JP2009156628A - Nuclear fuel assembly - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fuel assembly which can improve reactor shutdown margins while curbing the degradation in reactivity over a late period of an operation cycle in a reactor core whose output is improved or which is geared to longer operation. <P>SOLUTION: Refueling nuclear fuel assemblies for a boiling water reactor where fuel rods and at least one water rod or water channel are bundled like a tetragonal lattice and the number of refueling assemblies to be replaced at once and to be loaded at the same time in refueling are more than 30% of that of assemblies loaded into the whole core have two or more kinds of fuel assemblies with different numbers of fuel rods (B) containing burnable poison in most domains of the central section except for blankets at the upper and lower ends in the axial direction. One or more of the fuel rods (B) in the fuel assemblies have low-concentration domains of the burnable poison in a domain (C) just under the blanket at the upper end in the axial direction and the average concentration of the burnable poison in the domain (C) of the fuel assemblies with the most number of the fuel rods (B) is set to exceed that of the burnable poison in the domain (C) of the fuel assemblies with the least number of the fuel rods (B). <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a nuclear fuel assembly, and in particular, in a nuclear reactor core whose power to be introduced in the future is improved or in a nuclear reactor core whose operation cycle is longer than that of a current nuclear reactor. The present invention relates to a fuel assembly capable of preventing a decrease in reactivity at the end of a cycle due to an increase in unburned poisonous substances and improving a furnace shutdown margin.

  Generally, a fuel assembly used in a nuclear reactor is composed of a plurality of fuel rods filled with a large number of fuel pellets such as uranium oxide in a fuel cladding tube.

  An example of this fuel assembly is shown in FIG. 15 (a) is a longitudinal sectional view of the fuel assembly, FIG. 15 (b) is a sectional view taken along the line BB in FIG. 15 (a), and FIG. 15 (c) is a CC arrow in FIG. 15 (a). A cross-sectional view is shown.

  In the fuel assembly 1, a plurality of long fuel rods 2, short fuel rods 3, and one or two water rods 4 are bundled in a square lattice pattern by a plurality of spacers 5 arranged in the axial direction. .

  Among the plurality of bundled fuel rods, the long fuel rod 2 is fixed by the upper tie plate 7 and the lower tie plate 8 together with the external spring 6. The short fuel rod 3 is fixed to the lower tie plate 8. This bundle of fuel rods is surrounded by a channel box 9.

  Note that the length and number of the long fuel rods 2 and short fuel rods 3 and the water rods 4 in the channel box 9 and the number and shape and arrangement thereof differ depending on the reactor. The number is not limited to the fuel assembly 1 shown in FIG.

  In nuclear reactors, leakage of neutrons is large in the vicinity of the core and in the vicinity of the upper and lower parts, and in these regions, combustion of fissile materials and flammable poisons is generally delayed as compared with the core center.

  In particular, in a boiling water reactor, voids are generated in the coolant flowing in the channel box, and the void ratio is larger at the upper part of the core and smaller at the lower part of the core, so that neutron leakage at the upper part of the core is large.

  For this reason, the power of the upper part of the core is lower than that of the core, and the burning of fissile material and flammable poisons is delayed.

  For the above problem, a region where the enrichment of the fissile material contained in the upper end portion or the lower end portion or both of the fuel rods is lower than the central region has been provided, and the region is a low-reactivity region. Has been proposed.

  According to this method, the leakage of neutrons from the upper and lower ends of the fuel rod can be reduced, and the fissile material can be burned efficiently.

  At the same time, the method can reduce the unburned residue of the fissile material at the upper and lower ends of the fuel rod, and improve the fuel economy.

  However, even if a low reactivity region is provided at the upper and lower ends of the fuel rods, especially in the region immediately below the upper end, the output is lower than that of the central region and combustion does not proceed, so fissile materials and flammable poisons. Tended to remain unburned.

  For this reason, in the conventional fuel rod, a new fission material enrichment region having a lower concentration than the fission product enrichment in the central region is provided between the upper end region and the central region, and the axial fission product concentration is reduced. It has also been proposed to optimize the concentration distribution.

Similarly, for the flammable poison, a region where the concentration of the flammable poison is lowered is provided between the upper end region and the central region of the fuel rod to optimize the axially flammable poison concentration.
Japanese Examined Patent Publication No. 3-78954 Japanese Patent Publication No. 7-101237 JP 7-234293 A JP-A-8-248162 Japanese Patent Laid-Open No. 11-2111870 Japanese Patent No. 2996677

  However, in the case of the above conventional fuel assemblies, when the output is improved in the boiling water reactor in the future, or when the operation cycle is longer than that of the current boiling water reactor, the end of the operation cycle Therefore, it is expected that the furnace shutdown margin will decrease, or that the flammable poison will remain unburned at the end of the operation cycle, which will worsen the economy.

  In general, all control rods are inserted into the reactor core when the reactor is shut down, resulting in a subcritical state. However, a design condition is that a subcritical state can be achieved even if one control rod is not inserted. The subcriticality when the control rod with the highest reactivity value is not inserted as an index is called the furnace shutdown margin.

  In recent years, there has been an increasing demand for improving the economic efficiency of nuclear power generation, and it is planned to implement measures such as increasing the output per plant and extending the operating cycle. For these measures, it is necessary to increase the enrichment of fissile material in the fuel rods.

  However, increasing the concentration of fissile material increases the difference in reactivity between reactor operation and shutdown, increasing the effective multiplication factor when the reactor is shut down, resulting in a decrease in reactor shutdown margin. .

  In addition, long-term cycles and measures to improve power output increase the number of replacements of fuel assemblies at one time and have to load new fuel to the outer periphery of the core. In the case of fuel, combustible poisons remain unburned in the region immediately below the upper end portion, which causes the economy to deteriorate.

  Therefore, the problem to be solved by the present invention is that the reactivity at the end of the cycle due to an increase in the remaining amount of flammable poisons in a reactor core that is expected to be introduced in the future or has been operated for a long time. An object of the present invention is to provide a fuel assembly that improves a furnace shutdown margin while suppressing a decrease.

A replacement fuel assembly for a boiling water reactor according to the present invention is:
Boiling water in which a plurality of fuel rods and at least one water rod or water channel are bundled in a square lattice pattern, and the number of replacement bodies that are exchanged at the same time during fuel exchange and loaded at the same time is greater than 30% of the number of all core bodies. In replacement nuclear fuel assemblies for nuclear reactors,
Two or more types of fuel assemblies having different numbers of fuel rods (B) containing a flammable poison in the central region excluding the blanket at the upper and lower ends in the axial direction, and the flammability of the fuel assemblies At least one of the fuel rods (B) containing poisons has a low concentration region of combustible poisons in the region (C) immediately below the axial upper end blanket, and the fuel assembly having the largest number of fuel rods (B). An average flammable poison concentration in the axial region (C) of the body is higher than an average flammable poison concentration in the axial region (C) of the fuel assembly having the smallest number of fuel rods (B). And

  The axial length of the axial region (C) may be 4/24 or less of the effective length filled with fuel pellets of fuel rods.

  Further, the average concentration of the flammable poison in the central portion excluding the blanket at the upper and lower ends in the axial direction of the fuel assembly having the largest number of fuel rods (B) has the smallest number of fuel rods (B). It can be made to be lower than the average concentration of the flammable poison in the central portion excluding the blanket at the upper and lower ends in the axial direction.

  The fuel rod (B) having the highest flammable poison concentration in the region (C) immediately below the axial upper end blanket is the lattice position of (2, 2) in the fuel assembly, its symmetrical position, the water rod or It can be arranged at a position in contact with the water channel.

  In general, boiling water reactors are loaded with two types of replacement fuel assemblies that differ in the number of fuel rods filled with fuel pellets containing combustible poisons (hereinafter referred to as fuel rods containing combustible poisons). Therefore, the invention has been devised so that the surplus reactivity characteristics and the reactor shutdown margin from the beginning to the middle of the operation cycle can be appropriately controlled and designed flexibly.

  However, when measures such as increased output per plant and longer operating cycles are implemented, the number of replacement nuclear fuel assemblies that are replaced and loaded at one time increases, and the outer periphery of the core In some cases, new fuel is loaded.

  In such a loading position, even in the end of the cycle, the amount of flammable poisons remaining in the region C immediately below the upper end of the fuel assembly increases.

  In the present invention, the flammability of the region C immediately below the upper end portion of the fuel assembly with many fuel rods containing flammable poisons loaded in the central portion of the core having a high output is generally obtained regardless of the concentration of the flammable poison in the axially central portion. By setting the fuel assembly with a low toxic concentration and a low fuel output containing flammable poisons loaded on the outer periphery of the core, which has a low output, without reducing the excess reactivity even at the end of the operation cycle, It is possible to effectively secure a furnace shutdown margin during cold temperatures.

  Further, according to the present invention in which the axial length of the axial region (C) is 4/24 or less of the effective length filled with fuel pellets of the fuel rods, It is possible to ensure a sufficient furnace shutdown time.

  In general, in boiling water reactors, voids are formed in the coolant in the core during power operation, but the void ratio is higher in the upper part, and therefore the relative reactivity effect due to the negative reactivity effect due to the formed voids. There is a tendency that the output of the lower part is larger than that of the upper part. In view of this, the design is such that the axial concentration and the flammable poison concentration distribution are optimized so as to flatten the axial output distribution during the output operation.

  However, this conversely causes the output distribution to increase in the upper part at the time of the cold temperature where the void disappears.

  In recent years, the axial power distribution has a lower peak from the beginning to the middle of the cycle to promote the generation of plutonium in the upper axial direction with a high void fraction, and the upper peak is reversed at the end of the cycle to efficiently burn the generated plutonium. In addition, since the so-called spectrum shift operation is performed to increase the reactivity by lowering the void ratio, the axial output distribution at the time of cold temperature tends to be an upper peak at the end of the cycle.

  In the present invention, the axial length of the axial region (C) in which the concentration of the flammable poison is increased is 4/24 or less of the effective length in which the fuel pellets of the fuel rods are filled. The flammable poison concentration acts effectively in a region where the upper peak is noticeable when the temperature is low, and it is possible to effectively secure a furnace shutdown margin when the temperature is low.

  Further, the average concentration of the flammable poison in the central portion excluding the blanket at the upper and lower ends in the axial direction of the fuel assembly having the largest number of fuel rods (B) has the smallest number of fuel rods (B). According to the present invention, which is lower than the average flammable poison concentration in the central portion excluding the blanket at the upper and lower ends in the axial direction, it is possible to effectively ensure a furnace stop margin during cold as follows.

  In general, the two types of replacement fuel assemblies with different numbers of fuel rods containing flammable poisons have almost the same concentration of the flammable poison contained in the central portion in the axial direction, or the operation cycle period has changed. Even in some cases, in order to control the excess reactivity appropriately throughout the cycle, the concentration of the flammable poison in the fuel assembly with a small number of fuel rods with flammable poisons is high, and conversely the flammability of the fuel assembly with many fuel rods with flammable poisons The toxic substance concentration is set thin.

  On the other hand, the present invention can flexibly control the surplus reactivity and the furnace shutdown margin even when the operation cycle period changes, while the combustion is promoted and the amount of unburned poisonous burnout at the end of the cycle is small. By increasing the concentration of the flammable poison in the region C immediately below the upper end portion of the fuel assembly having many flammable poison-loaded fuel rods, it is possible to effectively secure the furnace shutdown margin at the cold temperature.

  The fuel rod (B) having the highest flammable poison concentration in the region (C) immediately below the axial upper end blanket is the lattice position of (2, 2) in the fuel assembly, its symmetrical position, the water rod or According to the present invention arranged at a position in contact with the water channel, it is possible to effectively secure a furnace stop margin at the time of cold temperature at the end of the cycle.

  That is, generally, when the fuel rod containing the combustible poison is disposed at (2, 2) or a symmetrical position thereof or a position in contact with the water rod or the water channel, combustion of the combustible poison is promoted.

  In the present invention, the fuel rod having the highest concentration of the combustible poison is disposed at the above position so that the combustible poison remains appropriately even at the end of the cycle, thereby effectively shutting down the furnace at the cold temperature even at the end of the cycle. A margin can be secured.

  As described above, according to the present invention, in a plurality of replacement nuclear fuel assemblies that are simultaneously loaded into the boiling water nuclear reactor at the time of fuel replacement, the output that is expected to be introduced in the future is improved or long-term operation is performed. In the converted core, it is possible to improve the reactor shutdown margin while suppressing a decrease in reactivity at the end of the cycle due to an increase in unburned poisonous burnout.

  An embodiment of the present invention will be described with reference to the drawings.

(First embodiment)
FIG. 1 is a diagram showing a first embodiment of the present invention, and is an axial distribution diagram of fuel and combustible poisons in a fuel rod.

  In this embodiment, as shown in FIG. 1, two types of replacement nuclear fuel assemblies (a first fuel assembly and a second fuel assembly) that are simultaneously loaded into a boiling water reactor at the time of fuel replacement are provided. Have.

  The first fuel assembly has five types of fuel rods U1, U2, U3, G1, and G2, and the second fuel assembly has four types of fuel rods U1, U2, U3, and G2.

  The fuel rods G1 and G2 contain a flammable poison in the most area of the central portion except the blanket at the upper and lower ends in the axial direction, and a low concentration area of the flammable poison is provided in the area C immediately below the upper blanket in the axial direction. Have.

  The two types of replacement nuclear fuel assemblies (the first fuel assembly and the second fuel assembly) are combustible in the axial region C of the first fuel assembly having a large number of fuel rods containing combustible poisons. The average concentration of toxic poison (g3 x 4 + g4 x 10) ÷ 14 is higher than the average flammable poison concentration g4 in the axial region C of the second fuel assembly with a small number of fuel rods containing flammable poisons. It is characterized by.

  Further, in the present embodiment, the number of replacement assemblies of fuel assemblies that are replaced at the same time during fuel replacement and loaded simultaneously is greater than 30% of the total number of core loads.

  FIG. 2 shows an example of the loading pattern of the fuel assembly according to the present invention.

  The fuel assembly according to the present invention is loaded with a high Gd fuel assembly on the outer periphery of the core shown in the text of Japanese Patent Publication No. 3-78954 and FIG. 12 and a low Gd fuel assembly in the center of the core. For example, if the core uses a fuel assembly until the fourth cycle, the fuel assembly of the fourth cycle is taken out at the regular inspection, and the fuel assembly of the third cycle becomes the mass for the fourth cycle. This is different from the loading pattern of the concept of minimum shuffling which is transferred to the eyes and minimizes the number of fuel replacement bodies at the time of regular inspection so that other fuel assemblies do not move. In the loading of the fuel assembly according to the present invention, the high Gd fuel assembly is loaded in the center of the core, and the low Gd fuel assembly is loaded in the outer periphery of the core.

  In the loading pattern of the minimum shuffling, since the new fuel is generally loaded to the outer periphery of the core, the reactivity at the end of the cycle is lowered and the fuel economy tends to be lowered.

  On the other hand, the fuel assembly according to the present invention is loaded with a high Gd fuel assembly at the center of the core and increases the number of replacements of the fuel assembly in response to the longer operating cycle. In order to make the new fuel come to the central part of the core, as shown in FIG. 2, the fuel assemblies of the first cycle and the second cycle are loaded in the central part of the core in a checkered pattern. Of course, the loading pattern in the center of the core is not limited to the checkered pattern, and any mixed arrangement including the checkered loading pattern may be used. However, according to the loading pattern of the present invention, the number of fuel replacement bodies is the number of all core loads. Even when it exceeds 30%, the decrease in reactivity at the end of the cycle can be appropriately controlled.

  Hereinafter, in order to compare the effects of the conventional fuel assembly and the fuel assembly of the present invention, FIGS. 3 and 4 show design examples 1 and 2 of the conventional fuel assembly.

  The design examples 1 and 2 of the conventional fuel assemblies shown in FIGS. 3 and 4 include two types of replacement nuclear fuel assemblies (the first fuel assembly and the first fuel assembly) that are simultaneously loaded into the boiling water reactor during fuel replacement. A fuel rod containing a flammable poison in a central region excluding a blanket at the upper and lower ends in the axial direction, each of the first fuel assembly and the second fuel assembly. G1 and a fuel rod G2, and the fuel rods G1 and G2 have a low concentration region of a flammable poison in a region C immediately below the axial upper end blanket.

  The conventional design example 1 shown in FIG. 3 is characterized in that the concentrations of combustible poisons provided in the region C immediately below the axial upper end blanket of the first fuel assembly and the second fuel assembly are the same.

  In the conventional design example 2 shown in FIG. 4, the average concentration g5 of the flammable poison in the axial region C of the first fuel assembly having a large number of fuel rods containing the flammable poison is the same as that of the fuel rod containing the flammable poison. It is characterized by being lower than the average flammable poison concentration g4 in the axial direction region C of the second fuel assembly with a small number.

  FIGS. 5 and 6 show the surplus reactivity and the change in combustion of the reactor shutdown margin when the first embodiment according to the present invention and the conventional design examples 1 and 2 are loaded with the same core loading pattern and control rod pattern.

  FIG. 7 shows, for example, a core, the average value of the fuel assembly relative output at the end of the cycle for each core radial layer, and the core when the new fuel is loaded in a checkered pattern from the center of the core. The results of evaluating the ratio of new fuel to the number of cores loaded with new fuel for the first time in each radial layer are shown. Three examples are shown with the core size of 1.35 million kWe class, 1.1 million kWe class and 800,000 kWe class. The layers in the core radial direction refer to those layers when virtual layers are assumed every predetermined distance from the center when the core is viewed from above.

  The excess reactivity needs to be controlled to an appropriate amount throughout the operation period, but it is generally necessary to set it to be greater than zero, particularly at the end of the operation cycle.

  As can be seen from FIG. 5, the surplus reactivity of the first embodiment of the present invention is slightly smaller than the conventional design example 1 over the entire operation cycle, but has a surplus reactivity substantially equal to that of the conventional design example 2, At the end of the cycle, it is almost the same as the conventional design example 1, and there is a margin compared to the conventional design example 2.

  In the conventional design example 2, the concentration of the flammable poison in the region C immediately below the upper end portion of the second fuel assembly in which the number of fuel rods containing the flammable poison loaded on the outer periphery of the core having a low output is generally high. Therefore, at the end of the cycle, the amount of flammable poison that remains unburned increases and the excess reactivity decreases. That is, in the conventional design example 2, since the number of replacement bodies is larger than 30% of the total number of cores loaded, the average relative output shown in FIG. It is thought that the new fuel of the second fuel assembly was loaded, and the amount of unburned poisonous burnout increased and the surplus reactivity decreased.

  On the other hand, in the first embodiment of the present invention, the amount of flammable poisons brought into the core is larger than in the conventional design example 1, and the surplus reactivity decreases from the beginning to the middle of the cycle. By increasing the concentration of the flammable poison in the region C immediately below the upper end of the first fuel assembly having a large number of fuel rods containing the flammable poison loaded in the center of the high core, it is flammable at the end of the cycle. The increase in the amount of unburned poison is minimized, and the surplus reactivity is not reduced.

  FIG. 6 shows that the furnace shutdown margin near the end of the cycle according to the first embodiment of the present invention is greatly improved.

  In the first embodiment of the present invention, the flammable poison in the region C immediately below the upper end portion of the first fuel assembly having a large number of fuel rods containing the flammable poison generally loaded in the core portion having a high output. This is because by increasing the concentration, the reactivity can be controlled effectively even at the end of the operation cycle, so that a sufficient furnace shutdown margin can be secured.

(Second Embodiment)
FIG. 8 is a diagram showing a second embodiment of the present invention, and shows an axial distribution diagram of fuel and combustible poison in the fuel rod.

  The second embodiment of the present invention has two types of replacement nuclear fuel assemblies (a first fuel assembly and a second fuel assembly) that are simultaneously loaded into a boiling water reactor during fuel exchange. The first fuel assembly and the second fuel assembly each have fuel rods G1, G2 and a fuel rod G2 containing flammable poisons. The fuel rods G1 and G2 are characterized in that the length of the axial region C immediately below the axial upper end blanket is 4/24 or less of the effective length filled with fuel pellets.

  In recent years, from the beginning to the middle of the cycle, the axial output distribution is the lower peak, and the void ratio is increased to promote the production of plutonium in the upper axial direction, and at the end of the cycle, the upper peak is reversed, and the generated plutonium is efficiently burned. In addition, since a so-called spectrum shift operation is performed to increase the reactivity by lowering the void ratio, the tendency of the axial output distribution at the cold end to become an upper peak at the end of the cycle is remarkable.

  FIG. 9 shows an example of the cold core average axial power distribution at the end of the cycle in a core constituted by two fuel assemblies with different numbers of fuel rods with the upper end blanket being 1/24 of the effective length.

  In both examples, it can be seen that the power distribution at the end of the cycle shows a high power distribution, particularly in the region of 1/24 to 5/24 of the effective fuel length.

  Therefore, as in the second embodiment of the present invention, by increasing the concentration of the combustible poison in the region of 4/24 or less of the effective fuel length, it is possible to effectively secure the furnace shutdown margin at the cold temperature.

(Third embodiment)
FIG. 10 is a diagram showing a third embodiment of the present invention, and shows an axial distribution diagram of fuel and combustible poisons in a fuel rod.

  The third embodiment of the present invention has two types of replacement nuclear fuel assemblies (a first fuel assembly and a second fuel assembly) that are simultaneously loaded into a boiling water reactor during fuel exchange. The first fuel assembly has a fuel rod G1 containing a flammable poison, and the second fuel assembly has a fuel rod G2 containing a flammable poison.

  Fuel rods G1 and G2 are the average concentration of flammable poisons ((g2 × 10) + (g3 × 9) + in the center, excluding the upper and lower blankets in the axial direction of the fuel assembly with the largest number of fuel rods containing flammable poisons. (G4 × 3)) ÷ 22 is the average concentration of flammable poisons in the center ((g1 × 10) + (g2 × 9) excluding the blanket at the upper and lower ends in the axial direction of the fuel assembly with a small number of fuel rods containing flammable poisons ) + (G5 × 3)) ÷ 22.

  As shown in FIG. 10, G1, g3, and g4 of G1 of the first fuel assembly have 10, 9, and 3 nodes, respectively, and g1, g2 of G2 of the second fuel assembly. , g5 have 10, 9, 3 nodes, respectively, and the above equation is the average concentration of the flammable poisons of the fuel rods G1 and G2, which are weighted, respectively.

  In the third embodiment of the present invention, the average concentration of the flammable poison in the axial region C of the first fuel assembly having a large number of fuel rods containing the flammable poison is small. Excluding the blanket at the upper and lower ends in the axial direction of the fuel assembly that is higher than the average combustible poison concentration in the axial region C of the second fuel assembly and has the largest number of fuel rods containing the combustible poison as described above. The average flammable poison concentration in the center is lower than the average flammable poison concentration in the center except for the upper and lower blankets in the axial direction of the fuel assembly with a small number of fuel rods containing flammable poisons. In general, the two types of replacement fuel assemblies with different numbers of fuel rods containing flammable poisons have almost the same concentration of the flammable poison contained in the central portion in the axial direction, or the operation cycle period has changed. Even in some cases, in order to control the excess reactivity appropriately throughout the cycle, the concentration of the flammable poison in the fuel assembly with a small number of fuel rods with flammable poisons is high, and conversely the flammability of the fuel assembly with many fuel rods with flammable poisons The toxic substance concentration is set thin.

  That is, when the operation cycle period becomes long, the number of replacement bodies increases, so the ratio of new fuel in the core increases. At this time, in order to reduce the amount of change in the amount of flammable poisons brought into the entire core, a large proportion of fuel assemblies with a small number of fuel rods containing flammable poisons will be loaded, but the excess reactivity will become longer. It is common to increase the concentration of flammable poisons in order to properly control over the life of the operating cycle.

  11 and 12, the first embodiment and the third embodiment according to the first embodiment of the present invention and the combustion change of the surplus reactivity and the furnace stop margin when the core is loaded in the first embodiment and the third embodiment. An example of

  As can be seen from FIGS. 11 and 12, when the operation cycle period is long in the first embodiment, the surplus reactivity increases after the middle of the cycle, and the furnace shutdown margin is deteriorated. On the other hand, when the third embodiment is used, the flammable poison average in the central portion excluding the blanket at the upper and lower ends in the axial direction of the fuel assembly with a small number of fuel rods containing the flammable poison loaded in the long-term cycle operation. Since the concentration is set high, the excess reactivity can be controlled to be relatively small even after the middle of the cycle, and as a result, the furnace shutdown margin is improved.

  The present invention can flexibly control the excess reactivity and the furnace shutdown margin even when the operation cycle period changes, while the combustion is promoted and the combustible loaded at the end of the cycle where the amount of unburned poison is small is loaded. By increasing the concentration of the flammable poison in the region C immediately below the upper end portion of the fuel assembly with many fuel rods containing the poisonous poison, it is possible to effectively secure a furnace stop margin at the cold temperature.

(Fourth embodiment)
13A and 13B are views showing a fourth embodiment of the present invention, in which FIG. 13A is a cross-sectional view showing the arrangement of fuel rods in a fuel assembly, and FIG. 13B is a diagram showing fuel rods and combustible poisons. An axial direction distribution diagram is shown.

  The fourth embodiment of the present invention has two types of replacement nuclear fuel assemblies (a first fuel assembly and a second fuel assembly) that are simultaneously loaded into a boiling water reactor during fuel exchange. In the first fuel assembly, the fuel rod G1 having the highest flammable poison concentration in the region C immediately below the axial upper end blanket is disposed at (2, 2), its symmetrical position, and the position in contact with the water channel. In the fuel assembly, the fuel rod G2 having the highest concentration of the flammable poison in the region C in the fuel assembly is disposed at (2, 2), its symmetrical position, and the position in contact with the water channel. That is, the fuel assembly of the present embodiment is characterized in that the fuel rods having the highest flammable poison concentration in the region C are arranged at (2, 2), the symmetrical position thereof, and the position in contact with the water channel.

  In general, excluding the outermost periphery of the fuel assembly, the output of the fuel rods at the positions (2, 2) and their symmetrical positions and in contact with the water rod or the water channel tends to increase, and combustion is promoted. Combustible poisons burn quickly.

  FIG. 14A shows an example of the combustion change in the local output peaking coefficient when the fuel assembly shown in FIG. 11 is taken as an example and the enrichment contained in all the fuel rods is made uniform. Similarly, FIG. 14 (b) shows an example of the combustion change of the local output peaking coefficient taking the fuel assembly of FIG. 15 as an example.

  From this result, it can be seen that the output of the fuel rod at the position in contact with (2, 2) and the water rod or water channel is relatively high. The symmetrical position of (2, 2) is not shown in FIGS. 14 (a) and 14 (b), but it is known that the local output peaking coefficient is high as in (2, 2).

  In general, the local output peaking coefficient depends on the enrichment distribution, but when measures such as improving the output or extending the operation cycle are implemented, the enrichment of almost all fuel rods except the outer periphery of the fuel assembly It is conceivable that there is a need to increase to the limit of the upper limit of handling regulations. (2, 2) as shown in FIGS. 14 (a) and 14 (b) and the fuel rod at the position in contact with the water rod or water channel It can be said that the output of the fuel rod at the symmetrical position tends to be higher.

  In the present invention, by increasing the concentration of the flammable poison in the region C immediately below the upper end portion of the fuel rod at the position where combustion is promoted and the amount of unburnable flammable poison at the end of the cycle is reduced, it is effective even at the end of the cycle. It is possible to secure a furnace shutdown margin when the temperature is cold.

The axial distribution map of the fuel of the fuel rod which comprises the fuel assembly of 1st Embodiment which concerns on this invention, and a combustible poison. The figure which shows the loading pattern of the fuel rod of the fuel assembly of 1st Embodiment which concerns on this invention. The axial distribution map of the fuel and the burnable poison of the fuel rod of the fuel assembly according to an example of the prior art (conventional design example 1). The axial distribution map of the fuel rod fuel and combustible poison in the fuel assembly according to an example of the prior art (conventional design example 2). The graph which showed the combustion change of the excess reactivity of 1st Embodiment of this invention and the conventional design example. The graph which showed the combustion change of the furnace stop margin of 1st Embodiment of this invention and the conventional design example. The graph which showed the relationship between a fuel assembly relative output and a new fuel ratio. The axial distribution map of the fuel of the fuel rod and combustible poison which constitute the fuel assembly of the second embodiment according to the present invention. The graph which shows the example of the power distribution of the core average axial direction at the time of the cold end of the cycle in the core comprised by the two fuel assemblies from which the number of fuel rods differs. An axial distribution diagram of fuel and combustible poisons of fuel rods constituting the fuel assembly of the third embodiment according to the present invention. The graph which showed the combustion change of the excess reactivity of 1st Embodiment and 1st Embodiment at the time of long-term cycle driving | operation of this invention, and 3rd Embodiment. The graph which showed the combustion change of the furnace stop margin of 1st Embodiment and 3rd Embodiment at the time of 1st Embodiment and long-term cycle driving | operation of this invention. The axial direction distribution map of the fuel of the fuel rod which comprises the fuel assembly of 4th Embodiment which concerns on this invention, and a combustible poison. The graph which shows the example of the combustion change of a local output peaking coefficient at the time of making the enrichment contained in all the fuel rods which made the fuel assembly of FIG. 11 an example uniform. The graph which shows the example of the combustion change of a local output peaking coefficient at the time of making the enrichment contained in all the fuel rods which made the fuel assembly of FIG. 15 an example uniform. The longitudinal cross-sectional view (a) of a fuel assembly, the BB arrow directional cross-sectional view (b) in a vertical cross-sectional view (a), and CC cross-sectional view (c) in a vertical cross-sectional view (a).

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Fuel assembly 2 Long fuel rod 3 Short fuel rod 4 Water rod 5 Spacer 6 External spring 7 Upper tie plate 8 Lower tie plate 9 Channel box

Claims (4)

Boiling water in which a plurality of fuel rods and at least one water rod or water channel are bundled in a square lattice pattern, and the number of replacement bodies that are exchanged at the same time and are loaded at the same time when the fuel is changed is greater than 30% of the number of all core bodies. In replacement nuclear fuel assemblies for nuclear reactors,
Two or more types of fuel assemblies having different numbers of fuel rods (B) containing flammable poisons in most of the central region excluding the blanket at the upper and lower ends in the axial direction, and the flammability of the fuel assemblies At least one of the fuel rods (B) containing the poison has a low concentration region of the combustible poison in the region (C) immediately below the axial upper end blanket, and the fuel assembly having the largest number of the fuel rods (B). The average combustible poison concentration in the axial region (C) of the body is higher than the average combustible poison concentration in the axial region (C) of the fuel assembly having the least number of fuel rods (B). A replacement fuel assembly for a boiling water reactor.   2. The fuel assembly according to claim 1, wherein the axial length of the axial region (C) is not more than 4/24 of an effective length filled with fuel pellets of fuel rods.   The fuel assembly with the largest number of fuel rods (B) has a combustible poison average concentration in the central portion excluding the blanket at the upper and lower ends in the axial direction of the fuel assembly. The fuel assembly according to claim 1 or 2, wherein the fuel assembly is lower than the average concentration of the flammable poison in the central portion excluding the upper and lower end blankets.   The fuel rod (B) having the highest flammable poison concentration in the region (C) immediately below the upper end blanket in the axial direction is the lattice position of (2, 2) in the fuel assembly or its symmetrical position, the water rod or The fuel assembly according to any one of claims 1 to 3, wherein the fuel assembly is disposed at a position in contact with the water channel.

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