Patent Number: 058728260
Section: summary

FIELD OF THE INVENTION The present invention relates to a fuel assembly for a nuclear power plant and particularly, to a fuel assembly for improving the economical efficiency of nuclear fuel by controlling output peaking and reducing nuclear thermal restrictions. DESCRIPTION OF THE PRIOR ART In the case of a conventional nuclear reactor, a fissile material represented by uranium 235 is sealed in a fuel rod and burned to take out and use the burnup energy. The fissile material sealed in the fuel rod generally uses enriched uranium obtained by enriching natural uranium. The enriched uranium is formed into a fuel pellet, sintered in a uranium dioxide sintered state, put in fuel cladding tubes arranged like a square checkerboard and used. A PLUTHERMAL project in which plutonium contained in used uranium fuel taken out of a light-water reactor is recycled and reused in a light-water reactor has been recently progressed in order to effectively use uranium resources. In the case of this project, MOX (Mixed Oxide) fuel assemblies obtained by replacing some or most of the uranium fuel rods in uranium fuel assemblies with MOX fuel rods obtained by enriching plutonium are loaded into a light-water reactor as its fuel and used. In this case, it is desirable that the characteristics of the MOX fuel assembly is close to that of uranium fuel. Moreover, because the design of uranium fuel tends to have a high burn-up and therefore, it is desirable to design a high-enrichment MOX fuel, that is, to maximize the loaded amount of plutonium per fuel assembly. However, when increasing the plutonium loading content in an MOX fuel assembly, a difference is produced between a uranium core and a plutonium core from the viewpoint of the reactor core characteristics due to the difference of nuclear characteristics between uranium and plutonium. That is, the neutron flux spectrum of MOX fuel becomes harder than that of uranium fuel because the thermal neutron absorption cross sections of Pu-239 and Pu-241 which serve as fissile materials are larger than that of U-235 and the neutron resonance absorption by Pu-240 is large, and therefore the neutron slowing down effect is deteriorated. In the case of a nuclear reactor, the reactor core is designed to have an excess reactivity so that the reactor can be operated for a certain operation period. To control the excess reactivity, a burnable poison (BP) represented by gadolinium is mixed into a fuel rod in its design. Also in the case of a nuclear reactor using MOX, the excess reactivity is controlled by using a plurality of fuel rods obtained by mixing burnable poison into fuel. In general, the thermal neutron absorption cross section has a 1/v dependence on neutron energy and there is a tendency that a neutron with a lower energy is more easily absorbed. Therefore, the absorbed quantity of neutrons by burnable poison increases as the neutron spectrum energy becomes soft, that is, as the system has more thermal neutrons. Therefore, the reactivity control effect by burnable poison lowers in a nuclear reactor core using MOX. Therefore, to obtain a reactivity control effect equivalent to the that of a uranium core, it is necessary to increase the number of fuel rods used and containing burnable poison. To cope with the above, it is considered to use the technique disclosed in Japanese Patent Laid-Open No. 146185/1985. This technique improves the reactivity value of gadolinium, decreases the number of gadolinium-contained fuel rods to be used, increases the plutonium inventory of a fuel assembly, and decreases the number of types of pellets used by noticing the importance of the fact that the outer peripheries of the fuel assemblies close to a water gap have a lot of thermal neutrons and the soft neutron spectrum is soft and arranging the gadolinium-contained fuel rods in the outer periphery. However, the above method cannot completely remove the burnable poison from the fuel in a fuel assembly and therefore, there arises a problem that the method is insufficient from the viewpoint of reducing plutonium inventory. It is considered to solve the above problem by the techniques disclosed in Japanese Patent Laid-Open Nos. 129790/1980 and 72087/1984. The latter techniques makes it possible to add burnable poison to a fuel pellet or make adjustment of uranium enrichment unnecessary by removably providing a reactivity control member to the outer periphery of the fuel channel box of a fuel assembly. In this case, the reactivity control member is a neutron absorber such as stainless steel or zirconium alloy, a member obtained by dispersing burnable poison such as gadolinium, silver, indium, boron, cadmium, or hafnium into stainless steel in the form of a simple substance or compound or cladding the burnable poison with stainless steel, a member obtained by cladding a reflector such as beryllium with stainless steel, or a member subjected to co-extrusion in which the above neutron poison, reflector, or natural or depleted uranium is sandwiched between stainless steel and rolled. Moreover, the technique disclosed in Japanese Patent Laid-Open No. 342091/1994 is proposed for the above problem. This is a technique for forming a slowing-down material rod loaded at the center of a channel box into a double tube of outside and inside tubes and providing burnable poison between the inside and outside tubes. In the case of the technique disclosed in Japanese Patent Laid-Open No. 72087/1984, however, a gap is produced between the channel box and the reactivity control member and therefore, crevice corrosion or galvanic corrosion easily occurs. Moreover, because the reactivity control member directly contacts with reactor water, the reactivity control member may be corroded. Furthermore, as described above, burnable poison is mixed in a nuclear reactor fuel in order to control the initial extra reactivity. As the result of comparison of the dependence of the absorbing cross section of uranium on the neutron energy with that of plutonium on the neutron energy, it is found that plutonium absorbs more neutrons, as shown in FIG. 22. Therefore, when using plutonium for a light-water reactor, the number of thermal neutrons absorbed by a reactivity control substance such as control rod material or burnable poison decreases, the control rod value or reactivity value of burnable poison lowers in the case of a reactor core loaded with MOX fuel assemblies and therefore, it is necessary to increase the number of fuel rods used containing burnable poison in the case of the reactor core loaded with MOX fuel assemblies. This means that the plutonium inventory per fuel assembly decreases and resultingly, the number of fuel assemblies to be manufactured for the consumption of the same amount of plutonium is increased. This causes the fuel manufacturing cost and the fuel transport cost to increase. Moreover, the nuclear reactor fuel must be so designed as to keep the local peaking coefficient at a proper value during the fuel service life and observe the thermal operation restriction value in order to maintain the soundness of the fuel. In general, in the case of a boiling-water nuclear reactor, a thermal neutron flux relatively increases at the outer periphery of a fuel assembly, that is, at the position close to the water gap and the output of fuel rods at the outer periphery tends to increase. Therefore, it is necessary to design the nuclear reactor fuel by increasing the number of types of enrichment and enrichment factors of a pellet in order to decrease the local peaking coefficient of fuel rods arranged at the outer periphery of the fuel assembly. It can be said that the technique disclosed in Japanese Patent Laid-Open No. 342091/1994 is not enough to solve the above problem because burnable poison is loaded at the center of the channel box. To manufacture MOX fuel, a fuel pellet is formed in a fully-closed vessel. Therefore, cleaning of a glove box when changing the enrichment of plutonium requires a lot of time compared to the case of uranium and therefore, the working ratio of manufacturing the MOX fuel is greatly lowered. Therefore, when the number of types of enrichment increases, there arises a problem that the clean-up frequency increases and resultingly, the fuel forming cost increases. SUMMARY OF THE INVENTION The present invention has been made to solve the above problems and its object is to provide a channel box of a fuel assembly for a nuclear reactor capable of properly controlling the excess reactivity without mixing a neutron absorber or burnable poison into fuel rods of a fuel assembly (MOX fuel assembly) containing plutonium. To achieve the above object, a fuel assembly of the present invention is provided with a fuel rod bundle in which a plurality of nuclear fuel rods containing uranium or plutonium and a channel box enclosing the fuel rod bundle, in which the channel box is provided with burnable poison and the burnable poison is embedded so that the burnable poison does not directly contact with reactor water, or coated with metal having a corrosion resistance higher than that of the burnable poison so that the burnable poison does not directly contact with the reactor water. Moreover, in a fuel assembly provided with a water rod, the water rod is provided with burnable poison, and the burnable poison is coated with metal having a corrosion resistance higher than that of the burnable poison so that the burnable poison does not directly contact with reactor water. The metal is, for example, zircaloy and as the above zircaloy it is preferable to use zircaloy 4 to be mentioned later. In a specific mode of the present invention, the burnable poison of the channel box and water rod is made of a metal, alloy, intermetallic compound, or ceramic and the metal, alloy, intermetallic compound, or ceramic contains at least one of cadmium, samarium, boron, gadolinium, silver, indium, and hafnium. It is preferable that the burnable poison is made of circonium or zircaloy and the content is preferably 10 wt. % or less. Furthermore, the burnable poison is made of metal, alloy, intermetallic compound, or ceramic obtained by adding at least one of cadmium, samarium, boron, gadolinium, silver, indium, and hafnium to zirconium or zirconium-base alloy as an alloying element to form a solid solution containing the added element in a dispersed or supersaturated state in the form of at least one of metal, intermetallic compound, oxide, hydride, and nitride. Furthermore, as a mode of arrangement of burnable poison in the channel box of a fuel assembly of the present invention, the burnable poison is provided unevenly in the cross section of the channel box viewed from its longitudinal direction, more nearby the corners of the box symmetrically, unevenly in the longitudinal direction of the box, more at the bottom of the box and less at the top of the box in its longitudinal direction. Furthermore, as a method of the present invention for manufacturing the channel box of the above fuel assembly, a recess is formed in one material plate of the channel box, a burnable poison plate is placed in the recess, the other material plate of the channel box is joined to the former material plate so that the burnable poison is embedded in the two material plates, the material plates are hot-rolled or hot-pressed without electron-beam-welding or welding the joints, and thereafter they are repeatedly cold-rolled and annealed a proper number of times. As another mode of the method of the present invention, a burnable poison plate is set between two material plates of a channel box and repeatedly hot-roll and contact-bond these three plates a proper number of times without electron-beam-welding or welding. The fuel assembly of the present invention constituted as described above makes it possible to decrease or eliminate the amount of gadolinium contained in the fuel and reduce the local peaking coefficient of a fuel assembly by providing neutron absorber or burnable poison in the fuel channel box of a fuel assembly used for a boiling-water nuclear reactor. Reactivity control and lowering of the local peaking coefficient by providing a BP member serving as neutron absorber or burnable poison in a channel box, particularly by using Gd.sub.2 O.sub.3 as the BP member will be described below. FIG. 23 shows the change of the control value resulting from the neutron dose. From FIG. 23, it can be seen that Gd.sub.2 O.sub.3 is optimum as a material for controlling the initial excess reactivity, that is, the reactivity of the first cycle (dose:.apprxeq.1.0.times.10.sup.22 nvt). Moreover, in a fuel assembly, the number of thermal neutrons in the fuel assembly is more than in the water gap portion and the neutron spectrum also becomes softer. This is caused by the two factors that the amount of water in the fuel assembly is relatively more than in the outer periphery and thermal neutrons are absorbed by the fissile material in the fuel assembly. Burnable poison and neutron absorber have a neutron-absorbing sectional area with a dependence of 1/v as shown in FIG. 24 and the reactivity control effect increases as the number of thermal neutrons increases. In the MOX fuel, because plutonium absorbs neutrons more than uranium, the neutron spectrum in a fuel assembly becomes harder and the reactivity control effect of burnable poison decreases. Therefore, it is possible to heighten more the reactivity control effect by providing burnable poison at a water gap portion, that is, in a channel box than by mixing the burnable poison into the fuel. Moreover, referring to FIGS. 15(a) and 15(b) showing the thermal-neutron flux distribution in a fuel assembly, thermal neutrons increase at the outer periphery of the fuel assembly including a relatively larger amount of water but the thermal neutron bundles decrease at the central portion of the fuel assembly. Therefore, the local peaking coefficient also tends to increase at the outer periphery of the fuel assembly. By mixing burnable poison or neutron absorber into a fuel channel box portion nearby the region where the local peaking coefficient increases, it is possible to effectively control the local peaking coefficient of the outer periphery of the fuel assembly. Furthermore, the function of distributing burnable poison or neutron absorber in the longitudinal direction of a channel box will be described below. In a boiling-water nuclear reactor, cooling water boils in the core of the nuclear reactor and flows from the bottom to the top of the nuclear reactor core. Therefore, water vapor bubbles (voids) are distributed in the axial direction of the nuclear reactor core and moreover, the number of voids tends to increase toward the top of the core. In a light-water-moderated nuclear reactor, the density of moderator (water) controls nuclear fission and the reactor is so designed that the nuclear fission is accelerated as the moderator density increases. Therefore, when considering the output distribution in the axial direction of the reactor core, the output tends to be high at the bottom of the reactor core where there are little voids compared to the top of the reactor core where there are more voids. It is possible to effectively cope with the above fact by distributing the amount of neutron absorber or burnable poison provided in the channel box so that it is large in the bottom region in the axial direction where the reactivity is high, and decreases toward the top region. Moreover, because the neutron spectrum is hard in the top region in the axial direction because the voidity is high in the region, the depletion of burnable poison or neutron absorber tends to be slow compared to the bottom region. Therefore, by distributing the neutron absorber in the axial direction, the depletion in the axial direction uniformly progresses. Furthermore, in the case of the present invention, burnable poison does not directly contact with reactor water because the burnable poison is embedded in a channel box or the burnable poison is coated with metal such as zircaloy. Therefore, the burnable poison does not corrode, or no crevis corrosion nor galvanic corrosion occurs between the burnable poison and the channel box. Furthermore, the present invention makes it possible to constitute a neat channel box from which no burnable poison is exposed to the outside by embedding the burnable poison in material plates at the manufacturing step before the step of forming the channel box and moreover, easily manufacture a channel box in which burnable poison is embedded and prevent the undesirable phenomenons such as separation of material plates of the channel box from each other by using the channel box manufacturing method of the present invention. From the above description, it can be understood that a fuel assembly (MOX fuel assembly) of the present invention containing plutonium makes it possible to properly control the excess reactivity without mixing neutron absorber or burnable poison into a fuel rod. Moreover, because no burnable poison is mixed into fuel, it is possible to use plutonium for a light-water reactor without decreasing the loaded amount of plutonium per fuel assembly. Furthermore, it is possible to reduce the local peaking of the outer periphery of a fuel assembly without increasing the number of types of pellet enrichment and thereby, decrease the number of types of enrichment of the pellets constituting the fuel assembly. Furthermore, by embedding burnable poison in a channel box or coating the burnable poison with metal such as zircaloy, the burnable poison does not corrode or crevis corrosion or galvanic corrosion does not occur between the burnable poison and the channel box and the burnable poison does not directly contact with reactor water. Therefore, it is possible to prevent the burnable poison from leaking into reactor water. Furthermore, it is possible to form a neat channel box from which burnable poison is not exposed to the outside and moreover, easily manufacture a channel box in which burnable poison is embedded and prevent material plates of the channel box from separating from each other.