Patent Number: 051868903
Section: description

DESCRIPTION OF PREFERRED EMBODIMENTS In the embodiment described hereinafter, the finding that a limitation of outer size of a wrapper tube becomes critical at a portion of the wrapper tube which faces to nearly the middle of a nuclear fuel stack in a nuclear fuel rod, and that both upper and lower portions from the critical portion have spaces to accommodate some expansion was obtained with fundamental consideration on thermal output power and removal of the thermal output power in a reactor core of a fast breeder reactor and investigation on elimination of useless space in a conventional fuel assembly for the fast breeder reactor, and a fuel assembly which comprises a wrapper tube having a reverse tapered shape in both outer and inner width from the portion which faces to the middle of the nuclear fuel stack in the nuclear fuel rod toward both upper and lower ends of the wrapper tube was devised. In considering of horizontal expansion of the wrapper tube caused by swelling etc. during the reactor operation, while a uniform gap between each wrapper tubes is necessary to be maintained at the middle portion of the reactor core fuel stack, that is a heat generation part of a nuclear fuel rods bundle, the gap between each wrapper tubes can be reduced in other portions. According to the understanding described above, a pressure drop of coolant can be reduced by enlarging a flow area of coolant by reducing the gap between the wrapper tubes at the portion other than the middle portion of the reactor core fuel stack and enlarging both of the outer and inner width of the wrapper tube larger than the width at the middle portion of the reactor core fuel stack in considering that the enlargement of the wrapper tube at a portion which faces to the gas plenum portion of the nuclear fuel rod, that is no heat generating part, does not have any problem in heat removal. Especially, a fuel assembly for a fast breeder reactor has a trend to be longer in length with advancement of nuclear fuel for high burnup, and the present invention can contribute preferably to the reduction of head loss of the fuel assembly described above. Hereinafter, embodiments of the present invention wherein the invention were applied to fuel assemblies for the fast breeder reactor are described in detail in referring FIG. 1 and 2. FIG. 1 is a vertical cross section of a fuel assembly related to an embodiment of the present invention. In FIG. 1, 271 fuel rods 1 having a small diameter are bundled and inserted into a wrapper tube 2, a means of fuel containment. In the bundling of the fuel rods, the external surface of the cladding 4 of each fuel rod 1 is wound with a wire spacer 5 to keep a constant gap between adjacent fuel rods. The fuel rod 1 having a small diameter is composed of a stack of fuel pellets 3 inserted into a cladding 4. A gas plenum portion to accommodate gas pressure which is generated with burning of the fuel pellets 3 is formed in the lower part of the cladding 4. While, fuel pellets 3 are consisted with two type of fuels, driver fuel pellets 10 and blanket fuel pellets 11, and the driver fuel pellets 10 are stacked in the reactor core region 12 which is a heat generating region and the blanket fuel pellets 11 are stacked in each of upper and lower blanket region 13, 14. At the upper end of the wrapper tube 2, an upper shield 6 for neutrons is fixed by welding, and a lower shield 7 is fixed by welding in a same way at the lower end of the wrapper tube 2. The lower shield 7 has an inlet for coolant conducting to the inside of the wrapper tube 2, and the upper shield 6 has an outlet for coolant. The outer diameter of the fuel rod 1 is 7.6 mm, and the minimum inner and outer width between faces of the portion of the fuel assembly 2 coincident to the portion of the fuel pellets stack (at the middle of the fuel pellets stack) are 150 mm and 158 mm. And the arrangement pitch between the wrapper tubes 2 which are composing the reactor core by standing together vertically in the reactor is 164 mm. As shown in FIG. 3, a wrapper tube 2 of a conventional fuel assembly has a uniform inner and a uniform outer width which are partly enlarged at positions of pads 8 for restriction of fuel assemblies at the outer side of upper blanket fuel portion. On the other hand, an embodiment of the present invention is characterized in a wrapper tube 2 of a fuel assembly having both of a partly larger inner width and a partly larger outer width at positions facing to upper and lower part of a reactor core region 12 than an inner and outer width at a position facing to the reactor core region 12. In the present embodiment, the inner and the outer width of the wrapper tube 2 at the portion facing to the gas plenum part 9 are 154 mm and 162 mm each, and the minimum inner and the outer width or diameter of the wrapper tube 2 at the portion facing to the reactor core region 12 are 150 mm and 158 mm. And the wrapper tube 2 has a shape having both of the inner and the outer widths or diameters enlarged continuously and gradually from the minimum inner and outer width or diameter to 154 mm, and 162 mm respectively, so that the wrapper tube may be considered to have an hourglass shape. Consequently, in the embodiment of the present invention, the pressure drop of coolant of the fuel assembly relating to the present invention could be reduced about 29% as compared with the pressure drop of coolant of the conventional fuel assembly for the fast breeder reactor as shown in Table 1. And a reactor core comprising the fuel assembly having the reduced pressure drop of coolant as an element consequently has a reduced pressure drop of coolant as a whole, and a capacity of a pump which circulates coolant to the reactor core can be reduced. Further, the reduction of the capacity of the pump makes it possible to use a smaller pump, and hence, a reduction in size of the reactor vessel of the fast breeder reactor including the small pump becomes possible. Moreover, the reduction of the pressure drop of coolant of the reactor core enhances natural circulation of coolant in the reactor core after stopping the operation of the pump, and the enhancement of the natural circulation of coolant can contribute to cooling of the reactor core and raises safety of the reactor. TABLE 1 ______________________________________ Present Fuel Assembly Prior art invention ______________________________________ Length in axial direction (mm) Upper blanket portion 300 300 Reactor core fuel stack portion 1000 1000 Lower blanket portion/ 1700 1700 Gas plenum portion Inner width of wrapper tube (mm) Upper blanket portion 150 154 Middle of fuel stack portion 150 150 Upper and lower ends fuel 150 154 stack portion Gas plenum portion 150 154 pressure drop of coolant (kg/cm.sup.2) 3.8 2.7 (%) 100 71 (Reduced 29%) ______________________________________ In the embodiment of the present invention, enlargement of the width by expansion of the wrapper tube 2 is done all over the outer fringe, but in case of a wrapper tube having a hexagonal horizontal cross section, it is possible to increase structural strength of the wrapper tube by leaving thick wall thickness only at portions of the hexagonal corners of the wrapper tube 2 as it is without an enlarging procedure. And in the embodiment of the present invention, expansion of the width of the wrapper tube 2 is done at the portion faced to the gas plenum portion 9, but as the expansion of the wrapper tube 2 by burning of nuclear fuel at the portion faced to the lower blanket portion 14 is smaller than the expansion at the portion of the wrapper tube 2 faced to the reactor core region 12, it is possible to enlarge the width of the wrapper tube 2 and the portion faced to the lower blanket portion just as same as the expansion at the portion faced to the gas plenum 9 portion. The present case is effective especially for the reduction of pressure drop of coolant at the region of 1.7 meters in 3 meters of the total length of the fuel rod. Further, in the embodiment of the present invention, the gas plenum portion 9 is provided at the lower part of the reactor core region 12, but in a case when the gas plenum portion 9 is placed at upper part of the reactor core region 12, the present invention also can be applicable. And the fuel assembly related to the present invention has an effect to enable to distribute coolant flow to the reactor core by being loaded into a part of the reactor core. That is, by using the enlarged wrapper tube 2 related to the present invention for the fuel assembly in the high power region of the reactor core and regulating of pressure drop of coolant at the fuel portion of the reactor core by changing the enlarging rate of the width of the wrapper tube depending on the change of thermal output, it becomes possible to distribute the flow of coolant most properly to each region of the reactor core. As the result, it is not necessary to use extra orifices for flow distribution which are conventionally installed at the inlet portion of coolant. FIG. 2 is a vertical cross section of the fuel assembly for the fast breeder reactor relating to the another embodiment of the present invention. The fuel assembly shown in FIG. 2 has a wrapper tube 2 having an upper shield 6 and a lower shield 7 which are welded to the wrapper tube the same as the fuel assembly shown in FIG. 1. The outer width of the fuel rod is 7.6 mm, and the minimum inner and outer width of the wrapper tube at the portion of narrowest width of the near middle of the fuel stack portion are 150 mm and 158 mm. And the pitch of arrangement of the wrapper tubes is 164 mm. In this embodiment, the width of the wrapper tube 2 has the minimum value at the middle of the reactor core fuel stack portion wherein the expansion of the wrapper tube 2 in horizontal direction is large, and as going up and going down to another portion (vertical direction) wherein the expansion of the wrapper tube is relatively small, the inner and the outer width of the wrapper tube 2 is continuously enlarged and the thickness of the wrapper tube 2 is reduced. The maximum inner and the outer width of the wrapper tube 2 at the portion faced to the upper and the lower blanket portion 13, 14 are 154 mm and 162 mm, and the inner and the outer width of the wrapper tube 2 at the portion faced to the most expanded gas plenum portion 9 are 158 mm and 162 mm. Consequently, in the embodiment of the present invention, pressure drop of coolant of the fuel assembly related to the present invention can be reduced about 42 % in comparison with a conventional fuel assembly for the fast breeder reactor by prior art as shown in Table 2. And the total pressure drop of coolant of the reactor core using the fuel assemblies relating to the present embodiment can be reduced as a whole. As a result, power for the coolant circulating pump can be saved, and reduction of pump size and reactor vessel becomes possible. TABLE 2 ______________________________________ Present Fuel Assembly Prior art invention ______________________________________ Length in axial direction (mm) Upper blanket portion 300 300 Reactor core fuel stack portion 1000 1000 Lower blanket portion/ 1700 1700 Gas plenum portion Inner width of wrapper tube (mm) Upper blanket portion 150 158 Middle of fuel stack portion 150 150 Upper and lower ends fuel 150 158 stack portion Gas plenum portion 150 158 pressure drop of coolant (kg/cm.sup.2) 3.8 2.2 (%) 100 58 (Reduced 42%) ______________________________________ In all of the present embodiments, a portion of a wrapper tube 2 faced to a reactor core region 12 is exposed to the environment of the highest temperature during reactor operation and is yielded the largest swelling. Therefore, in considering that the portion of the wrapper tube 2 faced to the reactor core region 12 has the largest swelling, the width of the portion is determined as nearly equal to a width of a conventional wrapper tube which is less than the maximum width of the wrapper tube. With the method described above, even though the width of the wrapper tube 2 is expanded horizontally at the portion faced to the reactor core region 12, any of mutual interference accident with adjacent wrapper tubes will not be occurred. And as the other portion of the wrapper tube 2 except the portion faced to the reactor core region 12 has small expansion coefficient, any of mutual interference with adjacent wrapper tubes will not be occurred even though the width of the wrapper tube 2 is enlarged. Accordingly, the reactor core having the fuel assembly relating to the embodiment of the present invention as an element, it is not necessitated to widen the gap between fuel assemblies, and enlarging of the reactor size is not necessary. In all of the present embodiments, an effect to use distributed coolant without any leakage, and to provide a fuel assembly for a fast breeder reactor which enable to reduce the pressure drop of coolant of the reactor core without any enlarging in size in composing of the reactor core of the fast breeder reactor is produced. By composing of a reactor core with a plurality of fuel assemblies, which are manufactured in any of the embodiments of the present invention, standing together vertically in the reactor core of a fast breeder reactor, an effect to provide the reactor core for the fast breeder reactor which enable to use distributed coolant to each of fuel assemblies without any leakage, and to reduce pressure drop of coolant of the reactor core without enlarging the size of the reactor core is obtained. By loading at least one fuel assembly which is manufactured in any of the embodiments of the present invention into a reactor core region, wherein the thermal output is relatively large, of the fast breeder reactor for increasing coolant flow at the region, an effect to average the thermal output of the reactor core is obtained. In the embodiments of the present invention, even though the minimum width portion of the wrapper tube, which is a means to contain fuel rods, has the same width as of the conventional wrapper tube, the width of another portion of the wrapper tube is larger than the width of the conventional wrapper tube, and an effect to provide a means of fuel containment which can contribute to achieve reduction of pressure drop of coolant of the fuel assembly for the fast breeder reactor without enlarging the size of the reactor core together with preventing the fuel assembly from contact with adjacent fuel assemblies by expansion with swelling of the wrapper tube during reactor operation is obtained. Especially in the embodiment of the present invention wherein the wall thickness of the means of fuel containment is reduced at the portion which faces to the gas plenum portion, in addition to an effect to provide a fuel assembly for the fast breeder reactor which is able to use distributed coolant for cooling of the reactor core without leakage and to reduce the pressure drop of coolant of the reactor core without enlarging the core size in the composition of the reactor core of the fast breeder reactor, another effect to provide the fuel assembly for the fast breeder reactor which is able to reduce further the pressure drop of coolant of the reactor core by enlarging of coolant flow area as much as equivalent to the reduction of wall thickness of the means of fuel containment are obtained. Especially in the embodiment of the present invention wherein the width of the means of fuel containment increases continuously from the middle of the axial length of the means to both ends of the means, in addition to an effect to provide a fuel assembly for the fast breeder reactor which is able to use distributed coolant for cooling of the reactor core without leakage and to reduce the pressure drop of coolant of the reactor core without enlarging the core size in the composition of the reactor core of the fast breeder reactor, another effect to provide the fuel assembly for the fast breeder reactor which is able to reduce the pressure drop of coolant of the reactor core even at the middle of the axial length of the means of fuel containment because of enlarging of the coolant flow area continuously from the middle of the means to both ends of the means of fuel containment are obtained. Especially in the embodiment of the present invention wherein the wall thickness of the means of fuel containment decreases continuously from the middle to both ends of the axial length of the means of fuel containment, in addition to an effect to provide a fuel assembly for the fast breeder reactor which is able to use distributed coolant for cooling of the reactor core without leakage and to reduce the pressure drop of coolant of the reactor core without enlarging the core size in composition of the reactor core of the fast breeder reactor, another effect to provide the fuel assembly for the fast breeder reactor which is able to reduce further the pressure drop of coolant of the reactor core even at the middle of the axial length of the means of fuel containment because of enlarging of the coolant flow area by reducing the extra wall thickness of the means of fuel containment are obtained. By using a fuel assembly manufactured in one of the embodiments of the present invention as an element of the reactor core of a fast breeder reactor and designing the fuel assembly so as to have different rate of enlarging of the coolant flow area in the means of fuel containment, an effect to regulate difference in thermal output between regions in the reactor core by regulating the distribution of coolant flow between fuel assemblies composing the reactor core with the shape of the fuel assembly itself without using any other members is obtained.