Abstract: The reactor core includes at least one module, a solid neutron moderator, and a liquid neutron moderators. Each module contains a casing, at least one heat pipe, one fuel element and thermal insulation. The heat pipe comprises a casing, a wick, and a coolant. The fuel element is made of nuclear fuel, arranged along an evaporation area of the heat pipe, around the heat pipe casing, in thermal contact with the heat pipe casing, and enclosed in a can. Liquid metals are used as the coolant of the heat pipe. Thermal insulation is arranged between the can and the module casing. At least one hole is made in the solid neutron moderator. Each module is arranged within a respective hole of the solid neutron moderator. The space between the module casing and the solid neutron moderator is filled with a liquid neutron moderator.

Abstract: A nuclear reactor core includes at least one module, a solid neutron moderator, and liquid neutron moderator. Each module comprises a housing, at least one heat pipe, at least one fuel element, casing, and thermal insulation. The heat pipe comprises a housing, wick, and evaporating coolant. The fuel element includes a shell and nuclear fuel. An evaporation zone of the heat pipe and the fuel elements are enclosed by the casing. The casing is filled with a liquid coolant. Liquid metal, for example, lithium, calcium, lead, and/or silver, is used as the heat pipe coolant and the liquid coolant. The thermal insulation is arranged in a space between the casing and module housing. The solid neutron moderator has at least one hole, wherein at least one module is located. A space between the solid neutron moderator and module is filled with the liquid neutron moderator.

Abstract: Disclosed embodiments include nuclear fission reactors, nuclear fission fuel pins, methods of operating a nuclear fission reactor, methods of fueling a nuclear fission reactor, and methods of fabricating a nuclear fission fuel pin.

Abstract: Disclosed embodiments include nuclear fission reactors, nuclear fission fuel pins, methods of operating a nuclear fission reactor, methods of fueling a nuclear fission reactor, and methods of fabricating a nuclear fission fuel pin.

Abstract: Porous UO2 sintered pellets to be fed into an electrolytic reduction process for the purpose of metallic nuclear fuel recovery, include one or more hollow spaces formed from the surfaces toward the interiors thereof. When the porous UO2 sintered pellets are used in the electrolytic reduction process, the efficiency increases.

Abstract: A nuclear fuel rod for a fast reactor is provided, in which a reactor core of the fast reactor can be designed compact-sized by reducing the length of the nuclear fuel rod to be smaller than the length of a conventional one. The nuclear fuel rod for a fast reactor includes a tubular fuel materials comprising a hollow portion formed therein, a tubular inner pipe inserted into the hollow portion of the tubular fuel materials to prevent collapse of the tubular fuel materials due to combustion of nuclear fuel, a tubular cladding pipe which surrounds the tubular fuel materials, and a liquid metal, or He gas or vacuum applied in a gap between the tubular fuel materials and the tubular cladding pipe, and the tubular inner pipe includes a collecting space formed therein to collect fission products such as fission gas which are generated due to combustion of the nuclear fuel.

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: 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: The present invention relates to a multi-core fuel rod for research reactor and, more particularly, to a multi-core fuel rod for research reactor in which monolithic fuel cores made of uranium-molybdenum alloy are disposed in an aluminum matrix in a multi-core form. The multi-core fuel rod in accordance with the present invention provides a minimized contact surface area between nuclear fuel and aluminum, and reduces the formation of pores and swelling by restraining formation of reaction layer to avoid excessive reaction between the fuel and aluminum. Therefore, improved stability of nuclear fuel can be obtained by minimizing temperature rise as well as achieving high density and thermal conductivity of the fuel.

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: An auction service is provided that stimulates competition between energy suppliers (i.e., electric power or natural gas). A bidding moderator (Moderator) receives bids from the competing suppliers of the rate each is willing to charge to particular end users for estimated quantities of electric power or gas supply (separate auctions). Each supplier receives competing bids from the Moderator and has the opportunity to adjust its own bids down or up, depending on whether it wants to encourage or discourage additional energy delivery commitments in a particular geographic area or to a particular customer group. Each supplier's bids can also be changed to reflect each supplier's capacity utilization. Appropriate billing arrangements are also disclosed. The technology required to facilitate forward delivery transactions, in which a buyer and seller agree to the terms of a transaction today but schedule the delivery for a future time, would be helpful to end users, resellers and suppliers.

Abstract: There are provided a light water reactor core which has the same levels in cost efficiency and degree of safety as those of an existing BWR under operation now, that is, which is oriented to plutonium multi-recycle having a breeding ratio near 1.0 or slightly larger and having a negative void coefficient with minimizing modification of the reactor core structure of the existing BWR under operation now, and to fuel assemblies used for the boiling water reactor. The light water reactor core having an effective water-to-fuel volume ratio of 0.1 to 0.

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 is concerned with the minimization of radiological doses during manufacturing processes of fuel assemblies, by employing non-irradiated fuel as a shielding material against the radiological output of re-processed fuel materials. The non-irradiated fuel can be employed in peripheral fuel rods and/or in the end portions of fuel rods with beneficial effects.

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.

Abstract: Nuclear fuel assembly configurations are provided for mixed oxide fuels. Neutron poisons are provided within the MOX of certain fuel rods in the fuel assembly, those fuel rods being preferentially grouped towards the periphery of the fuel assembly. In this way, optimized reduction of reactivity is provided during the initial part of the fuel cycle but the neutron poison is burnt out so as not to interfere with the reactivity during subsequent parts of the cycle.

Abstract: A new design concept of boiling water reactor fuel rod is disclosed. The new design is characterized by an upward shift in the location of the fuel pellet stack inside the fuel cladding. The resulting axial power shift upward decreases two-phase and total pressure drop and has a stabilizing effect. A device for affecting such fuel pellet displacement is a crushable tube placed under the fuel pellet stack, which also helps to mitigate fuel-clad mechanical interaction and reduces the likelihood of clad failure.

Abstract: The present invention relates to a fuel assembly with a substantially square cross section for a light-water reactor. The light-water reactor comprises a plurality of fuel rods (4) extending between a top tie plate (5) and a bottom tie plate (6). A fuel rod (4) comprises a cladding tube (7a) with a first and a second end which surround a column with fissionable material (7b). According to one aspect of the invention, at least one fuel rod (4) is provided with an axial gap (19) in the fissionable material (7b), such that fissionable material (7b) is arranged on both sides of the axial gap (19) in the fuel rod (4).

Abstract: Several unit loading patterns are arranged in the central area of an initial core to which the present invention is applied. The unit loading pattern is composed of one square-shaped unit cell and four cross-shaped control rods 3 which surround the unit cell. The unit cell is composed of one low enrichment fuel assembly 7, two high enrichment fuel assemblies 8 and one high enrichment fuel assembly 9. The low enrichment fuel assemblies 7 of each unit loading pattern adjoin each other and are arranged to constitute the first control cell 2a being square-shaped. The high enrichment fuel assemblies 9, obliquely adjoining the low enrichment fuel assembly 7 in each unit loading pattern, adjoin each other and are arranged to constitute the second control cell 2b being square-shaped.

Abstract: Each of short-length fuel rods 3 is arranged at a position other than in 3.times.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 nuclear fuel pellet having concentric layers of nuclear fuel in which an outer layer has less fissionable nuclear fuel per unit volume and substantially the same amount of fertile material per unit volume as an inner core.

Abstract: To provide a reactor core for light water cooled reactor, a fuel assembly and a control rod intended for Pu multi-recycling at a breeding ratio of about 1.0, or 1.0 or more while keeping the economical or safety performance to the same level as in BWR now under operation, that is, while minimizing the change for the incore structures and maintaining the void coefficient negative. A reactor core for a light water cooled reactor having an effective fuel-to-water volume ratio of 0.1 to 0.6 by the combination of a dense lattice fuel assembly constituted of fuel rods formed by adding Pu to degraded uranium, natural uranium, depleted uranium or low concentrated uranium, coolants at high void fraction of 45% to 70% and a cluster-type, Y-type or cruciform control rod.

Abstract: A reactor core for a boiling water reactor, a fuel assembly and a control rod intended for Pu multi-recycling at a breeding ratio of about 1.0, or 1.0 or more while keeping the economical or safety performance to the same level as in a boiling water reactor now under operation. The reactor has an effective water-to-fuel volume ratio of 0.1 to 0.6 by the combination of a dense lattice fuel assembly constituted of fuel rods formed by adding Pu to degraded uranium, natural uranium, depleted uranium or low concentrated uranium, and having coolants at a high void fraction of 45% to 70% and a cluster-type, Y-type or cruciform control rod.

Abstract: In an initial core of the present invention, a low enrichment fuel assembly having the lowest average enrichment factor and three fuel assemblies having a higher average enrichment factor than that of the low enrichment fuel assembly are arranged in a square shape, control rods of a cross shape are arranged at each of four corners of the square shape to constitute a unit loading pattern, and a plurality of the unit loading patterns are provided in the central region of the core, the fuel assemblies having the higher average enrichment factor are divided by a diagonal line into a first region of the side of the control rods and a second region of the side opposite to the control rods, and the number of the gadolinia-containing fuel rods is greater in the second region by at least two than in the first region.

Abstract: A fuel assembly has part length and full length fuel rods, and a pair of large-diameter water rods which occupy an area which can accommodate 7 fuel rods. Natural uranium regions are provided in the upper and lower end portions of the effective fuel zone of the fuel assembly. An intermediate region between these upper and lower natural uranium regions provides an enriched uranium region which has three axial sections: an upper section, a middle section and a lower section. The middle section has the highest average enrichment, the lower section has the medium average enrichment and the upper section has the smallest average enrichment. The difference in the average enrichment between the middle section and the lower section is smaller than that between the middle section and the upper section. The upper section has a lower concentration of burnable poison than other sections of the enriched uranium region.

Abstract: Gadolinium is provided which is adapted for nuclear fuel as a burnable poison, having a plurality of isotopes in an isotopic composition such that the content of at least one even mass numbered isotope is smaller than the content of the same isotope in natural gadolinium. A fuel assembly is also provided having a plurality of nuclear fuel rods arrayed as a lattice in which at least one of the fuel rods contains the gadolinium burnable poison of the present invention. Also, a fuel assembly is described which has a plurality of nuclear fuel rods arrayed as a lattice which includes at least a first group and a second group of nuclear fuel rods containing gadolinium. The content of Gd-157 in the gadolinium is larger than that found in natural gadolinium. Further, the gadolinium concentrations in the first and second groups are different from each other.

Abstract: A nuclear fuel bundle includes a square array of fuel rods each having a concentration of enriched uranium and plutonium. Each rod of an interior array of the rods also has a concentration of gadolinium. The interior array of rods is surrounded by an exterior array of rods void of gadolinium. By this design, usage of plutonium in the nuclear reactor is enhanced.

Abstract: A nuclear reactor core is provided with a fuel assembly which comprises channel boxes respectively separated with spaces into which control rods are arranged and a plurality of fuel rods arranged in the channel box, at least a part of the fuel rods being charged with uranium-plutonium mixture fuel and an enrichment of U-235 or a concentration of plutonium, or both being distributed in an axial direction of the fuel rods. The concentration of a burnable poison packed in the fuel rod is distributed in the axial direction of the fuel assembly. The ratio of the occupying region of a moderator of the fuel assembly is increased with respect to the fuel occupying region. The number of the fuel rods containing the burnable poison is variously changed with respect to the total number of the fuel rods and the arranging mode thereof is also varied with the concentration thereof.

Abstract: A fuel assembly for a boiling water reactor comprising an upper and lower tie plates, a plurality of fuel rods filled therein with a plurality of fuel pellets, a fuel bundle comprising the fuel rods retained with a plurality of spacers in spaced relation and disposed between the upper and lower tie plates, and a polygonal tube-like channel box which encloses the fuel bundle and constitutes flow paths for coolant. The fuel rods comprise long fuel rods and short fuel rods which are each shorter in the effective portion than each of the long fuel rods. The concentration of fissile material in each of the short fuel rods is lower than the mean concentration of fissile material in the cross section of the fuel bundle.

Abstract: A reactor core for a boiling water nuclear reactor comprises a plurality of vertical fuel assemblies (40), each one containing a plurality of fuel rods (10, 10a, 10b) with enriched nuclear fuel material, which are arranged between a bottom tie plate (11) and a top tie plate (12) in a surrounding vertical fuel channel (1). Each fuel assembly is designed with an inlet (3) for water for conducting water in through the bottom tie plate, through the vertical fuel channel, and out through the top tie plate. Further, each fuel assembly is arranged with intermediate gaps (37a, 37b) with respect to adjacent fuel assemblies and possibly with a channel (32, 50) arranged internally in the fuel assembly for conducting water through the gaps and through the internal channel (if any) in a vertical direction from below and upwards through the core.

Abstract: A core of a light-water reactor comprises a plurality of fuel assemblies each including a number of fuel rods. The fuel rod is provided with at least one area interposed between fuel areas in a clad of the fuel rod. The interposed area contains extremely reduced or substantially no fissile nuclide. At least two areas or layers with high enrichment of the fissile nuclide are formed in the axial direction of the reactor by the location of the interposed areas throughout the whole fuel assemblies arranged in the light-water reactor core.

Abstract: A fuel assembly for a boiling water nuclear reactor contains a plurality of vertical fuel rods, which are arranged between a bottom-tie plate and a top-tie plate (12) in a surrounding vertical casing part. The fuel rods extend through a number of spacers which are arranged in a spaced relationship in the vertical direction and which together with the bottom-tie plate and the top-tie plate retain the fuel rods in a spaced relationship in the lateral direction. The fuel assembly is designed for conducting water in through the bottom-tie plate, through the space between the fuel rods in the vertical casing part, and out through the top-tie plate.

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 fuel assembly of the type having a plurality of a fuel material characterized in that a water rod disposed in a channel has a large diameter so that it has a space into which a plurality of fuel rods could be loaded and liquid water boil flows through the water rod and which has a section filled with a fuel material comprising a natural uranium section defined at least one of the upper and lower ends of each fuel rod and an enriched uranium section comprising the uppermost, intermediate and lowermost sections each having a different degree of average enrichment across a cross section of the fuel assembly and the upper and lowermost sections have the degree of enrichment across a section of the fuel assembly is lower than that of the intermediate section, and the enriched uranium section has two sections each of which has a different content of a burnable poison per unit length in the axial direction and the content of the burnable poison per unit of length in the axial direction in the uppermost section is

Abstract: The fuel assembly has fuel arrangement of a square lattice of 10 lines by 10 rows. Four water rods having a large diameter are arranged in central region of horizontal cross section wherein arrangement of 12 fuel rods is possible. The four water rods having a large diameter are so arranged adjacently in a circle as to form an internal between each other. First coolant flow path which is extended toward axial direction is formed by surrounded with the water rods having a large diameter. The first coolant flow path leads to third coolant flow path which is formed around the fuel rods through second coolant flow path which is the interval between the water rods having a large diameter.The fuel assembly is able to optimize the moderator to fuel atom number density ratio and to reduce the pressure loss because a portion to be an excessively moderated region is utilized as the first coolant flow path.

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 high conversion nuclear reactor core has fuel assemblies made up from large numbers of axially-extending uranium-plutonium mixed oxide fuel rods. The fuel rods are densely packed so as to give a high conversion ratio of fissile substances, preferably approaching unity. The average plutonium enrichment in the assemblies is higher in their bottom, upstream halves, than in their top downstream halves. This has the effect of reducing a potentially dangerously high void coefficient in the core.

Abstract: The invention relates to a nuclear reactor with improved efficiency.The nuclear reactor comprises a massive partition (18) of material reflecting high energy neutrons at the circumference of the core (7) of the reactor, two layers (19 and 20) of material absorbing low energy neutrons and containing fertile material arranged one at the lower part and one at the upper part of the core (7) and an assembly of neutron energy spectrum variation rods (27). The rods (27) are associated with mechanisms permitting them to be either fully inserted into the core (7), or fully extracted. These rods (27) consist of a material absorbing low energy neutrons and permit a shift of the neutron spectrum towards the high energies.The invention is particularly applicable to pressurized water nuclear reactors.

Abstract: To increase the operating temperature of a reactor, the melting point and mechanical properties of the fuel must be increased. For an actinide-rich fuel, yttrium, lanthanum and/or rare earth elements can be added, as stabilizers, to uranium and plutonium and/or a mixture of other actinides to raise the melting point of the fuel and improve its mechanical properties. Since only about 1% of the actinide fuel may be yttrium, lanthanum, or a rare earth element, the neutron penalty is low, the reactor core size can be reduced, the fuel can be burned efficiently, reprocessing requirements are reduced, and the nuclear waste disposal volumes reduced. A further advantage occurs when yttrium, lanthanum, and/or other rare earth elements are exposed to radiation in a reactor, they produce only short half life radioisotopes, which reduce nuclear waste disposal problems through much shorter assured-isolation requirements.

Abstract: A nuclear fuel element for a high temperature gas nuclear reactor that has an average operating temperature in excess of 2000.degree. C., and a method of making such a fuel element. The fuel element is characterized by having fissionable fuel material localized and stabilized within pores of a carbon or graphite member by melting the fissionable material to cause it to chemically react with the carbon walls of the pores. The fissionable fuel material is further stabilized and localized within the pores of the graphite member by providing one or more coatings of pyrolytic carbon or diamond surrounding the porous graphite member so that each layer defines a successive barrier against migration of the fissionable fuel from the pores, and so that the outermost layer of pyrolytic carbon or diamond forms a barrier between the fissionable material and the moderating gases used in an associated high temperature gas reactor.

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: The present invention relates to nuclear reactors and more specifically to a fuel bundle arrangement for a boiling water nuclear reactor in which so-called mixed oxide fuels including plutonium and uranium are utilized in a nuclear fuel bundle together with a burnable absorber such as gadolinium to optimize the reaction of a nuclear fuel bundle.

Abstract: A gas cooled nuclear fuel element. A cylindrical tube having an open lower end contains nuclear fuel disks stacked one upon the other to define an annular space between the tube and disks. An upper and a lower end fitting in the tube support the stack of fuel disks. Each disk has an axial bore or channel for coolant flow. The disks may also be provided with grooves extending radially on the upper or lower face from the axial channel to the outer edge of each disk. The lower end fitting has a central bore that is coaxial with the axial channel in the disks. Coolant flows into the annular space, radially between disks, and then axially out the disks and lower end fitting.

Abstract: A fuel assembly for a nuclear reactor includes a number of fuel rods filled with a fuel material. A plurality of fuel rods have a partial effective fuel area filled with a fuel material and has a portion in which enrichment of a fissile nuclide is significantly reduced or the fissile nuclide does not exist at all on an axial level including a reactor shut-down zone at which subcriticality becomes small during a reactor operation period. The other fuel rods are filled with the fuel material throughout the entire axial length thereof. The first mentioned fuel rod may be provided with a partially interposed zone or may be constructed by a fuel rod having a length shorter than that of the other fuel rod. The tube means may be arranged in the fuel assembly so as to pass the moderator therethrough.

Abstract: The present invention provides a structural component for constituting a reactor core comprising one and the other metallic members arranged adjacent to each other in a reactor, and reduction means for reducing a difference in the swelling between the metallic members due to neutron irradiation thereto; a core of the reactor comprising a plurality of such constructural element; and a method for operating a reactor having metallic tubular members arranged in a core of the reactor and each enclosing a plurality of metallic cladding tubes in each of which substance heated by neutron irradiation thereto and cooled by passing coolant through the reactor core is filled, wherein the reactor is operated under a condition that an average used temperature of the cladding tubes is higher than an irradiation temperature that the swelling rate of the cladding material due to neutron irradiation thereto becomes maximum and an average used temperature of the tubular members is lower than an irradiation temperature that the

Abstract: In a boiling water reactor having discrete bundles of fuel rods confined within channel enclosed fuel assemblies, an improved fuel design of bundles of fuel rods interior of the channels is disclosed. Specifically, partial length rods are utilized which extend from the bottom of the channel only part way to the top of the channel. These partial length rods are shortened with respect to the remaining rods and are symmetrically distributed throughout the fuel bundle with the preferred disposition being in the second row of the bundle of fuel rods from the channel wall. The symmetrical distribution of the partial length rods is at spaced apart locations one from another. The partial length rods extend from the bottom of the fuel bundle and terminate within the boiling region. during shutdown of the reactor, an improved cold shutdown margin is produced at the top of the fuel assembly due to the improved moderator-to-fuel ratio and reduction in plutonium formation at the upper portion of the bundle.

Abstract: A fuel assembly for a nuclear reactor comprising a number of fuel rods arranged in a regular fashion in a channel box which is surrounded by water gap in a reactor core. Some of fuel rods disposed at portions facing the water gap through which no control blade is inserted or drawn out, have atomic number densities of fissionable material contained in the fuel rods. The atomic number density of each of these fuel rods is made smaller than that of the fuel rod disposed in the fuel assembly at portions other than the portions referred to above.

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 nuclear fuel rod has an elongated hollow cladding tube with a pair of end plugs and fuel pellets contained in the tube and extending between the end plugs. All pellets are composed of fissile material having a single enrichment. The pellets are provided in an axial stack having a length slightly less than the distance between the end plugs such that when the tube is disposed in a vertical orientation the stack of pellets at a lower end rests on the lower end plug and at an upper end is spaced from upper end plug by a gap. The gap is devoid of any structure restraining the stack from movement toward the upper end plug. The pellets in the stack are provided in an arrangement of axial regions which includes a pair of first opposite end axial regions and second and third tandemly-disposed middle axial regions between the first axial regions. The pellets in the first opposite end axial regions are identical in number and have annular configurations with an annulus of a first void size.