WO2014001714A1 - Device for detecting the level of a liquid contained in a container - Google Patents

Description

 DEVICE FOR DETECTING THE LEVEL OF A LIQUID CONTAINED IN

 A SPEAKER

TECHNICAL FIELD OF THE INVENTION

The invention relates to a device for detecting the level of a liquid contained in an enclosure. In particular, the invention relates to the level detection of a liquid contained in a pool of spent fuel deactivation of a nuclear reactor. Its main purpose is to detect a variation in the level of the liquid contained in such a chamber.

Bubble bubble sensor in the context of the present application means any sensor for performing the level measurement by insufflation, said level measurement being performed by blowing gas, preferably air, at a rate constant in a tube whose one end opens below the surface of the water.

STATE OF THE PRIOR ART

 Measuring the liquid level in an enclosure of a nuclear reactor is a particularly critical step, the securing must be optimal, because of the nature of the liquids concerned. A change in the level of the liquid, contained in a chamber of a nuclear reactor, must be detected as quickly and accurately as possible in order to be able to take, as efficiently as possible, the necessary measurements in response to a variation in the level. In the case of a spent fuel deactivation pool, it is necessary for the used fuel to be submerged under water at all times.

In the field of the detection of the level of a liquid in a chamber, and in particular in an enclosure of a nuclear reactor, different devices are known to those skilled in the art.

US Pat. No. 7,926,345 describes a device for level measurement in a pressurized chamber of a nuclear reactor using thermocouple level measurement. Such a level measuring device, in addition to its high cost and complexity of implementation due to the number of measurement tubes required, has a major drawback in terms of securing the measurement. Indeed, the thermocouples are arranged along the enclosure, perpendicular to the bottom of the enclosure, by level of control. Measurement sensing in this device is by level by means of a pair of thermocouples: a heated thermocouple and an unheated thermocouple; at each control level is arranged a pair of thermocouples. A difference in temperature between the two thermocouples of the same level of control indicates a decrease in the level of the liquid below this level of control. The failure of at least one thermocouple can shift the leak detection of a control level. This can lead to detrimental delays in taking the leak into account. In addition, in this device, all unheated thermocouples are arranged in one and the same tube. In case of failure of the tube containing all the unheated thermocouples, the entire device no longer works and the level measurement, and therefore the leak detection, is no longer assured.

Patent JP 2010085367 discloses a device for level measurement by bubbling in a storage chamber of a nuclear reactor. Such a device consists in inserting a bubbling cane vertically into the enclosure, which rod is blown at its upper end, air at a constant rate. The rate of insufflation of air in the cane is set to constantly get bubbles out of the lower end of the cane. The air pressure that must be applied to produce bubbles at the outlet of the rod is equal to the pressure of the liquid at the end of the cane and therefore proportional to the height of the liquid in the chamber. Such a device for level measurement by bubbling has the major disadvantage of not presenting optimal security of the measurement. Indeed, the tip of the bubbling cane can be blocked during an accident or as a result of accumulation of particles contained in the chamber and this implies a poor measurement of the level or even an absence of measurement of the level of the liquid in the enclosure. In case of failure of the device due to a plug at the end of the bubbling cane detection of liquid level change will be impossible until the end of the bubbling cane will not open. The operation to remove this plug can take a lot of time, in view of the specificity of the environment of use and the nature of the liquid contained in the enclosure, in the case of use in a chamber of a nuclear reactor, during which time the detection of change of level of the liquid in the enclosure will no longer be possible. SUMMARY OF THE INVENTION

 The invention aims to remedy all or part of the disadvantages of the state of the art identified above, and in particular to provide a level measurement of a liquid in a chamber, particularly in a chamber of a nuclear reactor such as a spent fuel deactivation pool, which is secured and thus ensure detection of liquid level change in this type of enclosure as quickly and efficiently as possible, without interruption.

In this embodiment, one aspect of the invention relates to a device for detecting the level of a liquid contained in an enclosure comprising:

 means for detecting the level of the liquid;

 a temperature sensor comprising an optical fiber adapted to receive a light signal having a broad frequency spectrum, said optical fiber comprising a plurality of Bragg gratings distributed along the length of the optical fiber, each Bragg grating being provided to provide backscattering of a particular wavelength signal, said optical fiber being arranged, along its length, substantially along the liquid level detection means, said sensor comprising means for determining the temperature of the liquid measured by the optical fiber in a plurality of areas from the wavelength of the signals backscattered by each of said Bragg gratings, each zone being located near each of said Bragg gratings; and

 an analysis means for determining the level of the liquid contained in the chamber from the temperature measured by the optical fiber in a plurality of zones.

Such a device has the advantage of providing diversified means of level measurement and ensure a secure measurement of the liquid level. Indeed, the level of the liquid contained in the chamber can be determined in several ways: by the liquid level detection means alone; by the analysis means alone or by the liquid level detection means coupled to the analysis means. Thus, in case of failure of one of the means, it is possible to do to switch the detection of the level of the liquid on the non-faulty means. This switching can take place time to repair the means having suffered a failure, for example. In the case of a determination of the level of the liquid by the analysis means coupled to the liquid level detection means, it is possible, for example, by comparing the two level values obtained to determine the failure of one of the means if the difference between the two values is too great. The fault may be that of the level detection means or the temperature sensor or the analysis means which determines the level from the temperature measured by the sensor. It is the temperature of the liquid measured by the optical fiber in a plurality of zones, even when the fiber is no longer immersed in liquid; the zone may be immersed in the liquid or immersed in the liquid.

 In addition, the presence of a temperature sensor also allows the temperature measurement and to ensure that the temperature of the liquid or the air contained in the enclosure does not exceed a threshold set by the specific conditions. operating the enclosure, particularly in the case of use in a reactor of a nuclear reactor. For example, in a spent fuel deactivation pool, the liquid in the pool must remain at a constant temperature. A change in temperature could lead to an unwanted restart of the spent fuel nuclear reaction and must be detected.

 In addition, such a detection device has the advantage of being installed in the speakers already comprising a liquid level detection means, to improve the security of the level measurement. The installation is done by inserting a fiber optic temperature sensor, a space-saving device, along the liquid level detection means already in place.

According to particular embodiments, used alone or in combination:

 • The means of analysis includes:

means for determining the slope of the temperature at a plurality of points distributed along the optical fiber; means for verifying that the values of said slope belong to an area of desired slope values (threshold);

 means for determining the liquid level when the slope value is outside said zone.

 Said analysis means is periodically triggered so that an alarm is triggered when the difference between two successively calculated level values exceeds a predetermined threshold;

The device comprises a means for detecting the failure of the liquid level detection means, the fault detection means engaging the operation of the analysis means in the event of failure of the liquid level detection means;

The liquid level detection means and the analysis means simultaneously determine the level of the liquid contained in the chamber;

 The liquid level detection means is a bubble-bubble sensor comprising a tube;

 • the optical fiber is fixed along the tube.

The invention also relates to an enclosure comprising the device according to the invention and such that the liquid level detection means is disposed along its length along an axis perpendicular to the bottom of the enclosure. The enclosure may be formed by a spent fuel deactivation pool of a nuclear reactor.

BRIEF DESCRIPTION OF THE FIGURES

 Other features and advantages of the invention will become apparent on reading the description which follows, with reference to the appended figure, which illustrates, by way of indication and in no way limiting:

FIG. 1 is a diagrammatic view of an enclosure and a device for detecting the level of a liquid according to one embodiment of the invention; For clarity, identical or similar elements are identified by identical reference signs throughout the figures.

DETAILED DESCRIPTION OF AN EMBODIMENT

In Figure 1 is illustrated, schematically, an embodiment of a liquid detection device 1 contained in a chamber 2.

In this chamber is disposed a level 3 detection means of the liquid. This detection means is disposed along its length perpendicularly to the bottom 21 of the chamber in order to measure the level of the liquid 1 i.e. to measure the height of liquid 1 contained in the chamber 2 relative to the bottom 21 of the enclosure. This level 3 detection means may be a thermocouple level detection means, bubbling level detection means, level detection means by means of magnetic or non-magnetized floats, or any other level detection means. of a liquid.

In addition, the device comprises a temperature sensor 4 of optical fiber type Bragg gratings. Such a temperature sensor comprises an optical fiber 41 on which is engraved a plurality of Bragg gratings 41 1. The Bragg gratings 41 1 are distributed along the length of the optical fiber. The optical fiber is adapted to receive a light signal with a broad frequency spectrum. The light signal can be transmitted by a source connected to an end 42 of the optical fiber 41. Each of the Bragg gratings 41 1 is provided for backscattering a particular wavelength signal under predefined temperature and mechanical stress conditions in the area 41 2 in which the network 41 1 is made. In case of modification of these conditions, the Bragg grating 41 1 reflects a signal of wavelength modified with respect to the initial wavelength. The ratio between the initial wavelength and the reflected wavelength is a function of the temperature and makes it possible to determine the temperature variation, and therefore the temperature, of the zone 41 2 surrounding the Bragg grating 41 1 corresponding to the studied wavelength. By associating with each Bragg grating 41 1 etched along the optical fiber a specific wavelength, it is possible to determine the temperature in a plurality of zones 41 2. The temperature is determined by means of the sensor 4, these means recover the wavelengths of the signals reflected by each said Bragg gratings and analyze them to determine the temperature of the liquid 1 in a plurality of zones 41 2.

 The optical fiber 4 is arranged along its length along the level 3 detection means, the optical fiber 4 can be fixed along the level 3 detection means. This means being arranged, as illustrated in FIG. FIG. 1, perpendicular to the bottom 21 of the enclosure, it is the same for the optical fiber 41. The Bragg gratings 41 1 being distributed along the length of the optical fiber 41, the temperature is determined in several zones 41 2 distributed along an axis perpendicular to the bottom 21 of the enclosure. It is thus possible to control the temperature of the liquid 1.

 The level detection device comprises an analysis means for determining the level of the liquid 1 contained in the chamber 2 from the temperature of the liquid 1 measured by the optical fiber 41 in a plurality of zones 41 2. The analysis means recovers the temperature determined in each of the zones 41 2 by the means of the sensor 4. The temperature sensor 4 is disposed in the chamber 2 which comprises liquid 1 and a medium 5 other than liquid. If a zone 41 2 is no longer immersed in liquid 1, the slope of the temperature around this zone 41 2 changes. This change in slope, significant of a medium change (liquid 1 - medium 5 other than liquid) is detected by the analysis means which deduces the level of the liquid 2 contained in the chamber 1. By way of example, the response time of the detection of the liquid level is 0.1 seconds for an optical fiber on which twenty Bragg gratings are etched. A calculation means could be connected to the analysis means in order to follow the evolution of the level of the liquid 2 contained in the chamber 1 over time.

The analysis means operates as follows for the calculation of the slopes between each of the successive temperature values. The calculating means performs a measurement of the temperature along the optical fiber, in the liquid or outside the liquid, in the latter case, in an area which is no longer immersed in liquid. A sequence {Pn} of temperature measurement points (ie acquisition points) along the fiber is thus performed (see Figure 2). An observation window centered on the acquisition points is defined; such a window is illustrated in FIG. 3: this window is of fixed size and defines a desired slope and a tolerance characterized by a dead band at the second order. The dead band and the desired slope thus define a zone of threshold slopes.

The detection method first consists in centering the detection window as illustrated in FIG. 3 on an acquired point P _k of the sequence {P _n } (an example of a series of points {P _n } is illustrated in FIG. 4). It should be noted that the sequence {P _n } does not necessarily have a fixed acquisition period. The detection method then consists in finding the point of intersection P | between the segment [P _k- i P _k ] and the observation window and the point of intersection P _r between the segment [P _k P _{k +} i] (following the segment [P _k- i P _k ]) and the observation window. The slope is given by the segment [Pi P _r ]. As illustrated in FIG. 5, the slope is here located in the threshold zone of FIG. 3 and is therefore in accordance with the value sought at the tolerance of the near dead band.

The observation window then propagates to the next point (figure 6) while remaining centered from point to point. The window is found here in a position that is close to the maximum slope but the slope is still in the threshold tolerance zone.

At the next point (figure 7), the slope [Pi P _r ] comes out of the criteria and is no longer in the threshold zone; the point of change of slope is then considered as the point Pk-i. Depending on the "shape" of the signal, it may be necessary to keep not P _k- i but Pi as the point of change of slope. A calibration curve is then used which gives the level as a function of the detected slope change.

On the basis of a periodic acquisition of the sequence of points, for each acquisition, a level is found. If the difference between two successive level values exceeds a predetermined threshold, then the level can be considered to have changed. An alarm can be triggered when the level change is detected.

Such level detection device offers the advantage of providing two measurements of the level of the liquid 1 and thus having access to a secure measurement of the level. It is possible to have an analysis means which compares the two level values provided and in case of inconsistency between the two values to detect a failure of the detection means 3 or the temperature sensor 4 or the analysis means which determines the level from the temperature measured by the temperature sensor 4.

 Advantageously, such a level detection device can be set up to secure the liquid level measurement in the speakers already comprising a single level 3 detection means. Indeed, the fiber temperature sensor 4 Optical is a compact device. The level 3 detection device can be implemented to secure the existing level measurements by level 3 detection means by arranging along the means 3 a temperature sensor 4 comprising a Bragg grating optical fiber.

 Determining the level of the liquid by the analysis means can be activated only in the event of failure of the level 3 detection means. The level detection device can comprise a means for detecting the failure of the detection means. if the means of detecting the failure indicates that the level measurement is no longer supplied or is no longer reliable, the analysis means is engaged in order to deduce the level of the liquid 1 from the temperature in each case. zones 412. The operation of the analysis means can be switched on by means of a switch. Thus the level measurement of the liquid in the enclosure is secured due to the possible diversification of the level measurement.

 In Figure 1 the level 3 detection means of the illustrated liquid is a bubble bubble sensor 3 comprising a tube 31, the tube 31 being disposed along its length along an axis perpendicular to the bottom 21 of the enclosure.

 This level detection device of a liquid can be installed in a spent fuel deactivation pool of a nuclear reactor. In this type of enclosure, the used fuel must be immersed permanently under a predefined quantity of water. Monitoring of the water level in a spent fuel deactivation pool requires a secure liquid level detection device as previously described.

Claims

1. Device for detecting the level of a liquid (1) contained in an enclosure (2) comprising:  liquid level detecting means (3);  characterized in that the device further comprises:  a temperature sensor (4) comprising an optical fiber (41) adapted to receive a light signal having a broad frequency spectrum, said optical fiber (41) comprising a plurality of Bragg gratings (41 1) distributed along the length of the optical fiber (41), each Bragg grating (41 1) being provided for backscattering of a particular wavelength signal, said optical fiber (41) being arranged, along its length, substantially along the liquid level detection means (3), said sensor (4) having means for determining the temperature of the liquid measured by the optical fiber (41) in a plurality of zones (412) to from the wavelength of the signals backscattered by each of said Bragg gratings (41 1), each zone (412) being located near each of said Bragg gratings (41 1); and  an analysis means for determining the level of the liquid contained in the chamber from the temperature measured by the optical fiber in a plurality of zones (412). 2. Device according to claim 1 characterized in that said analysis means comprises: means for determining the slope of the temperature at a plurality of points distributed along the optical fiber; means for verifying that the values of said slope belong to an area of desired slope values; means for determining the liquid level when the slope value is outside said zone. 3. Device according to the preceding claim characterized in that said analysis means is periodically triggered so that an alarm is triggered when the difference between two successively calculated level values exceeds a predetermined threshold. 4. Device according to any one of the preceding claims characterized in that it comprises means for detecting the failure of the liquid level detection means (3), the fault detection means engaging the operation of the means for detecting the analysis in case of failure of the liquid level detection means. 5. Device according to any one of claims 1 to 3 characterized in that the liquid level detection means (3) and the analysis means simultaneously determine the level of the liquid contained in the chamber (2). 6. Device according to any one of the preceding claims, characterized in that the liquid level detection means is a bubble bubble sensor (3) comprising a tube (31). 7. Device according to the preceding claim characterized in that the optical fiber (41) is fixed along the tube (31). 8. Enclosure (2) characterized in that it comprises the device according to any one of claims 1 to 7 and in that the means for detecting the level of the liquid (3) is disposed along its length along an axis perpendicular to the bottom (21) of the enclosure. 9. Enclosure according to the preceding claim formed by a pool of spent fuel deactivation of a nuclear reactor.