Abstract: A fuel assembly comprises fuel rods arrayed in a square lattice pattern of 10 rows and 10 columns, and three large-diameter water rods arranged along a diagonal line of the fuel assembly in such a region as able to accommodate 10 fuel rods. Partial length fuel rods are arranged in an outermost layer of the fuel rod array at fuel rod setting positions other than corners of the outermost layer. Ordinary fuel rods are arranged in a layer inside the outermost layer and adjacent to the outermost layer at positions adjacent to the partial length fuel rods in the outermost layer.The struction of the fuel assembly enables a reduction in the void coefficient and an improvement in the reactivity control capability. Also, the void coefficient can be reduced without lowering reactivity, and fuel economy is improved.

Abstract: A second fuel rod positioned at each corner of a channel box and second fuel rods adjacent to the former are formed to have a smaller outer diameter than that of ordinary first fuel rods, so that a pitch between the second fuel rods is narrower than a pitch between the first fuel rods. Making the outer diameter of the second fuel rods smaller than that of the first fuel rods reduces the power per unit length of the second fuel rods. The narrower pitch between the second fuel rods than the pitch between the first fuel rods provides two effects. First, a unit lattice cell becomes so small as to avoid an increase in the H/U ratio. Secondly, a new moderator region is formed between the second fuel rods and the first fuel rods adjacent thereto, the moderator region acting to intensify thermal neutron flux around those first fuel rods. These two effects enable a further reduction in the power per unit length of the second fuel rods.

Abstract: In a fuel loading method for a reactor core made up by high burn-up fuel, fuel assemblies loaded in a circumferential zone of the core are shuffled between two layers of the circumferential zone until residing in the core for two cycles and, after residing for two cycles, are moved to the fourth layer of the circumferential zone from an outermost layer of the core, thereby reducing the difference in exposure due to a different radial power level in the vicinity of the core boundary. Those fuel assemblies are moved to control cells in a central zone of the core after residing in the circumferential zone for three cycles, and also such fuel assemblies as having resided in the central zone for three cycles are moved to the core outermost layer, thereby reducing the difference in exposure between the fuel assemblies having resided in the central zone and the fuel assemblies having resided in the circumferential zone due to different power levels in the central and circumferential zones.

Abstract: A nuclear fuel assembly has an array of fuel rods comprising a plurality of first fuel rods each containing nuclear fuel material but not containing burnable poison and a plurality of further fuel rods each containing both nuclear fuel material and burnable poison. The further fuel rods comprise second fuel rods and third fuel rods, and each second fuel rod has at a lower region of the fuel assembly a burnable poison concentration which is a minimum burnable poison concentration in the further fuel rods. To increase the effectiveness of the second fuel rods in controlling axial power peaking, as seen in plan view on the array, the first fuel rods are the nearest neighbors of each further fuel rod.

Abstract: A fuel assembly has a plurality of first fuel rods and a plurality of second fuel rods having a shorter length in an axial direction than the first fuel rods. The second fuel rod is loaded with natural uranium in full length of its effective fuel length portion. The fuel assembly has a water rod having a larger horizontal cross sectional area at the upper region than the area at the lower region. The second fuel rods are arranged downward of the upper region of the water rod and adjacent to the lower region of the water rod. The width of the horizontal cross sectional area of the lower region of the water rod is set so as to locate the minimum values of both thermal neutron flux and resonance neutron flux in the horizontal direction of the fuel assembly at an outer side with respect the location of the second fuel rod in the horizontal direction.

Abstract: A nuclear fuel assembly for a nuclear reactor has a plurality of vertically extending fuel rods arranged side by side in a square array and containing fissile material. The array has two adjacent first sides which are next to a control rod region of the core and two adjacent second sides which are next to a non-control rod region of the core.

Abstract: A core of boiling water reactor is divided into a central region and a peripheral region surrounding it in the radial direction thereof. The loading fraction of new first fuel assemblies containing burnable poison and loaded in the central region is greater than the loading fraction of the new first fuel assemblies loaded in the peripheral region. The loading fraction of second fuel assemblies loaded in the central region of the core and operating in a second operation cycle is smaller than the loading fraction of the second fuel assemblies loaded in the peripheral region of the core and operating in the second operation cycle. The second fuel assemblies contain no burnable poison. In such a core, the reactivity of the peripheral region is greater than that of the central region in the beginning of an operation cycle. Contrary, the reactivity of the central region is greater than that of the peripheral region in the end of an operation cycle.

Abstract: A fuel assembly has a plurality of first fuel rods each of which contains nuclear fuel material but does not contain burnable poison, and a plurality of second fuel rods each of which includes nuclear fuel material and burnable poison. The amount of burnable poison in a lower region of the fuel assembly is smaller than that in an upper region thereof. When each of the second fuel rods is divided into an upper region and a lower region, a region of the divided regions in the second fuel rods containing a maximum burnable poison concentration Gmax and a region of the divided regions in the second fuel rods containing a minimum burnable poison concentration Gmin are located in the lower region of the fuel assembly. The burnable poison concentration of the upper region of each second fuel rod is between Gmax and Gmin. The fuel assembly may moderate a maximum linear heat rating with an increased spectral shift effect.