MOX fuel assembly

A reactor of a nuclear reactor loaded with a plurality of fuel assemblies, each of the fuel assemblies comprising:

• at least, a first fuel rod containing plutonium and not containing burnable poison;
• a plurality of second fuel rods containing uranium and burnable poison and not containing plutonium;
• a water rod;
• and
• a channel box having a rectangular shape in horizontal section and accommodating the first fuel rod, the plurality of second fuel rods, and the water rod in a bundle, wherein
• the plurality of second fuel rods is disposed on an outermost periphery and/or adjacent to the water rod, of a fuel rod array in the horizontal section,
• N2<N1 (N2 is a positive integer or zero) is satisfied where
• a number of the second fuel rods arranged on the outermost periphery is N1 and
• a number of the second fuel rods arranged adjacent to the water rod is N2, and
• of the second fuel rods arranged on the outermost periphery,
• W2<N2+WO<W1 (W2 is a positive integer) is satisfied where
• a number of the second fuel rods arranged without being vertically and/or horizontally adjacent to any of other second fuel rods in the horizontal section is WO,
• a number of the second fuel rods arranged vertically and/or horizontally adjacent to only one second fuel rod in the horizontal section is Wi, and
• a number of the second fuel rods arranged vertically and/or horizontally adjacent to two second fuel rods in the horizontal section is W2, and
• the second fuel rods arranged vertically and/or horizontally adjacent to the two second fuel rods in the horizontal section are arranged at a corner portion of the channel box.