Patent Number: 051006111
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to a fuel assembly for a light-water nuclear reactor 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-holes for conducting water through the bottom tie plate and into the spaces between the fuel rods. Experience shows that, for example in connection with repair and service of a nuclear reactor, debris may enter, which debris then moves with the water which circulates through the reactor core. The debris may, for example, consist of metal chips (borings) formed in connection with repair of, for example, a steam separator, pieces of metal wire, or other foreign particles which have entered the system from outside. The debris may give rise to abrasion damage, which has serious consequences if the damage arises 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 provide the bottom tie plates which already exist below the fuel rod bundle with a large number of holes to enable each bottom tie plate, in addition to each 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, through-holes in the bottom tie plate of a fuel assembly in a pressurized water or boiling water nuclear reactor are designed in a special way, which provides efficient capture of debris, especially if the debris consists of thin, elongated objects, such as pieces of wire and borings. The favourable result is obtained according to the invention by forming the through-holes in the bottom tie plate with parts, the centre lines of which are displaced in relation to each other in the lateral direction or make an angle with each other. As will be clear from the text below, it may be suitable to combine the use of a bottom tie plate according to the invention with another debris catcher to ensure that objects of debris of different shapes are taken care of. According to a preferred embodiment of the invention, in which through-holes in the bottom tie plate have parts whose centre lines are displaced in relation to each other, a part of a through-hole on the inlet side of the bottom tie plate for the water is common to several parts of through-holes on the outlet side of the bottom tie plate for the water. A part of a through-hole on the inlet side of the bottom tie plate thereby normally has a larger cross section than a part of a through-hole on the outlet side of the bottom tie plate. To increase the water flow through the bottom-tie plate, parts of through-holes in the bottom tie plate may be arranged in open communication with one or more edge sides of the bottom tie plate by way of transverse channels which at each respective edge side have orifices which are arranged in open communication with the spaces between the fuel rods in the same fuel assembly in a boiling water reactor or in the same and adjacent fuel assemblies in a pressurized water reactor. The open communication may advantageously be achieved via vertical recesses in the respective edge side on the bottom tie plate. A further increase of the water flow through the bottom tie plate may be obtained by arranging parts of the same kind of through-holes in the bottom tie plate in open communication with each other via transverse channels. According to another embodiment of the invention, in which through-holes in the bottom tie plate have parts, the centre lines of which make an angle with each other, a through-hole is arranged with a pocket in which one part changes into another part. To reduce the risk of clogging of the bottom tie plate, a part of the through-hole which extends from one side of the bottom tie plate may be arranged in open communication with at least two parts of through-holes which extend from the other side of the bottom tie plate. This results in redundant flow paths and a large open volume in the interior of the bottom tie plate. The invention will be explained in greater detail, by way of example, with reference to the accompanying drawings.