Patent Number: 
Section: description

Referring now to the figures of the drawings in detail and first, particularly, to FIG. 1 thereof, there is shown how a fuel element FA of a boiling-water reactor is inspected. The lateral box has been removed such that the fuel rod bundle FB including the schematically indicated fuel rods FR is visible. The fuel rods are held at a plurality of axial positions, in each case by a spacer FS at prescribed positions distributed regularly over the fuel element cross section. Located in the middle of the fuel element is a hollow tube or cladding tube (xe2x80x9cwater tubexe2x80x9d FW), on which the foot part FF and the head part FH of the fuel element are fastened. The fuel element bundle FB with the head part and the foot part is inserted into a positioning device P, in this case the foot part FF engaging in a centering plate PC and being fixed in its position via hydraulically pressed-on lateral jaws PB. The positioning device P further contains a frame, which is constructed here as a rack PG made from guide rails for a plane table MT. The guide rails PG define the z-axis of a reference system whose center point and x-, y-axes are given by the center point and the alignment of the centering plate PC. The plane table MT is part of a measuring device M which can be displaced in the z-direction through the use of drives PD, and is positioned at the level of a spacer of the fuel rod bundle FB. Such a plane table, which can be displaced along the fuel rod bundle relative to the fuel rods and their spacers, is already used for inspection devices and generally carries a video camera VC in order to undertake optical inspection of the fuel rods and the spacers. It is usual in this case for the video camera also to move in the x- and y-directions relative to the fuel rod bundle, in order to monitor the fuel element from all sides. In the present case, a plurality of video cameras are provided at the plane table MT in order to monitor the spacer completely without having to change the relative position of the plane table MT. In FIG. 1, the spacer to be inspected, which is a part of the fuel element bundle FB, is hidden by the measuring device M. However, it is possible to see two mutually opposite arms MA of the measuring device M, which run along the left-hand and right-hand outer surface of this spacer and carry probes US which are configured as ultrasonic probes. These ultrasonic probes US are directed partly toward the left-hand or right-hand outer surface of the spacer, and partly also toward the end faces of a calibration rod CS of known length d0. These ultrasonic probes US emit ultrasonic pulses which are reflected at the outer surfaces of the spacer or the end faces of the calibration rod CS. The reflected echo is received by the ultrasonic probes US and it is therefore possible to determine the spacing of the spacer from the propagation time of the pulse echo. The calibration rod CS, disposed on the front side and the rear side of the fuel rod bundle, is mounted in each case on a further measuring arm MAxe2x80x2, which likewise carries a plurality of ultrasonic probes USxe2x80x2. It is thereby also possible to measure the outer surfaces of the spacer on the front side (visible in FIG. 1) of the fuel rod bundle and the opposite rear side. The outermost ultrasonic probes of these further measuring arms MAxe2x80x2 are directed toward the end faces of further calibration rods (covered in FIG. 1) which are located below the measuring arms MA. In the cross section through the plane IIxe2x80x94II in FIG. 1, which is shown in FIG. 2, it may be seen that the three ultrasonic probes US1, US2 and US3 at three measuring points on the left-hand outer surface FSA of the spacer FS measure the spacing of this spacer from the corresponding, left-hand measuring arm MA. The outer ultrasonic probes US4 and US5, by contrast, measure the spacing of the end faces CA or CAxe2x80x2 of the corresponding calibration rod CS or CSxe2x80x2 extending from left to right. The spacings between these end faces CA and CAxe2x80x2, on the one hand, and the measuring probes US4 and US5 differ, but are known, and so these two probes US4 and US5 supply two calibration points for the relationship between the propagation time of the ultrasonic echo and the arc covered. Corresponding probes US1B to US5B are situated opposite the probes US1 to US5 on the right-hand arm MB. In the same way, these ultrasonic probes on the opposite outer surface FSB of the spacer report three measuring points for the spacing, and two calibration points on the corresponding end faces CB, CBxe2x80x2 of the calIbration rods. Since the length do of each calibration rod CS, CSxe2x80x2 is known, this also results in an exact value for the spacing of the opposite measuring arms in this plane, or a computational correction if the measuring arms are not strictly parallel to one another. In the same way, in the plane lying therebelow, in which the ultrasonic probes USxe2x80x2 shown in FIG. 1 are situated, three measuring points and two calibration points are formed in each case for the two other mutually opposite sides of the spacer. The signals from the sensors are fed to a computer CAL with a monitor MON which is provided outside the water reservoir in which the inspection takes place. This is an electronic evaluation system which selects the sensor signals in a suitable way, calibrates them and displays them as characteristic variables of the measured spacer, as is explained with the aid of an image (FIG. 3) output on a display screen. In this FIG. 3, firstly, the geometry G of an unbowed fuel element is demonstrated; the center GC thereof would be disposed at the coordinate origin of the x-y system of the measuring device M. D1, D2, D3 illustrate the three measuring points of an outer surface, and D1xe2x80x2, D2xe2x80x2, D3xe2x80x2 illustrate the corresponding, already calibrated measuring points at the opposite outer surface, which result from the ultrasonic echoes. The most important variables for assessing the relevant spacer (here: the third spacer, xe2x80x9cspacer 3xe2x80x9d) is the maximum spacing xcex94max between opposite outer surfaces. If the fuel element is not twisted, this is the maximum value of the variables xcex94(y1), xcex94(y2), xcex94(y3), xcex94(x1), xcex94(x2), xcex94(x3), wherein xcex94(y1) is the difference between the y-coordinates on the mutually opposite points D1, D1xe2x80x2. The differences xcex94(y2) and xcex94(y3) are assigned correspondingly to the respectively mutually opposite points D2, D2xe2x80x2 and, respectively, D3, D3xe2x80x2, and the differences xcex94(x1), xcex94(x2), xcex94(x3) of the x-coordinates are assigned to the further point pairs illustrated in FIG. 3. The value xcex94max can then be specified directly-in micrometers or as a percentage with respect to the ideal fuel element. The variable xcex94d(y), which describes a convex curvature of the outer surfaces, can be determined, for example, in accordance with xcex94d(y)=xcex94(Y2)xe2x88x92(xcex94(y1)+xcex94(y3))/2. A further interesting variable is the x-coordinate C(x) or y-coordinate C(y) of the center point C (which can be determined from the two measuring points D2, D2xe2x80x2) refer to the desired center point GC (origin of coordinates). The bowing of the entire bundle can be determined thereby. In order also to detect twisting of the spacer, it is possible, for example, to determine the angle between the straight line defined by the measuring points D1xe2x80x2 and D3 and the y-axis. In the display, illustrated in FIG. 3, on the display screen of the computer CAL, a curve of second order is drawn through the points D1, D2 and D3 by computation, and the same is done for the corresponding measuring points on the other outer surfaces. These curves and their point of intersection are illustrated as contour of the measured spacer. The coordinates C(x) and C(y) for the center point of the deformed spacer describe the bowing of the fuel element and result from the point of intersection of the connecting lines which are respectively calculated from the diametrical corners of the illustrated contour, calculated from the measuring points, of the deformed spacer. The angle xcex1 describes the twisting of the fuel element and corresponds to the mean value of the angle by which in each case a diagonal of the deformed spacer is rotated relative to the corresponding diagonal of the geometry G. The connecting straight lines between two neighboring corners describe the outer contour of an undented, but twisted spacer whose deviation from the geometry G can be described by the values xcex94d(x) and xcex94d(y). The maximum width of the spacer is described by the value xcex94max. As a rule, it suffices to display these calibrated, characteristic measured values on a display or to blend them into the display screen, while the remainder of the image can be used in order to make the video images of the camera VC available to the operating staff for the optical inspection of the spacer. The walls 1 of a fuel element storage rack in the fuel element cooling pond of a nuclear power plant are visible in FIG. 4. Also illustrated are only the foot part 5a, the control rod guide tubes 5b and the spacers 5c of the fuel element 5. Mounted on the top side of this storage rack is a frame 2 which forms a workstation with a frame part 11 and a platform 3 which laterally surround the fuel element. Various apparatuses which are provided for inspecting and/or maintenance can be mounted on positioning bolts 4. A pressurized-water fuel element 5 is transported to this workstation through the use of a fuel element handling machine, only the lower end 6 of the fuel handling machine mast with the centering bolt 7 being visible in FIG. 4. It is not explicitly shown in the following figures that the centering bolts 7 can be used to position the mast 6 on the frame 2 and then to place the fuel element in a defined position relative to the frame, which defines the reference system for measuring the fuel element. Mounted on the platform 3 is a base plate 12 which bears a plane table 20 which can be displaced via an x-drive 21 and a y-drive 22 along corresponding x- and y- guide rails 21xe2x80x2, 22xe2x80x2. The face of the plane table 20 is parallel in this case to the x-y plane of a reference system whose z-axis is given by the frame 2 and the mast 6 of the handling machine. These parts therefore constitute a positioning device which can simultaneously provide a coordinate system for evaluating measured values. Via the drives 21, 22, the plane table can be moved in the plane to a desired position in the x-direction and/or y-direction. A module 8 which contains a measuring device is fastened on the plane table. This module 8 is illustrated in FIG. 5, in which the plane of a drawing is parallel to the y-z plane of the reference system described above. Also shown is a shaft 13 which is provided chiefly for fixing the fuel element to be inspected in cases in which the foot of the fuel element is not fixed in the way shown in FIG. 1, but is still suspended in the mast 6 of the handling machine. The shaft can be fastened on the plane table or, via a plane part 11, on the base plate 12 or the platform 3. It includes shaft walls 14, which support the fuel element laterally and have on three sides a transverse slot 15 through which it is possible to access the three outer surfaces of the fuel element and/or the spacer 5c thereof. Positioned at the upper edges of the shaft walls are guide planes 16, which run in obliquely, from above, onto the edges of the shaft walls and serve the purpose of facilitating the introduction of the fuel element into the shaft. The measuring device has two mutually opposite measuring arms 30 at one end of which there is a probe 31 in each case. At their other end, the measuring arms are respectively connected to the remainder of the module via a feed drive 32 operating in the y-direction. The measuring arms are provided in this case such that their longitudinal axis is parallel to the y-axis of the reference system, and that the probes can respectively be laid through the transverse slots 15 against one of the two opposite outer surfaces 35, 36 of the fuel element or spacer 5c, which are virtually parallel to the longitudinal axis of the measuring arms. The arms are advantageously fitted such that their mutual spacing can be set, in which case it is then possible to use the device for fuel elements with different widths. The device can be used, for example, to measure the boxes and the spacers in the case of boiling-water fuel elements. FIG. 6 also shows further outer surfaces 37 of the spacer 5c, which can also be inspected. Optical monitoring via a video camera is provided for the purpose of remotely controlling the positioning of the measuring arms via the x- and y-drives. The camera 40 and the associated lighting 41 are therefore fitted on the plane table itself, or are components of the module mounted on the plane table. Also to be seen in FIG. 5 is a calibration rod 50, which is illustrated more precisely in FIG. 6. This FIG. 6 shows the shaft 13 with the walls 14, and the probes 31, which grip through the transverse slots onto two opposite outer surfaces of a spacer 36, and arms 30 of the probes. The plane of the drawing in FIG. 6 is parallel to the x-y plane of the reference system. In this exemplary embodiment, the calibration rod 50 is fastened on the plane table 20 via a holder 42 which is positioned by the x- and y-drive 21, 22 on the spacer 36 until it bears resiliently with a defined pressure. To render interpolation possible during calibration, the end faces of the calibration rod 50 are configured in three steps, that is to say they have a plurality of respectively mutually opposite subareas 52, 53. The steps are configured such that the calibration rod prescribes three linear measures, of which at least one is larger, and one smaller, than the mutual spacing of opposite outer surfaces of the fuel element to be measured. It is possible in this way to use interpolation to draw a calibration curve for the relationship between probe-measured data and the expansion of the fuel element. The measuring arms 30 can be moved synchronously in the y-direction via a feed 32. They can, for example, be stiff and mounted rotatably at one end such that the deviation of the probes in the x-direction can be detected by a rotary encoder on the rotatable bearing. A hydraulic drive can also be used, for example, instead of a y-feed for the purpose of extending the telescopic arms. In the exemplary embodiment illustrated in FIG. 5, a part 34 of the arm is constructed as a spring which bears strain gauges on both sides. The resistance of the strain gauges is measured via a Wheatstone bridge circuit for the purpose of determining the position of the probes. FIG. 7 shows such a measuring arm in detail, and FIG. 8 shows a circuit diagram of the bridge circuit. FIG. 7 shows the arm 30 with the probe 63, which slides with a camber or bulge 64 along the outer surfaces of the fuel element. The arm contains an approximately triangular spring 60. A rigid part 62 of the arm with the probe 63 is attached at one corner of this spring. The rigid part 65, connected to the y-drive 32, of the arm is fastened on the opposite side of the spring. The triangular shape of the spring is favorable, because in this way the spring tension is distributed uniformly over the entire length of the spring and exhibits a virtually linear dependence on the x-deflection of the probe. The two triangular faces of the spring are coated, and each form a strain gauge. The electrical resistance of each strain gauge depends approximately linearly on the spring tension. Consequently, it is possible to use the measurement of these resistances to calculate measured values for the x-position of the probe, which can be calibrated by the measured values which are obtained on the calibration rod. The strain gauges on the faces of the spring 60 are connected via connections 66 to the bridge circuit 68 of an electronic measuring system, for example an electronic evaluation device integrated in the computer CAL. A circuit diagram of the bridge circuit 68 is shown in FIG. 8. R1 and R2 are the resistances to be measured of the two strain gauges. They are connected in a Wheatstone bridge circuit to adjustable resistors R3 and R4. To balance the bridge, the current I is controlled to zero by suitable adjustment of R3 and/or R4. The voltages which are present across R1 and R2 or R3 and R4 are then respectively of the same absolute value. If the spring with the strain gauge is in a position of rest, R1 and R2 are of the same magnitude; R3 and R4 must then likewise become equal, so that balancing comes about. If the spring is deflected, one strain gauge is stretched while the other is compressed, that is to say one of the resistances becomes larger while the other becomes smaller. The ratio of the two adjustable resistors R3 and R4 then corresponds, with the bridge balanced, to the ratio of the resistances R1 and R2 to be measured. Effects which are to be ascribed to thermal expansion are largely eliminated with this method, since R1 and R2 change in the same sense. The measurement itself is advantageously carried out using AC voltage employing the known carrier frequency principle. A connected measuring amplifier can then be calibrated such that it directly specifies the tension of the spring and/or the deflection of the probe. The invention therefore renders it possible for the geometry of the fuel element to be measured in a simple way, and for deformations to be detected.