Patent Number: 052308618
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, the bottom tie plate being provided with through-going, vertical channels for conducting water through the bottom tie plate and into the spaces between 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) 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 may have serious consequences if it occurs on parts which are particularly easily damaged, such as fuel rods. To avoid damage of the above-mentioned kind in pressurized-water reactors, it is known to form the bottom tie plates, already existing under the fuel rod bundles, with a large number of straight vertical 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. SUMMARY OF THE INVENTION According to the present invention, water channels in the bottom tie plate of a fuel assembly for a nuclear reactor of pressurized-water or boiling-water type are formed in a special way which provides efficient capture of debris, particularly if the debris consists of narrow, elongated objects such as pieces of wire and borings. The favorable result is obtained according to the invention by providing the bottom tie plate, in addition to through-going vertical channels, with horizontal channels which are adapted to cross the vertical channels, and by arranging helical springs with horizontal symmetry axis in the horizontal channels, at least where these cross the vertical channels. A helical spring of the kind mentioned may advantageously be adapted to extend along at least substantially the entire length of the horizontal channel in which it is placed. At edge sides of the bottom tie plate, the horizontal channels are preferably arranged in open communication with the spaces between the fuel rods, but it is possible, per se, for the horizontal channels to be closed at their ends. Horizontal channels with helical springs may, according to a preferred embodiment of the invention, be arranged at different levels in the bottom tie plate. Preferably, horizontal channels with helical springs are arranged in one direction at one level in the bottom tie plate and at another direction, perpendicular to the first one, at another level in the bottom tie plate. The bottom tie plate may also be provided with channels free from helical springs, which channels are then preferably arranged at another level than those channels which contain helical springs.