Patent Number: 060350113
Section: claims

1. A reactor core for a boiling water nuclear reactor which includes four vertical fuel assemblies positioned around a control rod having arms that provide said control rod with a cruciform cross section, each of said four fuel assemblies being surrounded by a first and a second pair of gaps arranged transversely to and adjacent each other, which during reactor operation are filled with water, said first pair of gaps being adapted to encase two arms of said control rod, each fuel assembly containing a plurality of fuel rods containing enriched nuclear fuel material, said fuel rods being arranged between a bottom tie plate and a top tie plate, wherein at least one fuel assembly comprises a substantially pentagonally formed fuel channel with, in a cross section, four relatively long side portions and one relatively short side portion, said short side portion facing said second pair of gaps where they transversely communicate with each other. 2. A reactor core according to claim 1, wherein at least one of said four fuel assemblies comprises an integrally arranged channel in the form of a hollow tubular member connected to said top and bottom tie plates for conducting water in a vertical direction from the bottom tie plate upwards through the core. 3. A reactor core according to claim 2, wherein said internally arranged channel is a sub-channel-forming support member of cruciform cross-section spacing apart four sub-bundles of fuel rods within said one fuel assembly from each other, said cruciform support member constituting a central water passage in the space defined by removing one fuel rod from each of the corner portions of said sub-bundles directed towards the centre of a fuel channel box. 4. A reactor core according to claim 1, wherein said first pair of gaps are wider than said second pair of gaps and encase said two arms of a control rod. 5. A reactor core according to claim 1, wherein enrichment contents A in a fuel rod arranged adjacent to a centre of said control rod and the enrichment content B in the fuel rods arranged adjacent to said short side portion of the fuel channel is in accordance with the formula B=A.times.Fk(Fs(a/b-1)+1), where 0.72.ltoreq.Fk.ltoreq.0.92, Fs=0.72, a is a gap width of said first pair of gaps, and b is a width of said second pair of gaps. 6. In a boiling water nuclear reactor which includes a reactor core containing a plurality of vertical fuel assemblies, each fuel assembly including a bottom tie plate, a top tie plate, an outer channel, and a plurality of fuel rods within the outer channel and extending between said bottom and top tie plates, each fuel assembly being surrounded by first and second pairs of gaps arranged transversely to and adjacent each other, said gaps being filled with water when said reactor is operating, and a control rod having a cruciform cross section, two arms of which are located in said first pair of gaps, the improvement wherein the outer channel of one of said fuel assemblies has five sides, four of said five sides having a greater width than a fifth of said five sides, said fifth side facing said second pair of gaps where said second pair of gaps transversely communicate with each other.