Patent Number: 058728260
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIGS. 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 or sheets 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). FIG. 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. FIG. 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 assembly. FIGS. 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 plate is joined and electron-beam-welded in a vacuum state. Thereafter, they are hot-rolled, cold-rolled, and annealed several times at 600.degree. to 700.degree. 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.degree. 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.degree. to 700.degree. C. to pressure-weld them and electron-beam-welding both ends of plates 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. FIGS. 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 the whole 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. FIG. 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 FIG. 6(b) shows a case where the four laterals of the channel box 1 are clad with thin plates 5. FIGS. 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. FIGS. 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. FIG. 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". FIG. 11 shows a case where the width of a BP member 4 in the longitudinal direction of a channel box 1 is constant and the length of the BP member 4 in the longitudinal direction of the channel box 1 is 80 to 100% of the effective length of a fuel rod. FIGS. 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 an 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. FIG. 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. FIG. 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.degree. C.) equal to or higher than the melting point of Zr, and filling the voids of the BP powder with Zry. FIG. 12(2)(a) shows a method of coating a BP plate with Zry by one of plating or vacuum evaporation and FIG. 12(2)(b) shows a method of coating a Zry plate with a BP metal by the other one of the methods. FIG. 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 a HIP (hot isostatic process) at 1,000.degree. 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. FIG. 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 FIGS. 15(a) and 15(b), 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. FIGS. 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. FIG. 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 FIG. 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, FIGS. 17(a) and 17(b) show 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. FIGS. 18(a) and 18(b) show methods for attaching the BP members 4 to the channel box 1. FIG. 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. FIG. 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. FIG. 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. FIGS. 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 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. 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. %).