Fuel assembly

A fuel assembly, comprising:

• a lower fuel support member;
• an upper fuel support member;
• a channel box having a square shape in a cross section of the fuel assembly;
• the channel box having an upper end attached to the upper fuel support member, and extending toward the lower fuel support member, the channel box delimiting a space, wherein a plurality of first fuel rods, a plurality of second fuel rods, a plurality of third fuel rods and a plurality of fourth fuel rods are disposed in the space, the space having an outermost periphery adjacent to an inner surface of the channel box,
• the plurality of first fuel rods having a lower end supported by the lower fuel support member and an upper end supported by the upper fuel support member, containing a nuclear fuel material, and not containing a burnable poison;
• the plurality of second fuel rods having a lower end supported by the lower fuel support member and an upper end supported by the upper fuel support member, and containing the nuclear fuel material and the burnable poison;
• the plurality of third fuel rods having a lower end supported by the lower fuel support member and an upper end not supported by the upper fuel support member, having a length in an axial direction shorter than the plurality of first fuel rods and the plurality of second fuel rods in the axial direction, the plurality of third fuel rods containing the nuclear fuel material, and not containing the burnable poison;
• and
• the plurality of fourth fuel rods having a lower end supported by the lower fuel support member and an upper end supported by the upper fuel support member, and containing the nuclear fuel material and the burnable poison
• wherein two third fuel rods are disposed in the outermost periphery,
• wherein each of the plurality of second fuel rods are disposed in the outermost periphery and each of the plurality of second fuel rods is respectively disposed adjacent to at least one of two adjacent third fuel rods in the outermost periphery,
• wherein the plurality of fourth fuel rods are disposed at each corner of the channel box and adjacent to the outermost periphery, and
• wherein a concentration of the burnable poison of the plurality of fourth fuel rods is lower than a concentration of the burnable poison of the plurality of second fuel rods.