Element distrubution in nuclear fuel bundle

Advantageous heat transfer performance can be obtained by arranging an odd number of nuclear fuel rods in the outer row of lattice positions with more than two rows of lattice positions occupied by fuel rods. The rows are preferably concentric. The actual number of lattice positions in the inner rows may be an even number or an odd number but an odd number is preferred in every row. For example there may be 21 rods in the outer row, 15 lattice positions in the intermediate row and 9 lattice positions in an inner row. A still further inner row of 3 lattice positions may be provided or this may be a single auxiliary coolant pipe. (auth)