A reactor core for a boiling water nuclear reactor includes a plurality of fuel assemblies (40) with a plurality of vertical fuel rods (10) and possibly occasional vertical water-filled rods or channels (32, 48, 49, 50, 51) which are surrounded by a fuel channel (1). Between the fuel assemblies (40) there are arranged water gaps (37a, 37b). At least one fuel assembly (40) has at least one outer, reduced corner portion (41) facing a gap (37a, 37bb), and the number of fuel rods (10) in each such fuel assembly (40) is reduced by a number corresponding to the number of reduced corner portions (41) therein.