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 2014-41023 describes such a device. This comprises a hydrostatic equilibrium able well with the volume of liquid. The optical fiber is immersed in the liquid filling the well.
Such an arrangement is complex and difficult to implement on existing plants.
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 includes a sheath which is disposed in the or each optical fiber, the sheath being permeable to liquid.
The sheath acts as a straightener. It isolates the optical fiber or fibers from the environment and protects them mechanically. It can be easily installed in an existing installation, such as in a swimming pool.
Furthermore, the measuring device can have one or more of the characteristics below, considered individually or in all technically possible combinations:
- the sheath is pierced by a plurality of orifices;
- the measurement device comprises a ballast connected to one optical fiber section arranged to be disposed close to the bottom;
- the measurement device comprises a cage adapted to be disposed close to the bottom, the ballast being free in a longitudinal direction within the cage;
- the measurement device comprises a rigid support intended to be placed above the free surface, and at least one elongate member fixing the cage to the rigid support such that the weight of the cage is taken up by the rigid support without by the sheath;

- the or each optical fiber comprises first and second parallel branches to each other and engaged in the sheath, and a U-section connecting to one another the respective immersed ends of the first and second legs, the U-shaped section being adapted to be located near the bottom, the first and second legs having respective ends emerged provided to be located above the free surface;

- the ballast is a disc provided with a groove in which is engaged the U-section of the optical fiber;

- 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); and

- the analyzer is provided for determining the temperature profile along the or each optical fiber and to deduce therefrom the position of the free surface relative to the bottom.

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 sheath in which is positioned the or each optical fiber, the sheath being permeable to liquid.

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

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

- immersing the or each optical fiber into the liquid through the free surface; - 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.

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

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

- Figure 2 shows schematically the lower part of a measuring device according to a first embodiment of the invention;

- Figure 3 is a detail view of the measuring apparatus shown schematically in Figure 2;

- Figure 4 is a sectional view along the arrows IV in Figure 3;

- Figure 5 is a graph showing the temperature profile measured along the optical fiber of the device of Figures 2 and 3; and

- Figure 6 is a view similar to that of Figure 5 showing the temperature profile measured along the optical fiber to a second embodiment of the measuring device of the invention.

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

Detecting the liquid level is possible by measuring the temperature distribution along the optical fiber.

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 a light radiation backscattered by each optical fiber 15, and to derive the temperature distribution 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 returned 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. On the basis of complex reflection coefficients, the spectrum reflection is calculated based on the frequency. The reflectivity as a function of the fiber length is calculated by applying a Fourier transform to the spectrum of reflection. 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 strain applied to the optical fiber through a calibration and the presence of tabs in the analyzer 19. The light radiation backscattered by the optical fiber 15 is caused by random fluctuations of the profile index along the length of the optical fiber 15. for a given optical fiber, the signature of the optical fiber as a function of 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 temperature change or local mechanical stress, causing a change in the signature of the optical fiber as a wavelength shift of the reflected light radiation by the stretch of the fiber undergoing external stimulus. The analyzer 19 provides calibration 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. 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, which can be traced back to the localization and quantification of the disturbance applied to the optical fiber. The length of the unit is selected by the operator depending on the length of the fiber and to 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. Alternatively, the analyzer 19 analyzes the spectral signature of Raman scattering or Brillouin., Especially when the analyzer is type OTDR

Advantageously, the analyzer 19 and light source 17 are offset in a measurement room 27, 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 and in any case far enough to not be exposed to high temperatures and / or ionizing radiation, particularly severe accidents.

As shown in Figure 2, the measuring device comprises a sheath 29 in which is disposed the optical fiber or fibers. The sheath 29 is permeable to liquid. It is, as the one or more optical fibers immersed in the liquid through the free surface 13. The liquid fills the sheath 29 and runs through the optical fiber or fibers lying within the sheath 29. The sheath 29 extends 9 to the bottom, or substantially to the bottom 9.

It therefore protects the optical fiber over their entire lengths.

The sheath 29 is typically pierced with a multitude of small orifices 30 communicating the internal volume of the sheath 29 with the outside of the sheath. These orifices are shown enlarged in Figure 2.

The sheath 29 is typically of a plastic material resistant to temperature and radiation. For example, it is PEEK.

The sheath 29 is flexible and can be wound when the measuring device is not used, while having sufficient rigidity to protect the optical fibers from impact.

In the example shown in Figure 3, the sheath 29 is obtained by winding helically a ribbon, in the manner of a shower hose. The ribbon is delimited laterally by edges provided to be able to bind to one another, so that each turn of the helical ribbon is fixed by its edges to the previous turn and the next turn.

As shown in Figures 2 to 4, the measuring device 1 comprises, for each optical fiber 15, a weight 33, typically disc-shaped, bonded to a portion of the optical fiber 15 arranged to be disposed near the bottom 9 . This maintains the optical fiber 15 with a controlled voltage, and vertical. This is especially important to ensure that the optical fiber 15 does not contact the sheath 29, which could create a local stress on the optical fiber 15 and distort the measurement.

Furthermore, the measuring device 1 comprises a shaft or plunger 35 adapted to be disposed close to the bottom 9.

The cage 35 comprises for example two fixed concave half-shells to each other, the half shells having lights 37 allowing flow of liquid inside the cage 35.

The cage 35 defines internally a chamber in which are housed the or weights 33. The chamber is elongated in a longitudinal direction L, materialized in Figure 3. It has a form such that the weights 33 are free to move in the direction longitudinal L within the chamber but not in the other directions.

The measuring device 1 further comprises a rigid 39, visible form on Figure 5, intended to be placed above the free surface 13. The rigid support 39 is typically rigidly attached to the civil engineering structure 3, or a support frame rigidly attached itself to civil engineering.

The weight of the cage 35 also provides a necessary and sufficient voltage so that the sheath 29 be vertical rectilinear once immersed in the liquid.

The cage 35 is suspended from the rigid support 39, such that the weight of the cage 35 is taken up by the rigid support 39 without passing through the sheath 29. For this purpose, the measuring device 1 comprises at least one elongate member 41 rigidly fixed by a first end to the cage 35 and by a second end to the rigid support 39.

In the example shown, the measuring device comprises two elongate members 41. Alternatively, the measuring device comprises more than two elongate members 41.

The members 41 are, for example stainless steel cables.

cleats 43 can maintain the 41 bodies near the sheath 29.

The rigid support 39 comprises a conduit 45 to which an upper end 47 of the sheath 29 is rigidly fixed. The conduit 45 thus extends sheath 29.

The end of the duct 45 opposite to the casing is closable by a removable plug 49.

Furthermore, the cage 35 has, at one longitudinal end, a neck 51 defining a passage opening into the inner chamber defined by the cage. A lower end 53 of the sheath 29 opposite to the upper end 47, is

rigidly attached to the neck 51. The internal space of the sheath 29 thus communicates with the chamber 35 of the cage through said passage, and communicates with the internal volume of conduit 45.

Thus, the shroud 29 is arranged to continuously protect the optical fiber or fibers 15 from the rigid support 39 to an inlet through which the optical fiber or fibers 15 enter the cage 35.

According to a first embodiment of the invention shown in Figures 2 to 5, the or each optical fiber 15 comprises first and second branches 55 parallel to each other and engaged in the sheath 29 and that a U-section 57 connecting to each other the respective ends 59 immersed first and second arms 55. Thus, the optical fiber 15 follows a U-profile

The U section 57 is intended to be located near the bottom 9. This is shown in Figure 2.

The first and second legs 55 further comprise respective ends emerged 61, intended to be located above the free surface 13.

Thus, the U sections 87 of the or each optical fiber 15 is located outside the shroud 29, the first and second legs 55 being engaged in the sheath 29.

Typically, the portion U 57 is housed inside of the cage 35, and 61 emergent ends are located inside the conduit 45.

Emerged ends 61 are for example connected to an optical connector accommodated in the duct 45. The connector (not shown) is protected by the cap 49 when the measuring device is not used.

Emerged ends 61 are connected to the analyzer 19 by the intermediate optical fibers 63, as shown in Figure 1. The fibers 63 are for example in engagement with the connector.

Each ballast 33 is typically a disc provided with a groove 65, as shown in Figure 4. The U-shaped section 57 of the optical fiber is engaged in the groove 65.

The measuring device comprises at least two optical fibers 15, and in one embodiment includes four optical fibers 15.

The measuring device thus comprises several weights 33 disk-shaped. As shown in Figures 3 and 4, these disks are arranged in planes parallel to each other, and are stacked on each other. Each is free to move within the cage 35 independently of one another in the longitudinal direction so that each optical fiber 15 is tight.

Each ballast 33 weighs between five and fifty grams

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 or metal coating.

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.

The optical fibers are resistant to a temperature above 150 ° C, and at a higher irradiation to 1 MGy, preferably 5 MGy, more preferably to 10 MGy. Mean that resistant optical fibers are used as temperature sensors, without significant degradation of their performance.

5 illustrates the temperature profile along each of the optical fibers 15 of the measuring device when they are folded into U as shown in Figure 2. The curve indicates the temperature measured by the analyzer 19 as a function of the position along the optical fiber.

The curve initially present in the left part a plateau at a substantially constant temperature T2. This plateau corresponds to the portion of the first branch 55 of the optical fiber located above the free surface 13 of liquid.

The curve further comprises a central portion forming a plateau at a substantially constant temperature T1, with a central boss B. The part of the shelf to the left of the boss corresponds to the submerged portion of the first branch 55 of the optical fiber. The bump corresponds to the U-section 57 of the optical fiber housed in the groove 65 of the ballast. The temperature of return value of this portion of the optical fiber is different from the value returned by the submerged portions of the two branches, that this U-section undergoes a mechanical stress. This mechanical stress is derived from the arrangement in a semicircle of the optical fiber section.

The portion of the tray to the right of the bump B corresponds to the submerged portion of the second branch 55 of the optical fiber.

Furthermore, the curved part comprises right another tray at the temperature T2, corresponding to the part of the second part 55 located above the free surface 13. The two plates at the temperature T2 are connected to the plate at the temperature T1 by edges A, A ', corresponding to the free surface 13 of the liquid volume.

When the U-section 57 of the optical fiber 15 is located flush with the bottom 9, the distance between the edges A and A 'along the optical fiber corresponds to substantially twice the depth of liquid.

In this embodiment, the analyzer 19 is provided to determine the temperature profile along each optical fiber 15, and deduce therefrom the position of the free surface relative to the bottom as a function of this temperature profile.

To do this, the analyzer 19 is provided for determining the position of the two edges A, A 'along the optical fiber, that is to say the position of the two zones in which the temperature varies rapidly as a function of distance. These two edges correspond to the portions of the optical fiber located at the free liquid surface.

The analyzer 19 is arranged to then determine the distance between these two edges along the optical fiber, and halving this distance in order to deduce the position of the free surface relative to the bottom.

Typically, the analyzer made the same analysis from the temperature profile observed for each of the optical fibers, and thus obtains a plurality of position values ​​of the free surface relative to the bottom. These values ​​are then averaged, or exploited in all possible strategy.

The level measuring method using a device of the type shown in Figures 2-5 will now be described.

It is considered an initial state wherein the measuring device 1 is situated outside the volume of liquid.

During a first step, the or each optical fiber 15 is immersed in the liquid 7 through the free surface 13.

For example, place the sheath 29 and sheath 29 that is arranged substantially vertically so that the cage 35 is located within the volume of liquid in the immediate vicinity of the base 9. An upper portion of the sheath 29 is located outside the volume liquid, above the free surface 13. the rigid support 39 is then rigidly attached to a frame or civil engineering, so that the sheath 29 and the cage 35 are maintained in position, in particular in the vertical direction. The light source 17 and analyzer 19 are arranged in the measuring room 27, at a distance from the structure 3. The optical fibers 15 passing through the interior of the sheath 29 are then connected to the light source 17 and the analyzer 19 by the intermediate optical fibers 63.

To perform the measurement, the light radiation is sent from the light source 17 in each optical fiber 15.

Then, the light radiation scattered by each optical fiber 15 is analyzed by the analyzer 19.

To do this, as explained above, the analyzer 19 determines the position along the optical fiber, edges A, A 'to which the temperature measured by the optical fiber varies very quickly as a function of distance. It determines the distance, along the optical fiber, both fronts. It divides this distance by two, and thus deduce the liquid level from the bottom.

A second embodiment of the invention will now be detailed with reference to Figures 1 and 6. Only the points by which this second embodiment differs from the first will be described. The identical elements or perform the same function in both embodiments will be designated by the same references.

In the second embodiment of the invention, the optical fibers are not bent in U. Each optical fiber 15 thus comprises a single rectilinear portion, the upper end 67 of the optical fiber being located above the free surface 13 of the liquid and the lower end 69 of the fiber being located flush with the bottom 9, into the liquid. In this case, the ballast 33 is attached directly to the lower end 69.

6 shows the temperature profile identified by the analyzer 19 to an optical fiber 15 disposed vertically in the liquid volume. This graph shows the temperature determined by the analyzer 19 according to the distance from the lower end 69 of the fiber. Figure 6 shows the temperature curve comprises a first plate at a substantially constant temperature T1. This temperature corresponds to the temperature of liquid. This plateau corresponds to the submerged part of the optical fiber 15, from the lower end 69 to the free surface 13.

The curve has a second plateau at a substantially constant temperature T2. This temperature is the temperature of the air above the liquid volume 7. The plateau corresponds to the tip of the optical fiber from the free surface 13 to the upper end 67. The two plates are separated by a falling edge whose position indicates the level of the free surface relative to the bottom 9, that is to say the level of liquid.

The analyzer 19 is provided for determining the position of the trailing edge along the optical fiber. The falling edge is the region where the temperature varies rapidly as a function of distance.

The analyzer 19 is arranged to then determine the distance between the lower end 69 of the optical fiber of the falling edge. This distance indicates the liquid level relative to the bottom 9.

The measuring method using the device according to the second embodiment, having one or more optical fibers unfolded in U, will now be detailed.

The step of setting up the optical fibers or in the liquid and the step of sending the light radiation in the optical fiber from the light source are substantially identical to those described above for the the first embodiment.

Only the step of analyzing the light radiation scattered by each optical fiber is different.

Specifically, during this step, the analyzer 19 determines the position of the temperature front along the optical fiber, the front being the area where the temperature varies rapidly as a function of the position along the optical fiber. It then determines the position of the lower end 69 of the optical fiber 15 of the temperature profile. In the example shown in Figure 2, the lower end of the optical fiber is located at the abscissa 0.

The analyzer then determines the distance, along the optical fiber, the front of the lower end of the optical fiber, said distance corresponding to the liquid level from the bottom.

In the methods of measurement corresponding to the first and second embodiments, it is possible that the lower end 69 of the fiber, or the portion of U 57, is not located (e) exactly flush with the bottom 9. In this case, it corrects the level of liquid previously calculated to take account of the height at which is located the lower end or the U-shaped portion relative to the bottom.

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

First, because the optical fiber or fibers are disposed within a sheath permeable to the liquid, the arrangement of the measuring device in the structure is very simple. The sheath is immersed with the one or more optical fibers, the volume of liquid so that the optical fiber or fibers have a section located immediately close to the bottom 9, and a portion of each optical fiber is located au above the free surface. 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 the temperature inside the structure increases, and / or the humidity in the atmosphere above the liquid volume increases, and / or ionizing radiation -Dessus fluid volume increases, the analyzer is not affected.

CLAIMS

1. - Device for measuring liquid level by OTDR for a structure (3) of a nuclear installation containing a volume of liquid (7), the volume of liquid (7) being delimited by a bottom (9) and by a surface free (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 sheath (29) is arranged wherein the or each optical fiber (15), the sheath (29) being liquid permeable.

2. - A measuring device according to claim 1, characterized in that the sheath (29) is pierced by a plurality of orifices (30).

3. - A measuring device according to claim 1 or 2, characterized in that the measuring device (1) comprises a ballast (33) connected to a section of optical fiber (15) adapted to be arranged near the bottom ( 9).

4. - A measuring device according to claim 3, characterized in that the measuring device (1) comprises a cage (35) adapted to be disposed close to the bottom (9), ballast (33) being free in a direction longitudinally inside the cage (35).

5. Measuring device according to claim 4, characterized in that the measuring device (1) comprises a rigid support (39) intended to be placed above the free surface (13), and at least one elongate member (41) fixing the cage (35) to the rigid support (39) so that the weight of the cage (35) is taken up by the rigid support (39) without passing through the sheath (29).

6. Measuring device according to claim 5, characterized in that the sheath

(29) is arranged to continuously protect the or each optical fiber (15) to an inlet through which the or each optical fiber (15) enters the cage (35).

7. Measuring device according to any one of the preceding claims, characterized in that the or each optical fiber (15) comprises first and second branches (55) parallel to one another and engaged in the sheath ( 29), and a U-section (57) connecting to one another submerged ends (59) of respective first and second legs (55), the U-section (57) being adapted to be located near the bottom (9), the first and second legs (55) having respective ends emerged provided to be located above the free surface (13).

8. - A measuring device according to claim 7 in combination with any one of claim 3 to 6, characterized in that the ballast (33) is a disc provided with a groove (65) in which is engaged in the section U (57) of the optical fiber (15).

9. - A measuring device according to claim 7 or 8, characterized in that the U-section (57) is located outside the sheath (29).

10. 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.

January 1. - 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).

12. - A measuring device according to any one of the preceding claims, characterized in that the analyzer (19) is provided for determining the temperature profile along the or each optical fiber (5) and to deduce therefrom the position the free surface (13) relative to the base (9).

13. - A measuring device according to any one of the preceding claims, characterized in that the sleeve (29) is flexible and is likely to twist when the measuring device is not used.

14. - Structure (3) of a nuclear installation containing a volume of liquid (7), the volume of liquid (7) being delimited by a bottom (9) and a free surface (13), the structure (3) further comprising a device (1) level measurement by reflectometry comprising:
- at least one optical fiber (15) immersed in the liquid through the free surface
(13) ;
- a source (17) sending a light light radiation 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 sheath (29) is arranged wherein the or each optical fiber (15), the sheath (29) being liquid permeable.
15.- A method of level measurement by OTDR for a structure (3) of a nuclear installation containing a liquid volume, using a device according to any one of claims 1 to 12, the method comprising the following steps:
- immersing the or each optical fiber (15) into the liquid through the free surface (13);
- 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).