FUEL ASSEMBLY

PURPOSE:To assure a thermal margin and to attain log-term operation and higher burnup by forming the fuel assembly of 1st fuel rods packed with nuclear fuel and 2nd fuel rods packed with nuclear fuel contg. the transuranic elements at the ratio per unit mass higher than the ratio per unit mass thereof in the 1st fuel rods. CONSTITUTION:This fuel assembly 12 is constituted by alternately bundling the 1st fuel rods 11 packed with the 1st fuel 51 consisting of an oxide mixture composed of uranium and plutonium and the small-diameter 2nd fuel rods 12 packed with the 2nd fuel 52 consisting of the oxide mixture formed by enriching the transuranic elements in the uranium and the plutonium in a cladding pipe 8 consisting of a ferritic steel respectively and coating this pipe with a hexagonal wrapper tube 3 made of a ferritic steel. The fuel consisting of the transuranic elements and the ordinary fuel consisting of the uranium and the plutonium are separated in this case and, therefore, with the central temps. of the respective pieces of the fuel or the melting points of the fuel do not vary by the mixing ratio of the transuranic element over the entire part of the fuel assembly. The thermal margin of the fuel equiv. to the case of not enriching of the transuranic elements is assured.