WO2005027141A1

WO2005027141A1 - Device for the inspection of fuel assemblies

Info

Publication number: WO2005027141A1
Application number: PCT/SE2004/001289
Authority: WO
Inventors: Ulf Ahlberg; Peter Skoglund
Original assignee: Ahlberg Electronics Ab
Priority date: 2003-09-12
Filing date: 2004-09-09
Publication date: 2005-03-24
Also published as: SE0302431D0

Abstract

The present invention relates to a device for the inspection of a fuel assembly (9) of a nuclear reactor, said device comprising a rig (1; 101), on which a number of underwater camera units (12; 212) are mounted, that the number of a camera units (12; 212) corresponding to the number of longitudinal sides (8) of the fuel assembly (9) that is to be inspected, that the device comprises a number of underwater lighting units (13) that are mounted on the rig (1; 101), and that the device has means to study the images of the longitudinal sides (8) of the fuel assembly (9) that are generated by the camera units (12; 212). The invention also relates to a camera unit per se for the inspection of a fuel assembly of a nuclear reactor. It is significant of the device according to the present invention that the camera units (12; 212) constitute colour camera units with a digital sensor, and that the camera units (12; 212) are equipped with protection (16; 216) against radioactive radiation, said protection constituting a shielding (16; 216) that surrounds a camera body (18; 218), said shielding (16; 216) having a varying thickness.

Description

DEVICE FOR THE INSPECTION OF FUEL ASSEMBLIES

Technical Field of the Invention The present invention relates to a device for the inspection of a fuel assembly of a nuclear reactor, said device comprising a rig, on which a number of underwater camera units are mounted, that the number of a camera units corresponds to the number of longitudinal sides of the fuel assembly that is to be inspected, that the device comprises a number of underwater lighting units that are mounted on the rig, and that the device has means to study the images of the longitudinal sides of the fuel assembly that are generated by the camera units. The device according to the present invention is primarily intended to be used in connection with PWR (pressurized water reactor) but also in connection with other types of reactors. The device also relates to a camera unit per se for the inspection of a fuel assembly of a nuclear reactor.

Prior Art Within the nuclear industry it is extremely important to continuously check the status of the fuel assemblies that are inside the nuclear reactor. The parameters to be checked are for instance damages out of handling, oxide growth, the presence of extraneous objects and bending. For that reason visual inspections are carried out, e.g. by means of video camera units, whereby one side of the fuel assembly is filmed at a time. This visual inspection is preferably carried out in connection with the unloading of the reactor during the annual shutdown for maintenance and exchange of fuel when the fuel assemblies have been moved from the reactor building to the fuel building. By having only one side filmed at a time the time consumption is large and consequently also the cost high to carry out said inspection. The result of this is that only a random number of fuel assemblies are inspected.

Objects and Features of the Invention A primary object of the present invention is to present a device/a camera unit that is designed in such a way that it can resist the radiation from the fuel assembly that is to be inspected. A further object of the present invention is that the device allows a simultaneous inspection of all longitudinal sides of the fuel assembly. Still an object of the present invention is that the use of the device is to be integrated with necessary measures in the maintenance of the nuclear reactor. At least the primary object of the present invention is realized by means of a device/camera unit that has been given the features of the appending independent claims. Preferred embodiments are defined in the dependent claims.

Brief Description of the Drawings Below embodiments of the device/camera unit according to the present invention will be described, reference being made to the enclosed drawings, where:

Figure 1 shows a top view of a device according to the present invention, said device being in an operative position relative to an up-ender basket, in which a fuel assembly is under insertion or removal; Figure 2 shows a perspective view of the device, the up- ender basket and the fuel assembly according to figure 1;

Figure 3 shows an end view of an underwater camera unit that is included in the device; Figure 4 shows a longitudinal section along IV-IV in figure 3; Figure 5 shows a top view of an alternative embodiment of a device according to the present invention, said device being in operative position relative to a fuel assembly; Figure 6 shows a perspective view of the device and the fuel assembly according to figure 5;

Figure 7 shows a perspective view of an alternative underwater camera unit included in the device according to the invention; Figure 8 shows an end view of the camera unit according to figure 7 ; Figure 9 shows a section along IX-IX in figure 8; Figure 10 shows a section in the plane of the paper through the camera unit in figure 8 ; and

Figure 11 the shows a longitudinal section through a further alternative embodiment of a camera unit for inspection of fuel assemblies.

Detailed Description of Preferred Embodiments of the Invention The device according to the present invention, shown in figures 1 and 2, comprises a rig 1 that in the shown embodiment comprises a closed ring 3 that via a stationary cantilever 2 is mounted on a wall or the like. The cantilever 2 is normally included in the equipment of a nuclear plant.

In exemplifying and non-restricting purpose it is stated that the ring 3 may have a diameter of about 1000-1500 mm. In figures 1 and 2 it is also shown a fuel assembly 9 that is inspected by means of the device according to the present invention. In the shown embodiment the fuel assembly 9 has an externally square cross-section, i.e. the fuel assembly 9 has four longitudinal sides 8, of which two are visible in figure 2. In a conventional way the fuel assembly 9 consists of a number of fuel rods. The device also comprises four underwater camera units 12 and four underwater lightning units 13 that are mounted on the rig 1, and more precisely on the ring 3, according to the pattern that is evident from figures 1 and 2. Thus, the four camera units 12 are mounted in pairs diametrically opposite to each other and the four lightning units 13 are likewise mounted in pairs diametrically opposite to each other. The mutual relation between a camera unit 12 and two adjacent lightning units 13 is such that the image receiving opening of the camera unit 12 is generally located midway the adjacent lightning units 13, seen in the direction along the ring 3.

In the shown embodiment a mirror 17 is mounted in the opening. The corresponding is also valid for the location of a lightning unit 13, i.e. it is located midway two adjacent camera units 12, seen in the direction along the ring 3. The underwater camera units 12 are colour camera units that work with digital semiconductor technique, preferably CCD TV-cameras. The focal length of the optics is adjusted to the size of the inspected fuel assembly 9. An end view of a camera unit 12 is shown in figure 3 and a longitudinal section through the camera unit 12 is shown in figure 4. The cover 14 of the camera unit 12 is waterproof and preferably made out of stainless steel with a front glass 15 of acrylic plastic. As is evident from figure 3 the cover 14 generally has a circular cylindrical shape as well as the space that receives an underwater camera body 18. As is evident from both figures 3 and 4 the camera body 18 is eccentrically located relative to the cover 14. The camera unit 12 comprises a radiation protection that makes the camera unit 12 resistant to radiation. The radiation protection constitutes a shielding 16 that is manufactured from tungsten and lead and located inside the cover 14 of stainless steel. As is evident from figures 3 and 4 the shielding 16 has an essentially larger thickness on the side facing towards the fuel assembly 9 when the camera unit 12 is mounted in an operative position on the ring 3, than on the lower side in figures 3 and 4, i.e. on the side facing away from the fuel assembly 9 when the camera unit 12 is mounted in operative position on the ring 3. The thickness of the shielding 16 decreases continuously from the side facing towards the fuel assembly 9 to the side facing away from the fuel assembly 9. In exemplifying and non- restricting purpose it may be mentioned that the thickness of the shielding 16 on the side facing towards the fuel assembly 9 is at least ten times the thickness of the shielding 16 on the side facing away from the fuel assembly 9. The image from the object is projected via a mirror 17 into the camera body 18 of the camera unit 12, i.e. the light directed towards the camera unit 12 is subjected to a general change of direction of 90°, see the central light beam 19A in figure 4. Two side beams 19B are also shown in figure 4, said beams 19B also being reflected in the mirror 17. The light beams 19B converge towards each other in direction towards the mirror 17. In this connection it should be noted that adjacent to the mirror 17 the camera unit 12 is equipped with a bevel that creates a surface 6 that in the shown section forms an angle α_x with the central light beam 19A. When studying figure 4 it is realized that the bevelled surface 6 brings about that also the right side beam 19B in figure 4 may enter the mirror 17 without interfering with the radiation protection of the camera unit 12, said side beam 19B coinciding with the surface 6. The other side beam 19B forms an angle α₂ with the central light beam 19A. The camera unit 12 is connected to a recording unit (not shown) via a camera cable (not shown) that is resistant to radiation and approved for nuclear environment. Camera units of this type have been developed by AHLBERG ELECTRONICS AB to withstand high radioactive radiation and high water temperatures. The recording unit (not shown) preferably constitutes a multiplexer, a video hard disc and a video monitor. The video signals from the four camera units 12 passes through the multiplexer that converts the video signals to a serial video signal. The serial signal is then recorded on the video hard disc and shown on a monitor as four fields. In order to enable an inspection in real-time all four sides of the fuel assembly 9 is shown in a split image in the monitor. Each field may during the recording, alternatively afterwards, be shown in normal image size. The underwater lightning units 13 comprise a halogen lamp in order to achieve the right colour reproduction index. The luminous intensity is continuously adjustable. In figures 1 and 2 also the upper end of a so called up- ender basket 25 is shown, said basket 25 defining an internal space that in the shown embodiment has square cross-section, i.e. the internal space is dimensioned to receive the fuel assembly 9 that has an external square cross-section. The up- ender basket 25 is adjusted in respect to the rig 1 and more precisely to have the centre of the up-ender basket 25 to coincide with the centre of the ring 3. In figure 2 it is clearly illustrated how a fuel assembly 9 is inserted into the up-ender basket 25, i.e. the fuel assembly 9 is lowered into the up-ender basket 25, this being effected in a conventional way by means of a loading machine (not shown) . It is also feasible that figure 2 illustrates the removal of a fuel assembly 9 from the up-ender basket 25, i.e. the fuel assembly 9 is elevated from the up- ender basket 25, this normally being effected in the fuel building. Below a preferred method to inspect a fuel assembly 9, according to the present invention, will be described, a device according to the present invention being used to carry out the inspection. In this connection it should be pointed out for the sake of order that inspection of fuel assemblies in principle always is carried out under the water in the volume of water that is present in plant and fuel environment. Thereby, the radiation that the operative personnel and the material are subjected to decreases dramatically. The new basic principle of the present invention is that an inspection is carried out simultaneously of all longitudinal sides of the fuel assembly, the fuel assembly 9 of the shown embodiment having four longitudinal sides 8. According to a preferred use of the device according to the present invention that is schematically illustrated in figures 1 and 2, an inspection of the fuel assembly 9 is carried out when the fuel assembly 9 is inserted into respectively removed from the up-ender basket 25. The insertion of the fuel assembly 9 in the up-ender basket 25 is normally carried out in the reactor building in connection with the transfer of the fuel assembly 9 to the fuel building, this being effected in connection with the normal annual maintenance. The removal of the fuel assembly 9 from the up- ender basket 25 is normally carried out in the fuel building, to which the fuel assembly 9 has been transported in the up- ender basket 25 in connection with the maintenance. As is illustrated by the double arrow 26 in figure 2 the fuel assembly 9 will thus move relative to the device according to the present invention, the fuel assembly 9 may either be displaced downwards relative to the device or upwards relative to the device. Regardless in which direction the fuel assembly 9 is displaced relative to the device according to the present invention a simultaneous inspection of all longitudinal sides 8 of the fuel assembly 9 is carried out, the inspection being carried out by having the longitudinal sides of the fuel assembly 9 video filmed in the way that has been described above. Since the inspection of the fuel assembly 9 is carried out during a necessary measure in the maintenance, i.e. the insertion into respectively removal from the up-ender basket 25, said inspection consumes no extra time, which is extremely beneficial. Thereby, a complete inspection of all fuel in the reactor is achieved in connection with loading or unloading of the fuel in the reactor. No extra movements of the fuel assemblies are necessary, except the ordinary movements of the fuel assemblies in connection with loading or unloading. No time will be added to the critical path in connection with a shutdown. By critical path it should be understood the shortest time that the fuel assemblies are handled in connection with a shutdown. When using the method the movement of the fuel in connection with loading/unloading on the reactor is always on the critical path. In this connection added time means large expenses for the owner of the nuclear plant. In figures 5 and 6 an alternative embodiment of the device according to do present invention is shown. This embodiment also comprises a rig 101 that in its turn comprises a ring 103. Underwater camera units and underwater lightning units are mounted on the ring 103, said camera units and lightning units preferably being identical to the corresponding units in the embodiment that has been described above in connection with figures 1-3. For that reason the camera units have been allocated the reference numeral 12 and the lightning units have been allocated the reference numeral 13. The principal difference between the embodiment according to figures 5 and 6 and the embodiment according to figures 1-3 is that in the embodiment according to figures 5 and 6 the device is equipped with a guiding means in the shape of a, relative to the ring 103, centrally located collar 105 that is connected to the ring 103 by means of two diametrically located struts 107, the guiding means and the struts 107 being parts of the rig 101. The collar 105 is thus supported in the middle of the ring 103 by the struts 107. As is most evident from figure 5 the collar 105 defines an opening that generally corresponds to the external contour of a fuel assembly 9 that the collar 105 is intended to surround. The collar 105 comprises an entering portion 110 that generally is funnel shaped and consists of four surfaces that are inclined relative to the plane that the ring 103 is lying in. The collar 105 also comprises a guiding portion 111, see figure 6, that adjoins the external contour of the fuel assembly 9. As is evident from figures 5 and 6 the ring 103 of the device 101 according to the present invention is equipped with a number of fastening eyes 130 that each are equipped with a hole 131. As is most evident from figure 5 the wires 132 are received in the holes 131, said wires 132 thus supporting the rig 101 and the units of the device according to the present invention that are mounted on the rig 101. The wires 132 thus constitute an alternative to the cantilever 2 in the embodiment according to figures 1 and 2. The embodiment of a device according to the present invention shown in figures 5 and 6 may either be stationary, the fuel assembly 9 being displaced relative to the device, or the device may be displaced relative to a stationary fuel assembly 9. Generally, the device shown in figures 5 and 6 is simple to move, e.g. between different inspection activities and between different nuclear plants. In figure 5 the viewing beams that are associated with the camera units 12 and the light beams that are associated with the lightning units 13 are schematically shown. Thus, a centre line 21 of a light beam 20 that is emitted from a lightning unit 13 hits a corner of the collar 105 and thus also a corner of the fuel assembly 9. The light beam 20 is illustrated with dash dotted lines in figure 5. This means that each lightning unit 13 will illuminate two adjacent longitudinal sides 8 of the fuel assembly 9. As regards the camera units 12 each one registers in principal only one side of the fuel assembly 9, i.e. the four sides 8 of the fuel assembly 9 are each facing one camera unit 12. In figure 5 the viewing beam 22 that emanates from each camera unit 12 is illustrated by dash dotted lines. The corresponding arrangement with the viewing beams and light beams is also valid for the embodiment according to figures 1 and 2. The device according to the present invention that is shown in figures 5 and 6 is also normally used without having time added to the critical path. In figures 7-10 an alternative embodiment of a camera unit 212 is shown, said camera unit 212 may be a part of the device according to the present invention, i.e. the camera units 212 may replace the camera units 12. The cover 214 of the camera unit 212 is waterproof and preferably made of stainless steel with a front glass 215 of acrylic plastic. As is most evident from figure 8 the cover 214 has a generally circular cylindrical shape as well as a space 235, see figure 9, that carries a camera body 218. As is evident from figures 8-10 the camera body 218 is eccentrically located relative to the cover 214. In contrast to the camera body 18 of the above described the camera unit 12 the camera body 218 is not waterproof. This means that the space 235 is sealed by means of O-rings 236 that prevent water from entering into the space 235. At one end the space 235 is equipped with a rear element 237 that carries two of the O-rings 236. From the rear element 237 a camera cable (not shown) is extending, by which cable the camera unit 212 is connected to a recording unit (not shown) that is resistant to radiation and approved for nuclear environment. The camera unit 212 has a radiation protection that makes the camera unit 212 resistant to radiation. The radiation protection constitutes a shielding 216 that is manufactured from tungsten and lead and located inside the cover 214 of stainless steel. As is most evident from figures 9 and 10 the shielding 216 has an essentially larger thickness on the upper side in the figures 9 and 10, i.e. the side that faces towards the fuel assembly when the camera unit 212 is mounted in operative position on the ring, than the lower side in figures 9 and 10, i.e. the side that faces away from the fuel assembly when the camera unit 212 is mounted in operative position on the ring. As is most evident from the cross- section of the shielding 216 shown in figure 10 the thickness of the shielding 216 continuously decreases from the side facing towards the fuel assembly to the side facing away from the fuel assembly. The cross section shape of the shielding

216 is the result of the understanding that a fuel rod in principal constitutes a linear radiation source. Inspection of a fuel assembly is always carried out under the water, the water having a dampening effect upon the radiation. Further, the radiation decreases by the square of the distance, which means that the most intense radiation towards the camera units emits from the portion of a fuel rod that is at the same level as the camera unit. The portion of a fuel rod that is at a higher or lower level than the camera units thus emits less radiation towards the camera units. For that reason the thickness of the shielding 216 may continuously decrease from the side facing towards the fuel assembly to the side facing away from the fuel assembly. For safety reasons there should be at least a certain radiation protection also on the side of the camera unit 212 that faces away from the fuel assembly. The above reasoning concerning the cross-section shape of the shielding 216 is valid also for the shielding 16 that is included in the camera unit 12 according to figures 3 and 4. In exemplifying and non-restricting purpose it may be mentioned that the thickness on the shielding 216 on the side facing towards the fuel assembly is at least ten times the thickness of the shielding 216 on the side facing away from the fuel assembly. In a corresponding way as for the above described camera unit 12 the camera unit 212 is equipped with a mirror 217 and in connection thereto a bevel is provided that brings about the creation of a surface 206 that in the shown section forms a certain angle with the central light beam 19A. When viewing figure 9 it is realized that the bevelled surface 206 brings about that also the side beams 19B may reach the mirror

217 without interfering with the radiation protection of the camera unit 212. One side beam 19B coincides with the surface 206, which means that the side beams 19B form an angle α with the central light beam 19A. In figure 11 a section of a further alternative embodiment of a camera unit 312 for the inspection of a fuel assembly of a nuclear plant is shown. The camera unit 312 according to figure 11 is primarily intended to be used as a separate unit for more accurate inspection of a fuel assembly that has been removed in connection with the first inspection by means of a device according to the present invention. The camera unit 312 according to figure 11 is thus normally not intended to be a part of a device according to the present invention. The camera unit 312 has a structural design that in principal to a large degree corresponds to the camera unit 12 shown in figures 3 and 4. Thus the camera unit 312 has a cover 314 that is waterproof and preferably made out of stainless steel. Likewise the camera unit 12 the cover 314 generally has circular cylindrical shape as well as the space that receives an underwater camera body 318. As is evident from figure 11 the camera body 318 is eccentrically located relative to the cover 314. The camera unit 312 comprises a radiation protection, which makes the camera unit 312 resistant to radiation. The radiation protection constitutes a shielding 316 that is manufactured from tungsten and lead and located inside the cover 314 of stainless steel. As is evident from figure 11 the shielding 316 has an essentially larger thickness on the upper side in figure 11, i.e. the side that is intended to face towards a fuel assembly, than at the lower side in figure 11, i.e. the side that is intended to face away from the fuel assembly. The thickness of the shielding 316 decreases continuously from the side facing towards the fuel assembly to the side facing away from the fuel assembly. In exemplifying and non-restricting purpose it may be mentioned that the thickness of the shielding 316 on the side facing towards the fuel assembly is at least ten times the thickness of the shielding 316 on the side facing away from the fuel assembly. As is evident from figure 11 the shielding 316 surrounds the camera body 318 also on the right side in figure 11. The camera unit 312 is also equipped with a mirror 317 with principally the corresponding orientation as the mirror 17 of the camera unit 12. Adjacent to the mirror 317 the camera unit 312 is equipped with an opening that in the shown section tapers towards the mirror 317, said opening being defined by two towards the mirror 317 converging surfaces 306. This enables the camera unit 312 to handle light beams that converge towards each other.

Feasible Modifications of the Invention In the embodiments described above CCD TV-camera units are used. However, within the scope of the present invention, it is also feasible that other types of colour camera units are used, said units being equipped with a digital sensor. These camera units may on one hand be of the type that work with moving pictures and on the other hand of the type that work with still pictures. In the embodiments described above the fuel assembly 9 generally has a square cross-section. However, the invention is also suitable for fuel assemblies having a different cross- section, e.g. octagonal cross-section. In such a case the number of camera units and lightning units is adapted in order- to achieve a proper inspection of the fuel assembly 9.

Claims

1. Device for the inspection of a fuel assembly (9) of a nuclear reactor, said device comprising a rig (1; 201) , on which a number of underwater camera units (12; 212) are mounted, that the number of a camera units (12; 212) correspond to the number of longitudinal sides (8) of the fuel assembly (9) that is to be inspected, that the devi.ce comprises a number of underwater lighting units (13) that are mounted on the rig (1; 101), and that the device has means to study the images of the longitudinal sides (8) of the fuel assembly (9) that are generated by the camera units (12; 212), c h a r a c t e r i s e d in that the camera units (12; 212) constitute colour camera units with a digital sensor, and that the camera units (12; 212) are equipped with protection (16; 216) against radioactive radiation, said protection constituting a shielding (16; 216) that surrounds a camera body (18; 218), said shielding (16; 216) having a varying thickness.

2. Device according to claim 1, c h a r a c t e r i s e d in that the camera units (12; 212) constitute CCD TV-cameras.

3. Device according to any of the previous claims, c h a r a c t e r i s e d in that the camera units (12; 212) comprise a circular cylindrical cover (14; 214), that the camera body (18; 218) is eccentrically located in trie cover (14; 214), and that the shielding (16; 216) is located between the cover (14; 214) and the camera body (18; 218) .

4. Device according to any of the previous claims, c h a r a c t e r i s e d in that the rig (1; 101) comprises a closed ring (3; 103), on which the camera units (12; 212) and the lightning units (13) are mounted.

5. Device according to any of the previous claims, c h a r a c t e r i s e d in that the camera units (12; 212) comprise a mirror (17; 217) that brings about a general change of direction of 90° to the light approaching the camera units (12; 212) .

6. Device according to claim 3, c h a r a c t e r i s e d in that the cover (14; 214) is designed to avoid interference with the light that approaches the camera units (12; 212) .

7. Camera unit (12; 212; 312) for the inspection of a fuel assembly (9) of a nuclear reactor, c h a r a c t e r i s e d in that the camera units (12; 212; 312) constitute colour camera units with a digital sensor, and that the camera units (12; 212; 312) are equipped with protection (16; 216; 316) against radioactive radiation, said protection constituting a shielding (16; 216; 316) that surrounds a camera body (18; 218; 318), said shielding (16; 216; 316) having a varying thickness.

8. Camera unit according to claim 7, c h a r a c t e r i s e d in that the camera units (12; 212; 312) constitute CCD TV-cameras.

9. Camera unit according to claims 7 or 8, c h a r a c t e r i s e d in that the camera units (12; 212; 312) comprise a circular cylindrical cover (14; 214; 314), that the camera body (18; 218; 318) is eccentrically located in the cover (14; 214; 314), and that the shielding (16; 216; 316) is located between the cover (14; 214; 314) and the camera body (18; 218; 318).