Fuel assembly and fuel assembly channel box manufacturing method

Abstract: A channel box for nuclear-reactor fuel assemblies (MOX fuel assembly) containing plutonium is provided, capable of properly controlling excess reactivity without mixing neutron absorber or burnable poison into a fuel rod. In the channel box for a nuclear-reactor fuel assemblies, there is no gap between tne channel box (1) and members (4) containing burnable poison because the members (4) containing burnable poison are in the channel box. The burnable poison does not directly contact the reactor water. Alternatively, the burnable poison members are on a water rod (3).

Description: Detailed Description of the Invention [Technical Field of the Invention] The present invention relates to a fuel assembly for a nuclear power plant, particularly to a fuel assembly for improving the economic 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 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 a high burn-up and therefore, it is desirable to design a high-enrichment MOX fuel, that is, 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 previously 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 coping 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 has 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 technique 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. Summary of the invention (1)Object of the invention 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 decrease 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. 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. (2)Statement of the Invention To achieve the above object, there is provided a fuel assembly as disclosed in claim 1. The burnable poison may be 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, a method of the present invention for manufacturing the channel box of the above fuel assembly, is disclosed in claim 13. 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 2 O 3 as the BP member will be described below. Figure 23 shows the change of the control value resulting from the neutron dose. From Fig. 23, it can be seen that Gd 2 O 3 is optimum as a material for controlling the initial excess reactivity, that is, the reactivity of the first cycle (dose: ≃ 1.0×10 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 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 Fig. 15 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 as disclosed in claim 17. 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 is easily manufactured 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 each other. Brief description of the drawings. Figures 1(a) and 1(b) are a sectional view and a side view of a BP-member-embedded (nearby corners) channel box of the first embodiment of the present invention; Figure 2 is a sectional view and a side view of a BP-member-embedded (at corners) channel box of the second embodiment of the present invention; Figure 3 is a block diagram showing a process (pre-stage process) for manufacturing the channel boxes of the embodiments of Figs. 1 and 2; Figure 4 is a block diagram showing a process (post-stage process) for manufacturing the channel boxes of the embodiments of Figs. 1 and 2; Figure 5 is a block diagram showing another process for manufacturing the channel boxes of the embodiments of Figs. 1 and 2; Figures 6(a) and 6(b) are a sectional view and a perspective view of a BP-member-embedded (at corners) channel box of the third embodiment of the present invention; Figures 7(a) and 7(b) are a sectional view and a perspective view of a BP-member-embedded (at corners) channel box of the fourth embodiment of the present invention; Figures 8(a) and 8(b) are side views showing structures for arranging BP members of another embodiment of the present invention in a channel box; Figure 9 is a side view showing a structure for arranging BP members of still another embodiment of the present invention in a channel box; Figure 10 is a diagram showing the output distributions in the axial direction of a reactor core; Figure 11 is a side view showing a structure for arranging BP members of still another embodiment of the present invention in a channel box; Figures 12(1)(a) to 12(2) (b) are illustrations showing structures of a BP member of the present invention; Figure 13 is a block diagram showing a process for manufacturing a BP member of the present invention; Figure 14 is a block diagram showing a process for manufacturing another BP member of the present invention; Figures 15(a) and 15(b) are a sectional view and a side view of a channel box for BP members (nearby the corners) of still another embodiment of the present invention; Figures 16(a) and 16(b) are a sectional view and a side view of a channel box for BP members of still another embodiment of the present invention; Figure 17 includes a sectional view and a side view of a channel box for BP members of still another embodiment of the present invention; Figures 18(a) and 18(b) are a sectional view and a side view showing a structure for attaching BP members of the present invention to a channel box; Figure 19 is a sectional view and a side view showing another structure for attaching BP members of the present invention to a channel box; Figure 20 is a perspective view showing a structure for attaching BP members of the present invention to a water rod; Figure 21 is a perspective view showing a channel box made of a zirconium alloy containing BP of the present invention; Figure 22 is a diagram for comparison of the dependences of the absorption cross-sections of uranium and plutonium on the neutron energy; Figure 23 is an illustration showing the change of control value with the neutron dose; and Figure 24 is a diagram showing the relationship between the neutron absorption cross-sections of burnable poison and neutron absorber and the neutron energy; Figures 25(a) and 25(b) are thermal neutron flux distribution diagrams in a fuel assembly; and Figure 26 is a partial sectional view of a BWR fuel assembly using a channel box. Embodiments Figures 1(a) and 1(b) show the first embodiment of a fuel assembly A of the present invention, in which upper illustrations of Figs. 1(a) and 1(b) are sectional views of channel boxes viewed from their longitudinal direction and lower illustrations are side views of the channel boxes viewed from their longitudinal direction. The MOX fuel assembly A comprises a channel box 1, a bundle of many fuel rods 2, one or two water rods 3, and BP members 4 containing burnable neutron absorbing poison (BP) provided in the members at the four laterals of the channel box 1. The BP member 4 is embedded nearby the corners of four laterals of the channel box 1 in its longitudinal direction. Two water rods 3 are used in Fig. 1(a) and one water rod 3 is used in Fig. 1(b). The other parts are the same in Figs. 1(a) and 1(b). Figure 2 shows'the second embodiment in which BP members 4 are embedded at the corners of the laterals of a channel box 1 in its longitudinal direction. MOX fuel is made of uranium 238 fuel containing 1.5 to 10 wt% of Pu. It is preferable to provided MOX fuel containing much Pu at the inside of the channel box and MOX fuel containing less Pu at the outside of the channel box. Thus, by embedding the BP members 4 in the channel box 1, the BP members 4 do not directly contact with reactor water. Therefore, it is possible to prevent crevis corrosion and galvanic corrosion. Moreover, by arranging the BP member 4 at and nearby the corners of the channel box 1, it is possible to effectively control the local peaking coefficient of the corners of the fuel assembly A. Figure 26 is a partial sectional view of a BWR fuel assembly using the above channel box. As shown in Fig. 26, the BWR fuel assembly comprises many fuel rods 11 and spacers 12 for holding the fuel rods at predetermined intervals, a prismatic channel box 1 for storing the fuel rods and the spacers, a top tie plate 14 for holding'the upper ends of the fuel rods 11 having fuel pellets in fuel cladding tubes, a bottom tie plate 15 for holding the lower ends of the fuel rods 11, and a handle 13 for transferring the whole. Figures 3 to 5 show a method for manufacturing the channel box 1 of the fuel assembly A of the first embodiment. First, as shown in Fig. 3, a recess with a depth of 0.1 to 0.4 mm formed in a material plate made of zircaloy 4 serving as a channel box 1 in the longitudinal direction of the plate, a BP member 4 with the same size as that of the recess is fitted into the recess, and another thin zircaloy material plates is joined and electron-beam-welded in a vacuum state. Thereafter, they are hot-rolled, cold-rolled, and annealed several times at 600 to 700°C to form a complete plate. In stead of using the hot rolling, there is a method of heating the above materials up to 1,220°C and rolling them by hot press to form a plate. Moreover, when the materials are not separated from each other, it is possible to omit the electron-beam welding. Particularly, when rolling the materials by hot press, the electron-beam welding is often omitted. After forming the above one plate, the plate is bent to form a channel box 1 and two bent plates are butt-welded to form the rectangular channel box 1 and then, as shown in Fig. 4, the channel box 1 is completed by applying special heat treatment, heat-treatment shaping, and autoclave treatment to the channel box 1. The zircaloy 4 is a Zr-base alloy made of 1.20 to 1.70 wt% of Sn, 0.18 to 0.24 wt% of Fe, 0.07 to 0.13 wt% of Cr, 0.10 to 0.16 wt% of oxygen, and wt% of residual Zr. Moreover, as another method, as shown in Fig. 5 there is a method of hot-rolling a BP plate 4 sandwiched between material plates 1 and 1 made of zircaloy 4 at 600 to 700°C to pressure-weld them and electron-beam-welding the both ends of them in a vacuum state and thereafter, repeatedly cold-rolling and annealing them to form one plate. Also in this case, unless the material plates 1 and 1 are separated from each other, it is possible to omit the electron-beam welding in a vacuum state. Moreover, it is assumed that the BP member 4 is completely clad with material 1 of zircaloy-4 (Zry) 1 and thus, it does not contact with the outside as shown in Fig. 5. Thereafter, a channel box is manufactured by passing it through the same conventional processes as those shown in Figs. 3 and 4. Figures 6(a) and 6(b) are the third embodiment constituted by fitting a BP member 4 into a recess of a channel box 1 and cladding the outside with a thin plate 5 made of zircaloy (Zry). That is, the third embodiment is constituted by forming a recess at and around a corner outside of the channel box 1 and fitting the BP member 4 into the recess and thereafter, cladding a part of or whole the lateral of the channel box 1 with the thin plate 5 made of Zry and welding them so that the BP member 4 does not directly contact with the reactor water. Figure 6(a) shows a case where the BP member 4 is provided at a corner of the channel box 1, in which only the outside of the BP member is clad with a thin plate 5 and Figure 6(b) shows a case where the four laterals of the channel box 1 are clad with thin plates 5. Figures 7(a) and 7(b) show the fourth embodiment constituted by providing BP members 4 nearby the corners. The other parts are the same as those of the embodiment of Fig. 6. Moreover, when using a BP member 4 coated with another metal to be mentioned later, it is unnecessary to clad the member 4 with a thin plate made of Zry. Figures 8(a) and 8(b) show shapes of a BP member 4, in which Fig. 8(a) shows a case where the width of the BP member 4 at the bottom of a channel box is great discontinuously in its longitudinal direction compared with the width at the top of the box and Fig. 8(b) shows a case where the width of the BP member 4 at its bottom is increased continuously in its longitudinal direction compared to the width at its top. Figure 9 shows a case where a BP member 4 is divided in the longitudinal direction of a channel box 1. In this case, there are cases in which BP members 4 with an equal length are arranged at regular intervals and BP members 4 lengthened toward the bottom of the channel box 1 are arranged at narrow intervals. In this case, when considering the output distribution in the axial direction of a reactor core, the output distribution in the axial direction of the reactor core becomes large at the bottom of the reactor core where there are less voids compared to the top of the core where there are more voids as shown by curve "a" in Fig. 10 because water vapor bubbles (voids) are present in the axial direction of the core of the nuclear reactor and they increase toward the top of the reactor core. Therefore, as described above, by arranging less BP members 4 at the top and more BP members 4 at the bottom, it is possible to flatten the output distribution in the axial direction of the reactor core like a curve "b". Figure 11 shows a case where the width of a BP member 4 in the longitudinal direction of a channel box 1 and determining the length of the BP member 4 in the longitudinal direction of the channel box 1 to 80 to 100% of the effective length of a fuel rod 2. Figures 12 to 14 show a method for manufacturing a BP member 4. The BP member 4 contains at least one of cadmium (Cd), samarium (Sm), boron (B), gadolinium (Gd), silver (Ag), indium (In), and hafnium (Hf) and the element(s) is present in the form of one of metal, alloy, intermetallic compound, and ceramic. The content of BP metal in the BP member is required to be 2 to 8 wt% based on the total weight of the channel box in the case of gadolinium in order to control the initial reactivity. Moreover, an example of alloy composition when using Gd/Zry-4 alloy as a BP member, is tin: 1.20-1.70, iron: 0.18-0.24, chromium: 0.07-0.13, oxygen: 0.10-0.16, gadolinium: 5-80, and zirconium: the remainder (wt%). In this case, the content of gadolinium depends on the size of a BP member or the number of BP members. Figure 12(1) (a) shows a method of forming a BP member 4 by putting Zry powder on a BP metal (Gd or Cd) with a melting point lower than that of Zry, heating the Zry powder up to the melting point of the BP metal in a vacuum state, and filling the voids of the Zry powder with the BP metal. Figure 12(1) (b) shows a method of forming the BP member 4 by, contrarily, putting a BP metal with a melting point higher than that of Zry or BP oxide powder on a Zry plate or spongy zirconium, heating it up to a temperature (1, 860°C) equal to or higher than the melting point of Zr, and filling the voids of the BP powder with Zry. Figure 12(2) (a) shows a method of coating a BP plate 4 with Zry by plating or vacuum evaporation and Figure 12(2)(b) shows a method of coating a Zry plate with a BP metal contrary to the above method. Figure 13 shows a process for manufacturing a BP block 4 coated with zircaloy. As shown in Fig. 13, BP powder and Zry powder is strongly treated by an MA (Mechanical Alloying) method to produce MA alloy powder containing BP (metal oxide) in a supersaturated state in the form of a solid solution. In the case of the MA method, a planetary ball mill P-5/4 made by Fritch is used and the revolving speed of a disk is kept at 200 rpm to perform the treatment for 100 to 150 hr at room temperature in an Ar-gas atmosphere. Thereafter, the produced MA alloy powder is sintered by HIP (hot isostatic process) at 1,000°C or higher to complete a sintered-body BP member 4 shown in Fig. 13(A). When further coating the sintered BP member 4 with zircaloy, it is possible to manufacture a BP block 4 coated with zircaloy shown in Fig. 13(B) by putting the sintered body in a vessel 6 made of Zry and vacuum-sealing it and thereafter, compacting it by a HIP. By using the mechanical alloying method, it is possible to produce zircaloy containing BP metal and BP oxide in a supersaturated state in the form of a solid solution in such a way that the contents of the BP metal and BP oxides are more than those at room temperature. Figure 14 shows a method of manufacturing a coated BP block 7 by compacting a powder, obtained by mixing Zry powder and BP metal powder or BP oxide powder to form it into a block and thereafter, dipping the block in the liquid of a proper metal with a low melting point. Moreover, there is a method of alloying zirconium or zircaloy with a BP metal in addition to the above method of manufacturing the BP block 4. In this case, however, because the obtained alloy may be inferior to the existing zircaloy alloy in mechanical characteristics and corrosion resistance, it is necessary to improve the mechanical characteristics and corrosion resistance of the obtained alloy by adding an additional element to the alloy. Furthermore, there is a method of making an alloy by precipitating a BP metal serving as an intermetallic compound in zircaloy. Cadmium, boron, silver, and indium are enumerated as BP metals for producing an intermetallic alloy with zirconium. Though several embodiments of the present invention have been described above, the present invention is not restricted to the embodiments. Various modifications are allowed in design as long as they are not deviated from the gist of the present invention described in claims. In addition to the means for embedding the above BP members 4 in the channel box 1, there is the following method. That is, as shown in Fig. 15, when using the BP members 4 coated with another metal, means for attaching the BP members 4 to the laterals of the channel box 1 instead of embedding it can be used. Figures 15(a) and 15(b) show cases in which the above coated BP members 4 are arranged nearby the corners of the inner laterals of the channel box 1 in its longitudinal direction. Figure 16(a) shows a case in which the coated BP members 4 are arranged nearby the corners of the outer laterals of the channel box 1 in its longitudinal direction and Figure 16(b) shows a case in which the BP members 4 are arranged at the corners of the outer laterals of the channel box 1 in its longitudinal direction. Moreover, Fig. 17 shows a case in which the coated BP members 4 are arranged at the corners of the inner laterals of the channel box 1 in its longitudinal direction. Figures 18(a) and 18(b) show methods for providing the BP member 4 to the channel box 1. Figure 18(a) shows a method for attaching the BP members 4 by using rivets 8 made of zircaloy or stainless steel. In this case, it is possible to attach the BP members 4 to the inside or outside of the channel box 1 with rivets 8. Figure 18(b) shows a method for directly welding the coated BP members 4 to the laterals of the channel box 1. Furthermore, Fig. 19 shows a method for attaching the coated BP members 4 by fixtures 9 made of zircaloy or stainless steel. Figure 20 shows a case in which the coated BP members 4 are arranged in a water rod 3. As shown in Fig. 20, there are two cases: one case in which a tube 10 made of coated BP metal is placed in the water rod 3 to form a double structure, and the other case in which a tube 11 made of Zry is placed further inside of the BP metal tube 10 to form a triple structure. Figures 21(a) and 21(b) show a channel box 1 made of a Zry alloy containing BP. The shape of the channel box 1, as shown in Figs. 21(a) and 21(b), makes it possible to effectively control the local peaking coefficient of the corners of a fuel assembly A and moreover, simultaneously control the swelling of the corners of the channel box 1 due to creep deformation caused by irradiation with neutrons, by increasing the thickness of the corner. Moreover, by coating the surface of the channel box 1 with a high-corrosion-resistant alloy made of Zry containing no BP, it is possible to prevent burnable poison from directly contacting with reactor water and improve the corrosion resistance. As an example of the composition of the Gd/Zry-4 alloy with which the whole of a channel box is fabricated, is tin: 1.20-1.70, iron: 0.18-0.24, chromium: 0.07-0.13, oxygen: 0.10-0.16, gadolinium: 2.0-8.0, and zirconium: the remainder (wt%).

Claims: A fuel assembly provided with a fuel rod bundle in which a plurality of nuclear fuel rods (2) containing uranium or plutonium are arranged and at least one of a channel box (1) enclosing the fuel rod bundle and a water rod (3) in the fuel rod bundle, wherein burnable poison (4) is provided in at least one of the channel box and the water rod and the burnable poison is embedded so that it does not directly contact with reactor water. A fuel assembly according to claim 1, wherein the burnable poison is coated with metal having a corrosion resistance so that it does not directly contact with reactor water. The fuel assembly according to claim 1 or 2, wherein the burnable poison is made of metal, alloy, intermetallic compound or ceramic. The fuel assembly according to claim 3, wherein the burnable poison contains at least one of cadmium, samarium, boron, gadolinium, silver, indium and hafnium. The fuel assembly according to any one of claims 1 to 4, wherein the burnable poison is made by adding at least one of cadmium, samarium, boron, gadolinium, silver, indium and hafnium to zirconium or zirconium-base alloy as an alloying element and forming 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. The fuel assembly according to any one of claims 1 to 5, having the channel box with said burnable poison therein, wherein the burnable poison is unevenly distributed in the cross-section of the channel box looking in its longitudinal direction. The fuel assembly according to claim 6, wherein the burnable poison is arranged near corners in the cross-section of the channel box looking in its longitudinal direction. The fuel assembly according to claim 7, wherein the burnable poison is symmetrically arranged in the cross-section of the channel box looking in its longitudinal direction. The fuel assembly according to any one of claims 1 to 8, having the channel box with said burnable poison therein, wherein the burnable poison is unevenly distributed in the longitudinal direction of the channel box. The fuel assembly according to claim 9, wherein more of the burnable poison is at the bottom of the channel box and less at the top in the longitudinal direction of the channel box. A fuel assembly according to any one of claims 1 to 10, having the channel box with said burnable poison therein, wherein a part or the whole of the cross-section of the channel box viewed in its longitudinal direction comprises two or more layers, and at least one layer of the burnable poison is so extending provided as to face a region extending 80% or more of the effective length of the fuel rods. A fuel assembly according to claim 11, wherein at least one layer of burnable poison containing 5 wt% or more of at least one of cadmium, samarium, boron, gadolinium, silver, indium and hafnium is so provided as to face a region extending 80% or more of the effective length of the fuel rods. A method of making a fuel assembly channel box comprising forming a plate assembly by (a) the steps of forming a recess in a first plate of a channel box, providing a burnable poison plate in the recess, joining a second plate to the first plate so that the burnable poison is embedded in the two plates, and hot-rolling or hot-pressing the plates, or (b) the steps of providing a burnable poison plate between two plates and contact-bonding the three plates by hot-rolling them, and after (a) or (b) repeatedly cold-rolling and annealing the plate assembly, and assembling the plate assembly into the channel box. A method according to claim 13, wherein in (a) the joint between said plates is welded or electron-beam-welded before performing said hot-rolling or hot-pressing. A method according to claim 14, wherein in step (b) the ends of the three plates are electron-beam-welded in a vacuum state after hot-rolling of the plates. A method according to any one of claims 13 to 15, wherein the plates of the channel box are made of zircaloy and the burnable poison is made of metal, alloy, intermetallic alloy or ceramic. A channel box for use in a fuel assembly according to claim 1, wherein burnable poison is embedded in the channel box so that the burnable poison does not directly contact with reactor water in use. A channel box according to claim 17, wherein 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. A water rod for use in a fuel assembly according to claim 1, including burnable poison 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 in use.

Inventors: FUJIEDA TADASHI, INAGAKI MASAHISA, ITO KENICHI, NAKAJIMA JUNJIRO, NISHINO YOSHITAKA, SETO TAKEHIRO, TAKASE IWAO, YAMANAKA AKIHIRO, YAMASHITA JUNICHI, FUJIEDA, TADASHI, INAGAKI, MASAHISA, ITO, KENICHI, NAKAJIMA, JUNJIRO, NISHINO, YOSHITAKA, SETO, TAKEHIRO, TAKASE, IWAO, YAMANAKA, AKIHIRO, YAMASHITA, JUNICHI
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1997
Main classification: G21C3
IPC codes: G21C3, G21C7