rus
Issue #2/2015
D. Lyakhov
Apparatus for Electrooptical Measurement and Control of Components of Power Reactors Fuel Assemblies
Apparatus for Electrooptical Measurement and Control of Components of Power Reactors Fuel Assemblies
The technology of producing items by welding includes the operations of control of permanent joints on a mandatory basis. If there are elements with the complex shape surfaces incorporated in the items, the precision of their execution is determined using special measurement means –contact measurement devices. The article covers the control of welded joints of spacer grids and slab lattices of VVER power reactors fuel assemblies (FA).
The nuclear power facilities on the basis of double-circuit reactors with pressurized water (VVER) received the broad application in the world due to the technology simplicity, high reliability of construction and economy in use. The spacer grids (SG) and slab lattices designated for the intensification of heat carrier mixing (research reactor – RR) and created using the welding technologies refer to the nodes of VVER fuel assemblies. In their production the control of permanent joints with complex surface shapes is the compulsory procedure, during which it is important to measure the coordinates of individual points or form of surface sections. The universal measurement devices designed for the purposes of surface control refer to the complex mechanical devices, which have the probe; using the probe head the operator registers the absolute deviations of surface points from some set reference points. Besides the probe, the kits of measurement instruments can include rods, templates and other calibration elements but similar mechanical instruments are designed for the control of the specific products and they do not refer to the multi-purpose instruments. Since with the development of new materials and technologies the products for reactors are constantly modernized, for the economy the multi-purpose control and measurement instruments are required. Their complicated mechanical structure needs good protection against vibrations and thermal stabilization, careful use during operation. More requirements occur when the issues, which are connected with the measurement of some internal dimensions, distances between apertures, projections or different sections of the part with complex configuration, arise (Fig. 1). In general, all devices for contact measurement, which are known at the present time, need the bulky technological preparation for the execution of high-precision measurements of the products with complex configuration. And often, the technological preparation includes the creation of different mechanical or electrotechnical accessories, which, in turn, are designated for the specific particular applications and require large expenditures of time and money connected with certification and final adjustments.
Particularly, these reasons limit the capabilities of application of contactless optical control*. The contactless optical control is based on the use of two methods: measured object control method by the shaded image and object control method by the reflected light. The simple and multi-purpose optical devices for contactless control are offered in the article. In many aspects they do not have listed shortcomings; they are very simple in their implementation and possess the great universality and speed of needed information acquisition. Herewith, the set measurement precision is achieved. Also it should be specifically noted that the contactless optical control methods are often lowly connected with the mechanics and this fact makes them precise, reliable, universal, quick-operating, comparatively simple in the production and having high measurement accuracy (for example, for the measurement of the parameters of surface 100×100 mm they need fractions of second).
It is completely obvious that before we start measuring the distance between any objects by the image, first of all, it is necessary to determine the object boundaries. Our object is represented by the periodic structure with repeated elements (Fig. 2) having the distinguished boundaries, which must be controlled. The main boundary indicator at the image is the sudden drop in the intensity picture along the certain line crossing the element boundary (Fig. 2). Due to the laws of wave optics, the boundary will be always slightly blurred. For example, in Fig. 3 at the diagram the variation of the intensity Ii is shown at the image of object boundary along the certain coordinate direction Х, which is perpendicular to the boundary between the structure elements.
The intensity drop at the boundary is indicated by dashed line in the perfect case when there is no optical blurring. The solid line corresponds to the actual situation. It follows from the theory of optical images that the derivative from the image intensity in the direction X can be represented in the following form with the sufficient precision:
, (1)
where А is some constant which characterizes the maximum value of derivative; σ is the function which characterizes the boundary blurring; are the boundary coordinates. The approximate form of this function is given below (Fig. 4). In the flat case the intensity drop steepness is characterized by the module of intensity surface gradient.
. (2)
The analytical form of the function I (x1, x2) is unknown to us. We can use only numerical values of the intensity function which are registered by TV camera. Therefore, we will take the necessary partial derivatives from this numerical data. In the region of registering matrix of TV camera we select the segment from x1 to x2 corresponding to the area of four pixels (two by two) and approximate the intensity function by incompletely square-law quadratic multinomial from x1 and x2.
. (3)
Coordinates of the center of each of four pixels have the values (±1,±1). We know the intensity values in each pixel – Ii. Using the least square method we can determine the coefficient b. Then the values of the partial derivatives of the first order for both variables in the center of selected area are equal to the values of the coefficients b1 and b2.
Having calculated these coefficients, taking into account (3), the value of gradient module will have the following form:
. (4)
Further, using the methods of image recognition, for example, template matching technique, it is easy to find the image details, between which the distance must be measured or dimensions of which must be calculated. In the technology, the product boundary usually represents the straight line or circular arc. Unknown function coefficients (1) can be determined using the least square method (LSM) at nonlinear parameterization. Thus, in case of the variation of measured object form we only change the processing program of the image registered by TV camera but we do not change anything in the construction of the device itself.
For these purposes, using the initial file the whole image is scanned and the calculations on the basis of the formula (4) are performed sequentially for every four nearest pixels. And during the scanning one-pixel shift always takes places. We obtain the file with the image, on which the boundaries of all targeted objects are registered. The methods of preparation of the similar programs are well known. Basically, we can have the set of the programs for the most frequent, typical cases and use them from the general menu, when necessary. The lattice geometrics include the diameters of circle cells inscribed in the profile and distances between the centers of these circles. The positions of the boundaries (Fig. 5, 6) of welded lattices are calculated on the basis of shaded image (see Fig. 2).
Measurements of Geometrics of FA Spacer Grids
Works connected with the creation of the measuring bench OES-1, which is designated for the accomplishment of contactless measurements of geometrics of the spacer grids of power reactors VVER-440, were commenced in the late 90s. The first stage of these works ended with the design and commissioning of the pilot measurement facility for experimental and industrial operation in December 2001. Later in 2004–2007, at the Central Research Laboratory of OJSC "Machine Engineering Plant" the facility modernization was accomplished; it allowed transforming it into the measuring bench OES-1 for the measurement of geometrics of the spacer grids of power reactors VVER. The bench (Fig. 7) allows measuring the parameters of the grids of VVER-440 type without readjustment. Herewith, the bench grants the opportunity of analogous measurements for the grids of VVER-1000 type but its readjustment is required for this purpose. If the measuring bench is improved by equipping it with the lens with the diameter of 400 mm (as for now, the lens with the diameter of 200 mm is installed) the time needed for the measurement of parameters of grids VVER-1000 will be considerably shorter.
In order to increase the measurement accuracy of grid geometrics, these parameters should be measured in three-four different positions in the fastening unit. The grid shift relative to the previous position must be more than 3 mm. After the series of five consequent measurements in each position, the parameter calculations are performed. There should be no unidentified cells with different coordinates in the accomplished calculations. Only the variants of availability of unidentified cells with the same coordinates are permitted. Then, the obtained data is averaged with the help of this program. The files with the extension TV are used for summation.
Metrological Analysis
of Measurement Errors
Random Error
The random measurement error is distributed according to the normal law. Using the bench OES-1, with its correct adjustment and three consequent measurements the random error of the measurements of spacer grids geometrics is equal to – 10 μm (with the probability p = 0.95), and the measurement error of the grid engraved on the glass is equal to – 12 μm (with the probability p = 0.95). The great measurement error in the second case can be explained by the conditions of measurement execution: the picture lines partially transmit the light and in the resulting signal the ratio signal/noise considerably decreases. Besides, passing through the glass the light declines and covers the new interferences and this fact also causes the reduction of measurement precision.
Systematic Error
The systematic error of the measurement of inscribed diameter D grows with the increase of the angle of cell deviation α to the axis, which is directed perpendicularly to the grid plane, and always has the negative value. In other words, if we do not take into account the random errors the results of the measurements of inscribed circle diameters will always be lower than the value of the actual diameter or equal to it at this bench. The formula for the approximate evaluation of systematic error has the following form:
,
where L is the length of support along the cell axis in mm. It follows from this formula that at the angle values α = 0.25° and L = 5 mm the systematic error Δ is approximately equal to – 0.022 mm. In practice, this error was equal to – 0.020 mm at the average.
The mechanical method of measurements of cell diameters exists; this method implies the measurements of diameters with the help of the step cylindrical plug gages. Such gages consist of 4 plugs with the height of 40 mm located on the common longitudinal axis with the interval of 4 mm. The diameter of every consecutive plug in comparison with the previous one grows by 0.02 mm. The gages are dropped into every SG cell and diameter of the plug, which gets stuck in it, corresponds to the cell diameter. This control method takes a lot of time and, besides that, it is revealed that it has restrictions connected with the range of measured values (Fig. 8).
It is seen that the method with the use of gages is limited by the certain value of diameter. In the cases when the measurements are accomplished by the gages, the values are slightly overrated in comparison with the results executed by the optical-electronic method. The reason consists in the cell inclination (the systematic error occurs). Having accomplished two series of the measurements for the determination of diameter values (via optical-electronic method and method with the use of step cylindrical plug gages for which the cell inclination does not influence on the result) the value of module of the inclination angle can be estimated in radians for every cell according to the formula:
If due to the random errors Δ > 0 the inclination angle can be deemed equal to 0.
Quality Control of Execution of Welded Joints of Slab Lattices (RR)
When producing the slab lattices (RR) of FA of VVER-1000 reactor, the operation of laser welding quality control of plate junctions is carried out at the device OESK-4–1-B, which is equipped with the system of visualization of deviations from the required quality of welded joints, identification and assignment of evaluation status and coordination of the joints, which do not meet the requirements, for the lattice delivery to the repeated welding of these joints. This optical-electronic system displays, analyzes and assigns the certain status to each welded junction on the lattice.
During the execution of the whole complex of operations concerning the quality control of laser welded joints the lattice remains secured in the initial position (Fig.9) prior to the execution of the last action, and afterwards it is turned to the other side. On the reverse side the lattice goes through the analogous complex of control operations, and afterwards all obtained information is processed.
In order to receive the image of every welded joint from three sides and from the top, the original construction of TV camera location is applied for the lattice scanning (Fig. 10). When activating the program, it displays the list of the projects which have been scanned previously, registered with the help of lattice scanning program. Having selected the project of interest the operator can sequentially examine every welded joint from three sides: from the top and two frames at the angle of 45º from the bottom; in this way the operator can ascertain the quality of welded joint and assign the relevant status to it: good, suspicious and bad. Also, the program automatically assigns the status "has been examined by operator" to the joint in case if the operator did not determine the status of welded joint. This operation is introduced specifically in order to have the visual presentation of the number and location of the welded joints, which have been examined and which have not been examined previously, for the elimination of human factor. All information concerning the status of every joint (bad, good and suspicious welded joints) can be saved in the same project, which the operator has opened, and visually displayed in the graphic form at the lattice layout. Having reviewed all welded joints, by pressing the key of report generation the operator opens the generated file in Microsoft Office Word format where the information concerning the studied lattice with all coordinates of suspicious and bad joints is automatically entered. It is also possible to go back to the report generation at any moment selecting the project which is interesting for the operator. Also, after the study of lattice operator can generate the file containing the coordinates of welded joints in the format of the plant machine, which requires the improvement.
Interface Description
The table with the list of projects, which have been scanned previously with the help of the scanning program, is displayed at the interface of welded joint inspection (Fig. 11); the field displaying the properties of the project, which was selected by operator, is marked out: project name, information storage place, what side of the lattice is being inspected by the operator at the moment; the window, where detected suspicious welded joints and their names are displayed, where the status of welded joint is determined: "Good", "Suspicious", "Bad", is designed.
In the field of lattice layout (Fig. 12) the results of inspection state are visually displayed: grey node – inspection and review of welded joint have not been performed; blue node – the review has been performed but the determined status has not been assigned; green node – the status "Good" has been assigned; yellow node – the status "Suspicious’ has been assigned; red node – the status "Bad" has been assigned. The welded joint, which is marked out by big heavy point, means that this point with the relevant status at this moment is being reviewed and its photographs are being displayed at the screen (Fig. 13).
When starting the scanning program (Fig. 14), the main window for scanning control (it is marked with number 1) is displayed; images from four video cameras are given in the windows 2, 4–6; the schematic display representation of lattice and numbered cells are displayed in the window 5.
Conclusion
Reduction of the costs of the production of fuel assemblies of power reactors of VVER type among others covers the control operations connected with the confirmation of the requirements for the quality of technological operations on permanent joints, such as welding, and execution of the necessary measurements, which confirm the fulfillment of the requirements stipulated in the design documentation for the specified geometrics values. In this article, the issues connected with measurements and control of the components of fuel assemblies of power reactors of VVER type, such as spacer grids (SG) and slab lattices designated for the intensification of heat carrier mixing (RR), have been considered. These actions are performed with the use of optical-electronic systems, which allow carrying out the listed control and measurement operations by the contactless method; in turn, this fact allows improving the precision and fast response of the operations of measurement, registration, processing and result delivery and convenience of their further use.
Particularly, these reasons limit the capabilities of application of contactless optical control*. The contactless optical control is based on the use of two methods: measured object control method by the shaded image and object control method by the reflected light. The simple and multi-purpose optical devices for contactless control are offered in the article. In many aspects they do not have listed shortcomings; they are very simple in their implementation and possess the great universality and speed of needed information acquisition. Herewith, the set measurement precision is achieved. Also it should be specifically noted that the contactless optical control methods are often lowly connected with the mechanics and this fact makes them precise, reliable, universal, quick-operating, comparatively simple in the production and having high measurement accuracy (for example, for the measurement of the parameters of surface 100×100 mm they need fractions of second).
It is completely obvious that before we start measuring the distance between any objects by the image, first of all, it is necessary to determine the object boundaries. Our object is represented by the periodic structure with repeated elements (Fig. 2) having the distinguished boundaries, which must be controlled. The main boundary indicator at the image is the sudden drop in the intensity picture along the certain line crossing the element boundary (Fig. 2). Due to the laws of wave optics, the boundary will be always slightly blurred. For example, in Fig. 3 at the diagram the variation of the intensity Ii is shown at the image of object boundary along the certain coordinate direction Х, which is perpendicular to the boundary between the structure elements.
The intensity drop at the boundary is indicated by dashed line in the perfect case when there is no optical blurring. The solid line corresponds to the actual situation. It follows from the theory of optical images that the derivative from the image intensity in the direction X can be represented in the following form with the sufficient precision:
, (1)
where А is some constant which characterizes the maximum value of derivative; σ is the function which characterizes the boundary blurring; are the boundary coordinates. The approximate form of this function is given below (Fig. 4). In the flat case the intensity drop steepness is characterized by the module of intensity surface gradient.
. (2)
The analytical form of the function I (x1, x2) is unknown to us. We can use only numerical values of the intensity function which are registered by TV camera. Therefore, we will take the necessary partial derivatives from this numerical data. In the region of registering matrix of TV camera we select the segment from x1 to x2 corresponding to the area of four pixels (two by two) and approximate the intensity function by incompletely square-law quadratic multinomial from x1 and x2.
. (3)
Coordinates of the center of each of four pixels have the values (±1,±1). We know the intensity values in each pixel – Ii. Using the least square method we can determine the coefficient b. Then the values of the partial derivatives of the first order for both variables in the center of selected area are equal to the values of the coefficients b1 and b2.
Having calculated these coefficients, taking into account (3), the value of gradient module will have the following form:
. (4)
Further, using the methods of image recognition, for example, template matching technique, it is easy to find the image details, between which the distance must be measured or dimensions of which must be calculated. In the technology, the product boundary usually represents the straight line or circular arc. Unknown function coefficients (1) can be determined using the least square method (LSM) at nonlinear parameterization. Thus, in case of the variation of measured object form we only change the processing program of the image registered by TV camera but we do not change anything in the construction of the device itself.
For these purposes, using the initial file the whole image is scanned and the calculations on the basis of the formula (4) are performed sequentially for every four nearest pixels. And during the scanning one-pixel shift always takes places. We obtain the file with the image, on which the boundaries of all targeted objects are registered. The methods of preparation of the similar programs are well known. Basically, we can have the set of the programs for the most frequent, typical cases and use them from the general menu, when necessary. The lattice geometrics include the diameters of circle cells inscribed in the profile and distances between the centers of these circles. The positions of the boundaries (Fig. 5, 6) of welded lattices are calculated on the basis of shaded image (see Fig. 2).
Measurements of Geometrics of FA Spacer Grids
Works connected with the creation of the measuring bench OES-1, which is designated for the accomplishment of contactless measurements of geometrics of the spacer grids of power reactors VVER-440, were commenced in the late 90s. The first stage of these works ended with the design and commissioning of the pilot measurement facility for experimental and industrial operation in December 2001. Later in 2004–2007, at the Central Research Laboratory of OJSC "Machine Engineering Plant" the facility modernization was accomplished; it allowed transforming it into the measuring bench OES-1 for the measurement of geometrics of the spacer grids of power reactors VVER. The bench (Fig. 7) allows measuring the parameters of the grids of VVER-440 type without readjustment. Herewith, the bench grants the opportunity of analogous measurements for the grids of VVER-1000 type but its readjustment is required for this purpose. If the measuring bench is improved by equipping it with the lens with the diameter of 400 mm (as for now, the lens with the diameter of 200 mm is installed) the time needed for the measurement of parameters of grids VVER-1000 will be considerably shorter.
In order to increase the measurement accuracy of grid geometrics, these parameters should be measured in three-four different positions in the fastening unit. The grid shift relative to the previous position must be more than 3 mm. After the series of five consequent measurements in each position, the parameter calculations are performed. There should be no unidentified cells with different coordinates in the accomplished calculations. Only the variants of availability of unidentified cells with the same coordinates are permitted. Then, the obtained data is averaged with the help of this program. The files with the extension TV are used for summation.
Metrological Analysis
of Measurement Errors
Random Error
The random measurement error is distributed according to the normal law. Using the bench OES-1, with its correct adjustment and three consequent measurements the random error of the measurements of spacer grids geometrics is equal to – 10 μm (with the probability p = 0.95), and the measurement error of the grid engraved on the glass is equal to – 12 μm (with the probability p = 0.95). The great measurement error in the second case can be explained by the conditions of measurement execution: the picture lines partially transmit the light and in the resulting signal the ratio signal/noise considerably decreases. Besides, passing through the glass the light declines and covers the new interferences and this fact also causes the reduction of measurement precision.
Systematic Error
The systematic error of the measurement of inscribed diameter D grows with the increase of the angle of cell deviation α to the axis, which is directed perpendicularly to the grid plane, and always has the negative value. In other words, if we do not take into account the random errors the results of the measurements of inscribed circle diameters will always be lower than the value of the actual diameter or equal to it at this bench. The formula for the approximate evaluation of systematic error has the following form:
,
where L is the length of support along the cell axis in mm. It follows from this formula that at the angle values α = 0.25° and L = 5 mm the systematic error Δ is approximately equal to – 0.022 mm. In practice, this error was equal to – 0.020 mm at the average.
The mechanical method of measurements of cell diameters exists; this method implies the measurements of diameters with the help of the step cylindrical plug gages. Such gages consist of 4 plugs with the height of 40 mm located on the common longitudinal axis with the interval of 4 mm. The diameter of every consecutive plug in comparison with the previous one grows by 0.02 mm. The gages are dropped into every SG cell and diameter of the plug, which gets stuck in it, corresponds to the cell diameter. This control method takes a lot of time and, besides that, it is revealed that it has restrictions connected with the range of measured values (Fig. 8).
It is seen that the method with the use of gages is limited by the certain value of diameter. In the cases when the measurements are accomplished by the gages, the values are slightly overrated in comparison with the results executed by the optical-electronic method. The reason consists in the cell inclination (the systematic error occurs). Having accomplished two series of the measurements for the determination of diameter values (via optical-electronic method and method with the use of step cylindrical plug gages for which the cell inclination does not influence on the result) the value of module of the inclination angle can be estimated in radians for every cell according to the formula:
If due to the random errors Δ > 0 the inclination angle can be deemed equal to 0.
Quality Control of Execution of Welded Joints of Slab Lattices (RR)
When producing the slab lattices (RR) of FA of VVER-1000 reactor, the operation of laser welding quality control of plate junctions is carried out at the device OESK-4–1-B, which is equipped with the system of visualization of deviations from the required quality of welded joints, identification and assignment of evaluation status and coordination of the joints, which do not meet the requirements, for the lattice delivery to the repeated welding of these joints. This optical-electronic system displays, analyzes and assigns the certain status to each welded junction on the lattice.
During the execution of the whole complex of operations concerning the quality control of laser welded joints the lattice remains secured in the initial position (Fig.9) prior to the execution of the last action, and afterwards it is turned to the other side. On the reverse side the lattice goes through the analogous complex of control operations, and afterwards all obtained information is processed.
In order to receive the image of every welded joint from three sides and from the top, the original construction of TV camera location is applied for the lattice scanning (Fig. 10). When activating the program, it displays the list of the projects which have been scanned previously, registered with the help of lattice scanning program. Having selected the project of interest the operator can sequentially examine every welded joint from three sides: from the top and two frames at the angle of 45º from the bottom; in this way the operator can ascertain the quality of welded joint and assign the relevant status to it: good, suspicious and bad. Also, the program automatically assigns the status "has been examined by operator" to the joint in case if the operator did not determine the status of welded joint. This operation is introduced specifically in order to have the visual presentation of the number and location of the welded joints, which have been examined and which have not been examined previously, for the elimination of human factor. All information concerning the status of every joint (bad, good and suspicious welded joints) can be saved in the same project, which the operator has opened, and visually displayed in the graphic form at the lattice layout. Having reviewed all welded joints, by pressing the key of report generation the operator opens the generated file in Microsoft Office Word format where the information concerning the studied lattice with all coordinates of suspicious and bad joints is automatically entered. It is also possible to go back to the report generation at any moment selecting the project which is interesting for the operator. Also, after the study of lattice operator can generate the file containing the coordinates of welded joints in the format of the plant machine, which requires the improvement.
Interface Description
The table with the list of projects, which have been scanned previously with the help of the scanning program, is displayed at the interface of welded joint inspection (Fig. 11); the field displaying the properties of the project, which was selected by operator, is marked out: project name, information storage place, what side of the lattice is being inspected by the operator at the moment; the window, where detected suspicious welded joints and their names are displayed, where the status of welded joint is determined: "Good", "Suspicious", "Bad", is designed.
In the field of lattice layout (Fig. 12) the results of inspection state are visually displayed: grey node – inspection and review of welded joint have not been performed; blue node – the review has been performed but the determined status has not been assigned; green node – the status "Good" has been assigned; yellow node – the status "Suspicious’ has been assigned; red node – the status "Bad" has been assigned. The welded joint, which is marked out by big heavy point, means that this point with the relevant status at this moment is being reviewed and its photographs are being displayed at the screen (Fig. 13).
When starting the scanning program (Fig. 14), the main window for scanning control (it is marked with number 1) is displayed; images from four video cameras are given in the windows 2, 4–6; the schematic display representation of lattice and numbered cells are displayed in the window 5.
Conclusion
Reduction of the costs of the production of fuel assemblies of power reactors of VVER type among others covers the control operations connected with the confirmation of the requirements for the quality of technological operations on permanent joints, such as welding, and execution of the necessary measurements, which confirm the fulfillment of the requirements stipulated in the design documentation for the specified geometrics values. In this article, the issues connected with measurements and control of the components of fuel assemblies of power reactors of VVER type, such as spacer grids (SG) and slab lattices designated for the intensification of heat carrier mixing (RR), have been considered. These actions are performed with the use of optical-electronic systems, which allow carrying out the listed control and measurement operations by the contactless method; in turn, this fact allows improving the precision and fast response of the operations of measurement, registration, processing and result delivery and convenience of their further use.
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