METHOD AND DEVICE FOR INSPECTION OF FUEL ASSEMBLIES

Technical field of the invention

The present invention relates to a method and a device for inspecting a fuel assembly of a nuclear reactor, whereby inspection of longitudinal sides of the fuel assembly (F) is effected. The invention is intended to be used primarily in PWRs (pressurised water reactors) but also in other types of reactor.

State of the art

In the nuclear power industry it is extremely important to continuously monitor the status of the fuel assemblies used in a nuclear reactor. Examples of the parameters monitored are handling damage, oxide growth, presence of foreign objects and curvature .

A device known from WO 2005/027141 for inspecting a fuel assembly of a nuclear reactor comprises a rig on which a number of camera units for underwater use are mounted, and the number of camera units corresponds to the number of longitudinal sides of the fuel assembly which is to be inspected. The device also comprises a number of lighting units for underwater use mounted on the rig, and means for examining the images generated by the camera units of the longitudinal sides of the fuel assembly. The camera units take the form of colour camerci units with digital sensors and are provided with protection against radioactive radiation. The device according to WO 2005/027141 effects simultaneous inspection of all sides of the: fuel assembly. Since all the camera units are situated at the same level, it is not possible to inspect the lower end surface of the fuel assembly.

Objects and features of the invention A primary object of the present invention is to propose a method and a device for inspecting a fuel assembly in a nuclear reactor, whereby extremely complete inspection of the fuel assembly is effected.

A further object of the present invention is that the amount of time which the inspection takes should in principle not decrease but that the amount of information obtained should increase. Another object of the present invention is to integrate inspection with necessary stages in the maintenance of the nuclear reactor.

At least the first object of the present invention is achieved by a method and a device provided with the features indicated in the independent claims set out below. Preferred embodiments are defined in the dependent claims.

Brief description of the drawings

A preferred embodiment of the present invention is described below with reference to the attached drawings, in which: Fig. 1 depicts a plan view of a device according to the present invention;

Fig. 2 depicts a perspective: view, obliquely from above, of the device according to Fig. 1, with a fuel assembly cooperating with the device; Fig. 3 depicts a perspective view of the device, obliquely from below, with a fuel assembly cooperating with the device; Fig. 4 depicts a perspective: view of a camera which forms part of the device; Fig. 5 depicts a cross-section through the camera according to Fig. 4;

Fig. 6 depicts a plan view of an alternative embodiment of a device according to the present invention;

Fig. 7 depicts a perspective view obliquely from below of the device according to Fig. 6, with a fuel assembly cooperating with the device;

Fig. 8 depicts a side view of the device according to Fig. 6;

Fig. 9a depicts a perspective view of a further mirror which forms part of the alternative embodiment, with the further mirror situated at its lowest position; and

Fig. 9b depicts a perspectives view of the further mirror situated at its highest position.

Detailed description of a preferred embodiment of the invention

The device according to the present invention depicted in Figs. 1-3 comprises a rig 1 which in the embodiment depicted comprises generally a right-angled frame 3 which is generally situated in one plane. For exemplifying and non- limitative purposes it may be mentioned that the dimensions of the frame may preferably be 120 x 130 cm but will vary depending on the available space in connection to the nuclear power station where the device is to be used. The device also comprises four in principle identical camera units 5 for underwater use mounted on the rig 1, and more specifically on the frame 3, according to the pattern indicated in Figs. 1 and 2. The four camera units 5 are thus mounted in the region of the corners of the rectangular frame 3. The camera units 5 are mounted in pairs diametrically opposite one another. The camera units 5 for underwater use are colour camera units based on digital semiconductor technology, preferably CCD TV cameras. The camera units 5 are of a similar kind, particularly as regards radiation protection, to that described in WO 2005/027141 and more detailed information about their constructional configuration is to be found in WO 2005/027141, said document hereby being incorporated in the description for the present patent application.

The device according to the present invention also comprises in the embodiment depicted a fifth camera unit 6 which is situated at a lower level than the frame 3 when the device according to the present invention is in the position of use depicted in Fig. 2, i.e. when the frame 3 is in a horizontal plane. The fifth camera unit 6 is supported by a retainer means 4 which in the region of its one end supports the fifth camera unit 6 and in the region of its other end is fastened to the frame 3. For exemplifying and non-limitative purposes it may be mentioned that the length of the retainer 4, i.e. its extent transverse to the plane in which the frame 3 is situated, is of the order of 20-50 cm, preferably of the

order of 30 cm. The constructional configuration of the camera unit 6 is somewhat different from that of the camera units 5 but it has similar radiation protection. Two lighting units 34 are disposed close to the fifth camera unit 6 and their configuration is somewhat different from that of the lighting units 12 described below. The camera unit 6 is described in more detail below in relation to Figs. 4 and 5.

The device also comprises four arcuate brackets 7 which each have one of their ends fastened to the frame 3, the fastening being effected with advantage by, for example, riveted or threaded connections. In the embodiment depicted, the ends fastened to the frame 3 are situated centrally between two mutually adjacent camera units 5. From the fastening to the frame 3, the brackets 7 extend inwards towards the centre of the frame 3, and their ends pointing inwards from the edges of the frame 3 support a guide means 9 for the fuel assembly F, which guide means comprises a hopper 10 in which the fuel assembly F is intended to be accommodated, see Figs. 2 and 3. The hopper 10 defines a space which in plan view is square and adapted to the cross- sectional shape of the fuel assembly F. When the guide means 9 is in the position of use depicted in Fig. 2, it is situated at a level which is higher than the level at which the main plane of the frame 3 is situated, i.e. the guide means 9 is raised relative to the frame 3. The fifth camera unit 6 is directed towards the lower edςje of the hopper 10. This is illustrated in Fig. 3, which depicts a perspective view whereby the lower end surface G of the fuel assembly F is visible. For the sake of clarity, the fifth camera unit 6 has been omitted in Fig. 3, but the lower end surface G of the fuel assembly F is viewed in a manner corresponding to when it is filmed by the fifth camera unit 6.

Fig. 3 also shows the fuel assembly F bearing at its lower end surface G a support H which is provided with four legs I. The support H is permanently fastened to the fuel assembly F.

The hopper 10 forming part of the guide means 9 is rotatable about an axis which is perpendicular to the paper in Fig. 1. More precisely, parts of the guide means 9 are

rotatable relative to one another along a circular arc 11. The rotatability of the hopper 10 is symbolised by the double arrow R in Fig. 1.

A lighting unit 12 for underwater use is mounted on each of the arcuate brackets 7. The lighting units 12 are mounted in pairs diametrically opposite one another. All of the lighting units 12 are directed towards the guide means 9, more precisely towards the lower edge of the hopper 10. The lighting units 12 each comprise a halogen lamp to provide the correct colour reproduction index. The luminance is steplessly adjustable.

Figs. 4 and 5 depict the fifth camera unit 6, which comprises an outer casing 13, a rear element 14 and a mirror 15. As may most clearly be S€≥en in Fig. 5, the camera unit β is provided with radiation protection in the form of a shell 16 which is of varying thickness and made of tungsten and lead.

As may be seen in Fig. 4, a first locking screw 17 for the mirror 15 is accommodated in a first arcuate slit 18 in a flange 19. This arrangement means that the position of the mirror 15 can be adjusted by slackening the first locking screw 17 and pivoting the mirror 15 relative to the casing 13. As may be seen in Fig. 4, the rear element 14 is likewise pivotable relative to the casing 13 by means of a second locking screw 20 accommodated in a second arcuate slit 21 in the rear element. Slackening the second locking screw 20 makes it possible to pivot the rear element 14. The fact that both the mirror 15 and the rear element 14 are pivotable relative to the casing 13 makes it possible to ensure that the camera unit 6 is directed in a proper way at the lower end surface G of the fuel assembly F.

Each camera unit 5, 6 is connected to a recording unit (not depicted) by a camera cable (not depicted) which is radiation-tolerant and approved for use in reactor environments. Camera units of this kind have been developed by AHLBERG ELECTRONICS AB to tolerate high levels of radioactive radiation and high water temperatures.

The undepicted recording unit comprises with advantage a multiplexer, a video hard disk and a video monitor. The video

signals from the five camera units 5, 6 pass through the multiplexer, which converts the video signals to a serial video signal. The serial signal is thereafter recorded on the video hard disk and displayed on a monitor in the form of four part-images. To make inspection in real time possible, all four sides of the fuel assembly F can be displayed in a split image on the monitor. Each part-image can be magnified to normal image size during the recording process or later. In this context it should be noted that the device according to the present invention can be used together with a variety of recording units. It may happen that the user of the device according to the present invention is already in possession of a recording unit which can be used. The recording unit therefore does not necessarily form part of the device according to the present invention.

Using the device according to the present invention involves suitable suspension of the frame 3, i.e. the frame 3 will be stationary. The device according to the present invention will then be ready to perform an inspection. In this context it should be noted for the record that the inspection of fuel assemblies is in principle always effected under water in the water mass situated in the reactor/fuel environment, thereby greatly reducing the radiation to which operating staff and equipment are exposed. The fuel assembly F is caused to assume the position depicted in Fig. 2, whereby the fuel assembly F is normally supported by a gripper (not depicted) which engages with the upper end of the fuel assembly F. In the position depicted in Fig. 2, the lower end of the fuel assembly F is accommodated in the hopper 10. A first stage of an inspection of the fuel assembly F using the device according to the present invention is normally effected by the fifth camera unit 6 video-filming the lower end surface G of the fuel assembly F. This entails the fuel assembly F being rotated a full turn, i.e. 360°, accompanied by the hopper 10, i.e. the hopper 10 likewise rotates a full turn. In this context it should be noted that the means which support the fuel assembly F comprise means for rotating the fuel assembly F. When this first stage has been

completed, a full inspection of the lower end surface G of the fuel assembly F has been accomplished.

As a second stage, the fuel assembly F is lowered through the hopper 10 while a simultaneous inspection of all the longitudinal sides of the fuel assembly F is performed by their being video-filmed by the camera units 5, each camera unit 5 being directed towards a longitudinal side of the fuel assembly F. When the whole of the fuel assembly F has been lowered through the hopper 10, this second inspection stage is complete.

As a third inspection stage, the fuel assembly F is rotated approximately 45°, accompanied only by the hopper 10, i.e. the frame 3 remains statxonary. This means that each camera unit 5 is directed towards a corner of the fuel assembly F. The fuel assembly F is then raised relative to the hopper 10 while a simultaneous inspection of the corner portions of the fuel assembly F is effected by their being video-filmed by the camera unxts 5. When the whole of the fuel assembly F has been raised through the hopper 10, this third inspection stage is complete.

The three inspection stages described above provide extremely full information about the condition of the fuel assembly F.

Figs. 6-8 depict an alternative embodiment of a device for inspecting a fuel assembly according to the present invention. The device according to Figs. 6-8 comprises a rig 101 which in principle corresponds to the rig 1 according to Figs. 1-3. Accordingly, corresponding components of the two rigs 1 and 101 have been given the same reference numerals. The rig 101, however, comprises a further mirror 31 in addition to the first mirror 15 which is disposed close to the fifth camera unit 6. The further mirror 31 comprises a frame structure 32 and is fastened to the rig 101 by a first rod 30 which is connected to the frame 3 via the bracket 29. Two other lighting units 34 for underwater use are mounted on a second rod 33 which is fastened in the frame structure 32. The configuration of these other lighting units 34 is preferably similar to that of the lighting units 34 situated close to the fifth camera unit 6.

Fig. 8 illustrates the use of the further mirror 31. The fifth camera unit 6 can view the lower end surface G of the fuel assembly F via the further mirror 31, the viewing path being represented by Sl. It is of course also possible to direct the fifth camera unit 6 so that it points directly at the lower end surface G of the fuel assembly F, in which case the viewing path is represented by S2.

As may most clearly be seen in Figs. 9a and 9b, the further mirror 31 is pivotable relative to the first rod 30 and also movable a certain distance in its main plane relative to the first rod 30. This is made possible by the frame of the further mirror 31 being provided with slits 35.

It should be noted that the further mirror 31 is situated to one side of the centre of the rig 101 in order not to interfere with the fuel assembly F.

Conceivable modifications of the invention

In the embodiment described above, four camera units 5 are fitted at the corners of the rectangular frame 3. It is nevertheless possible within the scope of the present invention to conceive also of using more, but preferably fewer, camera units 5. It is possible, for example, for only two diametrically disposed camera units 5 to be mounted on the frame 3, in which case two longitudinal sides will be filmed as the fuel assembly F is lowered through the hopper 10.

Thereafter the fuel assembly F will be rotated 90° and the two remaining longitudinal sides will be filmed as the fuel assembly F is raised relative to the hopper 10. In this context, however, it should be noted that the device according to the present invention always comprises a further camera unit 6 which is situated at a lower level when the frame 3 is situated in a horizontal plane.