Patent Number: 055307299
Section: claims

1. A spacer for retaining, at a plurality of levels, elongated fuel rods surrounded by a fuel channel through which coolant is arranged to flow downstream in a fuel assembly for a boiling water reactor, said spacer comprising a plurality of cells including corner portions for accommodating the fuel rods which are surrounded by an external spacer frame which is provided all around with a plurality of openings through which the coolant is directed to the inside of the spacer frame, wherein the external spacer frame comprises a first portion in the fuel assembly arranged at a distance from the inner surface of the fuel channel and a second portion arranged at a shorter distance from the fuel channel compared with the first portion, and that on the inside of the frame, between the first and second portions, there is formed an edge which is obliquely positioned in relation to the longitudinal direction of the fuel assembly, said edge communicating with at least one of such openings located at the corner portions, the edge being inclined in a downstream direction toward each of the corner portions for directing the coolant toward the fuel rods arranged at the corner portions of the spacer. 2. A spacer according to claim 1, wherein at at the corner portions of the spacer frame, there are arranged recesses which are limited at top and bottom by projecting parts. 3. A spacer according to claim 1, wherein, at the lower edge of the spacer, guiding studs are arranged comprising a plate tab or the like, the plane of which is arranged parallel to the flow direction of the coolant and one side edge of which extends in an inclined manner from the lower edge of the frame in a direction upstream of the spacer towards the centre of the spacer. 4. A spacer according to claim 1, wherein the spacer comprises a further edge formed between the first and second portions extending in a direction transverse to the direction of flow. 5. A spacer according to claim 4, wherein the spacer comprises a guiding surface which comprises a long, flat plane inclined in a direction toward the centre of the spacer and extends downstream of the spacer starting from the further edge. 6. A spacer according to claim 4, wherein, in the second portion, there is arranged at least one bottle-like opening which constitutes an extension of one of said plurality of openings downstream of the spacer. 7. A spacer according to claim 1, wherein, in at least one corner, the spacer comprises a long, flat surface inclined inwardly toward the centre of the spacer. 8. A fuel assembly for a boiling water reactor with an upper part with coolant in two-phase state and a lower part with coolant essentially in single-phase state, said fuel assembly comprising an elongated fuel channel connected to a coolant intended to flow downstream through the fuel channel, a bundle of elongated fuel rods arranged in the fuel channel and retained by a plurality of spacers arranged at different levels along the bundle, each one of said spacers comprising a plurality of cells including corner portions for accommodating the fuel rods, said cells being surrounded by an external spacer frame which is provided all around by a plurality of openings through which the coolant is directed to the inside of the spacer frame, wherein the fuel assembly comprises at least one spacer which comprises a spacer frame with a first portion in the fuel assembly arranged at a distance from the fuel channel, and a second portion arranged at a shorter distance from the inner surface of the fuel channel compared with the first portion, and that on the inside of the frame, between the first and second portions, there is formed an edge which is obliquely positioned in relation to the longitudinal direction of the fuel assembly and which extends in an inclined manner in a direction downstream of the spacer (7) and toward the fuel rod (3) arranged in the corner of the spacer (7) said edge communicating with at least one of such openings located at the corner portions, the edge being inclined in a downstream direction toward each of the corner portions for directing the coolant toward the fuel rods arranged at the corner portions of the spacer. 9. A fuel assembly for a boiling water reactor with an upper part with coolant in two-phase state and a lower part with coolant essentially in single-phase state, said fuel assembly comprising an elongated fuel channel connected to a coolant intended to flow through the fuel channel, a bundle of also elongated fuel rods arranged in the fuel channel and retained by a plurality of spacers arranged at different levels along the bundle, each one of said spacers comprising a plurality of cells for accommodating the fuel rods, said cells being surrounded by an external spacer frame which is provided all around with a plurality of openings, wherein the upper part comprises at least one spacer according to claim 1 and the lower part comprises at least one low-pressure drop spacer. 10. A fuel assembly according to claim 9, wherein the low-pressure drop spacer comprises a first portion and a second portion, the first portion in the fuel assembly (1) being arranged at a larger distance from the inner side of the fuel channel than the second portion and wherein the first and second portions are arranged alternately and vertically positioned and that the low-pressure drop spacer both at top and bottom is provided with folded-in guiding studs comprising a plate tab or the like, the plane of which is arranged in parallel with the direction of flow of the coolant and one side edge of which extends in an inclined manner from the lower and upper edge of the frame, respectively, in a direction upstream and downstream of the low-pressure drop spacer, respectively, towards the centre of the low-pressure drop spacer.