Method and system for measuring position of translationally movable element of nuclear reactor

Abstract

The invention discloses a method (300) for measuring the position of a translationally movable element (111) of a nuclear reactor, characterized in that it comprises the steps of:-transmitting (301) kN optical signals (210) through kN so-called transmitter optical fibers (215) comprised in N detectors resistant to a main medium, k and N being integers greater than or equal to 1; receiving (302) kN optical signals through N main-medium-resistant tracks (22), each track receiving k optical signals, said tracks consisting of a reflective surface (222) and a diffusing surface (221); -receiving (303), through mN so-called receiver optical fibers (214) comprised in the N detectors (21), kN optical signals reflected (2221) or diffused (2211) by the N tracks (22), m being a natural number greater than or equal to k; -converting (304) the kN optical signals received by the mN receiver optical fibers (214) into a binary code. The sensor setup of the present invention is simpler than other methods discussed.

Description

Technical Field

[0001] The technical field of the invention is that of measurements classified in terms of safety aspects in nuclear reactors, in particular, but not exclusively, pressurized water reactors.

[0002] The present invention relates to position measurement systems and in particular to the measurement of the position of a translationally movable element in a nuclear reactor. A typical case is the position measurement of a nuclear reaction control absorber. Background Art

[0003] Several measurement methods make it possible to determine the position of movable elements in nuclear reactors, such as, for example, the position of movable control rods within the reactor vessel. These measurements are said to be classified in terms of safety, as they form part of the protection of facilities that can cause environmental pollution, inconvenience, or have a significant impact on the neighborhood and the public, adversely affect safety related to the public, the neighborhood, or personnel of the facility, or adversely affect the health and safety of employees at work. Due to their characteristics, nuclear facilities are classified as such.

[0004] Among the methods for measuring the position of a movable control rod there are, inter alia, inductive measuring techniques and reed relay measurements.

[0005] The principle of inductive position measurement consists of passing a magnetic or ferromagnetic target integral to a movable part through coils, the number of coils being equal to the number of measurement steps. When the control rod moves relative to at least one detection coil, an output signal is induced. The output signal includes identifiable characteristics representing one or more positions of the control rod inside the detection coils, which are then processed by a unit to deduce these positions. This measurement method has several disadvantages: it is difficult to manufacture coils resistant to the temperature and pressure conditions encountered in a reactor, and the measurement principle requires the removal of a large amount of wiring from the vessel. Finally, magnetic and ferromagnetic targets lose their effectiveness when subjected to the temperature conditions of a nuclear reactor vessel.

[0006] The principle of reed relay measurement involves placing a series of reed switches spaced along the length of the control rod's travel and connected to provide an output signal. When the control rod moves, a permanent magnet mounted on the rod applies an external magnetic field to the reed switch assembly, which switches a resistor on or off depending on the magnet's position. This reduces the amount of wire used compared to previous methods, but has several drawbacks, such as the limited effectiveness of the permanent magnet and reed relays under the temperature conditions of the host medium. Reed relays are guaranteed to operate at rated voltage up to ambient temperatures of 65°C to 200°C, depending on the model, while temperatures in the host medium vary between 290°C and 350°C. Furthermore, the service life of reed relays is significantly reduced in high-temperature environments. Finally, the AMDEC (Analysis of Failure Modes, Their Effects, and Their Criticality) service safety tool is not very effective for fault detection using this measurement principle. Other technologies for measuring control rod position, such as acoustic position sensors, have been investigated, but due to the difficulty of qualifying them for host medium, these technologies have not yet been developed on an industrial scale.

[0007] There are now fiber optic position sensors being developed for use in various environments, particularly industrial environments. A method for measuring the position of a movable element using a fiber optic sensor comprises placing one or more transmitter fibers and one or more receiver fibers on the same part. The one or more transmitter fibers send a light signal to the movable part, which comprises a reflective and a diffuse or absorptive surface. The received reflected or diffuse / absorbed signal is binary coded: the reflected signal will be coded as a "1" and the diffuse signal will be coded as a "0". The fiber optic position sensors known in the prior art make it possible to measure the absolute or relative position of a translationally movable part.

[0008] Fiber-optic sensors have an interesting characteristic: due to the small overall size of the optical fibers, they can be integrated into confined and difficult-to-access locations. Furthermore, radiation-compatible fiber-optic technology has been developed, allowing them to be used in radiation environments without dimming. However, despite their advantages, fiber-optic technology is not well suited to the corrosive and pressure conditions within nuclear reactor vessels. Therefore, there is an initial need to precisely determine the position of the movable rods of a control rod cluster using a system that is simpler to implement, has a small overall size, is compatible with the nuclear reactor vessel environment, and is reliable in terms of fault detection. In particular, there is a need to adapt position sensors of interest in the industrial sector, such as fiber-optic sensors, to the nuclear sector. Summary of the Invention

[0009] In this context, the object of the present invention is to provide an alternative to the aforementioned method making it possible to eliminate the above-mentioned problems by being adapted to the environment of the 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 translationally movable element in a nuclear reactor, characterized in that the method comprises the following steps:

[0011] - transmitting kN optical signals through kN so-called transmitter fibers included in N detectors against the host medium, k and N being natural numbers greater than or equal to 1;

[0012] - receiving kN optical signals, each track receiving k optical signals, through N tracks of the anti-main medium, the tracks consisting of a reflective surface and a diffusive surface;

[0013] - receiving kN optical signals reflected or diffused by the N tracks by mN so-called receiver fibers included in the N detectors, m being a natural number greater than or equal to k;

[0014] -Convert kN optical signals received by mN receiver fibers into binary codes.

[0015] The present invention implements a fiber-optic position sensor for measuring the absolute position of a movable element, for example, in a nuclear reactor. Setting up the sensor is simpler than the other approaches discussed: for example, there are no thin coils of wire to be manufactured. Furthermore, unlike conventional fiber-optic measurement methods used in industrial settings, the present invention implements a system comprising fiber-optic probes and rails adapted to the conditions and stresses of the primary medium in, for example, a nuclear reactor vessel. In a nuclear reactor's primary circuit, the primary medium refers to the medium whose heat transfer fluid is subjected to the following parameters:

[0016] - Particularly corrosive high-temperature pressurized water media

[0017] -Pressure between 120 bar and 180 bar

[0018] Temperature between -250℃ and 350℃

[0019] - Water velocity between 2m / s and 5m / s

[0020] - Radiation between 2 MGy and 100 MGy (gamma and neutrons)

[0021] Therefore, the optical fiber is included in the detector which is protected against the host medium and is not directly exposed to the high temperature and high pressure water.

[0022] According to one embodiment, the above method makes it possible to obtain a measurement of the position of the translationally movable element with an accuracy that depends on N and is equal to the length of the track divided by 2 N This embodiment allows, by choosing the number N, to select the desired accuracy.

[0023] In another embodiment of the method, a translationally movable element rotationally drives a cylindrical member, said rotating cylindrical member being surrounded by an auxiliary track consisting of P reflecting surfaces and P diffusing surfaces, P being greater than or equal to 1, said auxiliary track being resistant to the main medium, the method being characterized in that it comprises the following additional step for measuring the relative position of the translationally movable element:

[0024] - emission of two optical signals through two so-called transmitter fibers included in two auxiliary detectors against the primary medium, the auxiliary detectors being spaced apart by a non-zero angle;

[0025] - receiving two optical signals through the track;

[0026] - receiving, by means of at least two so-called receiver fibers comprised in the two auxiliary detectors, two optical signals reflected or diffused by the auxiliary track;

[0027] - Transmission and interpretation of the two optical signals received by the receiver fiber by the processing unit.

[0028] Advantageously, in addition to the first position measurement mentioned above, the invention implements a second relative measurement of the position of the translationally movable element, thereby making it possible to use a device resistant to the host medium, having two position measurements and improving the accuracy of the measurements. According to an alternative embodiment, the accuracy of measuring the relative position of the translationally movable element in a nuclear reactor depends on the total number of reflective and diffusive surfaces and is equal to 2P.

[0029] Further to the characteristics discussed immediately above, the measurement method may be characterized as follows: for each of the N detectors and the two auxiliary detectors, including a transmitter fiber and at least one receiver fiber:

[0030] - determining the reflection or diffusion properties of the optical signal received by at least one receiver fiber by means of the intensity of the optical signal received by the at least one receiver fiber according to the following steps:

[0031] o 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 receiver fiber of the detector is a so-called reflected optical signal;

[0032] o when the intensity of the received optical signal is greater than or equal to a lower threshold and strictly below an upper threshold, the lower threshold being strictly below the upper threshold, the optical signal received by the at least one receiver fiber of the detector is a so-called diffuse light signal;

[0033] Furthermore, the difference between the intensity of the reflected optical signal and the intensity of the diffused optical signal is equal to 13 dB, and the high threshold is such that the difference between the intensity of the reflected signal and the high threshold is between 0 dB and 13 dB.

[0034] The present invention enables easy detection of reflected or diffuse light signals by means of previously defined high and low threshold values.

[0035] Furthermore, in the N detectors and the two auxiliary detectors, a transmitter optical fiber and at least one receiver optical fiber are included:

[0036] - The optical signal received by at least one optical fiber is said to be defective when the intensity of the optical signal received by the receiver fiber is zero or positive and below a low threshold.

[0037] The difference between the intensity of the reflected optical signal and the low threshold is equal to 28 dB, and the difference between the high threshold and the low threshold is preferably greater than or equal to 15 dB.

[0038] Advantageously, the invention has a three-level coding instead of a binary coding, thereby allowing the detection of defect signals in addition to reflected or diffused signals, in order to detect possible anomalies or faults and guarantee the required safety level.

[0039] Another aspect of the present invention relates to a fiber optic probe for measuring the position of a movable element, the fiber optic probe being capable of measuring the position of a movable element in a nuclear reactor according to the method, the fiber optic probe comprising:

[0040] - Transmitter fiber and receiver fiber;

[0041] - A sheath for protecting optical fibers, including:

[0042] o at least one sealed flexible portion comprising a metal resistant to the host medium; the flexible portion providing both flexibility for transporting the probe in the internal arrangement of the reactor and flexibility to accommodate differences in expansion between the fiber and the metal envelope,

[0043] o At least one rigid part of the seal, consisting of metal or ceramic resistant to the host medium;

[0044] o A transparent window sealed against the primary medium, soldered to the end of the rigid metal part.

[0045] The sheath consists of a material that is resistant to the host medium, corrosion and oxidation, so it can be immersed in the main circuit without damaging the optical fiber. A transparent window allows the optical signal to pass therethrough.

[0046] According to one embodiment, the fiber optic probe is characterized by:

[0047] - a first end of the rigid portion is welded to a first end of the flexible portion;

[0048] - The second end of the rigid portion is welded to the transparent window.

[0049] This advantageous embodiment makes it possible to ensure the sealing of the end of the rigid portion in the host medium.

[0050] According to an alternative embodiment, the envelope of the fiber-optic probe includes a mirror tilted at 45° relative to the axis of the rigid part of the probe. Indeed, it is possible that the fibers used in the probe have a small angle of curvature and their radial edges cannot be placed facing a reflective or diffuse track, where the light signal would then be reflected by the mirror, thus enabling the fibers to be placed parallel to the movement of the movable element and reducing the overall dimensions.

[0051] According to an advantageous embodiment, the optical fiber included in the detector is made of a material resistant to radiation at certain wavelengths, for example, between 800 nm and 1000 nm for multimode fibers and between 1000 nm and 1150 nm for single-mode fibers. Thus, the darkening of the optical fiber at these wavelengths by radiation from a nuclear reactor is significantly reduced.

[0052] Another aspect of the invention relates to a system for measuring the position of a movable element in a nuclear reactor, the system comprising:

[0053] -N fiber optic detectors;

[0054] - N tracks consisting of diffusing and reflecting surfaces against the host medium;

[0055] The method according to the invention can be implemented.

[0056] This system makes it possible to measure the position of any movable element, generally and more precisely, the control rods in a nuclear reactor vessel. The rails can be made of resistant metals and coatings adapted to the main medium, which avoids their darkening.

[0057] According to the first embodiment of the aforementioned system, the first track of the N tracks comprises a diffusing surface, the second track of the N tracks comprises a reflective surface and a diffusing surface, and any additional track comprises twice the number of reflective tracks and diffusing tracks relative to the previous track.

[0058] Each additional track makes it possible to improve the position measurement accuracy.

[0059] According to one embodiment, the aforementioned system comprises a mechanical system comprising:

[0060] - a roller cooperating with the movable track and integral with the articulated arm;

[0061] - a spring cooperating with the articulated arm on the one hand and with the fixed support on the other hand;

[0062] Characterized in that one of the N detectors is attached to an articulated arm.

[0063] Optical signals are sensitive to the distance they travel, as they pass through several interfaces with different exponentials, and these exponentials vary with temperature. Therefore, the distance between the transparent window and the rails should be as small as possible and, above all, constant. The mechanical system described above makes it possible to keep this distance constant despite mechanical vibrations and play within the reactor.

[0064] According to one embodiment, the measurement system comprises:

[0065] - an auxiliary track consisting of P reflective surfaces and P diffusive surfaces;

[0066] - Two fiber optic detectors spaced apart at a non-zero angle.

[0067] Advantageously, the system comprises means for measuring relative position.

[0068] Another aspect of the invention relates to a nuclear reactor comprising a position measurement system as mentioned previously. BRIEF DESCRIPTION OF THE DRAWINGS

[0069] Further characteristics and advantages of the present invention will become apparent from the following description thereof, given by way of indication and not limitation, with reference to the accompanying drawings, in which:

[0070] Figure 1 It is a simplified diagram of a pressurized water nuclear reactor vessel;

[0071] Figure 2 The movable rod in its fixed housing is shown, in which the fiber optic detector and target track are placed;

[0072] Figure 3 The fiber optic detector is shown in detail;

[0073] Figure 4 is a cross-sectional view of the fiber optic detector;

[0074] Figure 5 A track assembly consisting of a diffusing surface and a reflective surface is shown;

[0075] Figure 6 A pair of surfaces consisting of a reflective surface and a diffusing surface is shown in detail;

[0076] Figure 7 A first embodiment of a system for measuring the absolute position of a translational control stick is shown;

[0077] Figure 8 A second embodiment of a system for measuring the absolute position of a translational control stick is shown;

[0078] Figure 9 One embodiment of a system for measuring the relative position of a translational control stick is shown.

[0079] Figure 10 is a block diagram illustrating a method for measuring the position of a translationally movable element in a nuclear reactor;

[0080] Figure 11 is a graphical representation of the intensity of an optical signal received by a fiber optic detector as a function of the position of a translationally movable element;

[0081] Figure 12 is a block diagram illustrating a method for measuring the relative position of a translationally movable element of a nuclear reactor. DETAILED DESCRIPTION

[0082] Unless otherwise stated, identical elements appearing in different figures have a single reference number.

[0083] Figure 1 is a diagram of a pressurized water reactor (PWR) vessel 1 showing an assembly 11 consisting of translation control rods 111, whose position is to be measured in their fixed parts 112, elements of the control rod bundle guide 13, and core elements 14. The water circulating in the vessel is subjected to the pressure, temperature and flow conditions of the primary circuit:

[0084] - Particularly corrosive high temperature pressurized water environment

[0085] -Pressure between 120 bar and 180 bar

[0086] Temperature between -250℃ and 350℃

[0087] - Water velocity between 2m / s and 5m / s

[0088] - Radiation between 2 MGy and 100 MGy (gamma and neutrons)

[0089] In the following, the environment of the primary circuit will be referred to as the primary medium. Therefore, any element introduced into the container should be resistant to the aforementioned primary medium.

[0090] Figure 2 The translation control rod 111 is shown in greater detail, the position of which is intended to be measured in its fixed part 112 (hereinafter referred to as the housing) located in the container 1. When it is moved, the translation control rod 111 moves relative to the housing 112 along the axis z, along which the translation control rod 111 extends. The illustration shows a fiber optic probe 21 attached to the housing 112, which emits a light signal 210 toward a target track 22 integral with the translation control rod 111.

[0091] Figure 3A detailed view of a fiber optic probe 21 is shown. The probe 21 comprises a protective sheath 220 consisting of a flexible portion 211, a rigid portion 212 having an axis A, and a transparent window 213. For example, the flexible portion can be a metal tube resistant to the host medium, having a length sufficient to allow it to exit the host medium via a vessel liner similar to that described in EP 33 17 883 and reach a less aggressive medium. The flexible portion 211 is connected to the rigid portion 212, which can also be a tube made of one or more host medium resistant metals, by welding, preferably soldering 216. The metal comprising the rigid and / or flexible portion can be 316L stainless steel, which is corrosion-resistant and water-compatible with the host medium. The inner wall of the rigid portion 212 is soldered to the copper, gold, or aluminum sheath of the optical fiber at surface 218 to prevent the optical fiber from sliding within the rigid portion. This soldering is made possible by the metal coating of the bare fiber and may require metal adapters depending on the geometry selected for the fiber and the rigid tube.

[0092] A transparent window 213, which enables transmission of optical signals, is connected to the rigid portion 212 by welding, preferably soldering, 217. The transparent window 213 is sealed and resistant to the host medium, oxidation, and corrosion. The window can be a viewing port made of glass, such as sapphire, selected to match the coefficients of expansion of the transparent window 213 and the rigid portion 212 to prevent the soldering, 217, from breaking during temperature transitions in the host medium. The transparent window 213, the rigid portion 212, and the flexible portion 211 have substantially the same diameter. These diameters are typically between 2 mm and 4 mm.

[0093] In this embodiment, solder joint 216 from a first end of rigid portion 212 to one end of flexible portion 211 and solder joint 217 from a second end of the rigid portion to transparent window 213 ensure sealing of both ends of rigid portion 212 .

[0094] The housing 220 of the detector 21 includes a plurality of optical fibers, protected from the host medium, including a transmitter fiber 215 and several receiver fibers 214. More generally, the housing 220 may include one or more receiver fibers 215.

[0095] Figure 4is a cross-sectional view of a detector showing: a transmitter fiber 215 and a plurality of multimode receiver fibers 214 surrounding the transmitter fiber 215 to optimize the collection of the received optical signal. The optical signal propagating within the optical fiber can have a wavelength between 800 and 1200 nm for multimode fibers and between 1000 and 1200 nm for single-mode fibers. For example, the optical fibers can be designed with a copper sheath and an ultrapure silica core with a low OH (hydroxyl) content to prevent them from darkening under the influence of nuclear radiation at the previously mentioned wavelengths.

[0096] Each detector 21 emits a light signal 210 toward the target track 22 . Figure 5 is an illustration of four tracks 22, each track preferably spaced 3 mm apart from each other. The tracks are composed of alternating reflective surfaces 222 and diffusing surfaces 221 or absorbing surfaces. More generally, a large number of tracks N can be selected to measure the 2 N Different positions, the measurement accuracy is equal to the travel of the translation control rod 111 divided by 2 N .exist Figure 5 In this case, the 2 of the translation drive rod 111 can be measured 4 = 16 different positions. The travel of the translation control rod 111 is typically between 2000 mm and 4000 mm, so the measurement accuracy is equal to: 1 / 2 of the total displacement distance of the movable part N The length of the track 22 is equal to the travel of the translation control rod 111 and the total width 225 of the four tracks is preferably 10 mm.

[0097] The tracks 22 are made of a material that is resistant to host media, oxidation and corrosive conditions so that their darkening is minimal. Figure 6 A portion of a track including a reflective surface 222 and a diffusing surface 221 is shown in detail. A metal piece 223, designed to be resistant to host media, such as 316L stainless steel, is used to form the reflective and diffusing surfaces 222 and 221. To achieve the reflective surface 222, the metal piece 223 is coated with a metal that is insensitive or only slightly sensitive to oxidation, such as hard chrome or gold. The diffusing surface 221 is created by machining a roughened surface onto the metal piece 223.

[0098] The rough surface is characterized by the parameter Ra, which measures the distance between the arithmetic mean of the absolute values ​​of the deviations between the peaks and valleys and the centerline. For the diffuse surface 221, Ra is preferably 3.6, and for the reflective surface, Ra is preferably 0.2. In addition, the depth of the diffuse surface 221 relative to the reflective surface 222 can be between 2 mm and 5 mm.

[0099] Figure 7A first embodiment of a system 2 for measuring the position of a translation control rod 111 is shown. The measurement system 2 comprises a track 22, a corresponding fiber-optic probe 21, and a follower system 23 consisting of an articulated arm 231, a roller 232, and a spring 233. The flexible portion 211 of the probe is partially integral with the housing 112, and the rigid portion 212 is integral with the articulated arm 231. The articulated arm 231 is integral with the roller 232, which contacts the track 22 attached to the translation control rod 111. The articulated arm 231 is also integral with the spring 233 attached to the housing 112.

[0100] In this embodiment, the rigid portion 212 is perpendicular to the translation control rod 111, and the optical signal emitted by the transmitter fiber included in the fiber-optic detector 21 encounters only one surface within the detector: the transparent window 213. The optical signal 210 emitted by the detector 21 then passes through the host medium, whose refractive index is sensitive to temperature and can reduce the intensity of the optical signal 210. Therefore, the distance d traveled by the optical signal 210 should be constant and as small as possible while still allowing for the clearance required for proper translation of the movable component. Typically, this distance is on the order of a few millimeters, for example, between 0.5 mm and 5 mm, to avoid loss of intensity of the optical signal 210. However, the operation of the CRDM (control rod drive mechanism) causes vibrations and mechanical play when moving the translation control rod 111, which can change the distance d traveled by the optical signal. This is why a follower system 23 has been implemented to keep this distance d constant. The follower system 23 is mechanically controlled by a spring 233 which extends or compresses when the roller 232 is driven by the translation lever 111 in a direction other than the direction of movement of the translation lever 111 along the axis z. The spring causes the articulated arm 231 to rotate about the axis x, thus allowing the roller to remain in contact with the rail 22.

[0101] exist Figure 8 In the second embodiment of the invention, a second embodiment of the invention is described. In this embodiment, the detector 21 is parallel to the translation control rod 111 and its housing 220 comprises a reflector 219 placed at 45° relative to the axis A of the rigid part 212 of the detector 21. The light signal emitted by the transmitter optical fiber 215 of the detector 21 is first reflected by the reflector 219 before being transmitted through the transparent window 213. A reflector is defined as any surface capable of reflecting a light signal. When small overall dimensions are required, or when the maximum curvature radius of the flexible part 211 is between, for example, 30 mm and 50 mm and does not allow manufacturing Figure 7 This approach is advantageous when the embodiment described in .

[0102] Figure 9A variant of the present invention is shown, in which, in addition to one of the two previously described embodiments, a second relative measurement subsystem 2b, complementary and distinct from the previous embodiments, makes it possible to measure the relative position of the translationally movable rod 111 by measuring the number of revolutions of the cylindrical member 15 driven by the translationally movable rod 111, using a screw-nut mechanism 16, and more precisely, a wheel 161 and a worm 162 having a pitch p expressed in mm. Mechanism 16 converts the movement of the translational rod 111 into a rotational movement of the cylindrical member 15, with the ratio p expressed in mm / rev. This mechanism 16 is described in detail in patent EP 3329 493 B1. Relative measurement system 2b includes an auxiliary track 22b surrounding the cylindrical member 15, consisting of P reflective surfaces 222b and P diffusive surfaces 221b, P being an integer greater than or equal to 1. The system 2b also comprises two auxiliary detectors 21b, identical to those previously described, facing the auxiliary track 22b, spaced apart by a non-zero angle α smaller than 360° / P.

[0103] Figure 10 FIG. 3 is a block diagram showing the sequence of steps of a measurement method 300 according to the present invention. Figure 7 and Figure 8 The fiber optic measurement system 2 described in embodies this method and makes it possible to obtain the absolute position of a movable element in the vessel of a nuclear reactor, for example a movable control rod 111 .

[0104] The first step of the method is to transmit 301 one or more optical signals, respectively, by the transmitter fiber 215 of one or more detectors 21, wherein the wavelength of the signal may lie between 800 nm and 1200 nm. In a second step 302, the signals are received by one or more tracks 22, each detector being located in front of a track 22 so that each optical signal is received by a single track. If an optical signal is received by and returned by a reflective track 222, it is said to be reflected, and if it is received by and diffused by a diffuse track 221, it is said to be diffused. Then, in a third step 303, the multimode receiver fiber 214 receives the so-called reflected signal 2221 or diffused signal 2211.

[0105] Then, in a fourth step 304, the optical signal received by the receiver fiber 214 is processed and converted in a unit (not shown). When the received signal is a reflected signal 2221, its corresponding code will be 1, and when the received signal is a diffuse signal 2211, its corresponding code will be 0. This type of coding is called binary coding. For the detector, the reflective or diffuse nature of the optical signal received by the receiver fiber 214 is determined based on the value of the intensity of the optical signal received by the receiver fiber 214.

[0106] Figure 11 is a graphical representation of the intensity of the optical signal received by the receiver fiber, depending on the position of the translationally movable element 111. When the intensity of the received optical signal is greater than or equal to a threshold value SH, also called the upper threshold, 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 value SB, also called the lower threshold, and strictly below SH, the received optical signal is said to be diffused and has an intensity SD. When the signal intensity is strictly below threshold SB, the signal is considered defective and has an intensity SI. The intensity difference between the intensity SR of the reflected signal 2221 and the intensity SD of the diffuse signal 2211 is preferably on the order of 13 dB, and the intensity difference between the reflected signal 2221 and the defect signal is preferably on the order of 28 dB. These values ​​therefore make it possible to determine the preferred minimum deviation between the upper threshold value SH and the lower threshold value SB: the difference SR - SH is less than or equal to 13 dB, and the difference SR - SB is equal to 28 dB, which then leads to the difference SH - SB being greater than or equal to 15 dB. This type of coding is a so-called three-level coding and enables the detection of faults and anomalies: in practice, limiting the coding to two levels (reflected / diffuse) would be dangerous, since the received signal could be mistakenly interpreted as diffuse, and any anomalies would not be detected in time. For example, a defect signal could be due to a misalignment between the fiber optic probe 21 and the track 22, or, for example, due to a malfunction of the probe 21.

[0107] When the measurement system 2 comprises N fiber optic detectors 21 and N rails 22 , the position of the translationally movable control rod 111 will be given in the form of a binary code, wherein each of the N digits is respectively comprised in the set {0, 1}.

[0108] Figure 12 is a block diagram showing a sequence of additional steps of a method 300 for obtaining a measurement of the relative position of a translationally movable element, eg a translationally movable control rod 111. The method is implemented by means of the previously mentioned fiber optic measurement system 2b.

[0109] The first of the additional steps of the method consists in transmitting 301b two optical signals, respectively, by two transmitter fibers 215 of two auxiliary detectors 21b, which are spaced apart by an angle α. The wavelength of the signals may lie between 800 nm and 1200 nm. In a second step 302b, the signals are received by the auxiliary track 22b. If an optical signal is received by the reflective track 222b and reflected by it, it is said to be reflected. If an optical signal is received by the diffuse track 221b and diffused by it, it is said to be diffused. Then, in a third step 303b, the receiver fiber 214 receives the so-called reflected signal 2221 or diffused signal 2211.

[0110] The optical signal received by the receiver fiber 214 is then processed by a unit. The offset of the angle α between the two auxiliary detectors 21b makes it possible, with the aid of the processing unit, to determine the number of times the cylindrical member 15 has rotated through the angle α and, using the ratio p expressed in mm / rev, to deduce the displacement of the translationally movable element 111. The phase shift between the two optical signals received by the receiver fiber 214 makes it possible to determine the direction of rotation of the cylindrical member 16 and, therefore, the direction of displacement of the translationally movable element 111. Relative measurements of the translationally movable element 111 require initial calibration. The measurement accuracy of the 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 162, and is equal to p / P. The pitch p is preferably 20 mm, and the number P is preferably between 5 and 10, so that the accuracy of the measured position of the translationally movable element 111 is preferably between 2 mm and 4 mm.

Claims

1. A method (300) for measuring the position of a translationally movable element (111) of a nuclear reactor, characterized in that The method comprises the following steps: - emitting (301) kN optical signals (210) through kN so-called emitter fibers (215) included in N detectors against the host medium, k and N being integers greater than or equal to 1; - receiving (302) kN optical signals, each track receiving k optical signals, through N tracks (22) of the anti-host medium, the tracks being composed of a reflective surface (222) and a diffusing surface (221); - receiving (303) kN optical signals reflected (2221) or diffused (2211) by the N tracks (22) via mN so-called receiver fibers (214) included in the N detectors (21), m being a natural number greater than or equal to k; - Converting (304) the kN optical signals received by the mN receiver fibers (214) into binary codes.

2. The method (300) according to claim 1, characterized in that The accuracy with which the position of a translationally movable element (11) is measured in a nuclear reactor depends on N and is equal to the length of the track divided by 2 N .

3. The method (300) according to claim 1 or 2, characterized in that A translationally movable element (111) rotationally drives a cylindrical member (15), said rotating cylindrical member (15) being surrounded by an auxiliary track (22b), said auxiliary track consisting of P reflective surfaces (222b) and P diffusing surfaces (221b), P being greater than or equal to 1, said auxiliary track (22b) being anti-main medium and receiving light signals emitted by the anti-main medium and by two auxiliary detectors (21b) spaced apart at a non-zero angle (α), the method being characterized in that it comprises the following additional steps for measuring the relative position of the translationally movable element (111): - emission (301b) of two optical signals via two so-called transmitter fibers (215) comprised in said two auxiliary detectors (21b), - receiving (302b) two optical signals from the track; - receiving ( 303 b ) two optical signals reflected ( 2221 ) or diffused ( 2211 ) by the auxiliary track ( 22 b ) by at least two so-called receiver fibers ( 214 ) included in the two auxiliary detectors ( 21 ); - Transmitting and interpreting (304b) the two optical signals received by the receiver optical fiber (214) by a processing unit.

4. The method (300b) according to one of the preceding claims, characterized in that The accuracy with which the relative position of a translationally movable element (111) is measured in a nuclear reactor depends on the total number of reflective surfaces (222b) and diffusing surfaces (221b) and is equal to 2P.

5. The measuring method (300) according to any one of the preceding claims, characterized in that For one of the N detectors (21) and the two auxiliary detectors (21b), comprising a transmitter optical fiber (215) and at least one receiver optical fiber (214): - determining the reflection or diffusion properties of the optical signal received by at least one receiver fiber (215) by means of the intensity of the optical signal received by at least one receiver fiber (215), according to the following steps: o the optical signal received by the at least one receiver 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 high threshold value (SH); When the intensity of the received optical signal is greater than or equal to a lower threshold value (SB) and strictly lower than a higher threshold value (SH), the lower threshold value (SB) is strictly lower than the higher threshold value (SH), and the optical signal received by the at least one receiving optical fiber (214) is a so-called diffuse optical signal (2211); And it is characterized in that the difference between the intensity (SR) of the reflected light signal (2221) and the intensity (SD) of the diffuse light signal is equal to 13dB, and the high threshold (SH) makes the difference between the intensity (SR) of the reflected signal and the high threshold (SH) between 0dB and 13dB.

6. The measuring method (300) according to the preceding claim, characterized in that For one of the N detectors (21) and the two auxiliary detectors (21b), comprising a transmitter optical fiber (215) and at least one receiver optical fiber (214): - When the intensity of the optical signal received by at least one receiver fiber (214) is zero or positive and below a lower threshold (SB), the optical signal is defective.

7. A fiber optic probe (21) for measuring the position of a movable element, characterized in that The fiber optic probe is capable of measuring a movable element (21) in a nuclear reactor according to the method as claimed in the preceding claim and comprises: - a transmitter optical fiber (215) and a receiver optical fiber (214); - a sheath (220) for protecting an optical fiber, comprising: o a flexible portion (211) of the seal, comprising a metal resistant to the primary medium; o a rigid portion of the seal (212), comprising metal and ceramic resistant to the primary medium; o A transparent window (213) sealed against the host medium.

8. The optical fiber detector (21) according to claim 7, characterized in that: - a first end of the rigid portion (212) is welded to a first end of the flexible portion (211); - The second end of the rigid portion (212) is welded to the transparent window (213).

9. The optical fiber detector (21) according to claim 7 or 8, characterized in that The envelope (220) comprises a reflector (219) inclined at 45° relative to the axis (A) of the rigid part (212) of the detector (21).

10. The optical fiber detector (21) according to any one of claims 7 to 9, characterized in that Optical fibers are made of materials that are resistant to nuclear radiation.

11. A system (2) for measuring the position of a movable element of a nuclear reactor, characterized in that The measurement system comprises: - N optical fiber detectors (21); - N tracks (22), consisting of a reflective surface (222) against the host medium and a diffusing surface (221), attached to the movable element; The method according to any one of claims 1 to 6 can be implemented.

12. The measuring system (2) according to claim 11, characterized in that A first track of the N tracks (22) includes a diffusing surface, a second track of the N tracks includes a reflective surface (222) and a diffusing surface (221), and any additional track includes twice the number of diffusing tracks and reflective tracks relative to the previous track.

13. The measuring system (2) according to claim 11 or 12, characterized in that The measurement system includes a mechanical system, and the mechanical system includes: - a roller (232) cooperating with the movable track and integral with the articulated arm (231); - a spring (233) cooperating on the one hand with the articulated arm (231) and on the other hand with the fixed support (12); Characterized in that one detector (21) among the N detectors (21) is attached to an articulated arm (231).

14. The measuring system (2) according to claim 11, characterized in that The measurement system comprises: - an auxiliary track (22b) consisting of P reflective surfaces (222b) and P diffusing surfaces (221b); - Two fiber optic detectors (21b), spaced apart by a non-zero angle (α).

15. A nuclear reactor, characterized in that: The nuclear reactor comprises a vessel (1) having a system for measuring the position of a movable element according to claims 11 to 14.

Images