A fuel assembly for a nuclear power boiling water reactor including a fuel channel defining a central fuel channel axis, fuel rods, each having a central fuel rod axis, at least 3 water channels for non-boiling water, each water channel having a central water channel axis and each water channel having a larger cross-sectional area than the cross-sectional area of (the average) fuel rod. The fuel rods include a first group of full length fuel rods and a second group of shorter fuel rods. The fuel assembly comprises 3 or 4 fuel rods which belong to said second group and which are positioned such that the central fuel rod axis of each of these 3 or 4 fuel rods is closer to the central fuel channel axis than any of the water channel axes of the water channels.

A rodlet for insertion into a fuel assembly guide tube of a fuel assembly in a pressurized water nuclear reactor is disclosed. The rodlet includes a capsule for housing a material to be irradiated, a backfill region disposed between the capsule and the material, and an outer tube comprising a plurality of apertures defined therein. The capsule defines a wall that surrounds the material. The outer tube is to house the capsule. The outer tube defines a first annulus and a second annulus. The first annulus includes a first width defined between the capsule and the outer tube. The second annulus includes a second width defined between the outer tube and the fuel assembly guide tube.