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.

Abstract: A fuel assembly including a plurality of fuel rods and water rods. The fuel rods are arranged with a predetermined pitch and two water rods are disposed adjacent to each other in a diagonal direction of the fuel assembly. More particularly, the fuel rods are arranged in a lattice of nine rows by nine columns and the two water rods are disposed inside a region corresponding to a space sufficient for disposing seven fuel rods at the center of the fuel assembly. Each of the water rods has an outer diameter greater than the pitch of the fuel rods.

Abstract: The reactor core is divided into a central region and a surrounding outer region in the radial direction. First fresh fuel unirradiated assemblies newly loaded onto the outer region contain a fissible material in amounts less than that of the fissible material of second fresh unirradiated fuel assemblies newly loaded onto the central region. If the amount of the fissible material in the upper region of the first fuel assembly is denoted by a, the amount of the fissible material in the lower region of the first fuel assembly by b, the amount of the fissible material in the upper region of the second fuel assembly by c, and the amount of the fissible material in the lower region of the second fuel assembly by d, then a relationship a/b<c/d is satisfied. The reactor core provides sufficient thermal margin and improved cold shutdown margin.

Abstract: A fuel assembly of the present invention comprises a plurality of fuel rods which are held by an upper tie plate and lower tie plate at the ends thereof and a moderating rod which is arranged between the fuel rods and held by the lower tie plate at its lower end. The fuel rods are arranged in a lattice form having 9 rows and 9 columns, and the moderating rod contains a passage for a coolant and has a cruciate cross-sectional form. The ratio A.sub.M /A.sub.C of the area A.sub.M of a moderator region in the moderating rod in the cross-sectional plane in which the moderator is present to the area A.sub.C of the coolant passages in said fuel assembly is within the range of 0.07 to 0.11, and the area A.sub.M is 75% or more of the total area of the fuel lattice units in which none of the fuel rods is arranged, but the moderating rod is arranged.

Abstract: A fuel assembly of the present invention comprises fuel rods which are arranged in 9 rows and 9 columns (9.times.9) in a channel box. The channel box has a width L between outer walls thereof and a width D between inner walls thereof, both of which satisfy the following equation:0.12.ltoreq.(P-L)/Dwherein P denotes the fuel assembly pitch in a reactor core. A sufficient cold shutdown margin for a reactor core can be secured by determining the widths L and D so as to satisfy the above-described equation, even if the average enrichment of the fuel assembly is increased to 4 wt % or more.

Abstract: In a light water moderation type nuclear reactor with the once-through method, the reactor core is divided into a central area and a peripheral area by a partition member, a first fuel assembly is arranged in the central area (high conversion area) and a second fuel assembly is arranged in the peripheral area. The ratio (r.sub.H/U) of the number of hydrogen atoms to that of uranium atoms in the central area is smaller than that of the ratio in the peripheral area and the second fuel assembly in the peripheral area is formed of fuel rods of the first fuel assembly having been previously burned in the central area and moved into the peripheral area. The plutonium production increases and uranium consumption is reduced during the first half of the lifetime of the fuel rods in the high conversion area with the take-up burn up increasing during the second half of the lifetime of the fuel rods in the burner area.

Abstract: A fuel assembly comprises a channel box, upper and lower tie plates fixed to the upper and lower portion of the channel box and a bundle of fuel rods enclosed in the channel box and retained by the upper and lower tie plates. The bundle of fuel rods includes ordinary fuel rods containing fissile material not containing gadolinia, and gadolinia-containing rods each containing gadolinia therein and having a larger outer diameter than that of the ordinary fuel rod. The gadolinia-containing rod has pellets enclosed in a closed cladding, and the outer diameters of the pellets also are larger than these of fuel pellets of the ordinary fuel rods.

Abstract: A fuel assembly comprises a plurality of fuel rods, moderator rods disposed among the fuel rods, upper and lower tie plates for holding both end portions of the fuel rods and the moderator rods, and fuel spacers for keeping distances between the fuel rods and the moderator rods constant. The fuel assembly is characterized in that there is provided a space region extending from the upper tie plate to the lower tie plate and having a space enough to dispose at least one of the fuel rods. At least one of the moderator rods being disposed adjacent to the space region. The fuel rods, the moderator rods, and the space region are disposed in a grid form.

Abstract: A fuel assembly is composed of first fuel rods containing nuclear fuel material and burnable poison material and second fuel rods containing nuclear fuel material but no burnable poison material. The fuel assembly is charged in a high conversion area of a boiling water type nuclear reactor having a reactor core having the high conversion area and a burner area surrounding the high conversion area. A ratio V.sub.C /V.sub.F is not greater than 1.5 in the fuel assembly where V.sub.C is the volume occupied by coolant flow paths and V.sub.F is the volume occupied by the nuclear fuel material per unit length in an axial direction of the fuel assembly. In addition, the burnable poison material is composed of neutron absorption nuclides having at least one resonant energy in a neutron energy region of one electron volt or less.

Abstract: A fuel assembly having a plurality of fuel rods arranged in a lattice state and being charged into a core of a nuclear reactor using light water as a coolant, which comprises the fuel rods consisting of a first group of fuel rods and a second group of fuel rods, an average concentration of U.sup.236 in the first group being lower than an average concentration of U.sup.236 in the second group, and the first group and the second group being arranged unevenly in the fuel assembly, preferably, a fuel assembly wherein the first group is arranged at the periphery of the fuel assembly, and the second group being arranged at positions other than the periphery, has a low reactivity penalty due to U.sup.236 and can reduce the requirements for natural uranium.

Abstract: In a nuclear reactor, gaps are defined between fuel assemblies so that moderators can be inserted. Hence, the nuclear characteristics vary between the central portion and periphery of the fuel assembly so that the local output peaking becomes greater on the periphery of the fuel assembly. The present invention flattens the local output peaking by providing that the quantity of a nuclear fuel material per fuel rod loaded in the fuel rods on the periphery of the fuel assembly is smaller than the quantity of the fuel material per fuel rod loaded in the fuel rods at the central portion of the fuel assembly. Any one of the following three constructions can be used:(1) The theoretical density of the nuclear fuel material pellets to be inserted into the fuel rod is reduced.(2) The nuclear fuel material pellets to be inserted into the fuel rod is made hollow.(3) The diameter of the fuel rod is reduced.

Abstract: The invention provides a fuel assembly comprising a plurality of fuel rods containing a fissionable material and a plurality of moderator rods filled with a burnable poison and a metal hydride as a solid moderator material. All of the burnable poison in the fuel assembly is filled in the moderator rods, and number of the moderator rods is at least 4 and not more than 15% of total number of the fuel rods and the moderator rods.

Abstract: In the operation of a boiling water reactor, when the core flow rate is decreased due to a trip of the circulating pump, the control rod pattern is suitably changed to avoid unnecessary scram of the control rods. The change of the control rod pattern is made by driving control rods which have been inserted to small depth deeper into a depth near 12/24 of the core height. In addition to this control rod operation, the control rods which have been inserted to large depth are withdrawn to a depth near 12/24 or, alternatively, the control rods which have not been inserted at all are inserted to the depth of about 12/24 of the core height.

Abstract: Provided is a fuel assembly for a boiling water reactor. The fuel assembly comprises fuel rod arranged in an 8-lines/8-columns square lattice, water rod and a channel box surrounding these fuel rods and water rods. A central portion corresponds to 80% of the fuel rod comprises fuel pellet having an outer diameter of 10.31 mm and inner diameter of 5.95 mm.