Device for measuring liquid level by optical reflectometry, comprising such a device structure and measurement method corresponding
The invention relates generally to liquid level measurements in nuclear installations, including storage pools for spent fuel assemblies.
More specifically, in a first aspect, the invention relates to a liquid level measuring device by optical reflectometry for a structure of a nuclear facility containing a volume of liquid, the liquid volume being delimited by a bottom and by a free surface, the measuring device comprising:

- at least one optical fiber adapted to be immersed in the liquid through the free surface;

- a light source sending a light beam into the optical fiber;

- an analyzer, arranged to analyze a light radiation backscattered from the optical fiber and to deduce the liquid level from the bottom.

JP 2014-41 023 describes such a device. That comprises a hydrostatic equilibrium able well with the volume of liquid. The optical fiber is immersed in the liquid filling the wells.

Such an arrangement is complex and difficult to implement on existing facilities.

In this context, the invention is to provide a measuring device that is easier to implement.

To this end, the invention relates to a measuring device of the aforementioned type, characterized in that the measuring device comprises a float floating on the free surface of the liquid, the float having a passageway wherein the or each optical fiber is received slidably, an optical fiber section engaged in the passage being pinched or deviated.

Such a measuring device can easily be installed in an existing installation, such as a swimming pool.

Furthermore, the measuring device can have one or more of the following characteristics, considered (s) individually or in all technically possible combinations:

- the measurement device comprises a straightener tube intended to be immersed in the liquid through the free surface, the floating float inside the tube straightener;

- the passage is arranged so that the optical fiber section engaged in the passage forms a circular arc of at least 45 °, preferably of at least 180 °;

- the passage is arranged so that the optical fiber section engaged in the passage forms at least a half S, and preferably one or more O;

- the measurement device comprises at least two optical fibers, one being coated with an acrylate coating and the other a polyimide or metal coating;

- the analyzer type is OFDR (optical frequency domain reflectometry) or OTDR (optical time domain reflectometry);

- the analyzer is provided for determining a mechanical stress profile along the optical fiber as a function of the light radiation backscattered from the optical fiber and for calculating the position of the free surface relative to the base in accordance with said profile; and

- the optical fiber is resistant to a higher irradiation than 1 MGy, preferably 5 MGy.

According to a second aspect, the invention relates to a structure of a nuclear facility containing a volume of liquid, the liquid volume being delimited by a bottom and by a free surface, the structure further comprising a level measuring device reflectometry comprising:

- at least an optical fiber immersed in the liquid through the free surface;

- a light source sending a light beam into the optical fiber; - an analyzer, arranged to analyze a light radiation backscattered from the optical fiber and to deduce the liquid level from the bottom;

the measuring device comprising a float floating on the free surface of the liquid, the float having a passageway in which the or each optical fiber is slidably received, an optical fiber section engaged in the passage being pinched or deviated.

The level measuring device is typically by reflectometry according to the first aspect of the invention.

According to a third aspect, the invention relates to a method for measuring by optical reflectometry liquid level to a structure of a nuclear facility containing a volume of liquid, using a measuring device as described above, the method comprising the steps of:

- immersing the or each optical fiber into the liquid through the free surface, the float floating on the free surface of the liquid, the or each optical fiber being slidably received in the passage of the float, the optical fiber section engaged in the passage being pinched or deviated;

- sending the light radiation into the optical fiber from the light source;

- analyzing the light radiation backscattered from the optical fiber and deduce the liquid level from the bottom.

Furthermore, the measurement method may further have the characteristics below:

- the liquid level relative to the base is derived by determining a mechanical stress profile along the optical fiber as a function of the light radiation backscattered from the optical fiber, and calculating the position of the free surface relative to the bottom depending said profile.

Other features and advantages of the invention will emerge from the detailed description is given below, for information only and in no way limiting, with reference to the attached figures, among which:

- Figure 1 shows a fuel assembly storage pool used in a nuclear reactor, equipped with a level measuring device by reflectometry according to the invention;

- Figures 2 and 3 represent the signal backscattered by the optical fiber to the analyzer as a function of the distance along the optical fiber, for analyzers OTDR and OFDR kind respectively; and

- Figure 4 is a simplified schematic representation of a float according to a variant of the invention.

The measuring device 1 is intended to be arranged in a structure 3 of a nuclear facility, which is a storage pool of spent fuel assemblies

5 in Figure 1. This structure comprises a volume of liquid 7. The liquid volume is delimited by a bottom 9 and side walls 1 1 and has upwardly a free surface 13.

The measuring device 1 is provided for measuring the liquid level, the liquid level corresponding to the height of liquid taken along a vertical direction from the bottom 9 to the free surface 13.

The liquid is typically water in the case of a spent fuel storage pool.

The measuring device 1 is operable to measure the liquid level in other structures of the nuclear reactor, e.g., other pools or tanks. It is also used in a structure located in a facility other than a nuclear reactor, such as a reprocessing plant for spent fuel assemblies or any other installation of the fuel cycle.

The liquid is not necessarily water, but may be any kind of aqueous or non-aqueous liquid.

As shown in Figure 1, the measuring device 1 comprises at least one optical fiber 15 arranged to be immersed in the liquid 7 through the free surface 13, a source 17 of light sending light radiation in the or each optical fiber 15 and an analyzer 19.

Each optical fiber 15 thus has a portion immersed in the liquid volume, and a tip portion extending above the free surface 13. Each optical fiber 15 extends to the bottom 9, or almost to bottom 9.

The analyzer 19 is provided for analyzing the light backscattered radiation at each point of each optical fiber 15, and for deriving the level of mechanical stresses along the optical fiber and the liquid level relative to the bottom 9.

The analyzer 19 is of the type OFDR (Optical Frequency Domain Reflectometry or optical frequency domain reflectometry). Alternatively, the analyzer type is OTDR (Optical Time Domain Reflectometry or optical time domain reflectometry).

The light source 17 is a laser or any other light source suitable for technical OFDR, OTDR or appropriate.

The OFDR technique is known, and will be only briefly described here. The light radiation from the light source 17 is divided by a first coupler between two arms of an interferometer: a reference arm and a measuring arm. The measurement arm is optically connected to the optical fiber 15 and transmits the light radiation to the optical fiber coupler 15. A second, located on the measuring arm, divides the light radiation to query the length of the optical fiber 15. the optical fiber 15 returns light radiation, referred to herein as light radiation backscattered in the measurement branch. The second coupler directs a portion of the backscattered light radiation in the reference arm. A third coupler, located on the reference arm, recombines the light radiation emitted by the source 17 and the light backscattered radiation. A polarization splitter and a polarization controller, both located in the reference arm, are used for dividing the recombined light radiation equally between two orthogonal polarization states. The interference between the backscattered light radiation and the two polarization states is recorded by the detectors. The analyzer 19 measures the complex reflection coefficient at each point of the optical fiber 15, depending on the wavelength of the light radiation emitted by the light source 17. From the reflection coefficients

complex, the reflection spectrum is calculated based on the frequency. The reflectivity as a function of the fiber length is calculated by applying a Fourier transform to the diffusion spectrum. The data detected by the two polarization states are used to perform the cross-correlation between the reference measurement and the measurement in real situations. This cross-correlation is a measure of the temperature or of the stress applied through a calibration and the presence of tabs in the analyzer 19.

The light radiation backscattered by the optical fiber 15 is caused by local and random fluctuations of the index profile along the length of the optical fiber 15. For a given optical fiber, the signature of the optical fiber according to the distance is a specific property. Is meant here by signature depending on the distance, the spectrum of the light radiation scattered by each point of the optical fiber. Each optical fiber has its own signature. The local changes in the refractive index caused by an external stimulus such as a change of temperature or local mechanical stress, causing a change in the signature of the optical fiber as a wavelength shift of the light radiation back-scattered by the section of the optical fiber undergoing external stimulus. In the case where the analyzer is OFDR type, it has calibrations tables, to derive the amplitude of the temperature change or strain from the spectral shift. The analyzer 19 couples this analysis with a measurement of flight time, thus allowing to measure temperature or stress continuously or almost continuously throughout the optical fiber.

Typically, measuring the reference signature of the optical fiber according to the distance, in a reference situation (at room temperature and at rest, that is to say without mechanical stress applied to the fiber). The reference signature is stored by the analyzer 19. The signature of the optical fiber as a function of distance is then measured in a real situation by the analyzer 19. The dispersion patterns from the two measurements are then compared by the analyzer 19 by a cross-correlation over the entire length of the fiber, the latter being divided into unit length. Thanks to the cross-correlation is measured the degree of similarity between these two signals making it possible to trace the location and quantification of the disturbance applied on the optical fiber, a length of the unit is selected by the operator depending the length of the fiber and consider other parameters such as the sensor operating conditions.

If, on the actual situation, an external parameter to the measuring device (temperature, mechanical stress) is modified with respect to the reference position at a point of the fiber, this change is recorded as a shift of the wavelength obtained by the cross-correlation at that point. The amount of shift depends on the magnitude of the change in the external parameter. The analyzer 19 typically comprises memory tabs, for determining the amplitude of the change in the external parameter depending on the wavelength shift.

The typical analyzer 19 analyzes the spectral distribution Rayleigh signing of the optical fiber. If the analyzer 19 is the type OTDR and not OFDR, Rayleigh signature is used to highlight the distribution of optical losses along the optical fiber. Mechanical stress applied at a point of the optical fiber contributes to the increase of the optical loss. The analyzer 19 is provided to detect the location of application of stress by detecting losses in excess generated locally.

Alternatively, the analyzer 19 analyzes the Brillouin spectral signature.

Advantageously, the analyzer 19 and light source 17 are offset in a measurement room 20, away from the liquid volume. Thus, the analyzer 19 is located in a different region of the space where is located the volume of liquid 7. This is local for example in another building, or a room of the same building as that in which the volume of liquid 7 .

The measuring device 1 further includes a float 21 floating on the free surface 13 of liquid, the float 21 having a passageway 23 wherein the or each optical fiber is slidably received. 25 a of the optical fiber section engaged in the passage 21 is pinched or deviated.

In the embodiment of Figure 1, 25 of the optical fiber section engaged in the passage 23 is deflected. By this is meant that it was not a straight shape, this rectilinear form corresponding to the baseline.

On the contrary, section 25 adopts the shape of the passage 23.

So as to generate an easy wavelength shift to be detected, the passage 23 is arranged so that the segment 25 of the optical fiber forms a circular arc of at least 45 °, preferably at least 90 ° and more preferably at least 180 °.

In the example shown, the passage 23 is arranged so that 25 of the optical fiber section engaged in the passage has the shape of an S. Thus, the segment 25 has two arcuate portions, each extending over 180 °, opposite curvatures.

Furthermore, the optical fiber 15 is arranged so as to present an upper portion 27 on the non-immersed volume of liquid 7, and a lower portion 29 immersed in the liquid volume 7. The sections 27 and 29 are connected to a to each other by the segment 25.

The measuring device 1 also includes a rigid support 31, rigidly secured to the civil engineering structure 3, or a fixed frame of the civil engineering. The non-immersed portion 27 of the optical fiber is suspended with its upper end 33 to the rigid support 31. The upper end 33 is for example connected to the light source 17 and analyzer 19 via an optical fiber 35 intermediate.

The immersed section 29 has a lower end 37 preferably located flush with the bottom 9. A ballast 39 is attached to the end 37, so as to maintain the immersed section 29 in a substantially vertical orientation.

The measuring device 1 advantageously comprises a tube Vanes 41 provided to be immersed in the liquid through the free surface 13, the float 21 floating inside the tube straightener.

The one or more optical fibers 15 are also disposed inside the tube straightener 41. This one is in a vertical orientation. It is for example rigidly secured to the structure 31.

Vanes the tube 41 provides a constant liquid level around the float even if the waves propagate to the free surface 13 outside the tube straightener. This helps to obtain a reliable measurement.

The measuring device 1 typically comprises a plurality of optical fibers 15. In this case, the float 21 comprises a plurality of passages 23, which are identical to each other and separated from each other. Each passage 23 slidably receives a segment of an optical fiber.

Alternatively, several optical fibers are engaged in the same way. For example, the measuring device comprises at least two optical fibers 15, and typically includes four optical fibers 15.

Advantageously, each optical fiber 15 is selected to withstand high levels of radiation and temperature. Typically, at least one of the optical fibers 15 is coated with an acrylate coating, and at least another is coated with a polyimide coating and / or metal. For example, an optical fiber is coated with an acrylate coating resistant to high temperatures, and three optical fibers are coated with a polyimide coating and / or metal.

Thus, the optical fibers 15 are resistant to a temperature above 150 ° C, and at a greater than 1 MGy irradiation, preferably 5 MGy, more preferably 10 MGy. resistant is understood that the optical fibers are used as strain sensor, without significant degradation of their performance.

FIG 2 illustrates the signal backscattered by each optical fiber 15 to the analyzer 19, depending on the position along the optical fiber 15 to an OTDR-type analyzer. This signal is representative of the level of optical losses induced in each point of the fiber, particularly those induced by the stress applied at the air-water interface. The curve shown in Figure 2 shows in the left part a first plate, corresponding to 27 of the fiber section located above the free surface 13, and possibly the fiber 35.

This curve at the right side a second plate, corresponding to 29 of the optical fiber section 15 immersed in the liquid.

The two plates are connected by a region forming a lump, denoted V, wherein the signal obtained by the analyzer 19 varies rapidly as a function of the position along the optical fiber. This zone corresponds to 25 of the optical fiber section engaged in the passage 23 of the float. This stretch is subjected to mechanical stress resulting in significant optical losses easily detected by the analyzer 19 because it is distorted to fit the shape of the passage 23.

3 illustrates the stress level for each optical fiber 15 to the analyzer 19, depending on the position along the optical fiber 15 for an OFDR type analyzer. The signal is similar to that of Figure 2, with some differences. The two plates are substantially the same level of stress.

The analyzer 19 is provided for determining a mechanical stress profile along the or each optical fiber 15 as a function of the light radiation backscattered from the optical fiber, and to deduce therefrom the position of the free surface 13 with respect to the bottom 9.

To do this, the analyzer 19 is provided for determining the position of the area V of rapid variation of the signal along the optical fiber, as a function of light radiation backscattered from the optical fiber.

The analyzer 19 is also provided for, depending on the position, calculating the position of the free surface 13 with respect to the bottom 9.

To do this, the analyzer is provided for determining the distance along the optical fiber 15 between a reference point and rapidly varying region of the signal. The reference point is typically the light source 17. In Figure 2, the origin of the abscissa axis corresponds for example to the light source 17.

The analyzer 19 is provided in order to deduce the liquid level from the bottom. To do this, the analyzer 19 is provided for the difference between the length along the optical fiber 15 and possibly of the intermediate fiber 35, between the light source 17 and the lower end 37 of the optical fiber, and the distance previously

determined. Said length is a data recorded in the analyzer memory. This result is adjusted if necessary to take into account the distance between the lower end 37 of the optical fiber and the bottom 9, if the lower end 37 is not flush with the bottom.

The level measuring method using a measuring device according to the invention will now be described.

It is considered an initial state wherein the measuring device is situated outside the volume of liquid.

During a first step, is dipped or each optical fiber 15 into the liquid through the free surface 13, the float being positioned floating at the free surface 13 of liquid, the or each optical fiber 15 is received slidably disposed in the passage 23 of the float 21.

Vanes 41 the tube is placed before or after the introduction of the optical fiber and the float.

The upper end 33 of the non-immersed portion 27 of the optical fiber is connected to the rigid support 31. The straightener 31 is connected also to the support 31.

The optical fiber 15 is arranged so that the non-immersed portions and immersed 27 and 29 are substantially vertical. The weight 39 maintains the lower end 37 flush with the bottom 9 and ensures a certain vertical tension so that the fiber 15 is rectilinear and allows the float 21 to slide freely along the fiber with the movement of the free surface of the liquid 13.

Then, the light source 17 sends a light radiation in the or each optical fiber 15. The light beam backscattered by the or each optical fiber 15 is analyzed by the analyzer 19, which deduces the liquid level relative to the bottom 9 .

To do this, the analyzer 19 first determines the mechanical stress profile along the or each optical fiber 15, depending on the light radiation backscattered by the or each optical fiber 15. The analyzer 19 then determines the position of the along the optical fiber 25 of each optical fiber section engaged in the passage 23, by using the profile of mechanical stresses. The analyzer, to do this, determines the fiber area in which the mechanical change very quickly. It then determines the distance, along the optical fiber 15 and possibly of the intermediate fiber 35, between the section 25 and the light source.

It deduce the liquid level from the bottom 9, as the difference between the total length along the fibers 15 and 35, between the lighting source 17 and the lower end 37 and the predetermined distance.

It is possible that the lower end 37 of the fiber is not exactly located flush with the bottom 9. In this case, the distance calculated previously is corrected to take account of the height at which is located the lower end 37 with respect basically.

According to an embodiment variant, shown in Figure 4, 25 of the optical fiber section engaged in the passage 23 is not deflected but pressed by members provided for this purpose and mounted on the float 21. For example, as illustrated in Figure 3, the section 25 is held between two rollers 41 rotatably mounted on the float 21, the rollers 41 rolling along the optical fiber when the float 21 moves vertically. The two rollers 41 pinch therebetween the optical fiber and thus create a stress in the segment 25.

The measuring device and the measuring method described above have many advantages.

Firstly, the fact that the measuring device comprises a float floating on the free surface of the liquid, and having a passage wherein a section of the optical fiber is slidably received, the arrangement of the measuring device in the structure is very simple. The float moves with the free liquid surface, which causes a displacement of the optical fiber in the passageway, the pinched or deformed section and varies with the liquid level. The measuring device can be easily implemented in existing systems without modification of civil engineering. Integration of the measuring device in a structure is not intrusive.

The electronics, including the analyzer can be easily arranged in a room located away from the liquid volume. Thus, in case of an accident causing an increase in the temperature inside the structure, and / or an increase in the humidity in the atmosphere above the liquid volume, and / or an increase in radiation ionizing above the liquid volume, the analyzer is not affected.

The measuring device thus remains functional under conditions of accidental atmosphere within the structure such that the liquid temperature reached 100 ° C, moisture in the atmosphere above the liquid reaches 100%, and elements of the measuring device located inside the structure are exposed to a cumulative dose greater than 1 MGY, preferably 5 MGy, more preferably 10 MGy.

Such an arrangement also allows to meet the strength requirements of the earthquake IEC 60068 and IEC 69180. This arrangement also helps meet the requirements of the RCC-E (design and construction rules for electrical equipment of nuclear islands) .
Moreover, tests have shown that the measuring device for determining the liquid level relative to the bottom with an accuracy of about one centimeter.
The analyzer is multiplexable, and allows to perform the analysis of signals from several optical fibers, which are not necessarily immersed in the same volume of liquid.

CLAIMS

1. - A level measurement by OTDR for a structure (3) of a nuclear installation containing volume (7) of liquid, the liquid volume being delimited by a bottom (9) and a free surface (13), the measuring device (1) comprising:

- at least one optical fiber (15) adapted to be immersed in the liquid through the free surface (13);

- a light source (17) sending a light beam into the optical fiber (15);

- an analyzer (19), provided for analysis of a light radiation backscattered from the optical fiber (15) and to deduce the liquid level from the bottom (9);

characterized in that the measuring device (1) comprises a float (21) floating on the free surface (13) of liquid, the float (21) having a passage (23) wherein the or each optical fiber (15) is slidably received, a section (25) of the optical fiber (15) engaged in the passage being pinched or deviated.

2. - Device according to claim 1, characterized in that the measuring device (1) comprises a tube straightener (41) adapted to be immersed in the liquid through the free surface (13), the float (21) floating inside the tube straightener (41).

3.- Device according to claim 1 or 2, characterized in that the passage (23) is arranged so that the segment (25) of the optical fiber (15) engaged in the passage (23) forms an arc of at least 45 °, preferably at least 180 °.

4. - Device according to any one of the preceding claims, characterized in that the passage (23) is arranged so that the segment (25) of the optical fiber (15) engaged in the passage (23) forms at least a half S and preferably one or more S.

5. - A measuring device according to any one of the preceding claims, characterized in that the measuring device (1) comprises at least two optical fibers (15), one being coated with an acrylate coating and the other of a polyimide or metal coating.

6. - A measuring device according to any one of the preceding claims, characterized in that the analyzer (19) is of type OFDR (optical frequency domain reflectometry) or OTDR (optical time domain reflectometry).

7. - A measuring device according to any one of the preceding claims, characterized in that the analyzer (19) is provided for determining a mechanical stress profile along the optical fiber (15) as a function of light radiation

backscattered by the optical fiber (15) and to calculate the position of the free surface (13) relative to the base (9) according to said profile.

8. - A measuring device according to any one of the preceding claims, characterized in that the optical fiber (15) is resistant to a higher irradiation than 1 MGy, preferably 5 MGy.

9. - Structure of a nuclear installation containing a volume of liquid, the volume of liquid (7) being delimited by a bottom (9) and a free surface (13), the structure (3) further comprising means ( 1) level measurement by reflectometry comprising:

- at least one optical fiber (15) immersed in the liquid through the free surface (13);

- a light source (17) sending a light beam into the optical fiber (15);

- an analyzer (19), provided for analysis of a light radiation backscattered from the optical fiber (15) and to deduce the liquid level from the bottom (9);

characterized in that the measuring device (1) comprises a float (21) floating on the free surface (13) of liquid, the float (21) having a passage (23) wherein the or each optical fiber (15) is slidably received, a section (25) of the optical fiber (15) engaged in the passage (23) being pinched or deviated.

10. - A method of level measurement by OTDR for a structure (3) of a nuclear installation containing a volume of liquid (7) using a measuring device

(1) according to any one of claims 1 to 8, the method comprising the steps of:

- immersing the or each optical fiber (15) into the liquid through the free surface (13), the float (21) floating on the free surface (13) of the liquid, the or each optical fiber (15) being received so sliding in the passage (23) of the float (21), the segment (25) of the optical fiber (15) engaged in the passage (23) being pinched or deviated;

- sending the light radiation into the optical fiber (15) from the light source (17);

- analyzing the light radiation backscattered from the optical fiber (15) and deduce the liquid level from the bottom (9).
January 1. - Measuring method according to claim 10, characterized in that the liquid level relative to the bottom (9) is derived by determining a mechanical stress profile along the optical fiber (15) as a function of the light radiation back-scattered by the optical fiber (15), and calculating the position of the free surface (13) relative to the base (9) according to said profile.