Patent Number: 044787863
Section: summary

TECHNICAL FIELD The present invention relates to a boiling water reactor fuel assembly comprising a plurality of vertical fuel rods surrounded by a fuel channel made with substantially square cross-section and positioned in the fuel channel with the aid of a plurality of spacers, the lower end of the fuel channel being hydraulically and mechanically connected to a nozzle which is provided with an inlet opening for water. The lower ends of the fuel rods rest on a grid device, which is supported by the nozzle. BACKGROUND ART A fuel assembly of the above kind is disclosed in U.S. Pat. No. 3,697,375. In this and other known fuel assemblies there is a tendency to creep deformations of the walls of the fuel channel. This phenomenon is caused, among other things, by the fuel channel being subjected to fast neutrons in combination with an internal overpressure, resulting in the walls being pressed outwards and acquiring a convex outer side. Creep deformations may thus result in a reduced gap width at some place in the control rod gaps, which may jeopardize the operability of the control rods. It is possible to counteract, at least to a certain extent, the above-mentioned pressing out of the walls of the fuel channel by providing the fuel channel with a centrally arranged stiffening device, which has a substantially cruciform cross-section along a predominant part of the vertical length of the fuel rods and is constructed in such a way that the fuel channel is divided into four vertical partial channels, each partial channel surrounding one partial bundle which comprises one-fourth of the total number of fuel rods in the fuel assemblies. Such a division into four is described in U.S. Pat. No. 3,164,530, where the stiffening device consists of four perforated dividing plates. Despite the perforation, it cannot be avoided that these dividing plates involve a considerable neutron absorption, which results in a reduced fuel economy. DISCLOSURE OF THE INVENTION With a fuel assembly according to the invention, the task is to stiffen the walls of the fuel channel by means of a stiffening device which has the mechanical advantages of the known stiffening device and which, in addition, has such a favourable effect on the neutron flux that the fuel economy is considerably better than with a corresponding fuel assembly without a stiffening device. This is achieved by using the stiffening device as a means for achieving in the reactor core a relatively evenly distributed moderator effect in horizontal direction, and thus a more even distribution of the neutron density, which means that the reactivity of the fuel assembly, at unchanged mean enrichment, is considerably increased, and that the differences between the fuel rods of the fuel assembly, as regards power load and burnup, are considerably reduced. The moderator effect of the stiffening device is achieved by providing said device with at least one through-going, vertical water passageway, running along a predominant portion of the length of the fuel rods and being furnished at its lower end with an inlet opening for water flowing through. In this specification, "predominant" and "dominant" are used in their commonly accepted sense to indicate prevalence over all others in magnitude. Since the water flowing through the vertical passageways of the stiffening device is subcooled and prevented from getting into touch with the fuel rods, its void content will be extremely small and its moderator effect the maximum one. What characterizes the invention will become clear from the appended claims .