Patent Number: 054715148
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to a fuel assembly for a nuclear reactor of light-water type, comprising a plurality of vertical fuel rods which are arranged, in spaced relationship, between a bottom tie plate and a top tie plate which are both provided with through-holes for inlet and outlet and for conducting coolant to the fuel rods. Experience shows that, for example in connection with repairs and service of a nuclear reactor, debris may enter and then move with the water which circulates through the reactor core. The debris may, inter alia, consist of metal shavings (borings or turnings) formed in connection with the repair of, for example, a steam separator, pieces of metal wire, or other foreign particles, which have entered the system from the outside. The debris may give rise to abrasion damage, which may have serious consequences if it occurs on parts which are particularly easily damaged, such as fuel rods. This may be the case if the debris adheres to a spacer. To avoid damage of the above-mentioned kind, it is known to form the bottom tie plates, already existing under fuel rod bundles, with a large number of bored holes in order for each bottom tie plate, besides its normal function, to function as a debris catcher in the form of a strainer and prevent debris from reaching the fuel rod bundle with control rod guide tubes and spacers. A type of bottom tie plate which is also intended to serve as a debris catcher is described in Swedish patent application 9100402-8. This bottom tie plate is provided with horizontal channels which cross vertical flow channels, and in at least some of the horizontal channels spiral springs with horizontal symmetry axis are arranged. The disadvantage of this solution is that the flow resistance across the bottom tie plate becomes too high. A type of separate debris catcher arranged below the bottom tie plate is described in Swedish patent 465 644. This debris catcher comprises several turns of a wound tape or several concentric rings with spacing elements arranged between the turns of the tape or the rings, which spacing elements maintain a mutual distance between the turns of the tape or the rings in the radial direction. The problem with hitherto known separate debris catchers is that they provide an impermissibly high flow resistance and/or are complicated and expensive to manufacture. SUMMARY OF THE INVENTION According to the present invention, a debris catcher is arranged as a separate unit below or above, close to or spaced from, the bottom tie plate in the flow path of the water through a bundle of rods. The debris-catching elements of the debris catcher consist of one or more springs, preferably helical springs. The use of helical springs as debris-catching elements allows an area reduction which is small in cross section and hence a moderate speed variation of the coolant flow when passing through the debris filter. The debris catcher also comprises a holder for fixing the spring/springs, which is permeable to the flow of cooling water to the fuel rod. The holder may, for example, consist of two or more substantially parallel plates, in which at least one of the flat sides of the respective plate is formed with a plurality of concentric annular slots in such a way that when two plates are arranged with the slotted flat sides against each other, they form a parallelepiped with annular channels. A number of springs, corresponding to the number of channels, are fixed in the channels between the plates. The springs are thus arranged in radial layers close to each other in spaced relationship and at one or more levels depending on how many plates are used. In the axial direction the plates are provided with through-holes for the flow of cooling water through the fuel assembly. The debris catcher is normally arranged at least substantially such that the symmetry axis of the springs is horizontal. In a boiling-water reactor, the debris catcher can be arranged spaced from the bottom tie plate. In at least certain types of pressurized-water reactors, it may be necessary to allow the debris catcher to make contact with the underside of the bottom tie plate or to arrange it at a very small distance from the underside. By designing the debris catcher in the manner described above, it may be given a very low flow resistance and hence constitute a minimal obstacle to the desired flow while at the same time the pins with intermediate springs are active during the debris catching. One advantage with the invention is that the debris catcher can be inserted into fuel assemblies without necessitating any major modifications and without significantly influencing the flow resistance. A debris catcher according to the invention is very simple to manufacture and very robust since it can be designed with few parts. This also means that the risk of small parts becoming detached in connection with the water flowing through is very small. An additional advantage is that it is possible to supplement, in a simple manner, already existing fuel designs with a debris catcher.