Patent Number: 054835658
Section: summary

TECHNICAL FIELD The present invention relates to a fuel assembly for a boiling-water nuclear reactor. Such a fuel assembly comprises a plurality of vertical fuel rods and possibly occasional vertical, water-filled rods or inner, for example circular, rectangular or cruciform water channels, which fuel rods are surrounded by a fuel channel of substantially square cross section. Associated with the lower end of the fuel channel is a transition section with a downwardly-facing inlet opening for reactor coolant. The transition section is provided with through-holes for a so-called by-pass flow outside the fuel assembly. The fuel channel is open at both ends such that the reactor coolant can flow through the fuel assembly. Each rod is included in two rows perpendicular to each other and each rod contains a stack of circular-cylindrical pellets of a nuclear fuel, the stack being arranged in a cladding tube. The fuel rods are arranged between a bottom tie plate and a top tie plate which are both provided with through-holes for an inlet and an outlet and passage of coolant to the fuel rods. The fuel assemblies are arranged in groups of four, forming a so-called supercell. Centrally in the supercell, a control rod of cruciform cross section is arranged. BACKGROUND ART, PROBLEMS A reactor core with fuel assemblies as described above is immersed into water which serves both as coolant and as neutron moderator. The space between the fuel assemblies is filled with flowing water for neutron moderation. The distance between two fuel assemblies is referred to as gap width. In addition to the water flowing along the fuel rods and the water being in contact with the fuel rods, the reactor core is traversed by a by-pass flow which is located at the above-mentioned gaps formed between the fuel assemblies. The major part of the water, typically 90%, is admitted into the fuel assemblies and allowed to boil therein. The remainder is discharged through the by-pass holes and is maintained in a nonboiling state. In certain cases, the fuel assemblies are provided with internal water channels. The water in these channels is not allowed to boil. To obtain a specified quantity of by-pass flow outside the fuel assemblies, that is, in the gaps, the transition section of the fuel assemblies are provided with so-called by-pass holes which allow the passage of the required quantity of water outside the fuel assemblies. Swedish printed patent application 424 236 discloses an example of a fuel assembly provided with continuous by-pass holes in the transition section. These holes are arranged at the center of the side surface of the fuel assembly and exactly opposite to each other, whereby the by-pass flow from one of the holes is always directed against the control rod. U.S. Pat. No. 5,697,376 discloses another example of a fuel assembly provided with continuous by-pass holes in the transition section. These two holes are arranged at the center of the side surface of the fuel assembly with an approximately 90.degree. angular adjustment. This location of the holes means that the fuel assembly can be arranged in one position only to prevent the by-pass flow from one of the holes from being directed against the control rod. In certain cases there is a need to turn the fuel assembly through an angle of 90.degree., 180.degree. or 270.degree. around its longitudinal axis. One such case is when bending the fuel channel of the fuel assembly. Bending of the fuel channel may occur after some time of use and means that the fuel channel changes its shape by bending. Bending of the fuel channel may, in certain cases, constitute a problem in symmetrical core lattices. In symmetrical core lattices, the control rod gaps, that is, the gaps into which the control rods are inserted, and the narrow gaps, that is, the gaps into which the control rods are not inserted, have the same gap width. In symmetrical lattices, the fuel channel is bent in a random direction when being irradiated with fast neutrons. The fast neutrons entail release of residual stresses, whereby the structure of the material is changed such that the fuel channel is extended. When the different sides of the fuel channel are extended to different degrees, the fuel channel becomes deflected towards the direction where the largest extension has taken place. In those cases where the fuel channel is bent in a direction towards the control rod in a supercell, the bending can be of such a magnitude that the control rod when being inserted into the core, for power control, adheres thereto. In those cases where this bending takes place in a direction towards the control rod, thus, the box should be turned 90.degree., 180.degree. or 270.degree. around its longitudinal axis to eliminate the risk of the bent fuel channel preventing the insertion of the control rod. When being bent, the fuel channel has a certain memory effect such that, if a fuel channel starts bending towards the control rod and the channel is turned such that it bends in a direction away from the control rod, the fuel channel will continue to bend in this direction, that is, away from the control rod. The memory effect is due to the fact that the gap towards which the fuel channel has started bending becomes smallest and is hence irradiated by more fast neutrons, whereby the bending continues towards this smallest gap. During refuellings, the fuel assemblies are picked out of the core and its fuel channels are inspected with respect to bending. In those cases where bending in a direction towards the control rod has been determined, the fuel assembly is turned 90.degree., 180.degree. or 270.degree. around its longitudinal axis. The problem is then that the holes which, according to the prior art, are provided centrally on the two sides of the fuel assembly, facing away from the control rod, are arranged to direct the by-pass flow against the control rod. A by-pass flow directed against the control rod may cause the rod to vibrate and hence wear out holes in the fuel channel, which may lead to a reduction of the safety margin with respect to so-called dryout since less water is passed into the fuel assembly when the pressure is lower outside thereof. SUMMARY OF THE INVENTION, ADVANTAGES The invention relates to a fuel assembly with holes for a by-pass flow arranged so as to enable a turning of the fuel assembly 90.degree., 180.degree. or 270.degree. around its longitudinal axis without the by-pass flow being directed against the control rod. The fuel assembly according to the invention comprises a cladding tube which is secured to a transition section. The transition section is formed with a guide member intended to guide the fuel assembly into an assembly supporting plate. The fuel assembly has a circular, downwardly-facing inlet opening which is surrounded by a substantially annular end surface of the transition section. The fuel assembly is intended to be arranged together with three identical fuel assemblies with a centrally placed control rod forming a supercell. Somewhere between the assembly supporting plate and the lowest part of a nuclear fuel pellets column, one or more continuous by-pass holes are arranged, preferably in the conical wall portion of the transition section. The by-pass holes are adapted such that, independently of how the fuel assembly is arranged in the core, the by-pass flow is directed away from the control rod. Since the transition section is provided with four holes distributed with a substantially 90.degree. pitch angle, the direction of the by-pass flow away from he control rod when turning the fuel assembly is achieved by the arrangement of a turning ring around the transition section, above the holes. The turning ring is provided with at east one hole arranged such that, irrespective of the location of the fuel assembly, the turning ring can be turned in a position such that at least one hole in the transition section is open for the passage of the by-pass flow directed away from the control rod. In those cases where the assembly is to be turned through 180.degree., two continuous by-pass holes are suitably provided which are diametrically opposed and placed such that both the holes have an equally large distance to the control rod. Possibly, more holes can be arranged in the same position but separated in the vertical direction. With this embodiment, no turning ring is needed. The size of the by-pass holes may vary depending on the desired quantity of water out from the fuel assembly. The fuel assembly according to the invention is intended for arrangement in boiling water reactors where the predominant part of the water in the reactor core passes through the lower part of the fuel assembly, that is, through the transition section of the assembly. The advantage of the invention is that in a simple manner a fuel assembly is obtained in which the holes for the by-pass flow are arranged such that they are not, neither before nor after a 90.degree., 180.degree. or 270.degree. turning, directed against the control rod in the above-mentioned supercell.