Patent Number: 051868903
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to a nuclear fuel assembly for a fast breeder reactor, a reactor core wherein the nuclear fuel assembly is used as one of components, and a regulating method of coolant distribution of the reactor core, especially, to the core which is preferable to reduce a pressure drop of coolant of the reactor core of the fast breeder reactor and to improve thermal characteristics of the reactor core. As illustrated in FIG. 3, a nuclear fuel assembly (hereinafter called a fuel assembly) for a fast breeder reactor of the prior art was generally of a type of a fuel assembly which was composed of a wrapper tube 2 wherein a plurality of nuclear fuel rods 1 were arranged in triangle lattices with wire spacers 5 or grid spacers between an upper shield 6 and a lower shield 7. The fuel rod was composed of a cladding tube 4 with a small diameter containing a stack of nuclear fuel pellets 3. The wrapper tube 2 has a uniform inner width in all through the axial direction (vertical direction) of the fuel assembly. The reactor core of the fast breeder reactor was composed of a plurality of the fuel assemblies which were standing together vertically in the core region. The nuclear fuel pellets 3 can be divided into a blanket fuel pellet for breeding and a driver fuel pellet for driving the reactor. The blanket fuel pellets are inserted into each of an upper and a lower blanket region of the cladding tube 4, and the driving fuel pellets are inserted into a core region of the cladding tube 4. A gas plenum portion is formed at lower portion lower than the lower blanket portion of the cladding tube 4. A fuel assembly having a wrapper tube free structure wherein a structural material other than a part of or all of the wrapper tube 2 is deleted is also known. A reactor core is composed of a plurality of fuel assemblies which are standing together vertically in coolant. The fuel assembly comprises a plurality of nuclear fuel rods and a wrapper tube as a means of fuel containment. As for a reactor other than a fast breeder reactor, a technique for a boiling water reactor was disclosed in the JP-A-1-98994 (1989) and the JP-A-59-180389 (1984). The technique described above is close to the present invention in a point of an altered inner width of a nuclear fuel rod container (which is called a channel box in the technical field of the boiling water reactor). In the technique disclosed in the JP-A-1-98994 (1989), an inner width of the channel box is enlarged at a down-stream side of coolant while an outer width is maintained uniform. In the technique disclosed in the JP-A-59-180389 (1984), both of an inner and outer width of a channel box is enlarged continuously from an up-stream side of the coolant to a down-stream side of coolant in a reverse tapered shape. As general characteristics of a type of fuel assembly which had a wrapper tube 2 like a fuel assembly for a fast breeder reactor, the fuel assembly had to have a structure to wrap around a plurality of nuclear fuel rods 1 having a small diameter with a wrapper tube 2 and to settle a small gap between the fuel rods in order to make a reactor core compact and to improve a nuclear characteristics of the reactor, hence a large powered circulating pump was used for coolant circulation because of a large pressure drop of coolant of coolant flowing through a fuel bundle portion, wherein a plurality of nuclear fuel rods 1 are bundled, of the wrapper tube 2. And the wrapper tube 2 had to have a thick wall to sustain the fuel assembly structurally and to depress expansion in horizontal direction caused by neutron radiation and pressure difference between the inner side and the outer side of the wrapper tube 2 in operation. Further, the wrapper tube had a structure to maintain a gap properly between each next wrapper tube 2 even though the fuel assembly expanded in horizontal direction as described above in composing the reactor core by standing a plurality of fuel assembly together vertically in the core region of the reactor. One of the distinguished features of a fast breeder reactor from other type reactors is a phenomenon to cause a dislocation of chemical elements in materials of the wrapper tube and expansion at a portion of the wrapper tube (a middle portion of the wrapper tube) which is faced to a reactor fuel region by the neutron irradiation as burning of the nuclear fuel proceeds. Once the expansion is caused, as the wrapper tube has a closed shape in a horizontal section, both of the inner and the outer width of the wrapper tube are enlarged and a situation wherein a wrapper tube contacts with adjacent wrapper tubes is easily caused. The phenomenon of expansion is called a swelling. The swelling is more distinguished in the fast breeder reactor than in other reactors, and the swelling is different from a channel creep caused by a stress which is generated by a pressure difference between an inner and an outer side of a wrapper tube in other type reactors. In the fast breeder reactor, the middle portion of the wrapper tube expands in a horizontal direction by both of the stress generated by the pressure difference and the swelling. Therefore, although the gap between each of wrapper tubes standing together vertically in the reactor core might be settled closer at regions near both ends of the wrapper tube, a large gap was settled actually in consideration of the expansion in horizontal direction at the middle portion of the wrapper tube in the fast breeder reactor. The gap between each of next wrapper tubes 2 had been maintained with a pad 8 having a larger width than the outer width of the wrapper tube 2 at the portions of the upper and the lower shield 6, 7, locations. The setting of the wrapper tube 2 and the gap between each of wrapper tubes 2 resulted in a reduction of a flow area in the fuel assembly, and had been a cause to increase pressure drop of coolant of the reactor core. If pressure drop of coolant at the portion of fuel rod bundle in the fuel assembly can be reduced, a large reduction of pressure drop of coolant of reactor core as a whole becomes possible. On the other hand, a fuel assembly of wrapper tube free type has less pressure drop of coolant as much as an amount of pressure drop of coolant caused by wrapper tubes, but leakage and unbalancing flow of coolant among fuel assemblies are concerned because wrapper tubes are not existing in the fuel assembly even though distribution of coolant to each fuel assemblies composing of the reactor core is regulated in corresponding to output power of the each fuel assemblies, and an investigation on the composition of the fuel assembly is necessary to resolve the concerning. In a case wherein a technique disclosed in the JP-A-1-98994 (1989) is applied to a wrapper tube for a fast breeder reactor, a plurality of wrapper tubes have to be standing together vertically in a core region with the same large gap as a conventional reactor core in consideration of swelling etc. as the wrapper tube has a uniform outer width all through the total length from the top to the bottom without any exception at middle, upper and lower portion of the wrapper tube. Therefore, a coolant flow area in the wrapper tube has to be reduced reversely as much as a plurality of wrapper tubes occupies a wider area for standing in a limited core region with a large gap between each of wrapper tubes, and consequently, an increasing of pressure drop of coolant of the reactor core is easily induced. In a case wherein a technique disclosed in the JP-A-59-180389 (1984) is applied to a wrapper tube for a fast breeder reactor, a lower end of the wrapper tube is reduced in width, and the reduced portion of the lower end of the wrapper tube is fixed to a member composing an inlet of coolant (equal to the lower shield in the fast breeder reactor), and the width of the reduced portion will not be enlarged even though burning of the fuel is proceeded. Therefore, the reduced portion in width is maintained all through the burning period of the nuclear fuel from the beginning to the end of the period, and will be easily a cause to increase the pressure drop of coolant of the reactor core. Further, a technique to enlarge a width of a middle portion of a wrapper tube by placing a reverse tapered tube is equal to abandon a margin to accommodate expansion in the horizontal direction by swelling etc., which will be caused in the middle of burning period, from the beginning of the burning of the nuclear fuel as much as an amount of enlarged width, and a gap between each of the wrapper tubes has to be enlarged in order to avoid contact with adjacent fuel assemblies even though the expansion in the horizontal direction is occurred. Accordingly, a large area for the reactor core becomes necessary, and the technique is not suitable for the reactor core having a limited area. Therefore, in the technical field of the fast breeder reactor, the development of a fuel assembly which enable to distribute flow of coolant exactly and to decrease pressure drop of coolant in a reactor core having a small area as possible has been desired. SUMMARY OF THE INVENTION The first object of the present invention is to provide a low pressure drop of coolant type fuel assembly which enables to distribute flow of coolant of a fast breeder reactor exactly and to decrease pressure drop of coolant of a reactor core without enlarging the area of the reactor core as possible; the second object of the present invention is to provide a reactor core of the fast breeder reactor which enables to distribute flow of coolant exactly and to decrease the pressure drop of coolant of the reactor core without enlarging the area of the reactor core as possible; the third object of the present invention is to provide a means of fuel containment of the fast breeder reactor for reduction of the pressure drop of coolant of the fuel assembly; and the fourth object of the present invention is to provide a method to regulate the distribution of coolant in the fast breeder reactor which is effective to reduce difference of thermal output power among each of reactor core regions by using the fuel assembly described above. To achieve the first object described above, there is provided a first feature of is a fuel assembly for a fast breeder reactor comprising a plurality of nuclear fuel rods which are arranged in a means of fuel containment characterized that the means of fuel containment is a fuel assembly having a shape which is narrow in both inner and outer width at a middle portion of the means, and on the contrary, relatively wide in both inner and outer widths at closer portions to each of both ends of the means of fuel containment. In the present invention, the middle portion of the means which is faced to the middle portion of the nuclear fuel rods wherein temperature is high and irradiation dose is large shows a tendency to have a larger expansion coefficient in the horizontal direction than the closer portions to each of both ends of the means in reactor operation, and in considering of relatively small expansion coefficient at both ends portion, a larger width than a conventional width can be applicable to the both ends portion for reducing flow resistance of coolant through the means of fuel containment in reactor operation, and as the width of the middle portion of the means will be expanded and enlarged during use in reactor operation, an effect to reduce flow resistance along whole length of the means of fuel containment is obtained, and moreover the exact distribution of coolant is maintained as same as the conventional means because that the coolant flows through the means of fuel containment does not disperse to the externals. A second feature to achieve the first object described above is a fuel assembly for a fast breeder reactor comprising a plurality of nuclear fuel rods which are contained in a means of fuel containment characterized that the means of fuel containment is a fuel assembly of which shape is narrow in width between both of each inner faces and outer faces at a portion of the means which has larger swelling than another portion of the means, and by the second feature, a middle portion of the means of fuel containment where swelling is easily caused has a narrower width than another portion for accommodating the expansion by swelling and for preventing a mutual interference between fuel assemblies which are used in standing together in the reactor core after the expansion, on the other hand, other portion of the means where the swelling is relatively difficult to be caused has a wider width than a conventional means by utilizing a space for preventing the fuel assembly from a mutual interference with adjacent fuel assemblies caused by the expansion with swelling of the middle portion of the means, and consequently the means of fuel containment has a reduced flow resistance for coolant which flow through the means of fuel containment. A third feature to achieve the first object described above is a fuel assembly for a fast breeder reactor comprising a plurality of nuclear fuel rods which are contained in a means of fuel containment characterized that the means of fuel containment is a fuel assembly of which shape is narrower in width between both of each inner faces and outer faces at least at a portion of the means facing to a reactor core fuel portion of the nuclear fuel rod with respect to another portion of the means not facing to the reactor core fuel portion of the nuclear fuel rod, and by the third feature, temperature at the reactor core fuel portion of the nuclear fuel rod will be raised to the highest in use of the fuel assemblies standing together in the reactor, and the portion of the means facing to the reactor core fuel portion of the nuclear fuel rod expands wider than another portion of the means, but even though the expansion is caused, the portion of the means where the expansion is caused has the same narrow width as a conventional means for accommodating the expansion and will not cause any interference with adjacent fuel assemblies, and the another portion of the means has previously wider width than a conventional means by utilizing a space for preventing the fuel assembly from a mutual interference with adjacent fuel assemblies caused by the expansion, and consequently the means of fuel containment has a reduced flow resistance for coolant which flow through the means of fuel containment. A fourth feature to achieve the second object described above is a reactor core of a fast breeder reactor comprising a plurality of nuclear fuel assemblies standing together vertically in a reactor vessel which contains coolant characterized that at least one of the fuel assemblies is a fuel assembly for the fast breeder reactor relating at least to one of the features described above, as the flow resistance of coolant which flow through the fuel assembly is reduced, the reactor core itself of the nuclear reactor which comprises the fuel assembly as an element of the invention has an effect to reduce a pressure drop of the coolant flowing through the reactor core, and a distribution balance of coolant is still maintained because the distributed coolant flow essentially in a space surrounded with the means of fuel containment. A fifth feature to achieve the second object described above is the reactor core of the fast breeder reactor relating to the fourth feature is characterized in being loaded with the fuel assembly which is related to at least one of the features described above to a region of the reactor core wherein a relatively large output power is generated, and in addition to the effect brought by the fourth feature, as the fuel assembly having the reduced pressure drop of coolant is loaded into the region of the reactor core wherein the relatively large output power is generated, an effect that cooling by coolant is intensified and keeping thermal balance with surrounded thermal output power becomes easy is obtained. A sixth feature to achieve the third object described a means of fuel containment for a fast breeder reactor characterized in having a shape that the width of the middle portion of the means is narrow in both of inner and outer widths, and on the contrary, portions closer to each of both ends have a wider width than the middle portion in both of inner and outer widths, and the means of fuel containment has an enlarged width which enables to have a large flow area for reducing pressure drop of coolant at a portion which has a relatively smaller thermal expansion coefficient than the middle portion of the means in reactor operation, and the middle portion of the means of fuel containment gradually expands the width during the reactor operation, but as the expansion is expected previously and the width at the middle portion of the means is not enlarged, any interference between adjacent fuel assemblies would not be occurred in reactor operation, and moreover, owing to the enlargement of the flow area of the coolant with the thermal expansion of the width of the middle portion of the means of fuel containment during the reactor operation, the pressure drop of coolant owing to the means is reduced and hence an effect to reduce the pressure drop of coolant of the fuel assembly is obtained by using the means of fuel containment relating to the present invention. A seventh feature to achieve the first object described above is the fuel assembly for the fast breeder reactor is characterized in having a thinner wall thickness at a portion of the means of fuel containment which faces to the gas plenum portion of the nuclear fuel rod than the wall thickness of the middle portion of the means whereof a width between both of the inner and the outer faces respectively are narrower than the width of another portion, and as the gas plenum portion has a lower temperature and less irradiation dose than the portion where the nuclear fuel exists, the wall thickness at the portion of the means of fuel containment faced to the gas plenum portion of the nuclear fuel rod could be less than the wall thickness of other portion, and the wall thickness at the portion faced to the gas plenum portion can be reduced and an effect to reduce pressure drop of coolant of coolant by enlarging the flow area of coolant in the means of fuel containment as much as equivalent to the reduction in the wall thickness is obtained. An eighth feature to achieve the first object described above is the fuel assembly for the fast breeder reactor is characterized in having a shape that the width of the means of fuel containment is enlarged wider continuously from the middle portion of the means as closing toward the both ends of the means of fuel containment, and as the thermal expansion coefficient becomes continuously smaller as closing toward the both ends of the means even in middle portion of the means, the width of the means of fuel containment can be enlarged as much as equivalent to the decreasing of the thermal expansion coefficient and an effect to reduce pressure drop of coolant of coolant flowing through the means of fuel containment is obtained in addition to the effect of the first feature. A ninth feature to achieve the first object described above is the fuel assembly for the fast breeder reactor is characterized in having a shape that a wall thickness of the means of fuel containment becomes thinner as closing toward the both ends of the means in middle portion of the means, and in as the wall thickness of the means is reduced continuously according to the thermal expansion coefficient of the portion is decreased, the flow area of coolant in the means can be enlarged and an effect to reduce pressure drop of coolant of coolant is obtained in addition to the effect of the first feature. An tenth feature to achieve the fourth object is a method to regulate flow distribution of coolant in a reactor core of a fast breeder reactor which comprises composing the reactor core with a plurality of fuel assemblies standing together vertically in a reactor vessel which contains coolant, and making a difference of thermal output of each reactor core region preferable by distributing flow of the coolant to each of the fuel assemblies in the reactor depending on the difference of thermal output of the each reactor core region, characterized in comprising the steps of enlarging the flow area of coolant of the means of fuel containment at both portions close to both ends of the means, reducing the flow area of coolant of the means of fuel containment relatively small at middle portion of the means facing to the portion of reactor core fuel portion of a nuclear fuel rod, and regulating the flow distribution of coolant by changing the enlarging rate of the flow area of coolant, and, as a value of pressure drop of coolant which flow through the means of fuel containment can be fixed by the enlarging rate of the flow area of coolant of the means of fuel containment, a flow rate of the coolant is fixed depending on the enlarging rate, and consequently, the flow distribution of coolant can be regulated. In utilizing the method to regulate the flow distribution as described above, the difference of thermal output of the each reactor core region can be fixed preferably.