Patent Number: 051924960
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to a fuel assembly and an upper tie plate thereof, and specially to the fuel assembly and the upper tie plate thereof which are preferable for effective utilization of fissile material and achieving high burnup in boiling water reactors. Improvement of fuel economy is able to achieved by increasing the degree of burnup of the fuel. For increasing the degree of burnup of the fuel, enrichment of uranium 235 in the fuel pellet may be increased. But, increasing of the enrichment without increasing of the moderator to fuel atom number density ratio (H/U ratio) causes hardening of a neutron spectrum. Therefore, an finite multiplication factor of the fuel assembly does not become the maximum value at the enrichment. FIG. 1 illustrates change of the relation between the H/U ratio and the infinite multiplication factor depending on increasing of the enrichment. For obtaining large infinite multiplication factor with a constant enrichment, it is necessary to achieve the most proper H/U ratio corresponding to the enrichment. That is, when the enrichment is increased in order to improve the fuel economy, the most proper H/U ratio is increased, and accordingly it becomes necessary to increase the number of water rods or to increase a horizontal cross sectional area of the water rods. And, when the enrichment is increased, power peaking in radial direction of the fuel assembly is increased and linear power density of fuel rod becomes large, and consequently the fuel rod is exposed to a more severe condition. Further, distribution of voids in axial direction of the reactor core is small at the lower end portion of the reactor core, and is large from the middle to the upper end portion of the reactor core. Therefore, as burning of the fissile material at the upper region of the fuel assembly is retarded, the concentration of uranium 235 becomes higher relatively than that in the other portion. And by effect of the void, fissile plutonium is produced and built up at the upper region of the fuel assembly. According to the reason mentioned above, power peaking becomes high at the upper portion in axial direction of the fuel assembly. As the increasing of the enrichment relates also to the increasing of power peaking in the axial direction, linear power density of the fuel rod becomes large as well, and the fuel rod is exposed to a more severe condition. On the other hand, a flow rate spectral shift operation of a nuclear reactor is currently considered, in which the void fraction is changed greatly by operating the nuclear reactor with smaller flow rate (the flow rate of the coolant which flows through the reactor core) in the reactor core than the designed flow rate value at the beginning of operation cycle and with larger flow rate in the reactor core than the designed flow rate value at the end of the operation cycle, and fissile plutonium is built up and burnt effectively. In performing the flow rate spectral shift operation, as the power peaking in axial direction becomes large, the linear power density of the fuel rod becomes larger and the fuel rod is exposed to a more severe condition. Accordingly, in order to decrease the linear power density of the fuel rod and to be sure to maintain thermal margin, it is necessary to reduce the power load per fuel rod by increasing number of the fuel rods in the fuel assembly by such method as changing the configuration of a fuel rods lattice from 8 lines by 8 rows to 9 lines by 9 rows etc. In view of the two aspects described above, increasing of the number of fuel rods in the fuel assembly by changing the configuration of the fuel rods lattice and increasing of the H/U ratio by increasing of horizontal cross sectional area of the water rod or number of the water rods are a current trend in the fuel assembly for boiling water reactor. For instance, in U.S. Pat. No. 4,781,885, a fuel assembly having a fuel rods lattice of 9 lines by 9 rows is disclosed, in which a large square water rod is installed at the central region which is equivalent to the 9 fuel rods arranged in a square lattice of 3 lines by 3 rows. Further, in JP-A-1-196593 (1989), a fuel assembly having fuel rods in diamond lattice of which bearings to the internal wall of the channel box is 45.degree. is disclosed, in which a cruciform large water rod is installed at the central region which is equivalent to a region for 12 fuel rods. More plutonium is built up generally at the upper region of the fuel assembly as described above, especially in case of the flow rate spectral shift operation, much plutonium are built up. When the quantity of plutonium built up at the upper region of the fuel assembly is increased so much, it becomes difficult to maintain surely the margin of the reactor shut down at cold shut down. The difficulty is caused by increasing of the infinite multiplication factor with disappearance of voids at the upper portion of the reactor core at the cold shut down. In order to solve the problem, in JP-A-64-88292 (1989), a plurality of water rods are installed at least in symmetrical positions to the diagonal line of the fuel assembly and fuel rods having shorter length in axial direction than the other fuel rods (partial fuel rod) are installed at least at the position between the water rods. In the fuel assembly, the void fraction of coolant at the space above the partial fuel rods where the fuel is not located, namely vanishing rods, becomes zero at the cold shut down of the reactor. The portion of the vanishing rod acts as a large water rod with the other water rods at the cold shut down. Therefore, the portion of the vanishing rod has an excessive neutron moderating effect and reversely a large neutron absorbing effect at the cold shut down. As the result, the difference between the infinite multiplication factors during the operation of the reactor and during the cold shut down becomes small, and shut down margin of the reactor is increased. Further, in the case of installing of the partial fuel rods, an additional effect such as reducing of pressure loss at two phase flow portion in the fuel assembly under the reactor operation is brought. As described above, increasing of the H/U ratio by increasing of the number of fuel rods, and further, increasing of horizontal cross sectional area of the water rods or the number of the water rods are the current tendency. Under such trend of the current technical development, a trial is performed which is aimed at high burnup by increasing the enrichment further. Such increment of the enrichment aiming at the increasing of the discharge burnup necessitates further enlarging of the horizontal cross sectional area of the water rods in order to make the H/U ratio the most proper. Nevertheless, as the horizontal cross sectional area of the large water rod is enlarged according to U.S. Pat. No. 4,781,885 and JP-A-1-196593 (1989), the pressure loss of the reactor core is increased by narrowing of the area of the coolant flow path which is formed between the fuel rods. The increasing of the pressure loss of the reactor core is a problem mainly in following points. (1) When the pressure loss of the reactor core is increased, the capacity of the pump has to be increased in order to compensate the increment. If the maximum flow in the reactor core is achieved by the maximum rotation of the pump under the condition without the increment of the pressure loss, the pump is not able to achieve the maximum flow in the reactor core when the pressure loss is increased. (2) Stability is lowered by increasing of the pressure loss. That is, as the pressure loss of the two phase flow in the upper portion of the fuel assembly is larger than the pressure loss of the single phase flow portion, when entering flow to the fuel assembly is increased, the resistance at the two phase flow portion is increased in order to reduce the entering flow. When the entering flow is reduced, the resistance at the two phase flow portion is reduced and the entering flow is increased again. By repeating of the phenomena, vibration of the flow is caused in the fuel assembly, and the stability is lowered. The larger the pressure loss at the two phase flow portion is, the easier the vibration of the flow is caused. On the other hand, the fuel assembly which is described in JP-A-64-88292 (1989) has a problem in aspect of fuel economy because optimization of the H/U ratio is not considered on the fuel assembly. Further, in case of installing partial fuel rods, which has two advantages such as the reduction of pressure loss at two phase flow portion and the secureness of the reactor shut down margin into the fuel assembly for high burnup as described in JP-A-64-88292, fuel inventory is decreased. The reduction of the fuel inventory increases the number of reload fuel assemblies and causes problems such as increment of the number of generated spent fuel assemblies. One of the methods for solving the problem is enlarging the diameter of the fuel rod for keeping the same fuel inventory as before installing of the partial fuel rod. But the method causes another problem of increasing pressure loss which is accompanied with the reduction of flow path area for the coolant. SUMMARY OF THE INVENTION One of the objects of the present invention is to provide a fuel assembly which is able to optimize the H/U ratio without increasing of pressure loss of the reactor core for achieving high burn up, and an upper tie plate thereof. Another object of the present invention is to provide a fuel assembly which enables partial fuel rods to be installed without increasing of the pressure loss of the reactor core and reducing of the fuel inventory. The characteristic of the present invention to achieve the objects described above is in the fuel assembly having: a plurality of fuel rods which are arranged in a lattice, a plurality of first means of water rods each of which has larger first cross sectional area than the area of a unit lattice of the fuel rods which corresponds to the each of the fuel rods described above and is arranged adjacently each other, first coolant flow path which is formed in a portion where is generated as an excessively moderated region in a second cross section when second means of water rods having the second cross section of same area as whole area of interior region of the outermost peripheral of a group of whole unit lattices of the fuel rods, which are occupied substantially by a plurality of the first means of water rods is assumed and also the second means of the water rods is assumed to be arranged in the fuel assembly instead of a plurality of the first means of water rods, and third coolant flow path connecting a plurality of the first means of water rods which are arranged substantially in the interior region in surrounding the first coolant flow path to second coolant flow paths which surround the first coolant flow path and the fuel rods and is located among the first means of water rods. By the present invention, a fuel assembly having a plurality of fuel rods which are arranged in a square lattice and a means of water rods is provided. The fuel assembly is characterized in having a plurality of the fuel rods comprising a plurality of the first fuel rods which are arranged in square lattices of 10 lines by 10 rows except the central region where the fuel rods are able to be arranged in a lattice of 4 lines by 4 rows, four second fuel rods each of which is arranged at each of four corners of the central region respectively, and means of water rods comprising a plurality of the water rods having a large diameter which are arranged adjacently each other in a circle with intervals in the central region, wherein 12 fuel rods are able to be arranged, except four corner portions. And, by the present invention, a fuel assembly having a plurality of fuel rods which are arranged in a square lattice and a means of water rods is provided. The fuel assembly is characterized in having a plurality of the fuel rods comprising a plurality of the first fuel rods which are arranged in a square lattice of 10 lines by 10 rows except the central region where the fuel rods are able to be arranged in a lattice of 4 lines by 4 rows, four second fuel rods each of which has a shorter axial length than the first fuel rod and is arranged at each of four corner portion of the central region respectively, and the means of water rods comprising a plurality of the water rods having a large diameter which are arranged adjacently each other in a circle with intervals in the central region, wherein 12 fuel rods are able to be arranged, except four corner portions. Further, by the present invention, a fuel assembly having a plurality of fuel rods which are arranged in a square lattice and a means of water rods is provided. The fuel assembly is characterized in having a plurality of the fuel rods comprising a plurality of the fuel rods which are arranged in a square lattice of 10 lines by 10 rows except the central region where the fuel rods are able to be arranged in a lattice of 4 lines by 4 rows, four fuel rods each of which is arranged at each of four corner portions of the central region respectively, and the means of water rods comprising a plurality of spectral shift water rods of which internal liquid level are adjustable by control of the coolant flow in the reactor core, which are arranged adjacently each other in a circle with intervals in the central region wherein 12 fuel rods are able to be arranged except four corner portions. Further, by the present invention, a fuel assembly having a plurality of fuel rods which are arranged in a square lattice and a means of water rods is provided. The fuel assembly is characterized in having a plurality of the fuel rods comprising a plurality of the first fuel rods which are arranged in a square lattice of 10 lines by 10 rows except the central region where the fuel rods are able to be arranged in a lattice of 4 lines by 4 rows, and four second fuel rods each of which is arranged at each of four corner portions of the central region respectively, and installing the means of water rods in the central region wherein 12 fuel rods are able to be arranged except four corner portions in surrounding the center of the central region, and forming a coolant flow path which leads to the coolant flow paths which are formed around the fuel rods is formed at the center of the central region. By the present invention, a fuel assembly having a plurality of fuel rods which are arranged in a square lattice and a means of water rods is provided. The fuel assembly is characterized in arranging a plurality of the fuel rods in a square lattice of 9 lines by 9 rows except the central region where the fuel rods are able to be arranged in a lattice of 3 lines by 3 rows, and having the means of water rods comprising four water rods having a large diameter which are arranged adjacently each other in a circle with intervals in the central region. Further, by the present invention, a fuel assembly having a plurality of the fuel rods which are arranged in a square lattice of at least 9 lines by 9 rows and are able to achieve the average discharge burn up of at least 45 GWd/t, and a means of water rod which is arranged at the central region of the square lattice is provided. The fuel assembly is characterized in comprising the means of water rods which have enough cross sectional area of water rods to give sufficient H/U ratio to make the infinite multiplication factor almost be saturated under the core-average void fraction and are so installed as to surround the center of the central region, and forming a coolant flow path which leads to the outer region of the means of water rods at the center of the central region. In order to achieve the objects described above, an upper tie plate comprising a plurality of first bosses each of which has a hole portion wherein upper end of the fuel rods is inserted, a plurality of second bosses each of which has a hole portion wherein upper end of the means of water rod is inserted, and a plurality of ribs each of which connects the bosses described above each other, is provided by the present invention. The upper tie plate is characterized in that the second bosses are installed at the center portion and that second opening which is formed among the four adjacent second bosses is larger than the opening which is formed among the four adjacent first bosses. The inventors found that a fuel assembly having a plurality of fuel rods in a square lattice of 10 lines by 10 rows and a water rod which has a large cross sectional area (for instance, the water rod having such a horizontal cross section as to occupy a region wherein 12 fuel rods are able to be placed as disclosed in JP-A-1-196593 (1989)) has an infinite multiplication factor which is hardly changed even though the H/U ratio is altered around 4.5 by changing of the horizontal cross section of the water rod at the average void fraction of the reactor core in case of aiming at discharge burn up of 55-60 MWd/t and is saturated to the increasing of horizontal cross section of the water rod (refer to line AB in FIG. 2). The saturation of the infinite multiplication factor is revealed to be caused by formation of an excessively moderated region at center of the horizontal cross section of the water rod with increasing of area of the cross section. The present invention is performed based on the finding described above, and the horizontal cross section of the water rod (simply called water rod area hereinafter) is so reduced as to optimize the H/U ratio of the fuel assembly by making the excessively moderated region, which is not contributable to improvement of moderating effect, an exterior region of the water rod, and further by making the region a coolant flow path wherein vapor-liquid two phase flow which leads to coolant path of around the fuel rods. Moreover, the present invention is aimed at reducing pressure loss of the fuel assembly by making the excessively moderated region the coolant flow path as described above. That is, in the present invention, a plurality of the first means of water rods each of which has a larger first horizontal cross section than the area of the fuel unit lattice corresponding to each fuel rod are arranged adjacently each other. Here a second means of water rod which has the second horizontal cross section equivalent to the whole area of the interior region of the outermost periphery of a group of whole fuel unit lattices, which is substantially occupied by the first means of water rods is assumed. The second means of water rod has a large horizontal cross sectional area which generates the excessively moderated region as described above. Therefore, the first coolant flow path is formed at the portion where the excessively moderated region is generated, and a plurality of the first means of water rods are arranged around the first coolant flow path. And the third coolant flow path which connects the first coolant flow path and the second coolant flow path which surrounds the fuel rods is formed among the first means of water rods. For instance, in the fuel assembly having a plurality of fuel rods arranged in a square lattice of 10 lines by 10 rows, a plurality of (for instance, four) water rods having a large diameter are arranged adjacently in a circle with intervals between each other respectively in the central region wherein 12 fuel rods are able to be arranged in a lattice of 4 lines by 4 rows except each of our corner portions (called the central region of 4 lines by 4 rows hereinafter). In case of arranging a plurality of water rods having a large diameter, the sum of the whole horizontal cross sectional area becomes smaller than the case when a large cruciform water rods is arranged in the region of the same area. The reduction of the total horizontal cross sectional area of the water rods is the result of exclusion of excessively moderated region in the saturated region of the infinite multiplication factor to the change of the H/U ratio. But, almost same infinite multiplication factor as the large cruciform water rods is obtained by the arrangement of a plurality of the water rods having a large diameter. The water rod having a large diameter has wider horizontal cross section than the area of a fuel unit lattice (refer to numeral 31 in FIG. 3), and has a larger outer diameter than the arrangement pitch of the fuel rods. On the other hand, by replacing the large cruciform water rods with a plurality of water rods having a large diameter, coolant flow paths are formed in the region of water rods having a large diameter. As the water rods having a large diameter are arranged with intervals between each other, the coolant flow path leads to coolant flow paths around the fuel rods through the intervals. Therefore, such composition as described above reduces pressure loss in comparison with the case in which the large cruciform water rod is arranged. Moreover, voids are flowed easily into the coolant flow path which is surrounded with each of the water rods from the coolant flow path which is formed around the fuel rods because the coolant flow path which is surrounded with a plurality of water rods is far from the fuel rods, which are heaters, and is surrounded with a plurality of water rods having a wider horizontal cross section than the fuel unit lattice, which are not heaters. Accordingly, in the upper region of the fuel assembly where the void fraction is high, void is gathered much to the coolant flow path which is surrounded with water rods, and consequently pressure loss of the fuel assembly is reduced. By the reason described above, the present invention is able to achieve making the fuel high enriched and high burn up, and also is able to optimize the H/U ratio without increasing of the pressure loss. In one of the examples of the present invention, each of four second fuel rods having short axial length is arranged at each of the four corner portions of the central region of 4 lines by 4 rows. In the case, as the coolant flow path which is surrounded with a plurality of the water rods having a large diameter reduces the pressure loss as described above, the increment of the pressure loss by increasing of the diameter of fuel rods is able to be compensated. Therefore, the arrangement of the partial fuel rods maintains certainly the effect of reduction of pressure loss and increment of shut down margin without reducing of the quantity of loaded fuel, and concurrently fuel economy is able to be improved. When a plurality of the water rods having a large diameter are arranged adjacently each other in a circle, there is a possibility to cause fretting by flow vibration of each other of the water rods in the upper region of the fuel assembly where two phase current flows, but, by arrangement of the partial fuel rods, an operation area of a small camera for inspection of the fretting is provided in the upper region of the partial fuel rods 20 and inspection of the water rods can be performed easily during regular inspection. In other example of the present invention, by using spectral shift water rods as the water rods having a large diameter, the fuel economy is further improved by the flow rate spectral shift operation. Further, the inventors found the saturation phenomenon of the infinite multiplication factor by the arrangement of the water rods even in the fuel assembly having a plurality of fuel rods in a square lattice of 9 lines by 9 rows, and by arranging of the four water rods having a large diameter adjacently in a circle in the central region where the fuel rods are able to be arranged in 3 lines by 3 rows, the H/U ratio is optimized and the pressure loss is reduced similarly.