EP4038640A1 - Visual inspection system and visual inspection method, namely for inspection of the external surface of nuclear components - Google Patents

Visual inspection system and visual inspection method, namely for inspection of the external surface of nuclear components

Info

Publication number
EP4038640A1
EP4038640A1 EP20789281.1A EP20789281A EP4038640A1 EP 4038640 A1 EP4038640 A1 EP 4038640A1 EP 20789281 A EP20789281 A EP 20789281A EP 4038640 A1 EP4038640 A1 EP 4038640A1
Authority
EP
European Patent Office
Prior art keywords
region
light pattern
light
inspection
deposit
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Pending
Application number
EP20789281.1A
Other languages
German (de)
French (fr)
Inventor
Pierre Guillermier
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Areva NP SAS
Original Assignee
Framatome SA
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Framatome SA filed Critical Framatome SA
Publication of EP4038640A1 publication Critical patent/EP4038640A1/en
Pending legal-status Critical Current

Links

Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/84—Systems specially adapted for particular applications
    • G01N21/88—Investigating the presence of flaws or contamination
    • G01N21/8806—Specially adapted optical and illumination features
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/84—Systems specially adapted for particular applications
    • G01N21/88—Investigating the presence of flaws or contamination
    • G01N21/94—Investigating contamination, e.g. dust
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/84—Systems specially adapted for particular applications
    • G01N21/88—Investigating the presence of flaws or contamination
    • G01N21/95—Investigating the presence of flaws or contamination characterised by the material or shape of the object to be examined
    • G01N21/952—Inspecting the exterior surface of cylindrical bodies or wires
    • G—PHYSICS
    • G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
    • G21C—NUCLEAR REACTORS
    • G21C17/00—Monitoring; Testing ; Maintaining
    • G21C17/06—Devices or arrangements for monitoring or testing fuel or fuel elements outside the reactor core, e.g. for burn-up, for contamination
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/84—Systems specially adapted for particular applications
    • G01N21/88—Investigating the presence of flaws or contamination
    • G01N21/94—Investigating contamination, e.g. dust
    • G01N2021/945—Liquid or solid deposits of macroscopic size on surfaces, e.g. drops, films, or clustered contaminants
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E30/00—Energy generation of nuclear origin
    • Y02E30/30—Nuclear fission reactors

Definitions

  • Visual inspection system and visual inspection method namely for inspection of the external surface of nuclear components
  • the present invention relates to an inspection system for visual inspection of surfaces, namely for visual inspection of the external surface of nuclear components.
  • Nuclear reactors such as pressurized water reactors (PWR) and boiling water reactors (BWR), comprise a reactor core formed of a plurality of nuclear fuel assemblies and a reactor coolant system for flowing reactor coolant, i.e. water, through the fuel assemblies for a heat exchange between water and the fuel assemblies.
  • reactor coolant i.e. water
  • a fuel assembly comprises a plurality of fuel rods.
  • Each fuel rod comprises an elongated tubular cladding containing a stack of nuclear fuel pellets, namely pellets of fissile material such as enriched uranium dioxide or a mixture of plutonium and uranium dioxides.
  • the fuel rod claddings are generally metallic, for instance made of zirconium or zirconium- based alloys.
  • a pressurized-water reactor cooling system conventionally comprises heat exchangers, also called ’’steam generators”, provided for heat exchange between the reactor coolant (“primary water”) and feedwater (“secondary feedwater”) which is heated and vaporized inside the steam generators by heat exchange with the reactor coolant.
  • a steam generator includes a bundle of tubes extending inside a shell. The reactor coolant circulates inside the tubes while the feedwater flows inside the shell and over the external surface of the tubes. Thus, heat exchange is possible between the reactor coolant and the feedwater by thermal conduction through the wall of the tubes.
  • corrosion products from the reactor systems and chemical species contained in the coolants deposit and accumulate on the heat transfer surfaces, for instance on surfaces made of zirconium alloy of the fuel rods of a nuclear reactor core or on surfaces of steam generator shell and tubes, generally made of stainless steel or of Ni-based alloys, thus forming a deposit layer on the heat transfer surfaces.
  • the deposit layer For the nuclear fuel rods, the deposit layer, commonly called CRUD, varies within its thickness and comprises an outer portion of low density loose CRUD, harbouring mostly water, which is in constant exchange with the circulating reactor water, and providing a metal oxide structure capable of attracting and retaining colloidal particulates. This portion of porous deposit is also called fluffy CRUD. Below the portion of fluffy CRUD, closer to the heat transfer surface, the deposit layer comprises an inner portion of higher density CRUD, called tenacious CRUD, stuck to the heat transfer surface. The dense tenacious CRUD forms on a metal oxide layer, which forms on heat transfer surface due to heating of heat transfer surface in water medium (i.e. general corrosion).
  • CRUD Due to closeness of CRUD to the nuclear reactions, CRUD is usually radioactive. CRUD on the fuel rods can become dislodged from the fuel rod surface by the water flow. Consequently, CRUD can travel in the water circuit of the nuclear reactor causing unintended irradiation of operators in parts of the plant that are normally not radiologically active.
  • CRUD deposition depends mainly on temperature, i.e. heat flux and resulting mass evaporation rate of the reactor coolant water, and on water chemistry, i.e. solubility of the corrosion products and chemical species contained in the water, it tends to deposit more rapidly at the locations of higher heat flux. Accordingly, on fuel rod surface, CRUD preferably deposits on the upper part of the fuel assemblies in PWRs and on the lower part in BWRs.
  • CRUD-lnduced Localized Corrosion refers to a mechanism of localized fuel rod cladding corrosion failure, i.e. increased oxide thickness leading to unacceptable fuel conditions that may impact fuel rod performance and reliability, such as hot spots and subsequent mechanical failures, and/or may result in exceeding oxide licensing limits leading to early removal of the nuclear fuel assembly from service.
  • CIPS CRUD Induced Power Shift
  • Axial Offset Anomaly refers to deviation of the measured neutron flux in the top half of the reactor core from the predicted values.
  • CIPS is believed to be caused by three related phenomena occurring in the reactor core while operating at full power: sub-cooled nucleate boiling concentrated mainly in the upper half of the reactor core, corrosion product deposition (i.e. CRUD) on the fuel rod surface, and concentration and precipitation within the porous CRUD in highly sub cooled boiling regions of species acting as a poison to the nuclear reaction, for instance boron or lithium species.
  • CIPS reduces the shutdown margin, and may force the nuclear power plant to reduce power output.
  • the fuel rods may be required to be cleaned, this cleaning process being usually conducted by methods such as ultrasonically or chemically cleaning of the fuel rod surface to remove the fluffy CRUD build-up.
  • Nuclear power plant fuel performance is influenced by CRUD deposition during normal plant operation, start-up and shutdown, as well as during refuelling and maintenance outages.
  • steam generator tubes are subject to the accumulation of feedwater impurities.
  • feedwater is maintained at a high level of purity, minute concentrations of corrosion products and/or feedwater impurities may be concentrated in the feedwater flow stream, consequently leading to build-up of these materials onto the external surface of the tubes of the steam generator as the feedwater is converted into steam or on the internal surface of the shell of the steam generator. These deposits will eventually degrade the thermal hydraulic performance of the steam generator.
  • One of the aims of the invention is to propose an inspection system that allows reliable and efficient inspection of a surface.
  • the invention proposes an inspection system for detecting a deposit on a surface, the inspection system comprising:
  • processing system configured to digitally process the image to determine the presence of a deposit on the region of the surface affecting light reflection properties of the surface as a function of the light pattern drawn onto the region of the surface.
  • the inspection system comprises one or several of the following optional features, taken individually or in any technically feasible combination: - the processing system is configured to determine presence of a deposit as a function of a comparison of the light pattern drawn onto the region of the surface with at least one other light pattern;
  • the comparison comprises comparison of the light patter drawn onto the region of the surface with at least one light pattern drawn onto a comparison region, the comparison region being another region of the same surface or a region of another surface, the comparison region being free of deposit, affected by deposit or with unknown presence of deposit.
  • the processing system is configured to compare light patterns drawn on the regions by comparing profiles of light intensity of the light patterns using the images of the light patterns;
  • the light pattern comprises one stripe or several parallel stripes
  • the inspection system is configured for inspection under water or in air of the surface of a nuclear component and/or the external surface of at least one nuclear fuel rod;
  • the light source system is configured to draw a respective light pattern onto the external surface of each fuel rod of a plurality of peripheral fuel rods of a fuel assembly and the image capturing system is configured to capture images of the light pattern drawn onto at least one fuel rod of the plurality of fuel rods.
  • the invention also relates to a computer implemented inspection method for detecting a deposit on a surface, comprising:
  • the inspection method comprises one or several of the following optional features, taken individually or in any technically feasible combination:
  • the step of determining the presence of a deposit on the surface comprises comparing the light pattern drawn onto the region of the surface with at least one other light pattern;
  • the comparison comprises comparing the light pattern drawn onto the region of the surface with at least one light pattern drawn onto a comparison region, the comparison region being another region of the same surface or a region of another surface, the comparison region being free of deposit, affected by deposit or with unknown presence of deposit;
  • - comparison of light patterns drawn onto regions comprises comparing profiles of light intensity of the light patterns using images of the light patterns;
  • the light pattern comprises one stripe or several parallel stripes
  • the inspection method is implemented underwater or in air on the surface of at least one nuclear component and/or on the external surface of a cladding of at least one nuclear fuel rod;
  • FIG. 1 is a diagrammatical side view of a section of a fuel assembly and of an inspection system
  • Figure 2 represents an image captured by the inspection system of Figure 1 ;
  • Figures 3 and 4 are light intensity graphics obtained from digital processing of the image of Figure 2.
  • Figure 1 diagrammatically illustrates a fuel assembly 2 and an inspection system 4 for the visual inspection of the fuel assembly 2.
  • the fuel assembly 2 is elongated along a longitudinal axis L which is intended to be vertical when in use. Only a section of the fuel assembly 2 is visible on Figure 1 .
  • the fuel assembly 2 comprises a bundle of elongated fuel rods 6 extending along the longitudinal axis L.
  • the fuel rods 6 are arranged in the bundle in a lattice.
  • the fuel rods 6 include internal fuel rods 6 located inside the bundle and peripheral fuel rods 6 at the periphery of the bundle.
  • the fuel assembly 2 has side faces 8 each formed of a row of peripheral fuel rods 6.
  • the fuel assembly 2 illustrated on Figure 1 comprises fuel rods 6 arranged in a 5x5 square lattice and the fuel assembly 2 has four side faces 8. Five fuel rods 6 forming a side face 8 are visible.
  • fuel assemblies 2 comprise fuel rods 6 arranged for example in a square lattice of greater dimensions or in a hexagonal lattice in which case the fuel assembly 2 has six side faces 8.
  • Each fuel rod 6 comprises a tubular cladding 10 extending along the longitudinal axis L and nuclear fuel pellets (not visible) stacked along the longitudinal axis L inside the cladding 10.
  • the cladding 10 is made for example of a zirconium (Zr) alloy.
  • the cladding 10 is usually cylindrical of circular cross-section.
  • the fuel assembly 2 comprises spacer grids 12 distributed along the bundle of fuel rods 6 for maintaining the fuel rods 6 longitudinally and transversely in a spaced relationship. Only two spacer grids 12 are visible in the section of the fuel assembly 2 illustrated on Figure 1. In practice, a fuel assembly 2 generally comprises more than two spacer grids 12.
  • the inspection system 4 comprises a light source system 14, an image capturing system 16 for capturing images and a processing system 18 for digitally processing images captured by the image capturing system 16.
  • the light source system 14 is configured to draw a specific light pattern 20 onto the external surface 24 of at least one peripheral fuel rod 6 of the side face 8.
  • the light source system 14 is configured to draw a specific light pattern 20 onto the surface 24 of more than one peripheral fuel rod 6 and preferably on each fuel rod 6 of a side face 8 of a nuclear fuel assembly 2.
  • the cladding 10 of each fuel rod 6 being usually cylindrical of circular cross-section, the surface 24 of each fuel rod 6 is thus curved, in particular convex.
  • the light source system 14 is configured to draw the light pattern 20 onto a circumferential portion of the surface 24 of the fuel rod 6 that is oriented outwardly with respect to the fuel assembly 2.
  • the light pattern 20 drawn on the fuel rod surface 24 is not uniform.
  • the light pattern 20 is varying in light intensity, from dark regions to bright regions depending on the local reflectance characteristics, i.e. the light reflection properties of each portion of the surface 24 of the fuel rod 6.
  • the light pattern 20 drawn on a surface 24 is the reflection onto the surface 24 of the light generated by the light source system 14.
  • the light source system 14 comprises at least one light source 28.
  • the shape of the light pattern 20 depends on the arrangement of one or several light sources 28 of the light source system 14 and the shape of the surface 24.
  • each light source 28 draws on the surface 24 of each fuel rod 6 of the side face 8 a light stripe 26 extending axially, i.e. along the longitudinal axis L, due to the convex shape of the surfaces 24.
  • the light source system 14 comprises n light sources 28 each with one different angle of illumination, in case of specular reflection
  • the light pattern 20 drawn on a surface 24 comprises n parallel elongated stripes 26 extending along the length of the illuminated portion of the fuel rod 6.
  • the stripes 26 are parallel to the longitudinal axis L.
  • a specular surface provides a bright defined light pattern 20, i.e. bright elongated stripes 26.
  • the light of the light sources 28 is scattered at many angles rather than at just one angle as in the case of pure specular reflection.
  • the light pattern 20 and light stripes 26 may become not visible.
  • the light source system 14 comprises at least one light source 28 and preferably two or three light sources 28 arranged to illuminate each surface 24 with different angles of illumination such as to drawn separate parallel light stripes 26 on each surface 24.
  • the light source system 14 comprises two light sources 28 configured to draw two stripes 26 in case of specular reflection and thus one combined resulting light pattern 20 onto the surface 24 of each fuel rod 6 of the side face 8.
  • the two stripes light pattern 20 is only one example of light pattern 20.
  • the man skilled in the art is capable of determining light sources arrangements for drawing specific light patterns that allows distinguishing regions with a deposit from regions deprived of deposit as explained below.
  • the image capturing system 16 is configured to capture an image 36 of the light patterns 20 drawn on the surfaces 24 of the fuel rods 6.
  • the image 36 is a digital matrix image formed of a matrix of pixels acquired by a matrix optical digital sensor.
  • the image 36 is a light intensity image. Each pixel is assigned a light intensity value.
  • the image capturing system 16 comprises one single image capturing device 30 comprising an optical digital sensor.
  • the image capturing device 30 is for example a video camera.
  • the optical digital sensor is for example a CMOS (Complementary Metal Oxide Semiconductor) optical sensor or CCD (Charge-Coupled Device) optical sensor.
  • CMOS Complementary Metal Oxide Semiconductor
  • CCD Charge-Coupled Device
  • the light source system 14 is provided as a single unit and the image capturing system 16 is provided as a single unit, the light source system 14 and the image capturing system 16 being arranged on either sides of the fuel assembly 2.
  • the light source system 14 may comprises several separated light sources 28 and the image capturing system 16 may comprise several single video cameras.
  • the light source system 14 and the image capturing system 16 are arranged facing the same side face 8 of the fuel assembly 2.
  • the processing system 18 is configured to digitally process images 36 captured by the image capturing system 16 to determine the presence of a deposit on the surfaces 24 of the fuel rods 6 affecting the light reflection properties of the surfaces 24 of the fuel rods 6 as a function of the light patterns 20 drawn on the surfaces 24.
  • the processing system 18 comprises a computer 32 for implementing the image digital processing and an image analysis software installed on the computer 32.
  • the image analysis software comprises software instructions for implementing the image digital processing when executed by the computer 32.
  • the processing system 18 is connected to the image capturing system 16 to receive the captured images 36.
  • the image digital processing comprises processing parameters of the image 36, more specifically of pixels of the image 36 that depend on the light reflection properties of the surface 24, more specifically on the light reflectance of the surface 24.
  • the processing system 18 preferably comprises a user interface 34 for producing inspection reports and allowing a user to visualize the inspection results.
  • the user interface 34 advantageously allows an operator to visualize images 36 captured by the image capturing system 16 and/or to enter instructions. Entering instruction may comprise in particular setting analysis parameters to be used during image digital processing.
  • the inspection system 4 and the fuel assembly 2 can be moved relative to one another along the longitudinal axis L to scan the fuel assembly 2 lengthwise, as illustrated by arrow F.
  • Figure 2 illustrates an image 36 captured by the image capturing system 16.
  • the sections 38 of the fuel rods 6 visible on the image 36 are partially affected by CRUD.
  • the visible lower portion 6A of each fuel rod 6 is unaffected by CRUD and the visible upper portion 6B of each fuel rod 6 is affected by CRUD.
  • CRUD specular.
  • CRUD affects the reflection properties of the surface 24 that becomes mostly a diffuse surface. Consequently, the intensity of light reflected by the surface 24 varies depending on whether the surface 24 is affected by CRUD or not.
  • the light pattern 20 is formed on the CRUD-unaffected lower portion 6A of each surface 24 and is also formed on the CRUD-affected upper portion 6B of the surface 24 but is much more diffracted due to the mat aspect imparted by CRUD.
  • the image digital processing comprises mathematically comparing the light pattern 20 drawn on at least one region of each surface 24 with at least one other light pattern 20 drawn on a comparison region by a light source system 14.
  • a comparison region is another region of the same surface 24 or a region of another surface 24, in particular a surface 24 of another fuel rod 6, in particular another fuel rod 6 of the same fuel assembly 2 or a fuel rod 6 not belonging to the same fuel assembly 2, for example a reference fuel rod, or even a surface 24 of a sample specifically prepared to be used as standard.
  • the comparison region is free of deposit, affected by deposit or has an unknown presence of deposit.
  • the light pattern 20 drawn on a region under inspection is compared to a reference light pattern obtained for a standard comprising a deposit-free and several different deposit-affected areas.
  • a reference light pattern may be stored in the computer 32 and used for inspection of different fuel assemblies 2 in the same or in different nuclear reactors.
  • Comparing the light pattern 20 of the region under inspection with the light pattern 20 of a region of the same surface with unknown presence or absence of deposit allows determining that one of the two regions is affected by CRUD and therefore that the surface 24 is affected by CRUD.
  • Comparing the light pattern 20 of the region under inspection with a reference light pattern 20 of a region with known presence or absence of CRUD allows determining presence or absence of CRUD in the region under inspection as a function of a mathematical correlation between the light patterns 20.
  • the mathematical comparison of two light patterns 20 is for example operated by mathematically comparing the light patterns 20 on images 36 captured by the image capturing system 16, e.g. by template matching.
  • the mathematical comparison of two light patterns 20 is operated by comparison of light intensity profiles of the two light patterns 20 calculated using images 36 captured by the image capturing system 16, the light intensity profiles being compared e.g. by template matching.
  • a light intensity profile is for example determined as the variation of the light intensity of the light pattern 20 along a first direction.
  • the light intensity profile along the first direction is obtained for example by determining at each position along the first direction the mean value of light intensity of the light pattern 20 along the second direction over the extension of the region along the second direction.
  • the individual light intensity profiles of the light patterns 20 drawn on regions of the fuel rods 6 at an axial location of the fuel assembly 2 are calculated by calculating an assembly light intensity profile in an analysis window of the image 36 having a height along the length of the fuel rods 6 and a width transversely to the length of the fuel rods 6.
  • the height of the analysis window may vary from one pixel to a few millimetres, depending on the conditions of the examination.
  • the analysis window comprises a matrix of pixels comprising columns extending along the length of the fuel rods 6 and rows extending transversely to the length of the fuel rods 6.
  • the assembly light intensity profile is determined by calculating the mean value of light intensity for each column and determining the assembly light intensity profile over the columns.
  • the light intensity is determined by the greyscale of the pixels in the analysis window of the image 36, using for instance an image processing program such as ImageJ.
  • Figure 2 illustrates a first analysis window 40 extending over the lower portions 6A unaffected by CRUD of the fuel rods 6 and a second analysis window 42 extending over the upper portions 6B affected by CRUD of the fuel rods 6.
  • Figure 3 is a graphic illustrating a first assembly profile 44 calculated for the first analysis window 40.
  • the graphic indicates mean greyscale in ordinate as a function of distance in image pixels in abscissa.
  • the first assembly profile 44 comprises a set of first peaks 46. Each first peak 46 corresponds to the individual light intensity profile of the surface 24 of a respective fuel rod 6. Each valley between two adjacent peaks corresponds to a gap between two adjacent fuel rods 6.
  • Figure 4 is a graphic analogous to that of Figure 3 and illustrating a second assembly profile 52 calculated for the second analysis window 42.
  • the second assembly profile 52 comprises a set of second peaks 54.
  • Each second peak 54 corresponds to the light intensity profile of the surface 24 of a respective fuel rod 6.
  • the first peaks 46 and the second peaks 54 have different shapes which allow discriminating between a CRUD-free surface 24 and a CRUD- affected surface 24. This discrimination does not depend on the greyscale of the image 36. It only depends on the shape of the peaks.
  • Determining presence or absence of CRUD of a region of a surface 24 of a particular fuel rod 6 at an axial location is operated as a function of the individual light intensity profile of the light pattern 20 drawn on this particular region of the surface 24 of the fuel rod 6 at this axial location, i.e. the peak corresponding to this particular region, isolated in the corresponding assembly profile.
  • the processing system 18 calculates a correlation coefficient as a function of the comparison between the light pattern 20 drawn on the region and the light pattern 20 drawn on the other comparison region e.g. by comparing the images of the light patterns 20 and/or comparing the light intensity profiles of the light patterns 20, and determines presence of CRUD as a function of the correlation coefficient.
  • the presence or absence of CRUD is determined as a function of the correlation coefficient and as a function of at least one criterion, depending in particular on whether the comparison region is a region which is CRUD-free, CRUD-affected or with unknown presence of CRUD.
  • a criterion is that if the light pattern of a region under inspection has a correlation coefficient with a reference light pattern of a CRUD-free region of the same surface or another surface which is higher than a threshold, the region under inspection is determined as being CRUD-free.
  • a criterion is that if the correlation between light patterns of two regions of the same surface with unknown presence or absence of CRUD is below a threshold, the surface is determined as being CRUD-affected in at least one of the two regions.
  • the inspection system 4 allows automated inspection of the fuel rods 6 and automated generation of an inspection report.
  • the inspection system 4 is reliable and efficient.
  • Peripheral fuel rods 6 of a side face 8 if a fuel assembly 2 are inspected simultaneously and rapidly. All the peripheral fuel rods 6 of a side face 8 or only a part of the peripheral fuel rods 6 of a side face 8 are inspected simultaneously.
  • the inspection system 4 and the fuel assembly 2 can be moved relative to one another along the longitudinal axis L to scan the fuel assembly 2 lengthwise, step by step or in a continuous movement. Preferably, the inspection is limited to the relevant portions of the fuel assembly 2.
  • the image capturing system 16 is distributed to capture in one shot images 36 covering one entire side face 8 of the fuel assembly 2.
  • One or several light source(s) 28 are provided to draw the light pattern 20 onto the side face 8 of the fuel assembly 2 in the field- of-view of the or each image capturing device 30 of the image capturing system 16.
  • the image capturing system 16 comprises for example at least one image capturing device 30 for each side face 8 to capture images 36 of each side face 8 in one shot.
  • the image capturing system 16 comprises for example at least one image capturing device 30 for only two side faces 8, and the fuel assembly 2 is rotated around its longitudinal axis L for inspection of all the side faces 8.
  • the invention is described above for the inspection of the external surface of nuclear fuel rods. It is however applicable to the inspection of an external surface of a steam generator tube or even, for instance, to the inspection of an internal surface of a steam generator shell. More generally, the invention is particularly suited for detecting presence of deposit on any nuclear component. The invention may be used more generally for detection of any deposit on any surface, in particular a surface of a nuclear power plant or a nuclear manufacturing plant, underwater or in the air.
  • the invention may be used for instance for under water detection of oxidation on control rod cladding tubes of rod cluster control assembly, for cleanness control, for instance in air on non-irradiated components in the manufacturing plant.
  • the inspection system 4 and the inspection method allow determining surface condition alteration affecting light reflection properties of a surface 24, in particular reflectance of the surface 24.
  • the light source 28 may comprise any type of light source leading to light reflection onto the surface 24 under inspection.
  • the light source is for example a halogen source, a fluorescent source, a LED source or a laser beam.
  • the arrangement of the light source 28 and the shape of the surface 24 under inspection determines the shape of light pattern 20.
  • the arrangement of the light source(s) 28 is chosen as a function of the surface 24 to be inspected to draw light patterns 20 that can be reliably discriminated by the processing system 18.
  • the surface under inspection is preferably curved, namely circular.
  • the surface under inspection is preferably concave or convex, and preferably convex.
  • Light sources draw a light pattern formed of light stripes on a cylindrical convex surface.

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Abstract

The inspection system for detecting a deposit on a surface (24) comprises: - a light source system (14) to draw a light pattern (20) onto at least a region of the surface (24); - an image capturing system (16) to capture a digital image (36) of the light pattern (20) drawn onto the region of the surface (24); and - a processing system (18) configured to digitally process the image (36) to determine the presence of a deposit on the region of the surface (24) affecting light reflection properties of the surface (24) as a function of the light pattern (20) drawn onto the region of the surface (24).

Description

Visual inspection system and visual inspection method, namely for inspection of the external surface of nuclear components
The present invention relates to an inspection system for visual inspection of surfaces, namely for visual inspection of the external surface of nuclear components.
Nuclear reactors, such as pressurized water reactors (PWR) and boiling water reactors (BWR), comprise a reactor core formed of a plurality of nuclear fuel assemblies and a reactor coolant system for flowing reactor coolant, i.e. water, through the fuel assemblies for a heat exchange between water and the fuel assemblies.
A fuel assembly comprises a plurality of fuel rods. Each fuel rod comprises an elongated tubular cladding containing a stack of nuclear fuel pellets, namely pellets of fissile material such as enriched uranium dioxide or a mixture of plutonium and uranium dioxides. The fuel rod claddings are generally metallic, for instance made of zirconium or zirconium- based alloys.
In the following, regarding the fuel rod cladding, “cladding external surface”, “cladding surface”, “fuel rod external surface” and “fuel rod surface” are used as equivalents.
A pressurized-water reactor cooling system conventionally comprises heat exchangers, also called ’’steam generators”, provided for heat exchange between the reactor coolant (“primary water") and feedwater (“secondary feedwater”) which is heated and vaporized inside the steam generators by heat exchange with the reactor coolant. Such a steam generator includes a bundle of tubes extending inside a shell. The reactor coolant circulates inside the tubes while the feedwater flows inside the shell and over the external surface of the tubes. Thus, heat exchange is possible between the reactor coolant and the feedwater by thermal conduction through the wall of the tubes.
During operation of the reactor core, corrosion products from the reactor systems and chemical species contained in the coolants deposit and accumulate on the heat transfer surfaces, for instance on surfaces made of zirconium alloy of the fuel rods of a nuclear reactor core or on surfaces of steam generator shell and tubes, generally made of stainless steel or of Ni-based alloys, thus forming a deposit layer on the heat transfer surfaces.
For the nuclear fuel rods, the deposit layer, commonly called CRUD, varies within its thickness and comprises an outer portion of low density loose CRUD, harbouring mostly water, which is in constant exchange with the circulating reactor water, and providing a metal oxide structure capable of attracting and retaining colloidal particulates. This portion of porous deposit is also called fluffy CRUD. Below the portion of fluffy CRUD, closer to the heat transfer surface, the deposit layer comprises an inner portion of higher density CRUD, called tenacious CRUD, stuck to the heat transfer surface. The dense tenacious CRUD forms on a metal oxide layer, which forms on heat transfer surface due to heating of heat transfer surface in water medium (i.e. general corrosion).
Due to closeness of CRUD to the nuclear reactions, CRUD is usually radioactive. CRUD on the fuel rods can become dislodged from the fuel rod surface by the water flow. Consequently, CRUD can travel in the water circuit of the nuclear reactor causing unintended irradiation of operators in parts of the plant that are normally not radiologically active.
Since CRUD deposition depends mainly on temperature, i.e. heat flux and resulting mass evaporation rate of the reactor coolant water, and on water chemistry, i.e. solubility of the corrosion products and chemical species contained in the water, it tends to deposit more rapidly at the locations of higher heat flux. Accordingly, on fuel rod surface, CRUD preferably deposits on the upper part of the fuel assemblies in PWRs and on the lower part in BWRs.
As thickness of CRUD increases and as its thermal conductivity is not favourable to thermal exchanges, CRUD may cause an increase of fuel rod cladding temperature and result in increased oxide thickness and increased concentrations of detrimental species that may lead to localized fuel rod cladding corrosion or mechanical failures. CRUD-lnduced Localized Corrosion (CILC) refers to a mechanism of localized fuel rod cladding corrosion failure, i.e. increased oxide thickness leading to unacceptable fuel conditions that may impact fuel rod performance and reliability, such as hot spots and subsequent mechanical failures, and/or may result in exceeding oxide licensing limits leading to early removal of the nuclear fuel assembly from service.
In PWRs, CRUD also may cause an unexpected change in core power distribution known as CRUD Induced Power Shift (CIPS). CIPS is one form of Axial Offset Anomaly and refers to deviation of the measured neutron flux in the top half of the reactor core from the predicted values. CIPS is believed to be caused by three related phenomena occurring in the reactor core while operating at full power: sub-cooled nucleate boiling concentrated mainly in the upper half of the reactor core, corrosion product deposition (i.e. CRUD) on the fuel rod surface, and concentration and precipitation within the porous CRUD in highly sub cooled boiling regions of species acting as a poison to the nuclear reaction, for instance boron or lithium species. Because the deposits are typically thicker in the top portion of the core, these poison species cause an abnormal power distribution along the axis of the core, reducing available margin for certain types of operating conditions. Among other difficulties, CIPS reduces the shutdown margin, and may force the nuclear power plant to reduce power output.
If one or a combination of these concerns (radiological concern, CIPS or CILC) exists then the fuel rods may be required to be cleaned, this cleaning process being usually conducted by methods such as ultrasonically or chemically cleaning of the fuel rod surface to remove the fluffy CRUD build-up.
Nuclear power plant fuel performance is influenced by CRUD deposition during normal plant operation, start-up and shutdown, as well as during refuelling and maintenance outages.
Plant operators routinely visually examine fuel rods during fuel assemblies’ movement at refuelling outages. The presence of CRUD on the fuel rod surface is observed with the use of video cameras. Since the captured videos are reviewed by an operator, the presence or absence of CRUD can be documented (qualitatively). However, this inspection is time consuming and the result is not fully reliable as conclusions may depend on the operator.
Similarly, steam generator tubes are subject to the accumulation of feedwater impurities. Although the feedwater is maintained at a high level of purity, minute concentrations of corrosion products and/or feedwater impurities may be concentrated in the feedwater flow stream, consequently leading to build-up of these materials onto the external surface of the tubes of the steam generator as the feedwater is converted into steam or on the internal surface of the shell of the steam generator. These deposits will eventually degrade the thermal hydraulic performance of the steam generator.
To conserve the heat transfer capabilities of the steam generator, the presence of the deposits, as well as the locations of the deposits, must be determined such that removal techniques may be implemented. Currently, visual inspections in air are utilized to ascertain locations. These techniques, however, are prone to error. Moreover, visual inspections introduce a significant radiological risk to workers as the steam generator is connected to a nuclear power generation system.
One of the aims of the invention is to propose an inspection system that allows reliable and efficient inspection of a surface.
To this end, the invention proposes an inspection system for detecting a deposit on a surface, the inspection system comprising:
- a light source system to draw a light pattern onto at least a region of the surface;
- an image capturing system to capture a digital image of the light pattern drawn onto the region of the surface; and
- a processing system configured to digitally process the image to determine the presence of a deposit on the region of the surface affecting light reflection properties of the surface as a function of the light pattern drawn onto the region of the surface.
In specific embodiments, the inspection system comprises one or several of the following optional features, taken individually or in any technically feasible combination: - the processing system is configured to determine presence of a deposit as a function of a comparison of the light pattern drawn onto the region of the surface with at least one other light pattern;
- the comparison comprises comparison of the light patter drawn onto the region of the surface with at least one light pattern drawn onto a comparison region, the comparison region being another region of the same surface or a region of another surface, the comparison region being free of deposit, affected by deposit or with unknown presence of deposit.
- the processing system is configured to compare light patterns drawn on the regions by comparing profiles of light intensity of the light patterns using the images of the light patterns;
- the light pattern comprises one stripe or several parallel stripes;
- the inspection system is configured for inspection under water or in air of the surface of a nuclear component and/or the external surface of at least one nuclear fuel rod;
- the light source system is configured to draw a respective light pattern onto the external surface of each fuel rod of a plurality of peripheral fuel rods of a fuel assembly and the image capturing system is configured to capture images of the light pattern drawn onto at least one fuel rod of the plurality of fuel rods.
The invention also relates to a computer implemented inspection method for detecting a deposit on a surface, comprising:
- drawing a light pattern onto at least a region of the surface under inspection;
- capturing a digital image of the light pattern drawn onto the region of the surface under inspection; and
- digitally processing the image for detecting a deposit on the region of the surface affecting the reflection properties of the surface as a function of the light pattern drawn onto the region of the surface.
In specific embodiments, the inspection method comprises one or several of the following optional features, taken individually or in any technically feasible combination:
- the step of determining the presence of a deposit on the surface comprises comparing the light pattern drawn onto the region of the surface with at least one other light pattern;
- the comparison comprises comparing the light pattern drawn onto the region of the surface with at least one light pattern drawn onto a comparison region, the comparison region being another region of the same surface or a region of another surface, the comparison region being free of deposit, affected by deposit or with unknown presence of deposit; - comparison of light patterns drawn onto regions comprises comparing profiles of light intensity of the light patterns using images of the light patterns;
- the light pattern comprises one stripe or several parallel stripes;
- it comprises scanning the surface under inspection by relatively displacing the light pattern and the field-of-view of an image capturing system along the surface;
- the inspection method is implemented underwater or in air on the surface of at least one nuclear component and/or on the external surface of a cladding of at least one nuclear fuel rod;
- it comprises drawing a respective light pattern onto the external surface of each fuel rod of a plurality of peripheral fuel rods of a fuel assembly and capturing a digital image encompassing at least a section of the external surface of at least one of the plurality of fuel rods.
The invention and its advantages will be better understood on reading the following description, given solely by way of non-limiting example and made with reference to the appended drawings, in which:
- Figure 1 is a diagrammatical side view of a section of a fuel assembly and of an inspection system;
- Figure 2 represents an image captured by the inspection system of Figure 1 ; and
- Figures 3 and 4 are light intensity graphics obtained from digital processing of the image of Figure 2.
Figure 1 diagrammatically illustrates a fuel assembly 2 and an inspection system 4 for the visual inspection of the fuel assembly 2.
The fuel assembly 2 is elongated along a longitudinal axis L which is intended to be vertical when in use. Only a section of the fuel assembly 2 is visible on Figure 1 .
In the following, “axial” and “longitudinal”, “axially” and “longitudinally” are used as equivalents.
The fuel assembly 2 comprises a bundle of elongated fuel rods 6 extending along the longitudinal axis L. The fuel rods 6 are arranged in the bundle in a lattice. The fuel rods 6 include internal fuel rods 6 located inside the bundle and peripheral fuel rods 6 at the periphery of the bundle. The fuel assembly 2 has side faces 8 each formed of a row of peripheral fuel rods 6.
For sake of clarity of the drawings, the fuel assembly 2 illustrated on Figure 1 comprises fuel rods 6 arranged in a 5x5 square lattice and the fuel assembly 2 has four side faces 8. Five fuel rods 6 forming a side face 8 are visible. Flowever, in practice, fuel assemblies 2 comprise fuel rods 6 arranged for example in a square lattice of greater dimensions or in a hexagonal lattice in which case the fuel assembly 2 has six side faces 8. Each fuel rod 6 comprises a tubular cladding 10 extending along the longitudinal axis L and nuclear fuel pellets (not visible) stacked along the longitudinal axis L inside the cladding 10. The cladding 10 is made for example of a zirconium (Zr) alloy. The cladding 10 is usually cylindrical of circular cross-section.
The fuel assembly 2 comprises spacer grids 12 distributed along the bundle of fuel rods 6 for maintaining the fuel rods 6 longitudinally and transversely in a spaced relationship. Only two spacer grids 12 are visible in the section of the fuel assembly 2 illustrated on Figure 1. In practice, a fuel assembly 2 generally comprises more than two spacer grids 12.
The inspection system 4 comprises a light source system 14, an image capturing system 16 for capturing images and a processing system 18 for digitally processing images captured by the image capturing system 16.
The light source system 14 is configured to draw a specific light pattern 20 onto the external surface 24 of at least one peripheral fuel rod 6 of the side face 8. In a preferred embodiment, the light source system 14 is configured to draw a specific light pattern 20 onto the surface 24 of more than one peripheral fuel rod 6 and preferably on each fuel rod 6 of a side face 8 of a nuclear fuel assembly 2. The cladding 10 of each fuel rod 6 being usually cylindrical of circular cross-section, the surface 24 of each fuel rod 6 is thus curved, in particular convex.
The light source system 14 is configured to draw the light pattern 20 onto a circumferential portion of the surface 24 of the fuel rod 6 that is oriented outwardly with respect to the fuel assembly 2.
The light pattern 20 drawn on the fuel rod surface 24 is not uniform. The light pattern 20 is varying in light intensity, from dark regions to bright regions depending on the local reflectance characteristics, i.e. the light reflection properties of each portion of the surface 24 of the fuel rod 6.
The light pattern 20 drawn on a surface 24 is the reflection onto the surface 24 of the light generated by the light source system 14.
The light source system 14 comprises at least one light source 28.
The shape of the light pattern 20 depends on the arrangement of one or several light sources 28 of the light source system 14 and the shape of the surface 24. In the present case, each light source 28 draws on the surface 24 of each fuel rod 6 of the side face 8 a light stripe 26 extending axially, i.e. along the longitudinal axis L, due to the convex shape of the surfaces 24. If for example, the light source system 14 comprises n light sources 28 each with one different angle of illumination, in case of specular reflection, the light pattern 20 drawn on a surface 24 comprises n parallel elongated stripes 26 extending along the length of the illuminated portion of the fuel rod 6. The stripes 26 are parallel to the longitudinal axis L.
A specular surface provides a bright defined light pattern 20, i.e. bright elongated stripes 26. Conversely, on a diffuse surface the light of the light sources 28 is scattered at many angles rather than at just one angle as in the case of pure specular reflection. The light pattern 20 and light stripes 26 may become not visible.
The light source system 14 comprises at least one light source 28 and preferably two or three light sources 28 arranged to illuminate each surface 24 with different angles of illumination such as to drawn separate parallel light stripes 26 on each surface 24.
In the illustrated embodiment, the light source system 14 comprises two light sources 28 configured to draw two stripes 26 in case of specular reflection and thus one combined resulting light pattern 20 onto the surface 24 of each fuel rod 6 of the side face 8.
The two stripes light pattern 20 is only one example of light pattern 20. The man skilled in the art is capable of determining light sources arrangements for drawing specific light patterns that allows distinguishing regions with a deposit from regions deprived of deposit as explained below.
The image capturing system 16 is configured to capture an image 36 of the light patterns 20 drawn on the surfaces 24 of the fuel rods 6.
The image 36 is a digital matrix image formed of a matrix of pixels acquired by a matrix optical digital sensor. The image 36 is a light intensity image. Each pixel is assigned a light intensity value.
In the illustrated embodiment, the image capturing system 16 comprises one single image capturing device 30 comprising an optical digital sensor. The image capturing device 30 is for example a video camera. The optical digital sensor is for example a CMOS (Complementary Metal Oxide Semiconductor) optical sensor or CCD (Charge-Coupled Device) optical sensor.
As schematically illustrated on Figure 1 , the light source system 14 is provided as a single unit and the image capturing system 16 is provided as a single unit, the light source system 14 and the image capturing system 16 being arranged on either sides of the fuel assembly 2. In practice, the light source system 14 may comprises several separated light sources 28 and the image capturing system 16 may comprise several single video cameras. Besides the light source system 14 and the image capturing system 16 are arranged facing the same side face 8 of the fuel assembly 2.
The processing system 18 is configured to digitally process images 36 captured by the image capturing system 16 to determine the presence of a deposit on the surfaces 24 of the fuel rods 6 affecting the light reflection properties of the surfaces 24 of the fuel rods 6 as a function of the light patterns 20 drawn on the surfaces 24.
The processing system 18 comprises a computer 32 for implementing the image digital processing and an image analysis software installed on the computer 32. The image analysis software comprises software instructions for implementing the image digital processing when executed by the computer 32. The processing system 18 is connected to the image capturing system 16 to receive the captured images 36.
The image digital processing comprises processing parameters of the image 36, more specifically of pixels of the image 36 that depend on the light reflection properties of the surface 24, more specifically on the light reflectance of the surface 24.
The processing system 18 preferably comprises a user interface 34 for producing inspection reports and allowing a user to visualize the inspection results. The user interface 34 advantageously allows an operator to visualize images 36 captured by the image capturing system 16 and/or to enter instructions. Entering instruction may comprise in particular setting analysis parameters to be used during image digital processing.
In order to inspect the fuel assembly 2 over its entire length, the inspection system 4 and the fuel assembly 2 can be moved relative to one another along the longitudinal axis L to scan the fuel assembly 2 lengthwise, as illustrated by arrow F.
Figure 2 illustrates an image 36 captured by the image capturing system 16. The sections 38 of the fuel rods 6 visible on the image 36 are partially affected by CRUD. The visible lower portion 6A of each fuel rod 6 is unaffected by CRUD and the visible upper portion 6B of each fuel rod 6 is affected by CRUD.
The reflection on a surface 24 unaffected by CRUD is mostly specular. CRUD affects the reflection properties of the surface 24 that becomes mostly a diffuse surface. Consequently, the intensity of light reflected by the surface 24 varies depending on whether the surface 24 is affected by CRUD or not.
As illustrated on Figure 2, the light pattern 20 is formed on the CRUD-unaffected lower portion 6A of each surface 24 and is also formed on the CRUD-affected upper portion 6B of the surface 24 but is much more diffracted due to the mat aspect imparted by CRUD.
The image digital processing comprises mathematically comparing the light pattern 20 drawn on at least one region of each surface 24 with at least one other light pattern 20 drawn on a comparison region by a light source system 14.
A comparison region is another region of the same surface 24 or a region of another surface 24, in particular a surface 24 of another fuel rod 6, in particular another fuel rod 6 of the same fuel assembly 2 or a fuel rod 6 not belonging to the same fuel assembly 2, for example a reference fuel rod, or even a surface 24 of a sample specifically prepared to be used as standard. The comparison region is free of deposit, affected by deposit or has an unknown presence of deposit.
In a preferred embodiment, the light pattern 20 drawn on a region under inspection is compared to a reference light pattern obtained for a standard comprising a deposit-free and several different deposit-affected areas. Such reference light pattern may be stored in the computer 32 and used for inspection of different fuel assemblies 2 in the same or in different nuclear reactors.
Comparing the light pattern 20 of the region under inspection with the light pattern 20 of a region of the same surface with unknown presence or absence of deposit allows determining that one of the two regions is affected by CRUD and therefore that the surface 24 is affected by CRUD.
Comparing the light pattern 20 of the region under inspection with a reference light pattern 20 of a region with known presence or absence of CRUD allows determining presence or absence of CRUD in the region under inspection as a function of a mathematical correlation between the light patterns 20.
The mathematical comparison of two light patterns 20 is for example operated by mathematically comparing the light patterns 20 on images 36 captured by the image capturing system 16, e.g. by template matching.
In alternative or in option, the mathematical comparison of two light patterns 20 is operated by comparison of light intensity profiles of the two light patterns 20 calculated using images 36 captured by the image capturing system 16, the light intensity profiles being compared e.g. by template matching.
A light intensity profile is for example determined as the variation of the light intensity of the light pattern 20 along a first direction.
When considering a region having an extension along a second direction perpendicular to the first direction, the light intensity profile along the first direction is obtained for example by determining at each position along the first direction the mean value of light intensity of the light pattern 20 along the second direction over the extension of the region along the second direction.
In the example illustrated on Figures 2 to 4, the individual light intensity profiles of the light patterns 20 drawn on regions of the fuel rods 6 at an axial location of the fuel assembly 2 are calculated by calculating an assembly light intensity profile in an analysis window of the image 36 having a height along the length of the fuel rods 6 and a width transversely to the length of the fuel rods 6. The height of the analysis window may vary from one pixel to a few millimetres, depending on the conditions of the examination. The analysis window comprises a matrix of pixels comprising columns extending along the length of the fuel rods 6 and rows extending transversely to the length of the fuel rods 6. The assembly light intensity profile is determined by calculating the mean value of light intensity for each column and determining the assembly light intensity profile over the columns. In the particular embodiment, the light intensity is determined by the greyscale of the pixels in the analysis window of the image 36, using for instance an image processing program such as ImageJ.
Figure 2 illustrates a first analysis window 40 extending over the lower portions 6A unaffected by CRUD of the fuel rods 6 and a second analysis window 42 extending over the upper portions 6B affected by CRUD of the fuel rods 6.
Figure 3 is a graphic illustrating a first assembly profile 44 calculated for the first analysis window 40. The graphic indicates mean greyscale in ordinate as a function of distance in image pixels in abscissa. The first assembly profile 44 comprises a set of first peaks 46. Each first peak 46 corresponds to the individual light intensity profile of the surface 24 of a respective fuel rod 6. Each valley between two adjacent peaks corresponds to a gap between two adjacent fuel rods 6.
Figure 4 is a graphic analogous to that of Figure 3 and illustrating a second assembly profile 52 calculated for the second analysis window 42. The second assembly profile 52 comprises a set of second peaks 54. Each second peak 54 corresponds to the light intensity profile of the surface 24 of a respective fuel rod 6.
As visible on Figures 3 and 4, the first peaks 46 and the second peaks 54 have different shapes which allow discriminating between a CRUD-free surface 24 and a CRUD- affected surface 24. This discrimination does not depend on the greyscale of the image 36. It only depends on the shape of the peaks.
Determining presence or absence of CRUD of a region of a surface 24 of a particular fuel rod 6 at an axial location is operated as a function of the individual light intensity profile of the light pattern 20 drawn on this particular region of the surface 24 of the fuel rod 6 at this axial location, i.e. the peak corresponding to this particular region, isolated in the corresponding assembly profile.
In one embodiment, the processing system 18 calculates a correlation coefficient as a function of the comparison between the light pattern 20 drawn on the region and the light pattern 20 drawn on the other comparison region e.g. by comparing the images of the light patterns 20 and/or comparing the light intensity profiles of the light patterns 20, and determines presence of CRUD as a function of the correlation coefficient.
The presence or absence of CRUD is determined as a function of the correlation coefficient and as a function of at least one criterion, depending in particular on whether the comparison region is a region which is CRUD-free, CRUD-affected or with unknown presence of CRUD.
In one embodiment, a criterion is that if the light pattern of a region under inspection has a correlation coefficient with a reference light pattern of a CRUD-free region of the same surface or another surface which is higher than a threshold, the region under inspection is determined as being CRUD-free.
In one embodiment, a criterion is that if the correlation between light patterns of two regions of the same surface with unknown presence or absence of CRUD is below a threshold, the surface is determined as being CRUD-affected in at least one of the two regions.
The inspection system 4 allows automated inspection of the fuel rods 6 and automated generation of an inspection report. The inspection system 4 is reliable and efficient. Peripheral fuel rods 6 of a side face 8 if a fuel assembly 2 are inspected simultaneously and rapidly. All the peripheral fuel rods 6 of a side face 8 or only a part of the peripheral fuel rods 6 of a side face 8 are inspected simultaneously. The inspection system 4 and the fuel assembly 2 can be moved relative to one another along the longitudinal axis L to scan the fuel assembly 2 lengthwise, step by step or in a continuous movement. Preferably, the inspection is limited to the relevant portions of the fuel assembly 2.
In view of expediting inspection of a fuel assembly 2, it is possible to provide image capturing devices 30 distributed along a side face 8 of the fuel assembly 2. In a specific embodiment, the image capturing system 16 is distributed to capture in one shot images 36 covering one entire side face 8 of the fuel assembly 2. One or several light source(s) 28 are provided to draw the light pattern 20 onto the side face 8 of the fuel assembly 2 in the field- of-view of the or each image capturing device 30 of the image capturing system 16. Similarly, it is possible to provide several image capturing devices 30 distributed around the fuel assembly 2 to capture simultaneously images 36 of different side faces 8. The image capturing system 16 comprises for example at least one image capturing device 30 for each side face 8 to capture images 36 of each side face 8 in one shot. Alternatively, the image capturing system 16 comprises for example at least one image capturing device 30 for only two side faces 8, and the fuel assembly 2 is rotated around its longitudinal axis L for inspection of all the side faces 8.
The invention is described above for the inspection of the external surface of nuclear fuel rods. It is however applicable to the inspection of an external surface of a steam generator tube or even, for instance, to the inspection of an internal surface of a steam generator shell. More generally, the invention is particularly suited for detecting presence of deposit on any nuclear component. The invention may be used more generally for detection of any deposit on any surface, in particular a surface of a nuclear power plant or a nuclear manufacturing plant, underwater or in the air.
The invention may be used for instance for under water detection of oxidation on control rod cladding tubes of rod cluster control assembly, for cleanness control, for instance in air on non-irradiated components in the manufacturing plant.
In a general manner, the inspection system 4 and the inspection method allow determining surface condition alteration affecting light reflection properties of a surface 24, in particular reflectance of the surface 24. The light source 28 may comprise any type of light source leading to light reflection onto the surface 24 under inspection. The light source is for example a halogen source, a fluorescent source, a LED source or a laser beam.
The arrangement of the light source 28 and the shape of the surface 24 under inspection determines the shape of light pattern 20. The arrangement of the light source(s) 28 is chosen as a function of the surface 24 to be inspected to draw light patterns 20 that can be reliably discriminated by the processing system 18.
The surface under inspection is preferably curved, namely circular. The surface under inspection is preferably concave or convex, and preferably convex. Light sources draw a light pattern formed of light stripes on a cylindrical convex surface.

Claims

1.- Inspection system for detecting a deposit on a surface (24), the inspection system comprising:
- a light source system (14) to draw a light pattern (20) onto at least a region of the surface (24);
- an image capturing system (16) to capture a digital image (36) of the light pattern (20) drawn onto the region of the surface (24); and
- a processing system (18) configured to digitally process the image (36) to determine the presence of a deposit on the region of the surface (24) affecting light reflection properties of the surface (24) as a function of the light pattern (20) drawn onto the region of the surface (24).
2.- Inspection system as in claim 1 , wherein the processing system (18) is configured to determine presence of a deposit as a function of a comparison of the light pattern (20) drawn onto the region of the surface (24) with at least one other light pattern (20).
3.- Inspection system as in claim 2, wherein the comparison comprises comparison of the light pattern (20) drawn onto the region of the surface (24) with at least one light pattern (20) drawn onto a comparison region, the comparison region being another region of the same surface (24) or a region of another surface (24), the comparison region being free of deposit, affected by deposit or with unknown presence of deposit.
4.- Inspection system as in claim 2 or 3, wherein the processing system (18) is configured to compare light patterns (20) drawn on the regions by comparing profiles of light intensity of the light patterns (20) using the images (36) of the light patterns (20).
5.- Inspection system as in any preceding claim, wherein the light pattern (20) comprises one stripe (26) or several parallel stripes (26).
6.- Inspection system as in any preceding claim, configured for inspection under water or in air of the surface (24) of a nuclear component and/or the external surface (24) of at least one nuclear fuel rod (6).
7.- Inspection system as in any preceding claim, wherein the light source system (14) is configured to draw a respective light pattern (20) onto the external surface (24) of each fuel rod (6) of a plurality of peripheral fuel rods (6) of a fuel assembly (2) and the image capturing system (16) is configured to capture images (36) of the light pattern (20) drawn onto at least one fuel rod (6) of the plurality of fuel rods (6).
8.- Computer implemented inspection method for detecting a deposit on a surface (24), comprising: - drawing a light pattern (20) onto at least a region of the surface (24) under inspection;
- capturing a digital image (36) of the light pattern (20) drawn onto the region of the surface (24) under inspection; and
- digitally processing the image (36) for detecting a deposit on the region of the surface (24) affecting the reflection properties of the surface (24) as a function of the light pattern (20) drawn onto the region of the surface (24).
9.- Inspection method as in claim 8, wherein the step of determining the presence of a deposit on the surface (24) comprises comparing the light pattern (20) drawn onto the region of the surface (24) with at least one other light pattern (20).
10.- Inspection method as in claim 9, wherein the comparison comprises comparing the light pattern (20) drawn onto the region of the surface (24) with at least one light pattern (20) drawn onto a comparison region, the comparison region being another region of the same surface (24) or a region of another surface (24), the comparison region being free of deposit, affected by deposit or with unknown presence of deposit.
11 .- Inspection method as in claim 9 or 10, wherein comparison of light patterns (20) drawn onto regions comprises comparing profiles of light intensity of the light patterns 20) using images (36) of the light patterns (20).
12.- Inspection method as in one of claims 8 to 11 , wherein the light pattern (20) comprises one stripe (26) or several parallel stripes (26).
13.- Inspection method as in one of claims 8 to 12, comprising scanning the surface (24) under inspection by relatively displacing the light pattern (20) and the field-of- view of an image capturing system (16) along the surface (24).
14.- Inspection method as in one of claims 8 to 13, implemented under water or in air on the surface (24) of at least one nuclear component and/or on the external surface (24) of a cladding (10) of at least one nuclear fuel rod (6).
15.- Inspection method as in one of claims 8 to 14, comprising drawing a respective light pattern (20) onto the external surface (24) of each fuel rod (6) of a plurality of peripheral fuel rods (6) of a fuel assembly (2) and capturing a digital image (36) encompassing at least a section (38) of the external surface (24) of at least one of the plurality of fuel rods (6).
EP20789281.1A 2019-10-03 2020-10-02 Visual inspection system and visual inspection method, namely for inspection of the external surface of nuclear components Pending EP4038640A1 (en)

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