Abstract: The improved fuel assembly has a plurality of elongated corner posts extending longitudinally between and releasably and rigidly interconnecting top and bottom nozzles so as to form a rigid structural skeleton of the fuel assembly. Additionally, a plurality of transverse grids are supported at axially spaced locations along the corner posts and a plurality of fuel rods are supported by the grids. Certain groups of the fuel rods are spaced apart laterally from one another by a greater distance than the rest of the fuel rods so as to define a number of elongated channels extending between the top and bottom nozzles. A cluster assembly having a cluster plate with a plurality of elongated rods is adapted to be removably supported on the top nozzle with its rods extending through the channels. The rods can be a plurality of guide thimbles in the case of one cluster assembly, or a plurality of oversized fuel rods in the case of another cluster assembly.

Abstract: A nuclear fuel spacer grid with improved outer straps. The grid has an egg-crate array of interleaved grid straps creating cells for enclosing the fuel rods. The grid also has four outer straps connected together in a square-shaped array surrounding the grid strap's heightwise edges. The outer straps have a central portion to which is attached the grid straps' heightwise edges. The outer straps also have a top and a bottom resilient lengthwise border portion extending vertically beyond and horizontally outwardly beyond their associated outer straps central portions.

Abstract: A nuclear fuel spacer grid with improved grid straps. The grid has an egg-crate array of interleaved grid straps creating cells for enclosing the fuel rods. Near beginning of life in the reactor, the grid straps bend in a spring-like manner to flexibly grip the fuel rods with their preferably rigid protrusions. In a cell, the protrusions on two non-opposing grid straps are closed to longitudinal coolant flow while the protrusions on the remaining grid straps associated with the cell are open to such flow. Near end of life the grid straps loose resiliency due to irradiation--induced stress relaxation and the fuel rod has a loose fit against the protrusions due to radiation effects. However, the fuel rod is held in position in the cell by the coolant flow therethrough which forces the fuel rod away from the closed protrusions and into contact with the open protrusions. The protrusions in one cell are coplanar. The protrusions in the cells lie in one of two longitudinally separated planes.

Abstract: A partial grid structure is provided for a nuclear reactor fuel assembly for selectively mounting either means for deflecting the upwardly flowing coolant or means for laterally supporting the fuel rods, or both, to substantially eliminate localized neutron flux peaking and thereby increasing the power output of the assembly. The grid structure extends across only a portion of the fuel assembly and is associated with a selective group of fuel rods defined by a predetermined number of the fuel rods contained within the fuel assembly. The grid straps form an egg-crate configuration creating cell openings for receiving the fuel rods of a predetermined fuel rod group. In an alternate embodiment, the fuel rods of the fuel assembly are partitioned into separate groups by the use of separate partial grid structures which are combined to form a grid structure assembly.

Abstract: A nuclear reactor fuel assembly spacer grid. The grid straps form an egg crate configuration creating standard cells to enclose the fuel rods. The standard cells have at least one of their straps with an integral spring portion extending into the standard cell and pushing against the fuel rod. A distinct backup spring is positioned transverse to and behind each of the spring portions of the straps and is positioned so as not to touch the fuel rods. The straps are made of a material having a low neutron capture cross-section while the backup spring is made of a material having good radiation stress relaxation properties.