Patent Number: 053596340
Section: abstract

A reactor core for a boiling water nuclear reactor comprises a plurality of vertical fuel assemblies (40), each one containing a plurality of fuel rods (10, 10a, 10b) with enriched nuclear fuel material, which are arranged between a bottom tie plate (11) and a top tie plate (12) in a surrounding vertical fuel channel (1). Each fuel assembly is designed with an inlet (3) for water for conducting water in through the bottom tie plate, through the vertical fuel channel, and out through the top tie plate. Further, each fuel assembly is arranged with intermediate gaps (37a, 37b) with respect to adjacent fuel assemblies and possibly with a channel (32, 50) arranged internally in the fuel assembly for conducting water through the gaps and through the internal channel (if any) in a vertical direction from below and upwards through the core. Each one of at least the main part of those fuel rods, central rods (10a), which in a fuel assembly are surrounded by fuel rods, edge rods (10b), which are located close to a water gap or close to an internal water channel, is arranged with a ratio of the enrichment in the central rod in question to the medium enrichment for the central rods and the edge rods, in a horizontal section, which is lower for an upper part than for a lower part of the central rod.