Abstract:
A device and a method for inspecting fuel elements of a nuclear reactor underwater are described. A remote controlled miniature endoscope forming part of an inspection device is introduced into the fuel element and the non-easily accessible parts of the fuel element located therein are inspected without having to disassemble the fuel element.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
         [0001]    This application is a continuation of copending International Application PCT/DE00/00373, filed Feb. 8, 2000, which designated the United States.  
         BACKGROUND OF THE INVENTION  
       FIELD OF THE INVENTION  
         [0002]    The invention relates to an apparatus for the inspection of a fuel element in a nuclear reactor. The apparatus has an endoscope, which is connected to an electronic image-receiving device, to a light source and to an actuating device, and is remotely controlled. The invention further relates to the use of the endoscope and a method using the endoscope for the inspection of a fuel element disposed under water in a nuclear reactor. For example, the surface of a fuel rod or of a barely accessible region of a bottom piece or a spacer can be inspected.  
           [0003]    In order to inspect the fuel element in the nuclear reactor, the fuel element is generally looked at closely and inspected visually. The visual inspection can be carried out before the first use of a fuel element in a nuclear reactor, after a use cycle or when the fuel element is being replaced. Because of the breakdown of the nucleides present in the fuel, radioactive radiation emerges from the fuel element. The visual inspection of fuel elements is therefore generally carried out under water in a basin belonging to a nuclear power station, mostly at a water depth of more than 10 m, to some extent down to 30 m.  
           [0004]    During the final acceptance in the fuel element production or during the quality testing of the fuel elements after they have been delivered to the nuclear reactor, an inspection can also be carried out in a test room and not under water. In principle, an apparatus for the inspection of the fuel elements in a nuclear reactor can also be used for these purposes.  
           [0005]    In addition to the inspection of the outer surfaces, in particular a visual inspection of internal regions is also advantageous.  
           [0006]    An apparatus for a boroscope—that is to say a pipeline system in which images are transmitted via mirrors—is described in U.S. Pat. No. 4,036,686. The boroscope can, for example, be introduced into the fuel element passages in a fuel element and can transmit images from the interior of a fuel element. However, it is not flexible and is too bulky to inspect any desired (in particular barely accessible) regions of the fuel element, for example of a bottom piece or of a spacer.  
           [0007]    U.S. Pat. No. 4,229,069 proposes an endoscope for use in ionizing radiation fields, in particular for observing highly radioactive material. However, it cannot be controlled remotely and is therefore not suitable for the inspection of a fuel element. However, measures are indicated here to make the endoscope particularly resistant to radioactive radiation. These include, for example, an advantageous composition of the glass fiber material. In addition, a heating apparatus for the temperature stabilization of part of the endoscope facing the fuel element is proposed. In addition, the endoscope is intended to be located in a fixed tube or a flexible metal housing in order to protect it against radioactive radiation.  
           [0008]    An endoscope according to U.S. Pat. No. 5,152,957 is part of a cleaning device for finding and eliminating foreign bodies, in particular in fuel element passages. It is rigid and conducts produced images of the foreign bodies and of a cleaning device via a long glass fiber to a camera above the water level. The endoscope is therefore not suitable for the inspection of all regions of a fuel element, in particular barely accessible regions. In addition, it is impossible to avoid severe damage to the glass fiber from radioactive radiation, under which the inspection of the fuel elements suffers. The glass fiber is too long and is guided along the entire length of the fuel element to be inspected. The fiber is therefore exposed to radiochemical breakdown arising from ionizing radiation over a large part of its length. This leads to a weakening of the image, which loses its brightness during its long propagation path through the fiber.  
         SUMMARY OF THE INVENTION  
         [0009]    It is accordingly an object of the invention to provide a miniature endoscope and a method for the inspection of fuel elements which overcome the above-mentioned disadvantages of the prior art devices and methods of this general type, which make it possible to inspect even internal, in particular barely accessible, regions of a fuel element, even under severe radioactive radiation.  
           [0010]    With the foregoing and other objects in view there is provided, in accordance with the invention, an apparatus for inspecting a region of a fuel element including a spacer and a bottom piece in a nuclear reactor. The apparatus contains an image production device having an electronic image receiving device; an endoscope having an intrinsically flexible end piece with an endoscope objective carried by the intrinsically flexible end piece; a light guide for images optically connecting the endoscope objective to the electronic image receiving device; an actuating device having an actuating motor for bending at least the intrinsically flexible end piece; and an illumination device having a light source sending light to be discharged from the intrinsically flexible end piece.  
           [0011]    The object is achieved by the apparatus for the inspection of a region of the fuel element in the nuclear reactor, in particular for the inspection of the surface of a fuel rod or of a barely accessible region of a bottom piece or of a spacer. In a first variant of the invention, the endoscope is provided, in particular a miniature endoscope, having an end piece that carries the endoscope objective and is interchangeable. In a watertight container, at least one electronic image-receiving device is connected to an image production device, to a light source and to an actuating motor of an actuating device disposed at another end of the endoscope. In addition, the components are disposed in the container so as to be protected against radioactive radiation.  
           [0012]    In the first variant, the invention is based on the finding that the endoscope can substantially not be adequately protected against radioactive radiation during visual inspection in the immediate vicinity of the fuel element.  
           [0013]    Although measures may be taken to increase the radiation resistance, as have already been described in principle in U.S. Pat. No. 4,229,069, the destruction of the endoscope by the radioactive radiation is unavoidable. At least that part of the endoscope which is mostly stressed, that is to say that part which projects into the fuel element, namely the end piece of the endoscope, is therefore interchangeable. The interchangeable part is preferably to be interchangeable simply and rapidly and can be replaced cheaply. Conventional endoscopes permit, for example, an adequately long net usable time for inspection with high quality of at least one hour in the immediate vicinity of the fuel element.  
           [0014]    Furthermore, according to the invention the endoscope is kept as short as just possible and is predominantly disposed at right angles to the fuel element. The radiochemical destruction of the endoscope, such as occurs to an enhanced extent when the endoscope is guided along the entire length of the fuel element as far as an eyepiece above a water surface, as described in U.S. Pat. No. 5,152,957, is therefore suppressed to the greatest possible extent.  
           [0015]    According to the invention, therefore, the container with the devices disposed in it, together with the endoscope, is led up to the fuel element for the purpose of inspection. While the endoscope is easy to replace after excessively long action of radiation, the aforementioned devices in the watertight container are protected against radioactive radiation and therefore do not need to be replaced regularly.  
           [0016]    In an advantageous development of the aforementioned first variant of the invention, an illumination device is provided which radiates light from a light source from an objective-side end of the endoscope. This has the advantage that, for the visual inspection of a region of the fuel element, the interesting region can be illuminated directly in a field of view of the endoscope. As compared with indirect illumination, for example by a lamp which radiates its light in from a direction that differs from that in the direction of view of the endoscope, shadowing is avoided in the case of this solution according to the invention, and therefore the quality of the visual inspection is improved.  
           [0017]    In particular, a development of the apparatus according to the invention contains an actuating device, which is provided at least to move and/or to rotate at least the end piece of the endoscope. This is because, in the aforementioned first embodiment of the invention, the use of a rigid endoscope is not ruled out. The movement, for example by tilting or pivoting at least the end piece of the endoscope by the aforementioned actuating device, therefore enlarges the viewing angle of the endoscope which can be achieved, and therefore the region of a fuel element inspected by the apparatus, even if the endoscope is rigid. Likewise, the rotation at least of the end piece of the endoscope has the effect of enlarging the viewing angle range, if the preferably rigid endoscope is equipped with a prism at the objective end.  
           [0018]    The aforementioned first variant of the invention is particularly advantageously equipped with an end piece, which is intrinsically flexible. In particular, an apparatus with an endoscope at least having a flexible end piece is provided for the inspection of a barely accessible region of a fuel element, for example of a bottom piece or of a spacer, since the endoscope may be bent in particular into spacer holding cells or grid cells of the bottom piece. The inspection can therefore be carried out without the fuel element having to be opened. In this case, depending on the requirement, a rigid endoscope can be interchanged for an endoscope in which at least the end piece is flexible.  
           [0019]    According to a second variant of the invention, this results in an apparatus which has an endoscope with an intrinsically flexible end piece carrying an endoscope objective, an actuating device, which is provided to bend at least the end piece, an illumination device, which is provided to discharge light from the end piece, and an image production device at another end.  
           [0020]    In the case of the second variant, the invention is based on the finding that when the endoscope, in particular a rigid endoscope, is inserted into the fuel element, both the fuel element and the endoscope can be damaged. This hazard can result from the accidental striking or canting of the endoscope in the fuel element, if relatively high mechanical forces act on both. This is sometimes virtually unavoidable. This is because the use of the endoscope frequently requires the endoscope to be adjusted accurately at a distance of 10 to 30 m or more with an accuracy of 2 to 3 mm. On the other hand, an intrinsically flexible endoscope can be inserted into the fuel element in a less susceptible manner and virtually adjusts itself as it is inserted, by deforming in accordance with the situation, for example during insertion into a fuel element passage. For this purpose, a flexible, for example metallic, tube, which is flexible up to a certain degree is advantageously used. The stiffness of such a tube is, for example, sufficient to carry the dead weight of the endoscope, so that the tube remains in its current form after being bent. The tube does not need to be protected to any noticeable extent against the radioactive radiation and can therefore be configured to be as flexible as necessary. Otherwise, the endoscope can be constructed like a medical endoscope. However, it is advantageous for the material used for the fiber bundle of such a known endoscope, which is as insensitive as possible to radioactive radiation. Such materials, such as glasses, are described in U.S. Pat. No. 4,229,069.  
           [0021]    The second variant of the invention further makes use of the fact that the bending of an intrinsically flexible endoscope can be controlled by an actuating device. With the aid of the actuating device to bend at least the end piece of the endoscope, it is possible, as already explained, for the achievable viewing angle of the endoscope to be enlarged substantially, in particular for the inspection of barely accessible regions of a fuel element. By bending the endoscope into niches or corners of the fuel element, even subregions of the fuel element, which cannot be reached with a rigid endoscope can also be moved into the field of view.  
           [0022]    Finally, a variant of the invention contains an illumination device, which is provided to discharge light from the end piece and therefore, for example by avoiding shadowing, is suitable for improving the inspection quality. An image production device at another end of the endoscope is used to record the image produced by the endoscope. This variant is primarily used in order also to track down foreign bodies, which have been caught at the bottom, in a spacer or between the fuel rods.  
           [0023]    The apparatus according to the invention advantageously provides an end piece, which is intrinsically flexible over a length of at least 10 mm, preferably about 50 mm. In particular, it is beneficial if the end piece is suitable to be bent with a radius of curvature greater than 10 mm. Furthermore, it is advantageous if the end piece is flexible in two directions, preferably on all sides. The flexible length of the end piece, the radius of curvature of the end piece and the directions of bending are, according to the invention, to be adapted to the requirements of the situation during the inspection, are made possible by choosing various interchangeable endoscopes.  
           [0024]    The aforementioned actuating device is preferably embodied, at least to bend the end piece of the endoscope, by a mechanical pulling device fixed to the endoscope objective. The mechanical pulling device may contain, for example, two pull cords or up to four pull cords. In order to operate the pulling device, use is made of at least one actuating motor. The latter is advantageously operated under remote control, for example via a control device, as described later. A further embodiment of the actuating device contains, for example, a rigid construction and an actuating motor for rotating the rigid construction together with the endoscope objective. The latter embodiment of the actuating device is fitted in particular in a rigid endoscope, preferably in the case of an endoscope with an endoscope objective or a prism at the front end of the end piece of the endoscope.  
           [0025]    A further embodiment of the invention contains a rigid part for guiding a center portion of the endoscope, so that the latter is expediently stabilized. To this end, a fixed ring or a guide rail, for example, is provided to support the endoscope. However, a preferably metallic hose, for example a corrugated hose, can also be provided, which is not completely rigid but may be bent and then remains in the bent form. This is provided in particular in the case of the endoscope that is flexible over its entire length. The further configuration of the apparatus according to the invention contains an electronic camera in the image production device to record an image supplied by the endoscope. This may be, for example, a black/white or a color camera. The camera is advantageous robust and of a low-noise configuration. A Vidicon camera, a CCD camera, or a CMOS camera are suitable. Also advantageous is any type of camera with at least  400  lines image resolution.  
           [0026]    Furthermore, the apparatus according to the invention preferably provides a front optical opening at the objective end of the endoscope. The opening is also used to discharge light from the end piece. The light is supplied by a light source belonging to an illumination device of the apparatus according to the invention. The illumination device and the image production device preferably contain at least one light guide, preferably at least one bundle of individual fibers. Furthermore, according to the invention it is beneficial for separate light guides to be provided for the transmission of images and light. Thus, for the transmission of images and light to achieve the object according to the invention, preferably at least one bundle of individual fibers is provided. These image and light conductors advantageously contain up to 10,000 individual quartz fibers. According to the invention, the illumination device is configured to discharge virtually white light, preferably light similar to daylight. This is implemented, for example, by a suitable selection of a gas pressure lamp, for example a xenon lamp, whose spectrum can be characterized by a temperature in the range from about 5,000 to 7,000 Kelvin. The power of the lamp should be about 100 W.  
           [0027]    The apparatus is preferably characterized by a flange for interchanging at least the end piece of the endoscope. The flange is advantageously also provided for coupling the endoscope to the image production device, to the illumination device and to at least one actuating motor. According to the invention, it is advantageous if the endoscope, the image production device and the illumination device and the actuating device are resistant to radioactive radiation. In particular, the endoscope is watertight. Furthermore, according to the invention it is beneficial if the image receiving device and/or the illumination device are protected against radioactive radiation by a shield. According to the invention, a container and a shield advantageously provide protection against radioactive radiation, at least up to a distance of 0.5 m from the fuel element. In addition to the endoscope, the image production device, the actuating device and the illumination device are also watertight. According to the invention, it is advantageous if the electrical parts of the image production device, of the actuating device and of the illumination device are protected against water, in particular at a water depth of more than 10 m, advantageously at least down to a water depth of 30 m. In this way, all the relevant components are additionally advantageously resistant or protected at a distance of about 0.5 m from the fuel element against a radioactive radiation power of about 10 8  to 10 10  mrad/h.  
           [0028]    For this purpose, low-noise components, for example relating to the image receiving device and illumination device, are used. In addition, radio-chemically resistant materials relating to the endoscope are used, for example cerium doped lenses in the endoscope objective and/or endoscope eyepiece.  
           [0029]    For example, lead plates or a radiation-proof box offer adequate protection. Here, it is preferable if fewer radiation-sensitive parts (for example the light source) are disposed relatively close to the fuel element, in order to create space for the camera or other sensitive parts, for which a greater distance is beneficial in the sense of shielding the beam.  
           [0030]    The flange is of a watertight configuration, at least for a water depth of more than 10 m, advantageously at least up to 30 m. In addition, for sufficiently good operation of the apparatus, heat dissipation from the watertight housing as a result of the discharge of heat from the gas pressure lamp is necessary.  
           [0031]    The apparatus preferably contains a mounting frame, which carries at least the image production device, the actuating device and the illumination device. The mounting frame is expediently fixed to a position manipulator, which is remotely controlled, for example from the rim of a fuel element storage basin in a nuclear reactor. In this way, therefore, all the devices needed directly for the operation of the endoscope, together with the endoscope, are led up to the fuel element, and are therefore under water during the inspection. An image recording device and/or an image display device are/is preferably also provided to record and/or to reproduce the images produced by the image production device. These are in particular disposed above the water surface. The devices are then viewed by the observer who is inspecting the fuel element, and the images of the subregions of a fuel element are, for example, displayed on a monitor or recorded, for example on a video cassette.  
           [0032]    Accordingly, the apparatus according to a development of the invention also contains a control device for the remote control of the endoscope. Likewise, the apparatus can contain an electrical power supply device, which is provided to feed a light source, an image production device and at least one actuating motor. The aforementioned control device and the aforementioned power supply device are preferably likewise disposed above the water surface and can be operated by the observer. It is advantageous if only one electrical line or one control line, which are virtually not susceptible to radioactive radiation, are led along the fuel element, underneath the water, to the aforementioned container.  
           [0033]    It is part of the invention to use an intrinsically flexible endoscope for the surface inspection of the region of the fuel element disposed under water in a nuclear reactor, in particular in conjunction with a camera disposed under water. As already explained, the use of intrinsically flexible endoscopes permits an enlargement of the inspectable region of a fuel element and an enhancement of the inspection quality. The comparatively inexpensive interchangeability of at least one endoscope end piece circumvents the increasing radiochemical breakdown of the endoscope material. This necessarily occurs, even in the case of endoscope materials that are protected in a complicated manner, such as those according to U.S. Pat. No. 4,036,686.  
           [0034]    Furthermore, the invention specifies a method for the inspection of a region of a fuel element disposed under water in a nuclear reactor, for example for the inspection of a surface of a fuel rod or of a barely accessible region of a bottom piece or of a spacer. According to the invention, the endoscope with the intrinsically flexible end piece carrying the endoscope objective, together with the actuating device, the illumination device and the image production device is brought up to the fuel element under water. In a further step, the end piece is led up to a subregion of the fuel element in such a way that the subregion comes into the field of view of the endoscope. The field of view of the endoscope is preferably determined by a depth of focus of about 1 to 3 cm and an objective aperture of up to 60°. In addition, the field of view of the endoscope can be expanded or set flexibly, for example by a zoom objective. A further device for changing the field of view is an interchangeable optical system, which is placed between a light guide and an image production device (“coupler”). This relates, for example, to a flexibly interchangeable eyepiece of the endoscope. Then, the aforementioned subregion and further subregions of the fuel element are inspected. According to the invention, these are those, which come alternately into the field of view of the endoscope as a result of displacement of the endoscope and/or as a result of curvature of the end piece, it being possible for the field of view to be illuminated by the illumination device. In this way, virtually all regions of a fuel element are accessible to visual inspection.  
           [0035]    Other features which are considered as characteristic for the invention are set forth in the appended claims.  
           [0036]    Although the invention is illustrated and described herein as embodied in a miniature endoscope and a method for the inspection of fuel elements, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.  
           [0037]    The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0038]    [0038]FIG. 1 is a diagrammatic illustration of an apparatus for inspecting a fuel element, which is brought up to a fuel element by a position manipulator according to the invention;  
         [0039]    [0039]FIG. 2 is an exploded, perspective view of an intrinsically flexible endoscope brought up to a barely accessible region of a bottom piece;  
         [0040]    [0040]FIG. 3 is an illustration of an intrinsically flexible, interchangeable endoscope with a watertight container, which is protected against radioactive radiation;  
         [0041]    [0041]FIG. 4 is a sectional view of an interchangeable, rigid end piece of the endoscope;  
         [0042]    [0042]FIG. 5 is a perspective view of the interchangeable, intrinsically flexible end piece of the endoscope;  
         [0043]    [0043]FIG. 6 is a front-elevational, partially broken away, view of the endoscope having an image production device and an illumination device;  
         [0044]    [0044]FIG. 7 is a perspective view of a method for inspecting the surface of a fuel rod;  
         [0045]    [0045]FIG. 8 is a perspective view of a method for inspecting a barely accessible region of a bottom piece. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0046]    In all the figures of the drawing, sub-features and integral parts that correspond to one another bear the same reference symbol in each case. Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1 thereof, there is shown the construction of an apparatus  1  for inspecting a fuel element  3 . The inspection apparatus  1  substantially contains an endoscope  5  and a watertight, radiation-protected container  7 , which are carried by a mounting frame  9 . The mounting frame  9  is fixed to a position manipulator  11 , which is provided to move the endoscope  5  and the watertight container  7  up to the fuel element  3 . In this embodiment, the endoscope  5  has an intrinsically flexible end piece  13 B which is held in a center part  15  by a rigid part  17 , in this case a guide rail  17 . The endoscope  5  is coupled to the container  7  in a watertight manner via a flange  19 . The flange  19  is used to interchange the endoscope  5  and to couple it to devices accommodated in the watertight container  7 , which are described in more detail in FIG. 3.  
         [0047]    For the purpose of inspection, the endoscope  5 , together with the watertight container  7 , is provided to be brought up to the fuel element  3  down to a water depth  21  via the manipulator  11 . All the necessary parts are accordingly protected against water and radioactive radiation  23 . The parts are at least an image receiving device  43 , an illumination device  45  (FIG. 3) and at least one actuating motor  47 A,  47 B (FIGS. 4, 5). The inspection apparatus  1  is therefore provided for the inspection of a region of the fuel element  3 , in particular a surface of a fuel rod  25 , or of another region of the fuel element  3 , for example a barely accessible region, as shown in FIG. 2.  
         [0048]    A line  27  for controlling the endoscope  5  and the electrical power supply to the devices in the container  7  advantageously forms, in this embodiment, the single connection to a power supply device  29  and to a control device  31  (FIG. 1). The latter are disposed, for example, at the rim of a fuel element storage basin of a nuclear reactor. Also advantageously disposed there is an image recording device and image display device  33 , so that the remote control of the endoscope  5  and the visual inspection of the same by service personnel can be performed from there.  
         [0049]    [0049]FIG. 2 shows two exemplary procedures for the inspection of barely accessible regions of the fuel element  3 , for example of a spacer  35  or of a bottom piece  37 . The inspection, for example, of a spacer cell  39  or a cell  47  of the bottom piece  37  is performed by the intrinsically flexible end piece  13 B of the endoscope  5  being bent into the corresponding cell  39  or  41 . Thus, for example, corroded surfaces can be made out or foreign parts can be found even in these barely accessible regions.  
         [0050]    [0050]FIG. 3 shows the endoscope  5  and the watertight, radiation-proof container  7  and also the devices accommodated in the container  7  in detail. It is again possible to see the lower part of the position manipulator  11  and of the mounting frame  9  which is fitted thereto and which carries the container  7  and the endoscope  5 . It is also possible to see, as in FIG. 1, the line  27  for control and the electrical power supply, which leads upward to the control and power supply devices  31 ,  33 ,  29  above the water surface.  
         [0051]    In this embodiment, the endoscope  5  again has an intrinsically flexible end piece  13 B and in its center portion  15  is supported by the rigid part  17 . Furthermore, in this configuration, the endoscope  5  is coupled to the housing  7  via the flange  19  in order to interchange the endoscope  5 . The flange  19  is watertight, as is the container  7 , at least down to a depth of 10 m, advantageously down to at least 30 m. The flange  19  is also used to couple the endoscope  5  to the light source  45 , to the electronic image-receiving device  43 , an electronic camera  43  in this case, and to two actuating motors  47 B.  
         [0052]    The actuating motors  47 B are provided to bend the intrinsically flexible end piece  13 B of the endoscope  5  via a mechanical pulling device  51 B. For this purpose, the mechanical pulling device  51 B is connected to an endoscope objective  53  (FIGS. 4, 5) at a front end  49  of the end piece  13 B of the endoscope  5  and is actuated via the actuating motors  47 B. A light guide  55 A for light leads away from the light source  45 . Together with a light guide  55 B for images, which opens into the electronic image receiving device  43 , the two light guides  55 A and  55 B are led as a bundle of individual fibers  57  as far as the endoscope objective  53  (FIGS. 4, 5). In this way, the images supplied by the endoscope  5  are led to the electronic camera  43  and, the light discharged by the light source  45  is led to a front optical opening  59  (FIG. 5) at the front end  49  of the end piece  13 B of the endoscope  5 .  
         [0053]    In addition to the container  7 , further protective apparatus or shields  61  (FIG. 3) within the container  7  are used to shield against radioactive radiation  23 , to protect at least the light source  45  and the electronic camera  43 . A lead plate, for example, can be used as the shield  61 . Furthermore, the devices are disposed within the container in such a way that the most sensitive parts, the electronic camera  43  in this case, are located at the greatest distance from the radiation source, that is to say the fuel element  3 .  
         [0054]    As already mentioned, use is made here of, for example, of a xenon gas pressure lamp as the light source  43 , with a spectrum which is characterized by a temperature of about 6,000 Kelvin, that is to say similar to daylight. The transparent window belonging to the optical conductors  55 A,  55 B,  57  used to transmit the light from the light source  45  and the images to the camera  43  must accordingly be of a broadband configuration. Furthermore, cooling ribs  63  are preferably fitted to the housing  7 , at least in the vicinity of the light source  45 , and are used for better heat dissipation of the heat power produced by the light source  45 . According to the configuration in FIG. 3, the guide rail  17  supports not only the center portion  15  of the endoscope  5  but, advantageously, also the fiber bundle  57 , continued in the housing, for the transmission of images and light.  
         [0055]    [0055]FIG. 4 shows, by way of example, an advantageous embodiment of an end piece  13 A of the endoscope  5  having an advantageous embodiment of an actuating device  67 A. The endoscope  5  with the end piece  13 A is enclosed by a rigid tube  69 , which is produced, for example, from metal or PVC material. In the embodiment shown in FIG. 4, the endoscope  5  is closed at the front end  49 , but for this purpose contains a lateral optical opening  75 . Through the lateral optical opening  75 , light for illuminating a field of view  65  is intended to emerge, and through the opening  75 , the image produced by the endoscope  5  of a partial view, for example of the fuel element  3 , is intended to be picked up. Within the tube  69 , the bundle of individual optical fibers  57  is used for the transmission of both images and light. In this embodiment, use is further made of a lens  53  and a prism  73  as the endoscope objective. The lateral optical opening  75  functions as an objective aperture. If required, instead of the objective there may also be, for example, a zoom objective, in order to adjust the optical properties, such as depth of focus or enlargement of the endoscope  5 , in a variable manner. At another end of the endoscope  5  there is located the flange  19 , to be coupled to the watertight housing  7 . The flange  19  in FIG. 4 bears schematically illustrated lead throughs  58  for coupling the glass fiber bundle  57 , a mechanical pulling device  51 A inside and outside the watertight, radiation-proof housing  7 . In the embodiment of the mechanical pulling device  51 A illustrated in FIG. 4, this is a rigid mechanical pulling device  51 A which, in the present example, is provided to rotate the end piece  13 A of the endoscope  5  with the endoscope objective  53  through an angle θ. As FIG. 4 shows, the rotation about the axis of the rigid endoscope  5  through the angle θ is effected by the actuating motor  47 A, as part of the actuating device  67 A. In this way, the field of view  65  of the endoscope  5  with a rigid, interchangeable end piece  13 A can be rotated, so that the viewing angle can substantially be varied by adjusting the rotational angle θ.  
         [0056]    In a similar way, the actuating apparatus  67 A can also be used to tilt the rigid endoscope  5 . Given the aforementioned tilt by a tilting angle θ, the viewing angle of the endoscope is likewise varied.  
         [0057]    [0057]FIG. 5 shows, by way of example, a beneficial embodiment of the endoscope  5  having the flexible, interchangeable end piece  13 B for the inspection of the fuel element  3 . A suitable actuating device  67 B for the flexible end piece  13 B is likewise shown. In this case, the flexible end piece  13 B is surrounded by a flexible hose  71 . This is preferably an intrinsically flexible PVC hose  71  or a metallic corrugated hose  71 . Furthermore, the endoscope  5  is closed off in a watertight manner by the hose  71 .  
         [0058]    In this embodiment, the flexible end piece  13 B carries at the front end  49  the front optical opening  59 . Through the optical opening  59 , the light led to the front end emerges, and the image produced by the endoscope  5  is picked up. The field of view  65  of the endoscope  5  with the flexible end piece  13 B therefore leads away from the front end of the endoscope. Disposed behind the front optical opening  59  of the endoscope  5  is the endoscope objective  53 , which is illustrated schematically here by a lens. This is followed by the bundle of individual glass fibers  57 , which are used for light guidance and image guidance. Also, guided in the intrinsically flexible hose  71  of the end piece  13 B of the endoscope  5 , in addition to the glass fiber bundle  57 , is the intrinsically flexible mechanical pulling device  51 B, which is fixed to the endoscope objective  53 . As opposed to the rigid mechanical pulling device  51 A, as illustrated in FIG. 4, the flexible pulling device  51 B is used to bend at least the end piece  13 B of the endoscope  5 . It is therefore connected to the endoscope objective  53  at four points  56  to bend the end piece  13 B on all sides. The fixing points  56  are each located at one end of two Cartesian axes oriented at right angles to one another on the endoscope objective  53 . In a way similar to that shown in FIG. 4, the flange  19  is also used in FIG. 5 to couple the endoscope  5  with the flexible end piece  13 B to the watertight, radiation-proof housing  7  or to interchange the endoscope  5  with the intrinsically flexible end piece  13 B. In addition, the flange  19  shown in FIG. 5 has suitable lead throughs  58  for coupling the glass fiber bundle  57  and the flexible mechanical pulling device  51 B to the devices inside the housing  7 .  
         [0059]    The advantageous configuration of the actuating device  67 B for the endoscope  5  with the flexible end piece  13 B is likewise illustrated schematically in FIG. 5. In this case, the actuating device  67 B contains the flexible, mechanical pulling device  51 B to bend the end piece  13 B on all sides. The mechanical pulling device  51 B in this configuration contains four pull cords, preferably made of metal, of which in each case one pair is set by the actuating motor  47 B. Alternatively, configurations with two pull cords are also provided. The two cables of a pair are fixed to opposite sides of the endoscope objective  53 , in each case on one of the axes disposed in Cartesian fashion in relation to each other. If the cables of a pair are each adjusted by one of the two actuating motors  47 B by a distance S v  and by a distance S h  with respect to each other, this has the effect of tilting the objective about a horizontal or vertical axis, and this accordingly effects the bending of the end piece  13 B of the endoscope  5 .  
         [0060]    A further advantageous configuration of the inspection apparatus  1  for fuel elements is shown in FIG. 6. The sketch shows the cross section of the intrinsically flexible end piece  13 B and, in schematic terms, the parts of an image production device  81  and of an illumination device  83 . The cross section of the end piece  13 B of the endoscope shows the intrinsically flexible endoscope sheath  71  and the cables  51 B, the light guides  55 A and the image conductors  55 B disposed along the endoscope axis, in section. Differing from the embodiments of the inspection apparatus previously shown, in particular of the endoscope  5 , an embodiment is illustrated here which, to bend the end piece of the endoscope on all sides, contains three cables  51 B and in which three separate glass fiber bundles serve as a light guide  55 A separate from the image conductor  55 B. The light guides  55 A are routed within the endoscope sheath  71  through the flange  19  (not shown here) as far as the light source  45  inside the container  7 . Accordingly, the image conductor  55 B is also routed to the image-receiving device  43 , likewise inside the housing  7 . The image receiving apparatus  43  is used to record an image  79  transmitted by the image guide  55 B. Likewise illustrated schematically in FIG. 6 is an eyepiece  77 , which is disposed between the image guide  55 B and image receiving device  43  and whose lens system is indicated schematically here by two lenses. Depending on the application, the eyepiece can advantageously be interchanged with a different eyepiece in order to optimize the field of view  65 , for example as regards the depth of focus or the focus.  
         [0061]    [0061]FIG. 7 shows, in schematic form, an exemplary procedure in the case of a method for the inspection of a region of the fuel element  3 , in this case the procedure for the inspection of the surface of fuel rods  25  inside the fuel element  3 .  
         [0062]    As already partly explained in FIG. 1 and FIG. 2, for this purpose, the endoscope  5  with the intrinsically flexible end piece  13 A or  13 B carrying the endoscope objective  53 , together with the actuating device  67 A or  67 B, the illumination device  83  and the image production device  81  are brought up to the fuel element  3  under water. As shown in FIG. 7, this can also be the endoscope  5  with a rigid end piece  13 A.  
         [0063]    In a second method step, the end piece  13 A is then guided up to a subregion  89  of the fuel element  3  in such a way that the subregion  89  comes into the field of view  65  of the endoscope  5 . This situation is recorded in FIG. 7.  
         [0064]    The subregion  89  and further subregions  85 , which come into the field of view  65  of the endoscope  5  as a result of displacement of the end piece  13 A of the endoscope  5  along a vertical position  87 , using the position manipulator  11 , are then inspected in a third method step, the field of view  65  being illuminated by the illumination device  83 . FIG. 7 illustrates the rigid endoscope  5  with the optical aperture  59  at the front end. However, it is also possible for example for the rigid endoscope  5  with the lateral optical aperture  75 , as in FIG. 4, to be used. In this case, as a result of rotation of the end piece  13 A of the rigid endoscope  5 , the viewing angle can be changed, which substantially predefines the direction of the field of view  65 , and thus the further subregions  85  of the fuel element can be inspected.  
         [0065]    In FIG. 8, a further beneficial alternative to the aforementioned third method step is shown schematically. In a similar way to the procedure explained previously, here the aforementioned third method step with the endoscope  5  with the intrinsically flexible end piece  13 B is sketched. Shown here is the inspection of the bottom piece cell  41  of the bottom piece  37 , which is not accessible to an inspection with the rigid endoscope  5 . However, since the endoscope  5  with the flexible end piece  13 B is used here, the barely accessible subregion  89  of the bottom piece is moved into the field of view  65  of the endoscope  5  by bending the end piece  13 B of the endoscope through an angle φ. The further subregions  85  are subsequently moved into the field of view  65  of the endoscope  5 , and inspected, by curving and bending the end piece  13 B of the endoscope  5  by further bending angles φ. In this case, the field of view  65  is illuminated by the illumination device  83 , not illustrated here.