Abstract: A hybrid fuel assembly for use in nuclear fuel reactors, such as a large pressurized water reactor, including a fuel assembly having a matrix of fuel rods of different sizes, wherein fuel rods of a first large size are disposed in island arrays which are located within the matrix of fuel rods of the second small size in the vicinity of the control rod guide tubes of the assembly, such that retention of the control rods normally associated with a reactor fuel assembly employing fuel rods only of the first size is possible while still allowing for the maximum linear heat generation rate (LHGR) for the larger first size rods to be reduced to the LHGR normal for the second smaller size fuel rods or lower.

Abstract: The fuel pellets inside the fuel rods of a boiling water reactor are made with a lower enrichment concentration in the axial radial region and a higher concentration near the peripheral radial region to reduce thermal expansion effects upon sudden rises in temperature of the pellet when the reactor power is abruptly increased by a rapid step-wise withdrawal of a control rod. At least rods immediately adjacent the control rods contain pellets of this type, but additional rods may also be of this construction.