o the electromagnetic radiation emitted by the radiatio source (6) is chosen in the range where the intensities o the irradiation reflected from the bare metal and from th corroded surface are different.

Furthermore, the invention relates to a method fo examining the local corrosion state of inner tube surface comprising the steps of irradiating the inner tube surfac with electromagnetic waves having a wavelength in the rang where the intensities of the irradiation reflected from th bare metal and from the corroded surface are different detecting the reflected irradiation by a radiation detectin means separated from the primary radiation and comparing th measured intensities with the intensity reflected from th bare metal surface.

According to a preferred method of the present inventio a small ring-shaped portion of the inner tube surface i irradiated and the intensity of reflected irradiation i measured near to the radiation source while excluding th primary radiation. From corrosion viewpoint the intensity o reflected irradiation is characteristic of the state of th inner tube surface. Repeating the measurement at severa discrete points parallel to the longitudinal axis of th tube information can be obtained about the corrosion stat of the surface both locally and in an average sense.

The measured reflected intensities can be processed digitally and stored by a computer, enabling a) the proper calibration of the sensing probe; b) the regular inspection of inner tube surfaces; c) graphical evaluation presenting light intensities in the function of distances along the longitudinal axis of the tube and in the function of time at a given position of the tube; and d) the determination of temporal tendencies and the actual state of inner tube surfaces and thereby the disconnection of dangerously corroded pipes from the he exchanger.

The parts of a preferred embodiments of the probe according to the invention are as follows: a) a tube supplied with slots in such a way that the radiation could leave the tube and the reflected radiation could enter to it; b) an element preferably of disc shape, which is opaque fo the radiation and divides the tube perpendicular to its longitudinal axis to two parts; c) a radiation source inserted with releasable or with permanent joint into one section of the tube; d) a radiation detecting means inserted with releasable or permanent joint into the other section of the tube. The wavelength of the electromagnetic radiation chosen in the range where the intensities of the irradiati reflected from the bare metal and from the corroded surfa are different. This means that the radiation source has emit radiation with wavelengths that overlap with t wavelength range where the reflectivity of the bare met surface is markedly different from that of the oxide lay formed at the metal surface. Moreover, the size a availability of the radiation source are also of cruci importance. According to these considerations a radiati source emitting visible light, for example an incandesce lamp or a light emitting diode, is preferable. The radiation sources are inexpensive, commercially availab and do not require maintenance. It is also possible to u UV light sources, first of all a discharge tube, whi should, however, be produced individually according to t requirements. Radiation sources emitting IR light may al be used. It is preferable to use a photodiode as radiati detecting means. This device consists of differe semiconducting elements depending on the wavelength radiation to be detected. The electrical conductance of t semiconductors depends on the intensity of radiation. Th intensity dependence can be converted into potenti dependence and can be measured as voltage. The advantage these radiation detecting means is that they are inexpensi and commercially available and require no maintenance. In a preferred embodiment of the probe of the prese invention a visible light source, preferably a white lig source, is applied, and in this case the light source preferably an incandescent lamp. The radiation detecti means is preferably a photodiode. Fig. 1 is a semi-sectional view diagram showing preferred embodiment of the sensing probe according to t invention operating in the range of visible light.

The sensing probe is explained in more detail by the a of Fig. 1. The stainless steel tube 1 is provided with four stadi curve shaped slots 2 cut parallel to the longitudinal ax of tube 1 at equal distances from both ends of tube anduniformly distributed on the surface of tube 1.

The opaque disc-shaped element 3 is preferably made plastic which divides tube 1 into two equal parts 4 and perpendicular to its longitudinal axis.

The light source 6 is inserted with releasable permanent joint into one section 4 of tube 1.

The photodiode 7 is inserted with releasable permanent joint into the other section 5 of tube 1.

The sensing probe is driven by cable 8, which consis of the steel wire 9 and of the twin wire 10 supplying power for the light source 6 and transmitting signals of t photodiode 7. The steel wire 9 is fixed to tube 1 by sp welding at point 17, providing also the electrical ground.

Two spacers 12 made of plastic hold the sensing probe the longitudinal axis of the pipe to be inspected. T spacers 12 are mounted on the stainless steel tubes 13 a 14 that are connected to tube 1 with clock springs providing the applicability of the sensing probe in curv pipes. The conical nose cowling 16 made of plastic is mount to the steel tube 14 which provides the safe driving of t sensing probe.

The plastic coverage 11 is mounted to the anti-sk shaped part of the steel tube 13. The sensing probe described in the present invention preferably produced in a size which is suitable for t pipes to be inspected and is compatible with manipulato usually applied in nuclear power reactors.

The calibration of the probe is done with well-defin specimens prior to the regular operation.

The reflected intensities measured by the sensing pro can be digitally processed and stored by computer. Th enables the tracking of corrosion processes and t quantitative comparison of tubes. For digital processing is necessary to normalize the measured intensities to t calibrations and to store the results together with t parameters of measurements. This procedure enables quantitative comparison of different data sets. It preferable to smooth data with an appropriate mathematic method to reduce electrical noise. The data evaluation c be done graphically, presenting light intensities in t function of distances along the longitudinal axis of t tube and comparing them for different tubes and al presenting the reflected intensities measured at a giv position of a single tube in the function of time. All these evaluations can be done either with absolu intensities or in the form of percentages.

The above description of the present invention applicable with suitable modifications if non-visible lig is used for the inspection procedure, without departing fr the scope of the novel concepts of the present invention.

The main advantages of the sensing probe and the meth according to the invention are as follows: a) the measurement is fast, simple and it can be performed directly in the nuclear power reactors; b) it is non-destructive; c) no additional chemical is required; d) it gives direct information about the local corrosion state of inner tube surfaces; e) tubes with small diameters can also be examined; f) it is safe, no radioactive material leaks into the environment.

Claims

What we claim is :

1. A sensing probe for examining the local corrosio state of inner tube surfaces, which a comprises a radiatio source (6) emitting electromagnetic radiation, a radiatio detecting means (7) , built into the same structural unit a the radiation source, measuring the irradiation reflecte from the inner surface to the tube, and an element (3 arranged between the radiation source (6) and the detectin means (7) , the said element (3) being opaque for th irradiation, wherein the wavelength of the electromagneti radiation emitted by the radiation source (6) is chosen i the range where the intensities of the irradiation reflecte from the bare metal and from the corroded surface ar different.

2. A sensing probe as claimed in claim 1, wherein th radiation source (6) emits visible light.

3. A sensing probe as claimed in claim 1, wherein th radiation source (6) emits white light.

4. A sensing probe as claimed in claim 1, wherein th radiation source (6) is an incandescent lamp.

5. A sensing probe as claimed in 1, wherein th radiation detecting means (7) is a photodiode.

6. A sensing probe as claimed in claim 1, wherein th element (3) , being opaque for the irradiation, is made o plastic.

7. A method for examining- the local corrosion state o inner tube surfaces, comprising the steps of irradiating th inner tube surface by electromagnetic waves having wavelength in the range where the intensities of th irradiation reflected from the bare metal and from th corroded surface are different, detecting the reflecte irradiation by a radiation detecting means separated fro the primary radiation and comparing the measured intensitie with the intensity reflected from the bare metal surface.

8. A method as claimed in claim 7, wherein visible preferably white, light is used as electromagnetic wave.

9. A method as claimed in claim 7, wherein a photodiod sensing visible light is used as radiation detection means.

10. A method according to claim 7, in which th radiation detection means is separated from the primar radiation by a disc made of plastic.