METHOD AND SYSTEM FOR MEASURING THE POSITION OF A MOVING ELEMENT IN TRANSLATION IN A NUCLEAR REACTOR

Abstract

METHOD FOR MEASURING THE POSITION OF A MOVING ELEMENT OF A NUCLEAR REACTOR The invention relates to a method (300) for measuring the position of a moving element in translation (111) of a nuclear reactor, comprising the following steps: Emission (301) of kN optical signals (210), k and N being natural integers greater than or equal to 1, by kN optical fibers called emitting fibers (215) included in N probes resistant to a primary medium; Reception (302) of the kN optical signals by N tracks (22) resistant to a primary medium, each track receiving k optical signals, said tracks being made up of reflective surfaces (222) and diffusing surfaces (221); Reception (303) by mN optical fibers called receiving fibers (214), m being a natural number greater than or equal to k, included in the N probes (21) of the kN optical signals reflected (2221) or scattered (2211) by the N tracks (22);Conversion (304) of the kN optical signals received by the mN receiving optical fibers (214) into binary code. Figure to be published with the abbreviation: Figure 7;

Description

Title of the invention: METHOD AND SYSTEM FOR MEASURING THE POSITION OF A MOVING ELEMENT IN TRANSLATION OF A NUCLEAR REACTOR. TECHNICAL FIELD OF THE INVENTION

[0001] The technical field of the invention is that of safety-classified measurements in a nuclear reactor, particularly but not limited to a pressurized water reactor.

[0002] The present invention relates to position measurement systems and, in particular, to the position measurement of a moving element in translation within a nuclear reactor. A typical case is the position measurement of the absorbers used to control the nuclear reaction. TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0003] Several measurement methods allow the position of moving parts in a nuclear reactor to be determined, such as the moving control rods within a reactor vessel, for example. These measurements are considered classified from a safety perspective because they fall under the protection of an installation capable of generating environmental pollution, causing significant inconvenience or impact on the surrounding area and the public, endangering the safety of the public, the surrounding area, or the personnel of the facilities, or harming the health and safety of employees at work. Given their characteristics, nuclear installations are classified installations.

[0004] Among the methods for measuring the positions of mobile control rods, we find in particular inductive measurement technologies and Reed relay measurements.

[0005] The principle of position measurement by induction consists of passing a magnetic or ferromagnetic target attached to the moving part in front of coils, the number of coils being equal to the number of measurement steps. When a control rod is moving relative to at least one detection coil, an output signal is induced. The output signal comprises identifiable characteristics representative of one or more positions of the control rod inside the detection coil and is then processed by a unit to deduce these positions. This measurement method has several drawbacks: coils resistant to the temperature and pressure conditions encountered in the reactor are difficult to manufacture, and this measurement principle requires a lot of wiring to be run out of the reactor vessel. Finally, magnetic and ferromagnetic targets lose effectiveness. when subjected to the temperature conditions of a nuclear reactor vessel.

[0006] The principle of measurement using Reed relays consists of placing a series of Reed switches spaced along the length of the control rod's stroke and connected to provide output signals. When the control rod moves, a permanent magnet mounted on the rod applies an external magnetic field to the Reed switch assemblies, which activate or short-circuit resistors depending on the magnet's position. This method reduces the number of wires used compared to the previous method, but has several drawbacks, such as the limited effectiveness of the permanent magnets and Reed relays under the temperature conditions of a primary environment. Indeed, for Reed relays, operation is guaranteed at the rated voltage up to an ambient temperature of 65°C to 200°C, depending on the model, whereas in a primary environment, the temperature varies between 290°C and 350°C.Furthermore, the lifespan of reed relays is significantly reduced in high-temperature environments. Finally, the FMEA (Failure Mode and Effects Analysis) tool for operational safety is not well-suited to fault detection using this measurement principle. Other technologies for measuring the position of a control rod have been studied, such as acoustic position sensors, but these have not been developed on an industrial scale due to the difficulties of qualification in primary environments.

[0007] Fiber optic position sensors are now available and are being developed for use in various environments, particularly industrial settings. A method for measuring the position of a moving element using fiber optic sensors involves placing one or more transmitting fibers and one or more receiving fibers on the same part. The transmitting fiber(s) send optical signals to a moving part, which contains reflective and diffusing or absorbing surfaces. The received signals, whether reflected or diffused / absorbed, are binary-coded: a reflected signal is coded as "1" and a diffused signal as "0". Fiber optic position sensors known in the prior art allow for the measurement of absolute or relative positions of a part in translation.

[0008] Fiber optic sensors offer some interesting characteristics: due to the small size of the fibers, they can be integrated into narrow and hard-to-reach places. Furthermore, fiber optic technologies adapted to radiation have been developed, theoretically allowing their applications to be expanded in a radiative environment without blackening. However, despite their advantages, fiber optic technologies are not suitable for the corrosion and pressure conditions inside a nuclear reactor vessel. Therefore, there is initially a need for The goal is to precisely determine the position of a moving rod in a control assembly using a simpler, more compact system, suitable for the environment of a nuclear reactor vessel and reliable for fault detection. This requires adapting position sensors commonly used in industry, such as fiber optic sensors, to the nuclear sector. Summary of the invention

[0009] In this context, the present invention aims to provide an alternative to previous methods, making it possible to overcome the aforementioned problems by adapting to the environment of a primary circuit in a nuclear reactor.

[0010] To this end, a first aspect of the invention relates to a method for measuring the position of a moving element in translation of a nuclear reactor, characterized in that it comprises the following steps: • Emission of kN optical signals, k and N being natural integers greater than or equal to 1, by kN optical fibers called emitting fibers included in N probes resistant to a primary medium; • Reception of kN optical signals by N tracks resistant to a primary medium, each track receiving k optical signals, said tracks being made up of reflective surfaces and diffusing surfaces; • Reception by mN optical fibers called receiving fibers, m being a natural number greater than or equal to k, included in the N probes of the kN optical signals reflected or scattered by the N tracks; • Conversion of kN optical signals received by mN receiving optical fibers into binary code.

[0011] The invention implements a fiber optic position sensor for measuring, for example, the absolute position of a moving element in the reactor. This sensor is simpler to implement compared to the other methods mentioned: for example, there are no thin winding wires to manufacture. Furthermore, unlike a conventional fiber optic measurement method used in industrial settings, for example, the invention implements a system comprising fiber optic probes and tracks adapted to the conditions and stresses of a primary environment in a nuclear reactor vessel. In the primary circuit of a nuclear reactor, the primary environment is defined as an environment whose heat transfer fluid is subjected to the following parameters: • High-temperature pressurized water environment, particularly corrosive • Pressure between 120 bar and 180 bar • Temperature between 250°C and 350°C • Water flow rate between 2m / s and 5m / s • Irradiation (gamma and neutrons) between 2 mgy and 100 mgy

[0012] Thus, the optical fibers are contained in protective probes resistant to a primary medium and are not directly exposed to water at high temperature and pressure.

[0013] According to one embodiment, the method described above makes it possible to obtain a position measurement of the moving object in translation with a measurement accuracy that depends on N and is equal to the length of the tracks divided by 2N. This embodiment allows the desired accuracy to be chosen by selecting the number N.

[0014] In another embodiment of the method, the moving element in translation drives a rotating cylindrical component, said rotating cylindrical component being surrounded by an auxiliary track made up of P reflective surfaces and P diffusing surfaces, P being greater than or equal to 1, said auxiliary track being resistant to a primary medium, the method being characterized in that it comprises the following additional steps enabling the measurement of a relative position of the moving element in translation: • Emission of two optical signals, by two optical fibers called transmitting fibers included in two auxiliary probes resistant to a primary medium, the auxiliary probes being spaced at a non-zero angle; • Reception of the two optical signals by the track; • Reception by at least two optical fibers called receiving fibers included in the two auxiliary probes of the two optical signals reflected or scattered by the auxiliary track; • Transmission and interpretation of the two optical signals received by the receiving optical fibers by a processing unit;

[0015] The invention advantageously implements a second relative position measurement of a moving element in translation, in addition to the first one mentioned above, thus allowing for two position measurements and improving measurement accuracy, with devices resistant to a primary medium. According to one embodiment, the relative position measurement accuracy of a moving element in translation in a nuclear reactor depends on the total number of reflective and diffusing surfaces and is equal to 2P.

[0016] In addition to the characteristics mentioned in the preceding paragraph, a characteristic of the measurement method may be, for one probe among the N probes and the two auxiliary probes, comprising a transmitting optical fiber and at least one receiving optical fiber: - The reflected or scattered nature of an optical signal received by the at least one receiving optical fiber is determined by the intensity of the optical signal received by the at least one receiving optical fiber according to the following steps: • When the intensity of the received optical signal is greater than or equal to a high threshold, the optical signal received by at least one receiving optical fiber of said probe is a so-called reflected optical signal; • When the intensity of the received optical signal is greater than or equal to a lower threshold and is strictly less than the upper threshold, the lower threshold being strictly less than the upper threshold, the optical signal received by the at least receiving optical fiber of the probe is a so-called diffuse optical signal; and the difference between the intensity of the reflected optical signal and the intensity of the scattered optical signal is equal to 13dB, and that the upper threshold is such that the difference between the intensity of the reflected signal and the upper threshold is between 0 dB and 13 dB.

[0017] The invention makes it easy to detect an optical signal of a reflected or scattered nature thanks to the upper threshold and lower threshold defined above.

[0018] Furthermore, among the N probes and the two auxiliary probes, comprising a transmitting fiber and a receiving fiber: • When the intensity of the optical signal received by the receiving optical fiber is zero or positive and below the lower threshold, the optical signal received by at least the optical fiber is said to be defective; The difference between the intensity of the reflected optical signal and the lower threshold is equal to 28 dB and the difference between the upper threshold and the lower threshold is greater than or equal to 15 dB.

[0019] The invention advantageously presents a three-level coding instead of a binary coding by allowing the detection of defective signals in addition to reflected or diffused signals, in order to detect possible anomalies or failures and to guarantee a required level of security.

[0020] Another aspect of the invention relates to a fiber optic probe for measuring the position of a moving element, enabling the position of a moving element to be measured in a nuclear reactor according to the method described above, comprising: - a transmitting optical fiber and a receiving optical fiber; - a protective casing for the optical fibers comprising: • at least one flexible, sealed section comprising a metal resistant to a primary medium; This flexible section ensures, on the one hand, the flexibility to allow the probe to be guided through the internal layout of the reactor, and to accommodate the difference in expansion between the fiber and the metal sheath, • at least one rigid, watertight part comprising a metal or ceramic resistant to a primary environment; • a transparent, watertight window resistant to a primary environment, brazed at the end to the rigid metal part

[0021] The casing comprises materials resistant to a primary medium, corrosion, and oxidation, and can therefore be immersed in a primary circuit without damaging the optical fibers. The transparent window allows optical signals to pass through.

[0022] According to one embodiment, a characteristic of the optical fiber probe is: - a first end of the rigid part is welded to a first end of the flexible part; - a second end of the rigid part is welded to the transparent window.

[0023] This advantageous embodiment makes it possible to guarantee the sealing of the ends of the rigid part in the primary medium.

[0024] According to one embodiment, the housing of the fiber optic probe includes a mirror inclined at 45° to the axis of the rigid part of the probe. Indeed, it is possible that the fibers used within the probe may have a small angle of curvature and that their radial edges may not be positioned opposite the reflective or diffusing tracks; the optical signals can then be reflected by a mirror, thus allowing the fibers to be positioned parallel to the movement of the moving element and reducing the overall size.

[0025] According to an advantageous embodiment, the optical fibers included in the probe are made of materials resistant to radiation at certain wavelengths, for example between 800 nm and 1000 nm for multimode optical fibers and between 1000 and 1150 nm for single-mode optical fibers. Thus, the blackening of the optical fibers is greatly reduced at these wavelengths under the effect of radiation in a nuclear reactor.

[0026] Another aspect of the invention relates to a position measurement system for a movable control rod of a nuclear reactor, comprising: • N fiber optic probes; • N tracks made up of diffusing surfaces and reflective surfaces resistant to a primary medium; implementing the process according to the invention

[0027] The system makes it possible to measure the position of any moving element, generally and more specifically within a nuclear reactor vessel. The tracks can be made of metals and a coating resistant to a primary environment, which prevents them from blackening.

[0028] According to a first embodiment of the system mentioned above, a first track among the N tracks comprises a reflective surface and a surface diffusing and any additional track includes twice the number of reflective and diffusing tracks compared to the previous track

[0029] Each additional track improves the position measurement accuracy.

[0030] According to one embodiment, the system mentioned above comprises a mechanical system including: • A wheel cooperating with the moving tracks and attached to an articulated arm; • A spring cooperating on one side with the articulated arm and on the other side with a fixed support; and characterized in that one probe among the N probes is fixed to the articulated arm.

[0031] Optical signals are sensitive to the distance traveled because they traverse several interfaces with different refractive indices, and these indices vary with temperature. Therefore, the distance between a transparent window and the tracks must be as small as possible and, above all, constant. The aforementioned mechanical system makes it possible to maintain this constant distance in the event of vibrations and mechanical play within a reactor.

[0032] According to one embodiment, the measuring system comprises: • An auxiliary track consisting of P reflective surfaces and P diffusing surfaces; • Two fiber optic probes spaced at a non-zero angle.

[0033] The system advantageously includes components enabling a relative position measurement to be performed.

[0034] Another aspect of the invention relates to a nuclear reactor comprising a position measurement system as previously mentioned. BRIEF DESCRIPTION OF THE FIGURES

[0035] Other features and advantages of the invention will become clear from the description given below, by way of example and not limitation, with reference to the accompanying figures, among which:

[0036] The [Fig. 1] is a simplified diagram of a pressurized water nuclear reactor vessel;

[0037] Fig. 2 represents a movable rod in its fixed sheath in which optical fiber probes and target tracks are placed;

[0038] Figure 3 shows in detail an optical fiber probe;

[0039] Fig. 4 is a cross-section of the optical fiber probe;

[0040] Fig. 5 represents a set of tracks made up of diffusing and reflective surfaces;

[0041] Figure 6 shows in detail a pair of surfaces consisting of a reflective surface and a diffusing surface;

[0042] Fig. 7 represents a first embodiment of a system for measuring the absolute position of a control rod in translation;

[0043] Fig. 8 represents a second embodiment of the absolute position measurement system of a control rod in translation;

[0044] Figure 9 represents an embodiment of a system for measuring the relative position of a control rod in translation.

[0045] The [Fig. 10] is a synoptic diagram, representing a method for measuring the position of a moving element in translation of a nuclear reactor;

[0046] The [Fig. 11] is a graphical representation of the intensity of an optical signal received by an optical fiber probe as a function of the position of a moving element in translation;

[0047] Figure 12 is a block diagram representing a method for measuring the relative position of a moving element in translation within a nuclear reactor. DETAILED DESCRIPTION

[0048] Unless otherwise specified, the same element appearing on different figures has a unique reference.

[0049] Figure 1 is a diagram of a pressurized water reactor (PWR) vessel 1, showing an assembly 11 consisting of a control rod in translation 111, whose position is to be measured, in its fixed part 112, an element of the control rod guides 13 and an element of the core 14. The water circulating in the vessel is subjected to the pressure, temperature and flow rate conditions of the primary circuit: High-temperature pressurized water environment, particularly corrosive Pressure between 120 bar and 180 bar Temperature between 250°C and 350°C Water flow rate between 2m / s and 5m / s Irradiation (gamma and neutrons) between 2 mgy and 100 mgy

[0050] The environment of the primary circuit will be referred to hereafter as the primary medium. Thus, any element introduced into the tank must be resistant to the aforementioned primary medium.

[0051] Figure 2 shows in more detail a translational control rod 111 whose position is to be measured, in its fixed part 112, referred to as the sleeve, located in the tank 1. When set in motion, the translational control rod 111 moves relative to the sleeve 112 along the z-axis, along which it extends. This representation shows schematic fiber optic probes 21 attached to the sleeve 112, emitting optical signals 210 towards target tracks 22 fixed to the translational control rod 111.

[0052] Figure 3 shows a detailed diagram of a fiber optic probe 21. The probe 21 comprises a protective housing 220 consisting of a flexible portion 211, a rigid portion 212 with axis A, and a transparent window 213. The flexible portion may, for example, be a metal tube, resistant to a primary medium, of sufficient length to allow it to exit the primary medium and reach a less aggressive medium via a tank penetration similar to the tank penetration described in patent EP 33 17 883. The flexible portion 211 is joined by welds, preferably brazes 216, to the rigid portion 212, which may also be a tube made of one or more metals resistant to a primary medium. A metal constituting the rigid and / or flexible portions may be 316L stainless steel, which is a corrosion-resistant stainless steel compatible with water in the primary medium.The inner wall of the rigid part 212 is brazed to the copper, gold or aluminum sheaths of the optical fibers at the surface 218 to prevent the optical fibers from slipping within the rigid part; This brazing is made possible by a metallic coating of the bare fibers and may require, depending on the geometry chosen for the fibers and the rigid tube, metallic adapter pieces.

[0053] The transparent window 213, which allows the transmission of optical signals, is connected to the rigid part 212 by means of welds, preferably brazes 217. The transparent window 213 is watertight and resistant to a primary medium, oxidation, and corrosion. The window may be a porthole made of glass, for example sapphire, chosen so that the coefficients of thermal expansion of the transparent window 213 and the rigid part 212 are matched in order to prevent breakage of the brazes 217 during temperature transitions in the primary medium. The transparent window 213, the rigid part 212, and the flexible part 211 are substantially of the same diameter, typically between 2 and 4 mm.

[0054] In this example, the brazing 216 from one end of the rigid part 212 to one end of the flexible part 211 and the brazing 217 from one end of the rigid part to the transparent window 213 ensure the sealing of the two ends of the rigid part 212.

[0055] The casing 220 of the probe 21 comprises a plurality of optical fibers, which it protects from aggressions of the primary environment, including a transmitting optical fiber 215 and several receiving optical fibers 214. More generally, the casing 220 may comprise one or more receiving optical fibers 215.

[0056] Figure 4 is a cross-section of the probe showing: a transmitting optical fiber 215 and several multimode receiving optical fibers 214 surrounding it to optimize the collection of received optical signals. The optical signals propagating within the fibers can have wavelengths between 800 and 1200 nm for the multimode fibers and between 1000 and 1200 nm for single-mode fibers. Optical fibers can, for example, be designed with a copper cladding and an ultra-pure silica core with a low OH (hydroxyl group) content to prevent them from blackening under the effect of nuclear radiation at the wavelengths mentioned above.

[0057] Each probe 21 emits optical signals 210 towards a target track 22. Figure 5 shows four tracks 22, each preferably spaced 3 mm apart. The tracks consist of alternating reflective 222 and diffusing 221 or absorbing surfaces. More generally, a number of tracks N can be chosen to measure 2N different positions of the control rod 111 in translation, the measurement accuracy being equal to the stroke of a control rod 111 divided by 2N. In the case of Figure 4, 24 = 16 different positions of the control rod 111 can be measured in translation. Since the stroke of a control rod 111 is typically between 2000 mm and 4000 mm, the measurement accuracy is then equal to: 1 / 2N of the total displacement distance of the moving part. The length of the tracks 22 is equal to the linear stroke of the control rod 111, and the total width 225 of the 4 tracks is preferably 10 mm.

[0058] The tracks 22 are made of materials resistant to the conditions of a primary environment, to oxidation and corrosion, so that their blackening is minimal. Figure 6 shows in detail a portion of a track comprising a reflective surface 222 and a diffusing surface 221. A metal part 223 resistant to the primary environment, made of materials such as 316L stainless steel, is used for the design of the reflective surface 222 and the diffusing surface 221. To obtain a reflective surface 222, the metal part 223 is coated with a metal that is not very susceptible or not susceptible to oxidation, such as hard chrome or gold, for example. The diffusing surface 221 results from machining a rough surface on the metal part 223.

[0059] A characteristic of a rough surface is the parameter Ra, which measures the distance between the arithmetic mean of the absolute values ​​of the deviations between the peaks and troughs and the center line. Ra is preferably 3.6 for diffusing surfaces 221 and 0.2 for reflective surfaces. Furthermore, the depth of the diffusing surfaces 221 relative to the reflective surfaces 222 can be between 2 mm and 5 mm.

[0060] Figure 7 shows a first embodiment of the system for measuring the position of a control rod 111 in translation. The measuring system 2 comprises a track 22, a corresponding fiber optic probe 21, and a tracking system 23 consisting of an articulated arm 231, a roller 232, and a spring 233. The flexible part 211 of the probe is partially attached to the sleeve 112, and the rigid part 212 is attached to the articulated arm 231. The articulated arm 231 is attached to a roller. 232 which is in contact with the track 22 fixed to the translational control rod 111. The articulated arm 231 is also integral with a spring 233, fixed to the sleeve 112.

[0061] In this embodiment, the rigid part 212 is perpendicular to the translating control rod 111. The optical signals emitted by a transmitting optical fiber included in the optical fiber probe 21 encounter only one surface within the probe: the transparent window 213. The optical signals 210 emitted by the probe 21 then pass through the primary medium, the refractive index of which is temperature-sensitive and can degrade the intensity of said optical signals 210. The distance d traveled by the optical signals 210 must therefore be constant and as small as possible while still allowing sufficient clearance for the proper translation of the moving part. Typically, this distance is on the order of a few millimeters, for example between 0.5 and 5 mm, in order to avoid loss of intensity of the optical signals 210.However, the operation of the CRDM (Control Rod Drive Mechanism) causes vibrations and mechanical play during the movement of the control rods 111, which can vary the distance d traveled by the optical signals. Therefore, the tracking system 23 was implemented to maintain this distance d constant. The tracking system 23 is mechanically controlled by means of the spring 233, which extends or compresses when the wheel 232 is driven by a moving control rod 11 in a direction other than the z-axis along which the control rod 111 extends. The spring causes the articulated arm 231 to rotate around the x-axis, allowing the wheel to remain in contact with a track 22.

[0062] Figure 8 shows a second embodiment of the invention. In this embodiment, the probe 21 is parallel to the translational control rod 111, and its housing 220 comprises a mirror 219 positioned at 45° to the axis A of the rigid part 212 of the probe 21. The optical signals emitted by the transmitting optical fiber 215 of the probe 21 are first reflected by the mirror 219 before being transmitted through the transparent window 213. A mirror is defined as any surface capable of reflecting optical signals. This embodiment is preferred when a small footprint is required, or when the maximum radius of curvature of the flexible part 211 is, for example, between 30 and 50 mm and does not allow for the embodiment shown in Figure 7.

[0063] Figure 9 represents a variant of the invention, in which, in addition to one of the two preceding embodiments, a second relative measurement subsystem 2b, complementary and differentiated from the former, makes it possible to measure the relative position of the moving rod 111 by measuring the number of revolutions made by a cylindrical component 15 driven by the moving control rod 111. thanks to a screw-nut mechanism 16, and more specifically a wheel 161 and a worm gear 162 with a pitch p, expressed in mm. The mechanism 16 transforms the translational movement of the control rod 111 into a rotational movement of the cylindrical component 15 with a ratio p expressed in mm / revolutions. This mechanism 16 is detailed in patent EP 3 329 493 Bl. The relative measurement system 2b comprises an auxiliary track 22b surrounding the cylindrical component 15, said auxiliary track 22b being made up of P reflective surfaces 222b and P diffusing surfaces 221b, P being an integer greater than or equal to 1. The system 2b also comprises two auxiliary probes 21b identical to those described above, located opposite the auxiliary track 22b, spaced at a non-zero angle α less than 360° / P.

[0064] Figure 10 is a block diagram illustrating the sequence of steps in the measurement method 300 according to the invention. The method is carried out using the optical fiber measurement system 2 described in Figures 7 and 8 and makes it possible to obtain the absolute position of a moving element in the vessel of a nuclear reactor, for example a movable control rod 111.

[0065] A first step of the process consists of the transmission 301 of one or more optical signals respectively by a transmitting fiber 215 from one or more probes 21, the wavelengths of the signals being between 800 and 1200 nm. The signals are received in a second step 302 by one or more tracks 22, each probe being opposite a track 22 so that each optical signal is received by only one track. If an optical signal is received by a reflective track 222 and is reflected back by said reflective track, then it is said to be reflected, and if an optical signal is received by a diffusing track and is diffused by said diffusing track 221, then it is said to be diffused. The so-called reflected signals 2221 or diffuse signals 2211 are then received by the receiving multimode fibers 214 in a third step 303.

[0066] The optical signals received by the receiving optical fibers 214 are then processed and converted into a unit (not shown) in a fourth step 304. When a received signal is a reflected signal 2221, its corresponding code will be 1, and when a received signal is a scattered signal 2211, its corresponding code will be 0; this type of coding is called binary coding. For a probe, the reflected or scattered nature of an optical signal received by the receiving fibers 214 is determined by the intensity value of the optical signal received by the receiving fibers 214.

[0067] Figure 11 is a graphical representation of the intensity of an optical signal received by a receiving fiber as a function of the position of the moving element 111 in translation. When the intensity of the received optical signal is greater than or equal to a threshold SH, also called the upper threshold, then the signal is said to be reflected and has an intensity SR. When the intensity of the received optical signal is greater than or equal to a threshold SB, also called the lower threshold, and strictly less than SH, then the received optical signal The signal is said to be diffuse and has an intensity SD. When the intensity I is strictly less than the threshold SB, the signal is considered defective and has an intensity SI. The intensity difference between the intensity SR of a reflected signal 2221 and the intensity SD of a diffused signal 2211 is preferably on the order of 13 dB, and the intensity difference between a reflected signal 2221 and a defective signal is preferably on the order of 28 dB. Thus, these values ​​allow us to determine the preferred minimum difference between the upper threshold SH and the lower threshold SB: since the SR-SH difference is less than or equal to 13 dB, and the SR-SB difference is equal to 28 dB, we deduce that the SH-SB difference is greater than or equal to 15 dB.This type of coding is called three-level coding and allows for the detection of faults and anomalies: indeed, limiting oneself to two levels of coding (reflected / scattered) would be dangerous because a received signal could be mistakenly considered as scattered, and any anomaly would not be detected in time. A defective signal can result from a misalignment between the optical fiber probes 21 and the tracks 22, for example, or from a malfunction of the probes 21, for example.

[0068] When the measuring system 2 includes N optical fiber probes 21 and N tracks 22, the position of the moving control rod 111 will be given in the form of an N-digit binary code, each digit included respectively in the set {0,1}.

[0069] The [Fig. 12] is a synoptic diagram illustrating the sequence of additional steps of the method 300 allowing to obtain a relative position measurement of a moving element in translation, for example the moving control rod in translation 111. The method is carried out using the optical fiber measurement system 2b mentioned previously.

[0070] A first step, among the additional steps of the process, consists of the transmission 301b of two optical signals respectively by two transmitting fibers 215 of two auxiliary probes 21b, spaced at an angle α, the wavelengths of the signals being between 800 and 1200 nm. The signals are received in a second step 302b by an auxiliary track 22b. If an optical signal is received by a reflective track 222b and is reflected back by said reflective track, then it is said to be reflected, and if an optical signal is received by a diffusing track 221b and is diffused by said diffusing track 221, then it is said to be diffused. The so-called reflected signals 2221 or diffused signals 2211 are then received by the receiving fibers 214 in a third step 303b.

[0071] The optical signals received by the receiving optical fibers 214 are then processed by a unit. The offset of an angle α between the two auxiliary probes 21b allows the processing unit to determine the number of times the cylindrical component 15 has rotated through an angle α and to deduce the displacement of the moving element 111 using the ratio p expressed in mm / revolution. The phase shift between the Two optical signals received by the receiving fibers 214 allow the direction of rotation of the cylindrical component 16 to be determined, and therefore the direction of movement of the moving element 111 in translation. The relative measurement of a moving element 111 in translation requires initial calibration. The measurement accuracy of this relative measurement depends on the number P of reflective surfaces 222b and the number P of diffusing surfaces 221b, and the pitch p of the worm gear 162, and is equal to p / P. The pitch p is preferably 20 mm and the number P is preferably between 5 and 10; therefore, the position measurement accuracy of a moving element 111 in translation is preferably between 2 and 4 mm.

Claims

Claims

1. Method (300) for measuring the position of a translationally movable element (111) of a nuclear reactor, characterized in that it comprises the following steps: - Emission (301) of kN optical signals (210), k and N being integers greater than or equal to 1, by kN so-called emitting optical fibers (215) included in N probes resistant to a primary medium; - Reception (302) of the kN optical signals by N tracks (22) resistant to a primary medium, each track receiving k optical signals, said tracks consisting of reflecting surfaces (222) and diffusing surfaces (221), the tracks being fixed to the movable element (111); - Reception (303) by mN optical fibers called receivers (214), m being a natural integer greater than or equal to k, included in the N probes (21) of the kN optical signals reflected (2221) or diffused (2211) by the N tracks (22);- Conversion (304) of the kN optical signals received by the mN receiving optical fibers (214) into binary code.;

2. Method (300) according to claim 1 characterized in that the precision of position measurement of a translationally mobile element (11) in the nuclear reactor depends on N and is equal to the length of the tracks divided by 2N.

3. Method (300) according to claim 1 characterized in that the translationally movable element (111) drives a cylindrical component (15) in rotation, said rotating cylindrical component (15) being surrounded by an auxiliary track (22b) consisting of P reflecting surfaces (222b) and P diffusing surfaces (221b), P being greater than or equal to 1, said auxiliary track (22b) being resistant to a primary medium and receiving optical signals emitted by two auxiliary probes (21b) resistant to a primary medium and spaced apart by a non-zero angle (a), the method being characterized in that it comprises the following additional steps making it possible to measure a relative position of the translationally movable element (111): - Emission (301b) of two optical signals, by two so-called emitting optical fibers (215) included in the two auxiliary probes (21b), - Reception (302b) of the two optical signals by the track; - Reception (303b) by at least two so-called receiving optical fibers (214) included in the two auxiliary probes (21) of the two optical signals reflected (2221) or diffused (2211) by the auxiliary track (22b); - Transmission and interpretation (304b) of the two optical signals received by the receiving optical fibers (214) by a processing unit;

4. Method (300b) according to claim 3 characterized in that the cylindrical component (15) is driven by the translationally movable element (111) by means of a wheel (161) and worm screw (162) mechanism with a pitch p, and in that the accuracy of measuring the relative position of a translationally movable element (111) in a nuclear reactor depends on the total number of reflecting surfaces (222b) and diffusing surfaces (221b) and the pitch p of the worm screw (162), and is equal to p / P.

5. Measuring method (300) according to one of claims 3 or 4 characterized in that for a probe, among the N probes (21) and the two auxiliary probes (21b), comprising a transmitting optical fiber (215) and at least one receiving optical fiber (214): - the reflected or diffused nature of an optical signal received by the at least one receiving optical fiber (215) is determined by the intensity of the optical signal received by the at least one receiving optical fiber (215) according to the following steps: • When the intensity of the received optical signal is greater than or equal to a high threshold (SH), the optical signal received by the at least one receiving optical fiber (214) is a so-called reflected optical signal (2221); • When the intensity of the received optical signal is greater than or equal to a low threshold (SB) and is strictly less than the high threshold (SH), the low threshold (SB) being strictly less than the high threshold (SH), the optical signal received by the at least one receiving optical fiber (214) is a so-called reflected optical signal (2221); receiving (214) is a so-called diffused optical signal (2211); And characterized in that the difference between the intensity (SR) of the reflected optical signal (2221) and the intensity (SD) of the diffused optical signal is equal to 13dB, and in that the high threshold (SH) is such that the difference between the intensity (SR) of the reflected signal and the high threshold (SH) is between 0 dB and 13 dB.

6. Method (300) of measurement according to one of the preceding claims characterized in that, for a probe, among the N probes (21) and the two auxiliary probes (21b), comprising a transmitting optical fiber (215) and at least one receiving optical fiber (214): - When the intensity of the optical signal received by the at least one receiving optical fiber (214) is zero or positive and lower than a low threshold (SB), the optical signal is defective.

7. Fiber optic probe (21) for measuring the position of a mobile element, characterized in that it measures a mobile element (21) in a nuclear reactor according to the method described in the preceding claims and comprising: - an emitting optical fiber (215) and a receiving optical fiber (214); - an envelope (220) for protecting the optical fibers comprising: • a flexible, sealed part (211) comprising a metal resistant to a primary medium; • a rigid, sealed part (212) comprising a metal or a ceramic resistant to a primary medium; • a transparent, sealed window (213) resistant to a primary medium.

8. Fiber optic probe (21) according to the preceding claim, characterized in that: - a first end of the rigid part (212) is welded to a first end of the flexible part (211); - a second end of the rigid part (212) is welded to the transparent window (213).

9. Fiber optic probe (21) according to one of claims 7 or 8, characterized in that the casing (220) comprises a mirror (219) inclined at 45° relative to the axis (A) of the rigid part (212) of the probe (21).

10. Fiber optic probe (21) according to one of claims 7 or 8, characterized in that the optical fibers are made of materials resistant to nuclear irradiation.

11. System (2) for measuring the position of a mobile element of a nuclear reactor characterized in that it comprises: - N optical fiber probes (21), said probes comprising one or more emitting optical fibers (215) and one or more receiving optical fibers (214); - N tracks (22) consisting of reflecting surfaces (222) and diffusing surfaces (221) resistant to a primary medium, fixed to the mobile element; implementing the method of one of claims 1 to 4.

12. Measuring system (2) according to the preceding claim, characterized in that a first track among the N tracks (22) comprises a diffusing surface, a second track among the N tracks comprises a reflecting surface (222) and a diffusing surface (221) and any additional track comprises double the number of diffusing tracks and reflective tracks compared to the previous track.

13. Measuring system (2) according to the preceding claim, characterized in that it comprises a mechanical system comprising: - A wheel (232) cooperating with mobile tracks and secured to an articulated arm (231); - A spring (233) cooperating on the one hand with the articulated arm (231) and on the other hand with a fixed support (12); and characterized in that a probe (21) among the N probes (21) is fixed to the articulated arm (231).

14. Measuring system (2) according to one of claims 11 to 13 characterized in that it comprises: - An auxiliary track (22b) consisting of P reflecting surfaces (222b) and P diffusing surfaces (221b), the auxiliary track (22b) surrounding a cylindrical component (15) in rotation, the rotating cylindrical component (15) being driven by the translationally movable element (111); - Two auxiliary fiber optic probes (21b) spaced apart by a non-zero angle (a), used to emit two optical signals by two so-called emitter optical fibers (215) included in the two auxiliary probes (21b), and to receive by at least two so-called receiver optical fibers (214) included in the two auxiliary probes (21) two optical signals reflected (2221) or diffused (2211) by the auxiliary track (22b).

15. Nuclear reactor characterized in that it comprises a tank (1) comprising a system for measuring the position of mobile elements according to one of claims 11 to 14.

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