Abstract: A method for calculating a PCI margin associated with a loading pattern of a nuclear reactor including a core into which fuel assemblies are loaded according to the loading pattern is implemented by an electronic system. The fuel assemblies include fuel rods each including fuel pellets of nuclear fuel and a cladding surrounding the pellets. This method includes calculating a reference principal PCI margin for a reference loading pattern of the fuel assemblies in the core; calculating a reference secondary PCI margin for the reference pattern; calculating a modified secondary PCI margin for a modified loading pattern of the fuel assemblies in the core, and calculating a modified principal PCI margin for the modified pattern, depending on a comparison of the modified secondary PCI margin with the reference secondary PCI margin.

Abstract: A coolant having a set temperature is fed into the nuclear reactor core of a medical neutron source, which is in a subcritical state. The nuclear reactor core is transitioned from the subcritical state to a critical state until the nominal power of the nuclear reactor is achieved. A neutron output channel is opened in order to conduct a neutron therapy session, and the operation of the reactor is maintained at nominal power while the neutron therapy session is conducted. At the end of the session, the neutron output channel is closed at the same time as the reactor core is transitioned to a subcritical state. The temperature of the coolant entering the core is maintained unchanged and equal to a set temperature, both when the core is transitioned to a critical state and during the operation of the nuclear reactor at nominal power.

Abstract: Nuclear fuel assemblies include non-symmetrical fuel elements with reduced lateral dimensions on their outer lateral sides that facilitate fitting the fuel assembly into the predefined envelope size and guide tube position and pattern of a conventional nuclear reactor. Nuclear fuel assemblies alternatively comprise a mixed grid pattern that positions generally similar fuel elements in a compact arrangement that facilitates fitting of the assembly into the conventional nuclear reactor.

Abstract: A fuel core for a nuclear reactor in one embodiment includes an upper internals unit and a lower internals unit comprising nuclear fuel assemblies. The assembled fuel core includes an upper core plate, a lower core plate, and a plurality of channel boxes extending therebetween. Each channel box comprises a plurality of outer walls and inner walls collectively defining a longitudinally-extending interior channels or cells having a transverse cross sectional area configured for holding no more than a single nuclear fuel assembly in some embodiments. A cylindrical reflector circumferentially surrounds channel boxes and is engaged at opposing ends by the upper and lower core plates. Adjacent cells within each channel box are formed on opposite sides of inner walls such that the cells are separated from each other by the inner walls alone without any water gaps therebetween which benefits neutronics for some small modular reactor designs.

Abstract: A fuel assembly for a pressure-tube nuclear reactor includes a fuel channel assembly. The fuel channel assembly has an outer conduit and an inner conduit received within the outer conduit. The conduits define an annular fuel bundle chamber for receiving a flow of a coolant in one direction. The inner conduit includes a central flow passage for receiving a flow of the coolant in an opposite direction. A fuel bundle positioned within the fuel bundle chamber consists of fuel elements arranged to form an inner ring surrounding the inner conduit, and an outer ring surrounding the inner ring. The coolant may be light water, and geometries of the fuel assembly may be selected so moderation by the volume of coolant promotes generally uniform power distribution in the fuel elements.

Abstract: A thermal-neutron reactor core includes: a solid moderator expanding to a lengthwise direction; a fuel in the moderator, parallel to the lengthwise direction of the moderator, the fuel containing a fissile material; a cooling tube parallel to the lengthwise direction of the moderator; and a plurality of kinds of burnable poison included in the fuel. The may contain a metal hydride. Furthermore, the plurality of kinds of burnable poison may include one burnable poison containing a concentration of one particular isotope of that one burnable poison.

Abstract: Fuel bundles for a nuclear reactor are described and illustrated, and in some cases includes fuel elements each having a first fuel component of recycled uranium, and a second fuel component of at least one of depleted uranium and natural uranium blended with the first fuel component, wherein the blended first and second fuel components have a first fissile content of less than 1.2% wt of 235U. Other fuel bundles are also described and illustrated, and include a first fuel element including recycled uranium, the first fuel element having a first fissile content of no less than 0.72 wt % of 235U; and a second fuel element including at least one of depleted uranium and natural uranium, the second fuel element having a second fissile content of no greater than 0.71 wt % of 235U.

Abstract: A basket assembly for receiving a plurality of fuel assemblies includes a basket having a grid defining spacing between fuel assembly compartments, the grid defining a first compartment for receiving a first fuel assembly and a second compartment for receiving a second fuel assembly, wherein the cross-sectional area of the second compartment is larger than the cross-sectional area of the first compartment. The basket assembly is configured to receive in the first compartment a first fuel assembly, the first fuel assembly being a regular fuel assembly, and the basket assembly configured to receive in the second compartment a second fuel assembly, the second fuel assembly being an irregular fuel assembly, wherein the irregular fuel assembly includes at least one irregular fuel rod.

Abstract: A method of making a fuel channel for a fuel assembly for a nuclear power boiling water reactor. The method includes providing at least one first sheet of a Zr-based material of a first thickness, and providing at least one second sheet of a Zr-based material of a second thickness which is less than said first thickness. It also includes assembling at least said at least one first sheet and said at least one second sheet, such that a fuel channel is formed and such that said at least one first sheet forms a lower part of the fuel channel. The at least one second sheet forms a higher part of the fuel channel and the lower part is joined with said higher part. The lower part constitutes 20-75% of the length of the fuel channel.

Abstract: A method is provided for operating a nuclear reactor. The method includes operating the nuclear reactor for at least one plutonium equilibrium cycle during which the core contains plutonium-equilibrium nuclear fuel assemblies; subsequently, operating the reactor for transition cycles, at least some of the plutonium-equilibrium nuclear fuel assemblies being progressively replaced with transition nuclear fuel assemblies and then with uranium-equilibrium nuclear fuel assemblies; and then operating the nuclear reactor for at least one uranium equilibrium cycle.

Abstract: Example embodiments may include a nuclear fuel rod and/or segment design using fuel element spacers. Fuel element spacers may be placed at intervals within fuel rods and/or segments in order to manipulate operating characteristics of the fuel rods and/or segments and/or decrease consequences of fretting of the fuel rod and/or segment. Example methods may include using fuel rods and/or segments having fuel element spacing elements by adjusting intervals of the spacing elements so as to affect the mechanical, neutronic, and/or thermal properties of the fuel rod and/or segment.

Abstract: A fuel rod for a nuclear reactor includes the fuel rod having a first axial zone positioned proximate to a bottom end, a second axial zone positioned adjacent to the first axial zone in the intermediate region, and a third axial zone positioned proximate to a top end. The first axial zone has an enrichment greater than the second axial zone and the second axial zone has an enrichment greater than or equal to the third axial zone. Also includes fuel assemblies having a plurality of fuel rods and methods of designing and manufacturing of fuel rods and fuel assemblies.

Abstract: In a fuel assembly having fuel rods U1 to U4, and P1 not containing gadolinia, and fuel rods G1-G3 containing gadolinia, the concentrations a of gadolinia in nuclear fuel materials A (8 wt % and 10 wt %) satisfy 0.7<a/amax?1.0, the concentrations b of gadolinia in nuclear fuel materials B (5 wt % and 6 wt %) satisfy 0.4<b/amax?0.7, the concentration c of gadolinia in a nuclear fuel material C (2 wt %) satisfies 0<c/amax?0.4, and L(A)/5.0?L(B) and L(B)/5.0?L(C) are satisfied. The gadolinia with concentrations a burns out during the end of cycle, the gadolinia with concentrations b during the middle of cycle, and the gadolinia with a concentration c during the beginning of cycle. L(A) is the total axial length of zones filled with the nuclear fuel materials A in the fuel rods G1-G3, L(B) for materials B in G2 and G3, and L(C) material C in G1.

Abstract: A fuel assembly with a plurality of fuel rods, extending along a uniform direction, disposed inside a channel box assuming a quadrangular duct shape, wherein: the plurality of fuel rods that are disposed on sides of a plurality of hypothetical concentric quadrangles of various sizes with shapes similar to a cross-sectional shape of the channel box viewed from an end along which the fuel rods extend, and include at least outermost fuel rods disposed on the sides of a largest hypothetical concentric quadrangle among the hypothetical concentric quadrangles and second layer fuel rods disposed on the sides of a second largest hypothetical concentric quadrangle among the hypothetical concentric quadrangles; and the outermost fuel rods are disposed so that consecutive outermost fuel rods are set apart from each other over equal intervals which are greater than intervals setting apart consecutive fuel rods among the second layer fuel rods.

Abstract: Fuel bundles for a nuclear reactor are described and illustrated, and in some cases includes fuel elements each having a first fuel component of recycled uranium; and a second fuel component of at least one of depleted uranium and natural uranium blended with the first fuel component, wherein the blended first and second fuel components have a first fissile content of less than 1.2 wt % of 235U. Other fuel bundles are also described and illustrated, and include a first fuel element including recycled uranium, the first fuel element having a first fissile content of no less than 0.72 wt % of 235U; and a second fuel element including at least one of depleted uranium and natural uranium, the second fuel element having a second fissile content of no greater than 0.71 wt % of 235U.

Abstract: A reactor fuel bundle includes both full-length fuel rods and part-length fuel rods. The part-length rods are clumped in two groups—a first rod group surrounds one or more water passages which are generally centrally disposed in a channel of the fuel bundle, and a second rod group is distributed about an inner perimeter wall of the channel.

Abstract: Fuel bundles for a nuclear reactor are disclosed, and in some embodiments include a first fuel element including thorium dioxide; a second fuel element including uranium having a first fissile content; and a third fuel element including uranium having a second fissile content different from the first fissile content. Nuclear reactors using such fuel bundles are also disclosed, including pressurized heavy water nuclear reactors. The uranium having the different fissile contents can include combinations of natural uranium, depleted uranium, recycled uranium, slightly enriched uranium, and low enriched uranium.

Abstract: A fuel assembly, comprising: a plurality of first fuel rods including uranium and not including a burnable poison; a plurality of second fuel rods including said uranium and said burnable poison; and a water rod; wherein said second fuel rods are placed at corners of an outermost layer of a fuel rod array; other second fuel rods are placed, in said outermost layer, adjacent to said second fuel rods placed at said corners; and other second fuel rods are placed adjacent to said water rod.

Abstract: A fuel assembly has a constitution in which plural fuel rods are arranged in 10 rows by 10 columns in a channel box, and includes plural fuel rods G containing gadolinium and plural partial length fuel rods P. In the fuel assembly, average enrichment of lower portion cross section is approximately 4.6 wt %, and average enrichment of upper portion cross section is approximately 4.7 wt %. The average enrichments at the outermost layer are approximately 5.6 wt % both in the upper portion and the lower portion. Ratios e/x of the average enrichment of the outermost layer e (wt %) to the average enrichment of the fuel assembly cross section x (wt %) are 1.19 in the upper portion and 1.22 in the lower portion, and the ratios satisfy equation (1). [ Equation ? ? 1 ] ? e x ? - 18.3 ? ( x 10 ) 5 + 68.766 ? ( x 10 ) 4 - 101.77 ? ( x 10 ) 3 + 74.428 ? ( x 10 ) 2 - 27.372 ? ( x 10 ) + 5.

Abstract: This fuel assembly for a pressurized water nuclear reactor comprises fuel rods which are arranged at the nodes of a substantially regular network which has a polygonal outer contour, the fuel rods containing uranium which is enriched in isotope 235 and not containing any plutonium before the assembly is used in a reactor. The rods are distributed in at least a first central group which is constituted by fuel rods which have a first level of nuclear reactivity, and an outer peripheral layer of fuel rods which have a level/levels of nuclear reactivity which is/are strictly less than the first level of reactivity.

Abstract: A fuel assembly for a boiling water reactor contains a plurality of fuel rods maintained laterally by spacers and surrounded by a fuel assembly case. The outer walls of a spacer are provided with projecting elements that maintain a minimum space between the outer surface of the spacer and the fuel assembly case, the opening span of a spacer being smaller than the internal width of the fuel assembly case. The fuel assembly is characterized in that the spacer is maintained in an off-center position by a force acting laterally thereon such that an external space located between an outer surface of the spacer associated with the outer surface of a central cell and the fuel assembly case is narrower than an internal gap opposite the external space and oriented towards the center or towards a control element disposed at the center.

Abstract: An apparatus for measuring perpendicularity of an upper tie plate of a nuclear fuel bundle includes a first comb having a first plurality of teeth; a second comb having a second plurality of teeth, the second comb being parallel to and spaced from the first comb; a comb connector that connects the first comb to the second comb, the comb connector having a second end extending beyond the first comb; a rigid finger provided at the second end of the comb connector; a rotatable gage provided on the second comb; and a mobile finger operatively connected to the gage.

Abstract: A pressurized water nuclear reactor fuel assembly designed to burn mixed oxide fuel that employs a fully annular fuel pellet stack in the fuel rods, and radial enrichment zoning within the assembly such that the intra-assembly rod power distribution is relatively smooth regardless of the characteristics of the adjacent assemblies.

Abstract: Example embodiments may include a nuclear fuel rod and/or segment design using fuel element spacers. Fuel element spacers may be placed at intervals within fuel rods and/or segments in order to manipulate operating characteristics of the fuel rods and/or segments and/or decrease consequences of fretting of the fuel rod and/or segment. Example methods may include using fuel rods and/or segments having fuel element spacing elements by adjusting intervals of the spacing elements so as to affect the mechanical, neutronic, and/or thermal properties of the fuel rod and/or segment.

Abstract: A spacer for a fuel assembly for a light water nuclear reactor contains a plurality of intercrossed segments, which form a grid. The segments are formed from first and second metal strips which are assembled and provided with protruding parts or corrugations in such a way that the adjacent protruding parts are embodied in such a way that a flow component perpendicular to a vertical central plane which extends between the metal strips is applied to cold water running out from the flow sub-channel.

Abstract: A reactor core (104) includes fuel assemblies (202) arranged in a loading pattern (1100) in response to corresponding power levels of the fuel assemblies (202). A crud deposition model (412) is used to predict crud deposition on the fuel assemblies (202) and fuel pins (300) within the fuel assemblies (202). The prediction of crud deposition enables generation of the loading pattern (1100) and design of lattice structures (1600) for the fuel assemblies (202) that results in the reduction of total crud deposition in the reactor core (104) and causes a substantially uniform crud deposition on the fuel assemblies (202) of the reactor core (104).

Abstract: A fuel assembly attains high burnup and increases reactor shut-down margin when loaded into a reactor core wherein a water gap width on a control rod side and a water gap width on a side opposite to the control rod side are almost equal to each other. The fuel assembly has a plurality of fuel rods arranged in a square lattice pattern, each fuel rod being filled with nuclear fuel pellets and also has at least one neutron moderator rod shifted toward one corner where a control rod is inserted, away from a cross sectional center of the fuel assembly.

Abstract: Separation devices are disposed in the vent volume above part-length rods and above one or more of the spacers above the upper ends of the part-length rods. The separation devices preferably comprise swirlers located above the lattice openings which would otherwise receive the rods but for the underlying part-length rods. In this manner, flow is directed laterally outwardly onto the surfaces and into the interstices of the full-length fuel rods for improved power performance while simultaneously adverse pressure drops across the spacers are minimized.

Abstract: A fuel assembly in accordance with the present invention comprises a plurality of first fuel rods and a plurality of second fuel rods having a length shorter than a length of the first fuel rod, and these two kinds of fuel rods are arranged in a fuel rod array of 10 rows by 10 columns. Two water rods are arranged in regions capable of arranging 8 fuel rods. The second fuel rods are not arranged in the outermost tier of the fuel rod array. Which satisfies the following conditions, that is, B≧60  (Equation 1) 15≦n≦20(n: integer)  (Equation 2) Awr/Ach≦0.149  (Equation 3) Lp/Lf≧11/24  (Equation 4) Awr/Ach≧(3.00×10−4×n2+6.00×10−4×n−1.2×10−2)×(Lp/Lf−1)+1.75×10−1  (Equation 5) Awr/Ach≦(8.63×10−4×n2−6.09×10−2×n+1.33×10−1)×(Lp/Lf−8.

Abstract: A fuel assembly having a plurality of fuel rods arranged in a square lattice array. The fuel rods include a plurality of short-length fuel rods each having a fuel active length shorter than that of each of remaining ones of the fuel rods. At least one water rod is arranged in a region in which one or more of the fuel rods are arrangeable in the array, and a plurality of fuel spacers are provided at a plurality of positions in the axial direction for holding the plurality of fuel rods and the at least one water rod, with mutual radial intervals therebetween being kept immovable. The plurality of short-length fuel rods include at least one first short-length fuel rod arranged in the outermost peripheral region of the square lattice array. Each of the plurality of fuel spacers include a plurality of cylindrical members.

Abstract: A fuel assembly attains high burnup and increases reactor shut-down margin when loaded into a reactor core wherein a water gap width on a control rod side and a water gap width on a side opposite to the control rod side are almost equal to each other. The fuel assembly has a plurality of fuel rods arranged in a square lattice pattern, each fuel rod being filled with nuclear fuel pellets and also has at least one neutron moderator rod shifted toward one corner where a control rod is inserted, away from a cross sectional center of the fuel assembly.

Abstract: A MOX nuclear fuel assembly employable either for a thermal neutron reactor employing UO2 as the nuclear fuel and light water as the moderator/coolant or for a thermal neutron reactor employing the MOX fuels as the nuclear fuel and light water as the moderator/coolant is provided with only one kind of MOX nuclear fuel rods each of which has relatively large magnitude of the enrichment grade of the fissionable Pu-s or Pu239 and Pu241, the quantity of the MOX nuclear fuel rods being relatively small.

Abstract: A fuel assembly attains high burnup and increases reactor shut-down margin when loaded into a reactor core wherein a water gap width on a control rod side and a water gap width on a side opposite to the control rod side are almost equal to each other. The fuel assembly has a plurality of fuel rods arranged in a square lattice pattern, each fuel rod being filled with nuclear fuel pellets and also has at least one neutron moderator rod shifted toward one corner where a control rod is inserted, away from a cross sectional center of the fuel assembly.

Abstract: For the redistribution of a coolant flow from a first region into a second region, in particular in boiling-water-reactor fuel elements having an eccentrically disposed water passage, a fuel-rod bundle is constructed in a mirror symmetry manner relative to a diagonal. A distance between adjacent fuel rods increases monotonically in particular along a diagonal. In addition, a fuel-rod bundle as an entity may be offset eccentrically along the diagonal. To compensate for asymmetry in the reactor core, a redistribution of coolant is provided which is advantageous in pressurized-water-reactor fuel elements.

Abstract: In a boiling water fuel assembly, some of the fuel rods are shortened. It is necessary to establish a sufficiently high maximum power for transition to boiling. To optimize this power, spacers are at a constant distance at a bottom and are at a shorter distance at a top. The spacers belonging to upper group have turbulence-generating vanes which, however, do not project above the shortened fuel rods.

Abstract: The invention provides fuel assemblies and reactor cores assembled from them which offer improved reactor performance and/or burnup rates and/or ease of manufacture. In particular, the invention provides a mixed oxide fuel assembly for a nuclear reactor in which the fuel assembly comprises a plurality of fuel rods, a largest diameter fuel rod type, a smallest diameter fuel rod type, and one or more intermediate size fuel rod types being provided, between 25% and 100% of the peripheral fuel rods being provided of the intermediate size or sizes fuel rod type, the corner rods of the fuel assembly being of the smallest size fuel rod type.

Abstract: A fuel assembly in accordance with the present invention comprises a plurality of first fuel rods and a plurality of second fuel rods having a length shorter than a length of the first fuel rod, and these two kinds of fuel rods are arranged in a fuel rod array of 10 rows by 10 columns. Two water rods are arranged in regions capable of arranging 8 fuel rods. The second fuel rods are not arranged in the outermost tier of the fuel rod array.

Abstract: A MOX nuclear fuel assembly employable either for a thermal neutron reactor employing UO2 as the nuclear fuel and light water as the moderator/coolant or for a thermal neutron reactor employing the MOX fuels as the nuclear fuel and light water as the moderator/coolant is provided with only one kind of MOX nuclear fuel rods each of which has relatively large magnitude of the enrichment grade of the fissionable Pu-s or Pu239 and Pu241, the quantity of the MOX nuclear fuel rods being relatively small.

Abstract: The invention relates to a fuel assembly for a boiling water reactor which is adapted, during operation of the reactor, to allow water to flow upwards through the fuel assembly while absorbing heat from a plurality of fuel rods, whereby part of the water is transformed into steam. The fuel assembly comprises a steam channel through which the steam flows through the fuel assembly. The steam channel (16a, 16b, 16c, 16d) consists of an empty volume which at least extends through part of the fuel assembly. The fuel assembly is designed such that the water and the steam are brought to rotate around the steam channel whereby the water is thrown away from the steam channel whereas the steam which is separated from the water flows upwards through the steam channel.

Abstract: A flow diverter is provided for attachment to a spacer cell above a part-length fuel rod. The flow diverter includes a tubular base having a plurality of laterally projecting tabs extending in an upstream direction for diverting flow from between the spacer cells into the volume above the spacer void of a fuel rod. A vortex generator is secured to or forms an integral part with the flow diverter for swirling the flow onto and into the interstices of laterally adjacent fuel rods. The flow diverter and vortex generator are formed of tubular stock. The vanes of the vortex generator are formed by providing slits along the tubular stock and twisting alternating edges of the stock inwardly to form the vanes.

Abstract: A method of operating a nuclear reactor reduces the number of reload fuels to be loaded into the nuclear reactor in the second and the following operation cycles. The nuclear reactor has a reactor core in which a plurality of reload fuel assemblies respectively having different infinite multiplication factors are arranged. The method operates the nuclear reactor with control rods inserted in control cells each comprising four reload fuel assemblies having relatively large infinite multiplication factors among the plurality of reload fuel assemblies for a period longer than half of a period of an operation cycle.

Abstract: A fuel assembly comprises a plurality of fuel rods bundled in grid pattern, a part of the fuel rods containing gadolinium as a burnable poison. At least one of the fuel rod having gadolinium contains gadolinium enriched in at least one kind of isotope of odd mass number more than an isotopic abundance of natural gadolinium. In the enriched gadolinium, a ratio of a content of Gd-155 to that of Gd-157 is 0.1 or less. An average concentration (wt %) G0 of enriched gadolinia is, with M denoting the number of month under rated power operation per one cycle of an equilibrium core, P power density of a nuclear reactor (kw/l unit) and W a sum of isotopic composition, is set in the range shown by the following expression. G0<0.25·P·M/W. Thereby, in the fuel assembly, residual reactivity of a burnable poison at a cycle end can be decreased and thermal performance can be improved.

Abstract: The invention concerns a spacer (14) for a nuclear boiling water reactor. The spacer (14) comprises a plurality of cells (16) for holding or allowing elongated elements (12) to pass through the cells. Between the cells (16) there are a plurality of flow channels (18). The spacer comprises at least a plurality of deflecting members (22). The deflecting member comprises a vane (24) which extends in a direction from a cell (16) into the neighbouring flow channel (18). The vane is inclined relative to a vertical plane (26) and is wider in its upper part than in its lower part. The invention also concerns a fuel assembly for a nuclear boiling water reactor, comprising a deflecting member with vane of similar construction.

Abstract: In a boiling water fuel assembly, some of the fuel rods are shortened. It is necessary to establish a sufficiently high maximum power for transition to boiling. To optimize this power, spacers are at a constant distance at a bottom and are at a shorter distance at a top. The spacers belonging to upper group have turbulence-generating vanes which, however, do not project above the shortened fuel rods.

Abstract: A fuel assembly, particularly, a fuel assembly including short-length fuel rods and fuel spacers, is used for a boiling water reactor, which is capable of sufficiently reducing the pressure loss of at least one of the fuel spacers positioned above the upper ends of the short-length fuel rods, irrespective of the arrangement of the short-length fuel rods, and also ensuring the structural strength of the fuel spacer. The fuel assembly includes fuel rods located in a square lattice array, two water rods arranged in a region in which seven of the fuel rods are arrangeable, two fuel spacers for holding the fuel rods and the water rods with mutual intervals kept immovable. Each of the fuel spacers includes cells which are connected to each other and in which the fuel rods are to be inserted, respectively, and a band for surrounding the outermost peripheries of the cells. The short-length fuel rods include four first short-length fuel rods arranged in the outermost peripheral region of the square lattice array.

Abstract: A fuel assembly for a boiling water reactor comprising full-length fuel rods (3a, 3c) with a first length extending over the height of the entire fuel assembly, and part-length fuel rods (3b) with a second length extending only in the lower part (10) of the fuel assembly, the fuel rods which have positions immediately adjacent to the part-length fuel rods being adapted so as to be bent inwards, in their upper part, towards the open region (15) which is formed above the part-length fuel rods. At least half of the fuel rods in the fuel assembly are straight.

Abstract: A fuel assembly for a boiling water reactor comprising a plurality of fuel units, stacked on top of each other, each of which comprising a plurality of fuel rods extending vertically between a top tie plate and a bottom tie plate, and means for keeping the fuel elements together. The fuel elements are surrounded by a fuel channel with a substantially square cross section. At least two of the fuel units differ from each other in regard to fuel distribution or free flow area.

Abstract: In each of a number of fuel assemblies loaded in a core of a boiling water reactor, a fuel holding portion of a lower tie plate holds lower end portions a plurality of fuel rods and at least one water rod. The water rod includes a rising pipe opened to a space in the lower tie plate below the fuel holding portion and introducing upward a coolant introduced to the rising pipe, and a falling pipe communicated with the rising pipe and introducing downward the coolant introduced through the rising pipe. The falling pipe has a coolant outlet opened to a second coolant passage defined between the fuel rods above the fuel holding portion. The rising passage is filled with the coolant during a period of rated power operation of the reactor, and a surface of the coolant is formed in the rising pipe during a period of non-rated power operation in which a flow rate of the coolant supplied to the fuel assemblies is lower than that during the period of rated power operation.

Abstract: Each of short-length fuel rods 3 is arranged at a position other than in 3×3 corner regions 6 to 9 in such a manner as not to be simultaneously adjacent to a water rod 5 and others of the short-sized fuel rods. Gd fuel rods 4 are arranged at positions excluding the outer periphery, and the number of those of the Gd fuel rods 4 adjacent to the short-length fuel rods 3 is one-half or less the total number of the Gd fuel rods. At a transverse cross-section of a region upward from upper ends of the short-sized fuel rods 3, the amount of burnable poison contained in a polygonal region 10 whose vertexes are located at centers of those of the first fuel rods 3 arranged at the outermost layer is smaller than the amount of burnable poison outside the region 10. With this configuration, a critical power can be improved in consideration of both a distribution of the flow of coolant and a distribution of a thermal power in the fuel assembly.

Abstract: A fuel assembly for a boiling water reactor comprising a plurality of fuel units (3a, 3b, 3c, 3d), stacked on top of each other, each one comprising a top tie plate (5), a bottom tie plate (6) and a plurality of fuel rods (4a, 4b, 4c) arranged between the top tie plate and the bottom tie plate. The fuel units are surrounded by a fuel channel (9) with a substantially square cross section. At least some of the fuel units comprise fuel rods with different diameters and different fuel quantities. The fuel rods are adapted such that fuel quantity and lattice space are optimized laterally and axially in the fuel assembly.