Patent Number: 039403130
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawing, FIG. 1 shows the upper end of a fuel rod which comprises tubular jacket 1, the nuclear fuel charge 11 and the end cap 2 for sealing off the end of the jacket 1. In this embodiment, a non-magnetic capsule-like housing or capsule 3 is inserted between the fuel charge 11 and the end cap 2. The capsule 3 has inlet means in the form of bores 31 and, within the capsule, there is provided a compact body 4. The compact body 4 comprises a pressed mixture of aluminum powder and a water-soluble organic or inorganic mercury compound such as mercury nitrate. The compact body 4 also contains ferromagnetic material. In the event of a leak in the jacket 1, moisture enters into the interior of the fuel rod and, therefore, also through the holes 31 of the capsule 3. This causes the pressed compact body 4 to disintegrate and undergo an increase in volume and fill practically the entire interior of the capsule 3 as shown in FIG. 1b. This means that the ferromagnetic component of the body 4, which component consists, for example, of iron powder, has a substantially lower spatial concentration or density than it had before the moisture entered the capsule; this new condition can be determined by magnetic means, for instance, by measuring the magnetic reluctance from the outer side of the fuel rod. A further embodiment of the invention is shown in FIGS. 2a and 2b wherein, for clarity, the parts of the fuel rod surrounding the capsule 3 have been omitted. However, the same situation applies as in FIG. 1. In this embodiment a cup-like container or cup 32 is located in the capsule 3. The cup 32 is filled with chips or powder of a non-rusting magnetic material. The cup 32 is closed by a covering lid 42 which consists of a pressed mixture of aluminum powder and a mercury compound. If the fuel rod is intact, the situation according to FIG. 2a obtains wherein the condition of the cup 32, or more precisely, its charge 41 can be measured magnetically, as discussed in the embodiment of FIG. 1. However, it is here also possible to lift this cup within the capsule 3 with the application of a strong magnetic field and then to drop it. This action can be detected, for instance, by a suitable pick-up microphone responsive to the noise developed thereby inside the fuel rod. If, however, damage occurs to the jacket 1, cover 42, in response to moisture, decomposes as shown in FIG. 2b and the entire interior of the capsule 3 becomes filled. Therefore, there is no longer any tolerance space in which the cup 32 can move, so that the magnetic test no longer produces noise. If, therefore, no noise is detected in the testing operation, a positive and reliable indication for a defect within the fuel rod in question is provided. Whereas, in the embodiment according to FIGS. 1a and 1b, a decomposition of the ferromagnetic body and a change in the concentration of the ferromagnetic particles take place, FIG. 3 shows a third embodiment wherein, as in the embodiment of FIG. 2, a decomposition or change of the magnetic reluctance is unnecessary. The compact body 4 consists of two individual pellets 44 and 43 which are disposed within the capsule 3 as shown in FIG. 3a; this configuration provides a free space above the pellets. The lower pellet 43 again consists here of aluminum powder and a mercury compound, whereas the upper pellet 44 consists of a solid of ferromagnetic material which does not decompose. If a defect in the jacket occurs, the condition illustrated in FIG. 3b develops, wherein, the pellet 43 has decomposed and increased in volume to urge the ferromagnetic pellet 44 upward. The pellet 44 has been therefore moved to a new physical location that can be ascertained from outside the fuel rod by magnetic or noise measurements. The composition of the material, as discussed above, can be caused not only by water in liquid form, but also, by steam. The material of which the capsule 3 is made and also that of the cup 32 should not be attacked by this moisture; this material must, of course, be non-magnetic. The material of the jacket 1 can be, for example, a zircon compound or stainless steel; however, a ceramic material can also be used. The capsule 3 can at the same time be used here for the mechanical support of the jacket 1. It is also possible to provide thermal insulation between the capsule 3 and the fuel charge 11 in the form of a ceramic pellet. The physical size of the capsule 3 is advantageously chosen so that a collecting space is provided for the fission gas to limit the internal pressure of the fuel rod.