Patent Number: 060884207
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to a reactor core which is applied to a water cooling reactor, and in particular, to a reactor core which has improvements in core internal structural materials, fuel design and core arrangement structure. In general, a light water reactor, which is a water cooling reactor, is classified into a boiling water reactor and a pressurized water reactor. A number of fuel assemblies are charged in a reactor core of a boiling water reactor in four groups. In each fuel assembly, a fuel coating cladding) tube is filled with a fissile material as a nuclear fuel, and a heat generated by a nuclear reaction of the fissile material is removed by a coolant. A water is used as a coolant for removing the heat in the light water reactor. Hydrogen contained in water has a high neutron moderation ability, and therefore, the conventional water cooling reactor has a high ratio of water, and a high energy neutron (fast neutron) generated by the nuclear fission is greatly moderated. Thus, a low-energy thermal neutron (slow neutron) occupies most of neutrons. In the case where the fissile material absorbs the low-energy neutron, a fissile reaction of newly generating about three neutrons is not caused, but a ratio of a neutron capture of absorbing the neutrons in an atomic nucleus without causing the nuclear fission, becomes great. Therefore, the number of neutrons generated per neutron absorption is reduced in the nuclear fission reaction by a low-energy neutron. On the other hand, in a high-energy neutron (fast neutron), since a ratio of neutron capture reaction is low and the fissile reaction is great, two or more average neutrons per neutron absorption can be generated inclusive of the neutron capture effect. One of two or more fast neutrons newly generated is used for maintaining chain reaction, and on the other hand, the reminder thereof is absorbed in a parent material (nuclear material) such as .sup.238 U (U-238), thus, a fissionable material being effectively produced. In a case where a ratio of production and annihilation of the fissionable material is 1 or more, it has been found that fuel breeding is performed and a resource energy can be secured. This is the reason why various countries have made a research and development of a breeder reactor which newly produces fissionable materials by a speed more than the development of consuming a nuclear material. However, in a conventional water cooling reactor, the ratio of water, which is coolant, to fuel ranges from about 2.0 to 2.5, and accordingly, a fast neutron generated by a fissile reaction is moderated, and then, becomes a low energy. Thus, the breeding is not performed, and a ratio of production and annihilation of the fissile material was 1 or less, for example, a value of about 0.5. Therefore, in a breeder reactor, an uranium resource, which can be theoretically converted into a thermal energy at 100%, has mot been effectively utilized, and the uranium resource effectively utilized has been merely about 1%. In the case of making use of a high-energy spectral neutron, in the conventional large-scale fast breeder reactor, there is the possibility that a reactivity (void reactivity) becomes positive due to the boiling of a coolant. However, in a water cooling reactor, it is important to make negative the void reactivity in view of stability and safety of a reactor core. SUMMARY OF THE INVENTION An object of the present invention is to substantially eliminate defects or drawbacks encountered in the prior art described above and to provide a reactor core capable of increasing a breeding ratio and improving a utilization factor of an uranium fuel. An another object of the present invention is to provide a reactor core which has a breeding ratio of at least about 1 to improve a utilization factor of an uranium resource and which can hold a void reactivity at a negative value so as to achieve an environmental protection and to further improve stability and reliability of a reactor core. A further object of the present invention is to provide a reactor core which has a breeding ratio of at least about 1 so that a utilization factor of the uranium resource is 100 time as much as a conventional reactor core and which can hold a void reactivity at a negative value so as to improve stability and economy of the reactor core, even if the reactor core has the same diametrical direction size as that of a conventional reactor core. These and other objects can be achieved according to the present invention by providing, in one aspect, a reactor core including a number of fuel assemblies charged therein, wherein each of the fuel assemblies comprises a fuel bundle in which adjacent fuel rods are arranged so as to provide a triangular shape and a ratio of a coolant channel cross section to a fuel cross section is set to be 1 or less. In this aspect, the fuel assemblies have an arrangement pitch of about 300 mm or more. The fuel assembly is composed of a cylindrical channel box and the fuel bundle disposed in the channel box and having substantially square cross section, the channel box being provided with a protrusion (projection) for removing a coolant at an outer side thereof. The fuel assembly is composed of a cylindrical channel box and the fuel bundle disposed in the channel box, the channel box being provided with a support pad at an upper portion thereof in an operating state so as to mutually support adjacent fuel assemblies in a transverse direction. The fuel bundle may be constructed in a manner that a number of fuel rods are arranged and held by means of a grid-shape spacer so as to provide a triangular shape. The grid-shape spacer is provided with grid and a spring means attached to the grid for preventing vibration of the fuel. The vibration preventive spring and the grid are formed integrally with each other, and the grid is formed of a stainless steel or inconell. The fuel bundle may be formed by assembling in a bundle a number of fuel rods each having a fuel cladding tube filled with a fuel material by means of a fuel spacer and at least either one of the channel box and the fuel cladding tube is formed of a stainless steel. Pultonium and recovery uranium are used as the fuel material. The fuel assemblies comprise at least a normal fuel assembly and a partial fuel assembly having an exothermic portion length shorter than that of the normal fuel assembly, and the fuel assembly has an exothermic portion of a maximum length of 2 m or less and the partial fuel assembly has an exothermic portion of a maximum length of 1 m or less. The normal fuel assembly has upper and lower portions to which shaft brackets are provided and the partial fuel assembly has upper and lower portions to at least one of which a shaft bracket is not provided. In another aspect, there is provided a reactor core including a number of fuel assemblies each having a cylindrical channel box and a fuel bundle disposed therein, wherein a control rod is further provided between the fuel assemblies or in the fuel assembly to be freely withdrawal, the control rod being provided with a control rod absorber having a hollow follower at upper or lower portion thereof for preventing a coolant from flowing when the control rod is withdrawn. In this aspect, the fuel assemblies have an arrangement of four fuel assemblies adjacent to each other and the control rod has a cross-shaped cross section and is disposed between the four fuel assemblies so as to be freely withdrawal from a lower side thereof, the control rod being formed with a follower at an upper portion thereof which forms a coolant removal space, and a ratio of the number of control rods to the number of fuel assemblies charged in the reactor core is set to be substantially 1:1. In a further aspect, there is provided a reactor core including a number of fuel assemblies charged therein, wherein the fuel assemblies comprise a normal fuel assembly having a predetermined fuel effective exothermic portion and a partial fuel assembly having an exothermic portion having a length shorter than that of the normal fuel assembly, the partial fuel assembly being provided, at an upper portion thereof, with a hermetic container which is filled with a sealed gas. In this aspect, the hermetic container is formed of aluminum, zirconium or zircaloy having a small neutron absorption cross section. The fuel assembly is composed of a cylindrical channel box and a fuel bundle disposed in the channel box, the fuel bundle including a plurality of fuel rods and being provided with a coolant removal rod at a central portion between adjacent three or four fuel rods. The coolant removal rod has an inner hollow structure, and the coolant removal rod is formed of aluminum, zircaloy or zirconium having a small neutron absorption cross section. The fuel assembly is provided with a support pad disposed at four corner portions of an upper portion of the channel box so as to ride on each of the corner portions from an outer side thereof and upper portions of adjacent fuel assemblies are supported by means of the support pads in a transverse direction. The fuel assembly is composed of a cylindrical channel box and a fuel bundle disposed in the channel box, the fuel bundle including a number of fuel rods formed into a bundle by means of a fuel spacer so as to provide a triangular shape, the channel box being provided with an inner side portion covering the fuel bundle, and a protrusion which corresponds to unevenness of an outer periphery of the fuel bundle is formed to the inner side portion of the channel box. The channel box is provided with an outer side portion to which a protrusion for removing a coolant is formed. In a still further aspect, there is provided a reactor core including a number of fuel assemblies charged therein, wherein the fuel assemblies are composed of cylindrical channel boxes each having an inner space being divided into a normal fuel element region and a partial fuel element region by means of a coolant channel partition wall. The normal fuel element region is formed so that a normal fuel element having a predetermined fuel effective length is arranged and the partial fuel element region is formed so that a short-dimension fuel element having a fuel effective length shorter than that of the normal fuel element is arranged and a distribution of coolant flow rate to the normal fuel element region and the partial fuel element region is carried out by means of an orifice provided on a lower portion of the fuel assembly. According to the present invention of the characters and structures mentioned above, the fuel pins constituting the fuel bundle are arranged so as to provide a triangular shape, so that the arrangement density of fuel pins can be improved. Further, the ratio of the coolant channel cross section to the fuel cross section is set to 1 or less, which is considerably smaller as compared with the conventional ratio, so that the average neutron energy can be made close to the sodium cooling water type fast breeder reactor. As a result, a ratio of neutron capture reaction of the fissionable material is small, and the number of neutrons generated per neutron absorption is increased. Thus, the number of neutrons absorbed in the parent material becomes much, so that the breeding ratio can be set to about 1, and also, a utilization factor of uranium resource can be greatly improved. The arrangement pitch of the fuel assembly is about 300 mm or more so that the fuel assembly has a large diameter. Accordingly, it is possible to lower the ratio of water in the gap between the fuel assemblies to the overall volume and to lower the ratio of water to fuel, so that the breeding ratio can be increased. The channel box is provided with the protrusion (projection) at the outer side thereof, so that the water removal space is ensured in the reactor core and the ratio of water to fuel can be lowered and also provided with the support pad at the upper portion thereof, so that the upper lattice plate can be dispensed and the gap between the fuel assemblies can be made small. Therefore, the fuel volume ratio is increased, and the ratio of water to fuel is lowered, thus increasing the breeding ratio. Since the fuel bundle housed in the channel box is supported by means of the grid spacer so that fuel rods are arranged to provide a triangular shape, the fuel assembly is safely secured and the fuel rods are closely arranged. Thus, the fuel volume ratio is increased and the ratio of water to fuel is increased, also increasing the breeding ratio. Since the grid of the grid spacer is provided with the vibration preventive spring and the fuel rod is elastically and stably held by means of the vibration preventive spring, the fuel assembly can be safely secured. Further, the grid and the vibration preventive spring are formed integrally with each other, and thereby, the number of components can be reduced, and molding process is easily performed. Since the grid is formed of stainless steel or inconell, the grid can be made thin and mechanical and physical strength can be sufficiently maintained even if the grid is made thin. Since at least one of the channel box and the fuel coating tube is formed of stainless steel, the wall thickness is made thin, and the fuel volume ratio is increased, and the ratio of water to fuel is lowered to increase the breeding ratio. Since plutonium and recovery uranium are used as the fuel material, the number of neutron generated from materials other than plutonium can be increased, thus increasing the breeding ratio. Since the normal fuel assembly and the partial fuel assembly are arranged, in the case where the output power of reactor core raises up to increase the void reactivity, the neutron generated in the reactor core leaks out on the upper portion (or lower portion) of the reactor core through the streaming path, so that the void reactivity can be made negative. Therefore, inherent stability and environmental protection can be achieved. Since the normal fuel assembly has a maximum exothermic portion of the length which is 2 m or less, and the partial fuel assembly has a maximum exothermic portion of the length which is 1 m or less, even if the reactor core has the same core diametrical direction size as the conventional light water cooling reactor, the void reactivity can be made negative. Since the normal fuel assembly is provided with the shaft brackets so as to absorb the neutron leaking from the reactor core and, on the other hand, the shaft bracket is not provided on at least one of upper and lower portions of the partial fuel assembly so as to increase the neutron leakage when the void reactivity increases, whereby the breeding ratio can be increased and the void reactivity can be made negative even if the reactor core has the same core diametrical direction size as the conventional reactor core. Therefore, environmental protection, safety and reliability can be improved. Since the control rod is provided with a hollow follower for preventing a coolant from flowing into the reactor core when being taken out, it is possible to lower the ratio of water in the gap between fuel assemblies or the gap in the fuel assemblies to the overall volume and the ratio of water to fuel is lowered, thus increasing the breeding ratio. The control rod having a cross-shaped cross section is provided between four fuel assemblies adjacent to each other so as to be freely withdrawal from the lower portion thereof, and each control rod is formed with a follower which forms a water removal space at the upper portion thereof, and further, a ratio of the number of control rods to the number of fuel assemblies charged in the reactor core is set so as to be substantially 1:1. Accordingly, the fuel assembly has a large diameter, and the fuel volume ratio is increased and the ratio of water to fuel is lowered, so that the breeding ratio can be increased to at least about 1. Since the partial fuel assembly is provided with a hermetic container which is filled with a sealed gas, at the upper portion thereof, the streaming path is formed on the upper portion of the partial fuel assembly so that the leakage of neutron in the axial direction of the reactor core can be facilitated. Therefore, the breeding ratio is increased, and simultaneously, the void reactivity is lowered, so that the void reactivity can be made negative. Since the hermetic container provided on the upper portion of the partial fuel assembly is formed of aluminum, zirconium or zircaloy having a small neutron absorption cross section, the neutron absorption in the hermetic container is decreased and the neutron absorption of U-238 of the nuclear material is relatively increased, so that the breeding ratio can be increased and the void reactivity can be lowered. Since the coolant removal rod is provided on the gap between the fuel rods of the fuel assembly, the coolant can be removed by means of the coolant removal rod, and the volume ratio of water to fuel is decreased, so that the void reactivity can be lowered. Since the coolant removal rod has an inner hollow structure, the neutron absorption is decreased by the coolant removal rod, and the neutron is relatively absorbed in the nuclear material such as U-238, so that the breeding ratio can be increased and the void reactivity can be lowered. Further, the coolant removal rod is formed of aluminum, zircaloy or zirconium having a small neutron absorption cross section, so that the breeding ratio can be further increased and the void reactivity can be lowered. Since the support pads are provided at four corner portions of the upper portion of the cylindrical channel box so as to ride on each corner portion from an outside portion thereof, and the upper portions of the adjacent fuel assemblies are supported by means of the support pads in a transverse direction, this serves to eliminate a reactor core lattice plate. The gap between the fuel assemblies is made narrow, and as a result, the fuel volume ratio is increased and the breeding ratio can be increased. The void reactivity can be lowered. Further, the support pad serves to previously prevent the fuel assemblies from being in contact with each other and to secure a gap for inserting the control rod. The fuel bundle housed in the fuel assembly is constructed in a manner that a number of fuel rods are made into a bundle by means of a fuel spacer so as to provide a triangular shape, and an inner side of the channel box is provided with a protrusion. Accordingly, the fuel rods are closely arranged, and the fuel volume ratio is increased. On the other hand, the protrusion serves to lower the ratio of coolant to fuel, so that the breeding ratio can be increased and the void reactivity can be lowered. Since the outer side of the channel box is provided with a protrusion, in addition to the inner side thereof, the ratio of water to fuel is further lowered, so that the breeding ratio can be increased and the void reactivity can be lowered. The upper portion of the partial fuel element region is voided and the streaming path of neutron is formed, so that the void reactivity can be lowered and made negative. The distribution of coolant flow rate to the normal fuel element region and the partial fuel element region is properly carried out by means of an orifice provided on an lower portion of the fuel assembly. The nature and further characteristic features will be made more clear from the following descriptions made with reference to the accompanying drawings.