A fuel assembly includes full length fuel rods which contain a plutonium fissile (Puf) but do not contain a burnable poison, full length fuel rods which contain the fissile uranium and the burnable poison, and partial length fuel rods which contain Puf but do not contain the burnable poison in a channel box. The plutonium enrichment is decreased in an order of the full length fuel rods. The concentration of the burnable poison of the full length fuel rod is higher than the concentration of the full length fuel rod. In each side of a rectangular outermost periphery adjacent to the inner surface of the channel box in a horizontal cross-sectional view of the fuel assembly, two partial length fuel rods are adjacently disposed, and the full length fuel rod containing the burnable poison is disposed to be adjacent to each partial length fuel rod.

A nuclear fuel assembly comprising a plurality of control rod guide assemblies. At least one of the control rod guide assemblies includes a guide tube having an axial dimension, the guide tube being supported by the plurality of grids and extending axially between the top nozzle and the bottom nozzle, the guide tube having an upper portion having a first radius and a lower portion having a second radius less than the first radius, and an external dashpot tube disposed around a portion of the lower portion in an area beginning at the bottom grid and extending toward the top nozzle.

A nuclear fuel assembly comprising a plurality of control rod guide assemblies. At least one of the control rod guide assemblies includes a guide tube having an axial dimension, the guide tube being supported by the plurality of grids and extending axially between the top nozzle and the bottom nozzle, the guide tube having an upper portion having a first radius and a lower portion having a second radius less than the first radius, and an external dashpot tube disposed around a portion of the lower portion in an area beginning at the bottom grid and extending toward the top nozzle.

A holding fixture for assisting in assembly of a support grid for nuclear fuel rods and including a plurality of straps each having a plurality of slots extending approximately half a height of the straps and tabs formed beside or between the slots. The holding fixture includes an actuation plate, a support plate having a plurality of receiving members structured to receive therein straps of the support grid and having a plurality of cells, and a plurality of cam assemblies structured to move to deflect every other tab of the straps received in the plurality of receiving members. The cam assemblies are disposed in every other cell of the support plate.

A holding fixture for assisting in assembly of a support grid for nuclear fuel rods and including a plurality of straps each having a plurality of slots extending approximately half a height of the straps and tabs formed beside or between the slots. The holding fixture includes an actuation plate, a support plate having a plurality of receiving members structured to receive therein straps of the support grid and having a plurality of cells, and a plurality of cam assemblies structured to move to deflect every other tab of the straps received in the plurality of receiving members. The cam assemblies are disposed in every other cell of the support plate.

A nuclear fuel assembly comprising a plurality of control rod guide assemblies. At least one of the control rod guide assemblies includes a guide tube having an axial dimension, the guide tube being supported by the plurality of grids and extending axially between the top nozzle and the bottom nozzle, the guide tube having an upper portion having a first radius and a lower portion having a second radius less than the first radius, and an external dashpot tube disposed around a portion of the lower portion in an area beginning at the bottom grid and extending toward the top nozzle.

A nuclear fuel assembly comprising a plurality of control rod guide assemblies. At least one of the control rod guide assemblies includes a guide tube having an axial dimension, the guide tube being supported by the plurality of grids and extending axially between the top nozzle and the bottom nozzle, the guide tube having an upper portion having a first radius and a lower portion having a second radius less than the first radius, and an external dashpot tube disposed around a portion of the lower portion in an area beginning at the bottom grid and extending toward the top nozzle.

A fuel assembly includes: a square cylinder channel box having a square cross-section; and fuel rods disposed in a square lattice shape and filled with a nuclear fuel material. The fuel rods includes: a first rod disposed in an outer layer in the box and a second rod disposed in an inner layer in the box and having a larger diameter than the first rod. An absolute value of a difference between a hydraulic equivalent diameter around the first rod and a hydraulic equivalent diameter between a surface in the box and the first rod is more than or equal to a value obtained by multiplying an absolute value of a difference between the hydraulic equivalent diameter around the first rod and the hydraulic equivalent diameter around the second rod by a ratio of a cross-sectional area of the inner layer to a cross-sectional area of the entire box.

A fuel assembly includes: a square cylinder channel box having a square cross-section; and fuel rods disposed in a square lattice shape and filled with a nuclear fuel material. The fuel rods includes: a first rod disposed in an outer layer in the box and a second rod disposed in an inner layer in the box and having a larger diameter than the first rod. An absolute value of a difference between a hydraulic equivalent diameter around the first rod and a hydraulic equivalent diameter between a surface in the box and the first rod is more than or equal to a value obtained by multiplying an absolute value of a difference between the hydraulic equivalent diameter around the first rod and the hydraulic equivalent diameter around the second rod by a ratio of a cross-sectional area of the inner layer to a cross-sectional area of the entire box.