JP2005098924A - Mox fuel assembly - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce a manufacturing cost per one body of a fuel assembly, while reducing local output peaking. <P>SOLUTION: Water rods 14 are arranged in a substantially central part area of a square lattice-like array of n-number of lines and n-number of columns, each fuel rod 12 is constituted of a fuel rod having a usual fuel effective length and a fuel rod having a short length, the uranium fuel rods 12 (fuel numbers 1-4) are arranged in the outermost peripheral layer of the square lattice-like array, the uranium fuel rods 12 (fuel numbers 1, G1) are arranged in four corner positions in the second layer out of areas including the second layer counted from the outermost peripheral layer of the square lattice-like array, and located in an inner side thereof, and the plurality of uranium fuel rods 12 (fuel numbers G1) with a flammable poison, and a plurality of MOX fuel rods 12 (fuel number P1) same each other in plutonium enrichment degrees are arranged in the residual positions. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a fuel assembly loaded in a boiling water reactor (hereinafter referred to as BWR as appropriate), and more particularly to a MOX fuel assembly having MOX fuel rods mixed with plutonium oxide.

(1) Increasing the number of flammable poison fuel rods and the amount of poisons due to a decrease in neutron absorption effect As a nuclear fuel recycle for nuclear power plants, uranium / plutonium mixed oxide fuel in which plutonium extracted by reprocessing is mixed with uranium (hereinafter referred to as appropriate) , MOX fuel). In recent years, there is a need for higher burnup of MOX fuel in order to improve fuel economy.

  In the MOX fuel assembly, the thermal neutron absorption of plutonium 239 and plutonium 241 which are fission nuclides is larger than that of uranium 235, and the neutron absorption of plutonium 240 is larger than that of uranium 238. The neutron absorption effect of combustible poisons such as gadolinia is lower than the body. In addition, in order to increase the burnup of the fuel, it is necessary to increase the reactivity of the fuel. Therefore, by increasing the plutonium enrichment of the MOX fuel, the hardening of the neutron energy spectrum and the effect of neutron absorption can be improved. The decline increases.

  The reactivity control in BWR uses a reactivity suppression effect by mixing a flammable poison such as gadolinia into the fuel rod in addition to the reactivity suppression effect by the control rod. Therefore, as the uranium enrichment or plutonium enrichment of the fuel increases with the increase in burnup, the reactivity of the fuel increases, so the number of fuel rods that contain flammable poisons and the combustibility to suppress the reactivity Although the amount of the toxic poison tends to increase, in MOX fuel, this tendency is promoted by the effect of reducing the neutron absorption effect of the flammable poison due to the hardening of the neutron spectrum.

(2) Addition of flammable poisons to uranium fuel rods Generally, increasing the number of fuel rods containing flammable poisons will reduce the infinite multiplication factor at the beginning of combustion. Further, if the concentration of the flammable poison to be mixed is increased, the time when gadolinia burns out is delayed, and as a result, the maximum value of the infinite multiplication factor can be suppressed. By using these effects, it is possible to appropriately control the surplus reactivity and the axial power distribution of the core by the combination of the number of fuel rods mixed with the flammable poison and the concentration of the mixed fuel rods. However, in the case of the MOX fuel assembly, mixing a flammable poison such as gadolinia into the MOX fuel rod is not practical because it makes the molding of the fuel complicated and causes a high cost. For this reason, it is usually considered that a flammable poison such as gadolinia is mixed only in the uranium fuel rod (see, for example, Patent Document 1).

  In this prior art, in particular, the value of the flammable poison is determined by placing the uranium fuel rod containing the flammable poison at the four corner positions on the outermost periphery of the fuel assembly having the softest neutron spectrum adjacent to the large water gap. Is increasing. The MOX fuel rods are arranged at all the lattice positions other than the above four corner positions.

  At this time, in this prior art, as the MOX fuel rods, a high enrichment fuel rod with a relatively large plutonium enrichment, a relatively small low enrichment fuel rod, and an intermediate enrichment fuel rod in the middle of them. There are different types. In order to reduce the local power peaking in the radial direction in the fuel assembly, the low enriched fuel is provided in the outermost peripheral layer of the fuel assembly that is adjacent to the water gap and has a relatively soft neutron spectrum and tends to have a large local output. A rod and a medium enrichment fuel rod are arranged, and a low enrichment fuel rod is placed at a position adjacent to the uranium fuel rod containing a flammable poison at the above four corner positions where local power tends to increase. Has medium enrichment fuel rods. Also, from the same point of view, medium enrichment fuel rods are arranged at the four corners of the second layer in the region including the second layer from the outermost layer and inside it, and the enrichment is performed at other positions. Fuel rods are arranged.

JP 59-46587 A (FIG. 7)

  However, the following problems exist in the above-described conventional technology.

  In other words, MOX fuel is used to reprocess spent fuel that was not considered to be reused in the past, extract plutonium, and mix and use it effectively. The molding process becomes more complicated than fuel, and the manufacturing cost tends to increase. Therefore, in order to further utilize and promote the MOX fuel designed from the viewpoint of effective use of nuclear fuel materials, it is necessary to eliminate any additional factors that increase the production cost as much as possible in terms of energy policies in Japan, which has limited resources. Is also very important.

  In order to reduce the manufacturing cost of the MOX fuel, it is effective to increase the average plutonium enrichment of the MOX fuel rod as much as possible. This is because by loading as much plutonium as possible into one MOX fuel rod, the number of MOX fuel rods for loading a whole predetermined amount of plutonium can be reduced, and as a result, manufacturing per MOX fuel assembly is integrated. This is because the cost can be reduced.

  In the above prior art, consideration from the viewpoint of cost reduction by increasing the average enrichment is not sufficient, and three kinds of enrichment MOX fuel rods, that is, in order to reduce local output peaking, that is, As described above, the low enrichment fuel rod, the medium enrichment fuel rod, and the high enrichment fuel rod are provided. Thus, in addition to high enrichment fuel rods, there are three enrichment fuel rods, medium enrichment fuel rods and low enrichment fuel rods, resulting in an increase in the average enrichment of all MOX fuel rods. Therefore, it is difficult to sufficiently reduce the manufacturing cost per unit MOX fuel assembly.

  The objective of this invention is providing the MOX fuel assembly which can reduce the manufacturing cost per fuel assembly integral, reducing local output peaking.

  (1) In order to achieve the above object, according to the present invention, a plurality of MOX fuel rods containing a uranium / plutonium mixed oxide fuel and a plurality of uranium fuel rods containing a uranium oxide fuel are arranged in a square lattice of n rows and n columns. In the MOX fuel assembly for boiling water reactors arranged in the form of water, a water rod or a water channel is arranged in a substantially central region of the square lattice arrangement, and the fuel rod is a normal fuel having a normal active fuel length. Rods and short fuel rods having an effective fuel length shorter than usual, and the uranium fuel rods are arranged in the outermost circumferential layer of the square lattice arrangement, and two layers from the outermost circumferential layer of the square lattice arrangement (2, 2), (2, n-1), (n-1, 2), (n-1, n-) which are the four corner positions of the second layer in the region including the inside of the eye. In 1), the uranium fuel rods are arranged, and the normal fuel rods in the remaining positions are as follows: A plurality of the uranium fuel rods containing a flammable poison and a plurality of the MOX fuel rods having the same plutonium enrichment are arranged.

  In the fuel assemblies arranged in a square lattice shape, the outermost peripheral layer faces the water gap portion, so that the output peak is most likely to occur. For this reason, in the MOX fuel assembly, it is necessary to reduce the plutonium enrichment of the MOX fuel rods arranged at the outermost periphery of the square lattice array. Here, in order to increase the average plutonium enrichment of the MOX fuel rods, it is effective to replace the MOX fuel rods with low enrichment with uranium fuel rods among the many MOX fuel rods arranged in the MOX fuel assembly. It is.

  Accordingly, in the present invention, corresponding to the above, the uranium fuel rods are arranged in the outermost circumferential layer of the square lattice array without arranging the MOX fuel rods, and the most relative relative to the second layer from the outermost circumferential layer. In particular, uranium fuel rods are arranged at four corners near the water gap where output peaks are likely to stand. In the remaining positions other than the above, all the MOX fuel rods having the same plutonium enrichment (in other words, one kind of enrichment) are arranged. Thus, by making the enrichment level of all the MOX fuel rods the same and making the enrichment level one type, for example, by setting the enrichment level of that one type relatively high, Unlike conventional structures that provide settings, the average plutonium enrichment of MOX fuel rods can be increased. As a result, as much plutonium as possible can be loaded into a single MOX fuel rod, the number of MOX fuel rods for loading a certain predetermined amount of plutonium can be reduced, and as a result, per MOX fuel assembly. The manufacturing cost can be reduced. In addition, by making the kind of enrichment one kind, a production line for performing fuel pellet molding can be simplified as compared with the three conventional structures, and the production cost can also be reduced.

  On the other hand, when the outermost circumferential layer of the square lattice arrangement is replaced from the MOX fuel rods to the uranium fuel rods as described above, the more the number of replacements increases, the more the fuel rods in the fuel assembly at the time of combustion progress. Local output peaking increases. Accordingly, in the present invention, a water rod having a substantially circular cross-sectional shape or a water channel having a substantially rectangular cross-sectional shape is disposed in a substantially central region of the square lattice array, and the water channel is relatively left as it is. A water region appears anew near the central region where the output is low far from the gap, thereby relatively increasing the output of the central region. As a result, the increase in the local output peaking due to the replacement can be offset, and the local output peaking can be reduced when viewed from the fuel assembly. For example, since the output is particularly high near the water rod or the water channel, this may not be used as the MOX fuel rod but may be replaced with a short uranium fuel rod, for example.

  As described above, in the present invention, the enrichment type of the MOX fuel rods can be unified into one type while reducing the local output peaking, and the manufacturing cost per unit fuel assembly can be reduced.

  (2) In the above (1), preferably, the plurality of uranium fuel rods containing the flammable poison are all arranged in the second layer from the outermost peripheral layer of the square lattice array.

  (3) In order to achieve the above object, the present invention also provides a plurality of MOX fuel rods containing uranium / plutonium mixed oxide fuel and a plurality of uranium fuel rods containing uranium oxide fuel in a square of n rows and n columns. In a MOX fuel assembly for a boiling water reactor arranged in a grid, a water rod or a water channel is arranged in a substantially central region of the square grid arrangement, and the fuel rod has a normal active fuel length. The fuel rod is composed of a fuel rod and a short fuel rod having an effective fuel length shorter than usual. The plurality of MOX fuel rods are composed of a low-load MOX fuel rod having a relatively low plutonium load and a plutonium load. A high-load MOX fuel rod having a relatively high level, and the uranium fuel rod is disposed on the outermost circumferential layer of the square lattice array, and includes the second layer from the outermost circumferential layer of the square lattice array. Of the inner area, the low wealth The second layer including the degree MOX fuel rods including at least four corner positions (2, 2), (2, n-1), (n-1, 2), (n-1, n-1) And a plurality of the high load MOX fuel rods are arranged as the normal fuel rods at the remaining positions.

  In the fuel assemblies arranged in a square lattice, the outermost peripheral layer faces the water gap portion, so that the output peak is most likely to occur. For this reason, in the MOX fuel assembly, it is necessary to reduce the plutonium enrichment of the MOX fuel rods arranged at the outermost periphery of the square lattice array. Here, in order to increase the average plutonium enrichment of the MOX fuel rods, it is effective to replace the MOX fuel rods with low enrichment with uranium fuel rods among the many MOX fuel rods arranged in the MOX fuel assembly. It is.

  Accordingly, in the present invention, corresponding to the above, the uranium fuel rods are arranged in the outermost circumferential layer of the square lattice array without arranging the MOX fuel rods. The remaining positions other than the above are composed of a low-load MOX fuel rod having a relatively low plutonium load and a high-load MOX having a relatively high plutonium load (in other words, an enrichment level). Two types of MOX fuel rods are arranged, and at this time, the low-load MOX fuel rods are placed at the four corners that are most close to the water gap and have the highest output peak in the second layer from the outermost layer. It is arranged. By limiting the enrichment of the MOX fuel rods to two in this way, the average plutonium enrichment of the MOX fuel rods can be set higher than in the conventional structure in which three enrichment settings are provided. It becomes possible. As a result, as much plutonium as possible can be loaded into a single MOX fuel rod, the number of MOX fuel rods for loading a certain predetermined amount of plutonium can be reduced, and as a result, per MOX fuel assembly. The manufacturing cost can be reduced. Further, by using two kinds of enrichment types, the production line for forming fuel pellets can be simplified as compared with the three types of conventional structures, thereby reducing the production cost.

  On the other hand, when the outermost circumferential layer of the square lattice arrangement is replaced from the MOX fuel rods to the uranium fuel rods as described above, the more the number of replacements increases, the more the fuel rods in the fuel assembly at the time of combustion progress. Local output peaking increases. Accordingly, in the present invention, a water rod having a substantially circular cross-sectional shape or a water channel having a substantially rectangular cross-sectional shape is disposed in a substantially central region of the square lattice array, and the water channel is relatively left as it is. A water region appears anew near the central region where the output is low far from the gap, thereby relatively increasing the output of the central region. As a result, the increase in the local output peaking due to the replacement can be offset, and the local output peaking can be reduced when viewed from the fuel assembly. For example, since the output is particularly high near the water rod or the water channel, this may not be used as the MOX fuel rod but may be replaced with a short uranium fuel rod, for example.

  As described above, in the present invention, the MOX fuel rod enrichment type can be reduced to two types while reducing the local output peaking, and the manufacturing cost per unit fuel assembly can be reduced.

  (4) In the above (3), preferably, the plurality of uranium fuel rods containing the flammable poison are all arranged in the outermost peripheral layer of the square lattice arrangement.

  As a result, fuel rods containing flammable poisons are arranged at positions where the spectrum is soft near the water gap region outside the fuel assembly, thereby increasing the value of the flammable poisons and reducing the number of fuel rods containing flammable poisons. The amount of plutonium loaded per fuel assembly can be increased.

  (5) In the above (4), more preferably, the plurality of uranium fuel rods containing the flammable poison are provided at positions further adjacent to positions adjacent to the four corner positions in the outermost peripheral layer. Yes.

  (6) In the above (1) or (3), and preferably, four corner positions among the four sides formed from the outermost peripheral layer of the square lattice array to the approximate center of each side. The fuel rods to be used are the short fuel rods.

  (7) In the above (1) or (3), preferably, the fuel rod located at the approximate center of each of the four sides formed by the outermost peripheral layer of the square lattice arrangement and the water in the innermost layer. A fuel rod located in the vicinity of the rod or the water channel is referred to as the short fuel rod.

  According to the first aspect of the present invention, the type of enrichment of the MOX fuel rods can be unified into one type while reducing the local output peaking, and the manufacturing cost per unit fuel assembly can be reduced.

  According to the invention described in claim 3, while reducing the local output peaking, the enrichment types of the MOX fuel rods are limited to two, and the manufacturing cost per unit fuel assembly can be reduced.

  Embodiments of the present invention will be described below with reference to the drawings.

A first embodiment of the present invention will be described with reference to FIGS.
FIG. 1 is a longitudinal sectional view showing the overall structure of the fuel assembly of the present embodiment.

  In FIG. 1, the fuel assembly 11 of this embodiment includes a plurality of (74 in this example) fuel rods 12 and two water rods 14 in n rows and n columns (n (laughs) 9 in this example). (9 rows and 9 columns) A fuel bundle is formed by arranging in a square lattice shape, and the upper and lower axial ends of the fuel bundle are supported by the upper tie plate 15 and the lower tie plate 16, and a plurality of axial intermediate portions are fuel spacers. 17, the gap between the fuel rod 12 and the water rod 14 is appropriately maintained, and the outer peripheral side thereof is covered with a channel box 13 having a substantially rectangular tube shape.

  The channel box 13 is attached to the upper tie plate 15 and surrounds the outer periphery of the bundle of fuel rods 12 held by the spacer 17. The cross-shaped control rod 19 is inserted between the channel boxes 13 of the four adjacent fuel assemblies 11 at a ratio of one to the four fuel assemblies 11 so as to be adjacent to the channel box 13.

  The fuel rod 12 is configured such that a large number of fuel pellets are filled in a cladding tube sealed at both ends by an upper end plug and a lower end plug, and the fuel pellet is pressed up and down by a spring disposed in a gas plenum region in the cladding tube. Is.

There are three types of fuel pellets: MOX fuel pellets made of uranium / plutonium mixed oxide fuel, uranium fuel pellets made of uranium oxide fuel, and uranium fuel pellets containing gadolinia mixed with uranium oxide fuel and gadolinia as a flammable poison. There is. The MOX fuel pellet is composed of PuO _{2 as a} fuel material and UO _{2 as} a fuel base material, and includes ²³⁹ Pu, ²⁴¹ Pu and ²³⁵ U as fission materials. The uranium fuel pellet is composed of UO ₂ which is a fuel material and contains ²³⁵ U which is a fission material. The uranium fuel pellet containing gadolinia is composed of UO _{2 as} a fuel material and gadolinia (Gd ₂ O ₃ ) as a combustible poison contained therein, and includes ²³⁵ U as a fission material.

  FIG. 2 is a cross-sectional view showing the detailed structure of the fuel assembly according to the present embodiment. In FIG. 2 and FIG. 1 described above, 74 fuel rods 12 and two water rods 14 are arranged in a square grid of 9 rows and 9 columns (hereinafter, each grid position is represented by (row number, Column number)).

  The water rod 14 is not filled with fuel material, and the cooling water that does not boil inside passes therethrough. Of the array of 81 fuel rods in 9 rows and 9 columns, the bundle rod has a substantially central region. Two are arranged to replace seven fuel rods.

  The fuel rod 12 is provided with a total of six types of fuel pellets of fuel numbers 1, 2, 3, 4, P1, and G1.

  Fuel numbers 1, 2, 3, and 4 are uranium fuel pellets that do not contain gadolinia, and are loaded on a total of 34 fuel rods, 32 long fuel rods (described later) and 2 short fuel rods (described later). The fuel rods 12 (hereinafter referred to as the fuel rods 12 (fuel numbers 1 to 4)) having the uranium fuel pellets of the fuel numbers 1 to 4 are the outermost periphery of a square lattice array when viewed in the horizontal direction. It arrange | positions in the 2 corner position of the 2nd layer from a layer and an outermost periphery part.

  At this time, the uranium fuel enrichment relationship is fuel number 1> fuel number 2> fuel number 3> fuel number 4. And in order to reduce local output peaking, the arrangement is as follows. That is, the fuel rods 12 (fuel number 4) having the smallest enrichment are provided at the four corner portions (1, 1), (1, 9), (9, 9), and (9, 1) of the square lattice arrangement. 8 places (1, 2), (1, 8), (2, 1), (2, 9), (8, 1), (8, 9), (9, 2) adjacent to these , (9, 8) are arranged next to the fuel rods 12 (fuel number 3) with the smallest enrichment, and further, eight locations (1, 3), (1, 7) adjacent to the corners on the opposite side. , (3,1), (3,9), (7,1), (7,9), (9,3), (9,7) are the fuel rods 12 with the next smallest enrichment (fuel number) 2) is arranged. In the remaining part of the outermost peripheral layer and the two corners (2, 8), (8, 2) of the second layer from the outermost peripheral layer, the fuel rod 12 (fuel number 1) having the highest enrichment is provided. Has been placed.

  Fuel number G1 is a uranium fuel pellet containing gadolinia, loaded on a total of 14 long fuel rods (described later) and 2 short fuel rods (described later), and two layers from the outermost circumferential layer in a square lattice array Of the region including the eye and the region inside it (the region indicated by the alternate long and short dash line), it is arranged in the outermost peripheral layer (in other words, the second layer from the outermost peripheral layer in a square lattice array). Specifically, as shown in FIG. 2, (2, 2), (2, 3), (2, 6), (2, 7), (3, 2), (3, 8), (4 , 8), (6, 2), (7, 2), (7, 8), (8, 3), (8, 4), (8, 7), (8, 8) The rod is loaded. The uranium enrichment is appropriately set according to the burnup control mode.

  The fuel number P1 is MOX fuel pellets that do not contain gadolinia and all have the same plutonium loading, and a total of 26 long fuel rods (described later) and 4 short fuel rods (described later), Specifically, the portion other than the above-described gadolinia-containing uranium fuel rod 12 (fuel number G1), MOX fuel rod 12 (fuel number P1), and water rod 14 in the region including the second layer from the outermost layer and the inside thereof. The fuel rods 12 are loaded.

  Of the fuel rods 12 described above, the fuel rods 12 (fuel number G1) located at the corner positions (2, 2) and (8, 8) among the four sides formed from the outermost layer to the second layer, Fuel rod 12 (fuel number 1) located at corner position (8,2), (2,8), center of each side (2,5), (5,2), (5,8), (8, The total of 8 fuel rods 12 (fuel number P1) located in 5) are defined as short fuel rods (represented by ◎ in FIG. 2) whose effective fuel length is shorter than usual (in contrast to this, normally effective) The long fuel rod is appropriately referred to as a long fuel rod).

  The fuel assembly 11 of the present embodiment configured as described above has the following operational effects.

  Generally, in a fuel assembly arranged in a square lattice pattern, the output peak is most likely to occur because the outermost peripheral layer faces the water gap. For this reason, in the MOX fuel assembly, it is necessary to reduce the plutonium enrichment of the MOX fuel rods arranged at the outermost periphery of the square lattice array. Here, in order to increase the average plutonium enrichment of the MOX fuel rods, it is effective to replace the MOX fuel rods with low enrichment with uranium fuel rods among the many MOX fuel rods arranged in the MOX fuel assembly. It is.

  Therefore, in the present embodiment, corresponding to the above, the uranium fuel rods 12 (fuel numbers 1 to 4) are arranged in the outermost circumferential layer of the square lattice array without arranging the MOX fuel rods. In addition, the four corner positions (2, 2), (2, 8), (8, 2), (8, 8) also includes uranium fuel rods (more precisely, uranium fuel rods 12 (fuel number 1) without gadolinia and uranium fuel rods 12 with gadolinia (fuel number G1)).

  FIG. 3 is an explanatory diagram showing the output peaking reduction effect of this configuration. On the premise that the MOX fuel rods with a predetermined high plutonium richness are arranged in the region including the second layer from the outermost layer, the above four corner positions of the second layer from the outermost layer are the uranium fuel rods. When replaced (a uranium fuel rod arrangement case, corresponding to this embodiment), when replaced with a MOX fuel rod having a lower load than the above-mentioned predetermined load (low load MOX fuel rod arrangement case), what Are compared with each other (high load-rich MOX fuel rod placement case), with the horizontal axis representing the burnup (GWd / t) and the vertical axis representing the burnup peaking. Yes. The MOX fuel assemblies used in the evaluation at this time maintained the same burn-up burnup and the same MOX fuel rod average plutonium enrichment in the core loaded state. As shown in FIG. 3, a large burnup peaking is observed in the “high enrichment MOX rod arrangement case” in which MOX rods having the same enrichment as the inner region are arranged at the four corner positions from the outermost layer to the second layer. On the other hand, in the “low enrichment MOX rod arrangement case” in which MOX rods with lower enrichment than the inner region are arranged at the four corner positions and the “uranium rod arrangement case” in which uranium rods are arranged, the appropriate burnup It can be seen that peaking is obtained and the local output peaking is greatly reduced.

  In the present embodiment, the remaining positions other than the above (outermost peripheral layer and second corner portion) have the same plutonium enrichment (in other words, one kind of enrichment) MOX fuel rods 12 ( All fuel numbers P1) are arranged. In this way, by setting the enrichment level of all the MOX fuel rods 12 (fuel number P1) to be the same and the enrichment level to be one type, the enrichment level is set to be relatively high, so that three types of wealth are obtained. Unlike the conventional structure in which the degree of change is provided, the average plutonium enrichment of the MOX fuel rod can be increased. As a result, as many plutonium as possible can be loaded into one MOX fuel rod 12 (fuel number P1), the number of MOX fuel rods 12 for loading a predetermined amount of plutonium as a whole can be reduced. As a result, the manufacturing cost per MOX fuel assembly 11 can be reduced. In addition, by making the kind of enrichment one kind, a production line for performing fuel pellet molding can be simplified as compared with the three conventional structures, and the production cost can also be reduced.

  On the other hand, when the outermost circumferential layer of the square lattice arrangement is replaced from the MOX fuel rods to the uranium fuel rods (fuel numbers 1 to 4) as described above, the more the number of the replacements increases, the more the combustion progresses. The local output peaking of the fuel rod 12 in the fuel assembly 11 is increased. Accordingly, in the present embodiment, correspondingly, the water rods 14 and 14 having a substantially circular cross-sectional shape are arranged in the substantially central region of the square lattice array, and the output is relatively far from the water gap as it is. A water region newly appears in the vicinity of the central region where the temperature becomes low, thereby relatively increasing the output of the central region. As a result, the increase in the local output peaking due to the replacement can be offset, and the local output peaking when viewed from the whole fuel assembly 11 can be reduced.

  As described above, in the present embodiment, while reducing the local output peaking, the enrichment type of the MOX fuel rods 12 (fuel number P1) is unified to one type, and the manufacturing cost per fuel assembly is reduced. Can be reduced.

  Further, in the region including the second layer from the outermost peripheral layer and inside it, all of the fuel rods 12 containing the flammable poisons are relatively close to the water gap region outside the fuel assembly 11 and have the relatively softest spectrum. Since the fuel number G1) is arranged, the value of the combustible poison can be increased, the number of the fuel rods 12 containing the combustible poison can be suppressed, and the amount of plutonium loaded per fuel assembly 11 can be increased accordingly.

  In the first embodiment having the above-described configuration, for example, the output in the vicinity of the water rods 14 and 14 is particularly high. Therefore, this may not be used as the MOX fuel rod but may be replaced with a short uranium fuel rod, for example. Such a modification will be described with reference to FIG.

  FIG. 4 is a cross-sectional view showing the overall structure of the fuel assembly 11 'according to this modification.

  4, in this modification, in the configuration of the fuel assembly 11 of the first embodiment shown in FIG. 2, (4, 4), (6, 6) 2 in the vicinity of the water rods 14, 14. The MOX fuel rod 12 (fuel number P1) is replaced with a short uranium fuel rod (fuel number 1). Correspondingly, the fuel rod 12 (fuel number G1) located at the corner position (2, 2), (8, 8) of the second layer from the outermost circumferential layer, which was a short fuel rod, the corner position ( 8, 2), fuel rod 12 located at (2, 8) (fuel number 1), at the center of each side (2, 5), (5, 2), (5, 8), (8, 5) The total of eight fuel rods 12 (fuel number P1) are normally effective fuel rods without changing the type of fuel pellets. The four uranium fuel rods 12 (fuel number 1) of (5), (5, 1), (5, 9), (9, 5) are short fuel rods. As a result, the number of short fuel rods is reduced from a total of 8 in the first embodiment to a total of 6.

  Also according to this modification, the same effect as the first embodiment is obtained.

  A second embodiment of the present invention will be described with reference to FIG. This embodiment is an embodiment in which gadolinia-containing uranium fuel rods are arranged in the outermost peripheral layer. Portions equivalent to those in the first embodiment are denoted by the same reference numerals, and description thereof is omitted as appropriate.

  FIG. 5 is a cross-sectional view showing the detailed structure of the fuel assembly 11-2 according to the present embodiment. In FIG. 5, 74 fuel rods 12 are provided with a total of six types of fuel pellets of fuel numbers 1, 2, 3, P1, P2, and G1.

  The fuel number G1 is a uranium fuel pellet containing gadolinia, loaded on eight elongate fuel rods, and arranged in the outermost peripheral layer of a square lattice arrangement. Specifically, as shown in FIG. 5, (1,3), (1,7), (3,1), (3,9), (7,1), (7,9), (9 , 3) and (9, 7) are loaded on eight fuel rods 12. The uranium enrichment is appropriately set according to the burnup control mode.

  Fuel numbers 1, 2, and 3 are uranium fuel pellets that do not contain gadolinia and are loaded on 24 long fuel rods. Fuel rods 12 having these uranium fuel pellets of fuel numbers 1 to 3 (fuel numbers 1 to 3 are arranged in the outermost peripheral layer of a square lattice array. Number 1> Fuel number 2> Fuel number 3. In order to reduce local output peaking, the arrangement is as follows: four corners (1, 1 in a square lattice arrangement) 1), (1,9), (9,9), (9,1) are provided with the fuel rods 12 (fuel number 3) having the smallest enrichment, and adjacent to these eight locations (1,2) , (1,8), (2,1), (2,9), (8,1), (8,9), (9,2), (9,8) are the next least concentrated A fuel rod 12 (fuel number 2) is arranged, and the remaining portions (1, 4), (1, 5), (1, 6), (4, 1), (4) of the outermost peripheral layer. , 9), (5 1), (5,9) (6,1), the (6,9) is the most enrichment of large fuel rods 12 (fuel No. 1) is disposed.

  Fuel numbers P1 and P2 are MOX fuel pellets that do not contain gadolinia, and are loaded on a total of 42 long fuel rods and 8 short fuel rods, respectively. Fuel rods 12 having the fuel numbers P1 and P2 of MOX fuel pellets (the fuel numbers P1 and P2 are arranged in a region including the second layer from the outermost peripheral layer in a square lattice array and inside thereof. The plutonium enrichment magnitude relationship of the MOX fuel is as follows: fuel number P1> fuel number P2, that is, P1 is a fuel with a relatively high load and P2 is a fuel with a relatively low load. In order to reduce the output peaking, the arrangement is as follows: That is, the four corners (2, 2), (2, 8), (2, 8, 2), (8, 8) and the center position (2, 5), (5, 2), (5, 8), (8, 5) of each side of the quadrangle formed in the second layer Low load fuel rods 12 (fuel number P2) having a relatively low load are combined as short fuel rods (represented by ◎ in FIG. 5). It is arranged eight, the remaining Tominido the parts relatively high Takatomi acetonide fuel rods 12 (fuel numbers P1) is arranged.

  Also in the fuel assembly 11-2 of the present embodiment configured as described above, the following operational effects can be obtained as in the fuel assembly 11-1 of the first embodiment.

  As described above, the outermost peripheral layer of the square lattice arrangement is most likely to have an output peak, and it is necessary to reduce the plutonium enrichment of the MOX fuel rods arranged on the outermost peripheral portion. In order to increase the plutonium enrichment, it is effective to replace the low enrichment MOX fuel rod with a uranium fuel rod.

  In the present embodiment, corresponding to the above, the uranium fuel rods 12 (fuel numbers 1 to 3) are arranged in the outermost circumferential layer of the square lattice array without arranging the MOX fuel rods. In the remaining positions other than the above (outermost peripheral layer), a low load MOX fuel rod 12 (fuel number P2) with a relatively low plutonium enrichment and a high load with a relatively high plutonium enrichment. MOX fuel rods 12 (fuel number P1) are arranged, and at this time, four corners (2, 2) where the output peak is most likely to be relatively close to the water gap in the second layer from the outermost circumferential layer, In (2, 8), (8, 2), and (8, 8), a low-richness MOX fuel rod 12 (fuel number P2) is arranged. Thus, by limiting the enrichment of the MOX fuel rods 12 (fuel numbers P1, P2) to two types, the plutonium average enrichment of the MOX fuel rods is more than that of the conventional structure in which three enrichment settings are provided. The degree can be set higher. As a result, as in the first embodiment, as much plutonium as possible can be loaded per MOX fuel rod 12 (fuel number P1 or P2), so that a whole predetermined amount of plutonium can be loaded. The number of MOX fuel rods 12 can be reduced, and as a result, the manufacturing cost per MOX fuel assembly 11-2 can be reduced. In addition, by making the enrichment types two types, it is possible to simplify the production line for performing fuel pellet molding compared to the three types of conventional structures, thereby reducing the production cost.

  On the other hand, when the outermost circumferential layer of the square lattice arrangement is replaced from the MOX fuel rods to the uranium fuel rods (fuel numbers 1 to 3) as described above, the more the number of the replacements, the more the combustion progresses. The local output peaking of the fuel rod 12 in the fuel assembly 11-2 is increased. Accordingly, in the present embodiment, correspondingly, the water rods 14 and 14 having a substantially circular cross-sectional shape are arranged in the substantially central region of the square lattice array, and the output is relatively far from the water gap as it is. A water region newly appears in the vicinity of the central region where the temperature becomes low, thereby relatively increasing the output of the central region. As a result, the increase in the local output peaking due to the replacement can be offset, and the local output peaking when viewed from the whole fuel assembly 11 can be reduced.

  As described above, also in the present embodiment, the enrichment types of the MOX fuel rods 12 (fuel numbers P1 and P2) are limited to two types while reducing local output peaking as in the first embodiment. In addition, the manufacturing cost per unit fuel assembly can be reduced.

  Further, in the square lattice arrangement, all the burnable poison-containing fuel rods 12 (fuel number G1) are concentratedly arranged in the outermost peripheral layer closest to the water gap region outside the fuel assembly 11 and having the softest spectrum. Therefore, the value of the combustible poison is particularly increased, and the number of the fuel rods 12 containing the combustible poison is reduced to eight. As a result, since the number of MOX fuel rods 12 can be increased from 26 to 42 in the first embodiment, the amount of plutonium loaded per fuel assembly 11-2 can be further increased accordingly.

  In the second embodiment having the above-described configuration, for example, the output in the vicinity of the water rods 14 and 14 is particularly high. Therefore, this may not be used as the MOX fuel rod but may be replaced with a short uranium fuel rod, for example. Such a modification will be described with reference to FIG.

  FIG. 6 is a cross-sectional view showing the overall structure of the fuel assembly 11'-2 according to this modification.

  6, in this modification, in the configuration of the fuel assembly 11-2 of the second embodiment shown in FIG. 5, (4, 4), (6, 6) in the vicinity of the water rods 14, 14. The two MOX fuel rods 12 (fuel number P1) are replaced with short uranium fuel rods (fuel number 1). Correspondingly, at the four corner positions (2, 2), (8, 8), (8, 2), (2, 8) in the second layer from the outermost peripheral layer, which were short fuel rods. MOX fuel rod 12 (fuel number P2) located and MOX fuel rod 12 (fuel number P2) located at the center of each side (2,5), (5,2), (5,8), (8,5) ) Are normally effective length fuel rods, and MOX fuel rods 12 located at the center (2, 5), (5, 2), (5, 8), (8, 5) of each side As for (fuel number P2), the richness is increased and replaced with the high richness MOX fuel rod 12 (fuel number P1).

  Then, instead of the above-described elongated fuel rods, the four positions of the midpoint positions (1, 5), (5, 1), (5, 9), (9, 5) of each side of the quadrangle of the outermost peripheral layer The uranium fuel rod 12 (fuel number 1) is a short fuel rod. As a result, the number of short fuel rods is reduced from a total of 8 in the second embodiment to a total of 6.

  Also by this modification, the same effect as the second embodiment is obtained.

  In the above description, the case where the two water rods 14 and 14 having a substantially circular cross-sectional shape are arranged in the substantially central region of the square lattice array has been described as an example, but the present invention is not limited to this. That is, you may arrange | position one large diameter water rod or three or more small diameter water rods. Moreover, it is not restricted to a water rod, You may arrange | position the water channel of cross-sectional shape substantially square. FIG. 7 is a cross-sectional view showing such a modification. 7, in this example, in the configuration of the fuel assembly 11′-2 shown in FIG. 6, instead of the water rod 14, the fuel rods 12 in the substantially central region of the square lattice array of the fuel assemblies 11A. Water channels 14A having a substantially square cross-sectional shape are provided in nine spaces. Accordingly, the two short uranium fuel rods 12 (fuel number 1) in the vicinity of the water rod 14 in FIG. 6 are omitted, and the center (1, 5), (5, 1), The short uranium fuel rods 12 (fuel number 1) located at (5, 9) and (9, 5) are made long without changing the fuel. In such a case, the same effects as those of the first and second embodiments are obtained.

It is a longitudinal section showing the whole fuel assembly structure of a 1st embodiment of the present invention. It is a cross-sectional view showing the detailed structure of the fuel assembly according to the first embodiment of the present invention. It is explanatory drawing showing the output peaking reduction effect | action in 1st Embodiment of this invention. It is a cross-sectional view showing a modification in which the vicinity of the water rod is replaced with a short uranium fuel rod instead of the MOX fuel rod. It is a cross-sectional view showing the detailed structure of the fuel assembly according to the second embodiment of the present invention. It is a cross-sectional view showing a modification in which the vicinity of the water rod is replaced with a short uranium fuel rod instead of the MOX fuel rod. It is a cross-sectional view showing the modification using a water channel.

Explanation of symbols

11, 11 'Fuel assembly 11-2 Fuel assembly 11'-2 Fuel assembly 11A Fuel assembly 12 Fuel rod 14 Water rod 14A Water channel

Claims (7)

MOX fuel assembly for boiling water reactors in which a plurality of MOX fuel rods containing uranium / plutonium mixed oxide fuel and a plurality of uranium fuel rods containing uranium oxide fuel are arranged in a square grid of n rows and n columns In
A water rod or a water channel is disposed in a substantially central region of the square lattice arrangement;
The fuel rod is composed of a normal fuel rod having a normal fuel active length and a short fuel rod having a fuel active length shorter than normal,
The uranium fuel rods are arranged on the outermost peripheral layer of the square lattice arrangement,
(2, 2), (2, n−1), (n−) which are the four corner positions of the second layer in the region including the second layer from the outermost peripheral layer of the square lattice arrangement. 1, 2), (n-1, n-1), the uranium fuel rods are arranged, and the remaining normal fuel rods include a plurality of uranium fuel rods containing flammable poisons, plutonium A MOX fuel assembly comprising a plurality of the MOX fuel rods having the same enrichment degree. The MOX fuel assembly according to claim 1, wherein
The MOX fuel assembly, wherein the plurality of uranium fuel rods containing the flammable poisons are all arranged in the second layer from the outermost peripheral layer of the square lattice array. MOX fuel assembly for boiling water reactors in which a plurality of MOX fuel rods containing uranium / plutonium mixed oxide fuel and a plurality of uranium fuel rods containing uranium oxide fuel are arranged in a square grid of n rows and n columns In
A water rod or a water channel is disposed in a substantially central region of the square lattice arrangement;
The fuel rod is composed of a normal fuel rod having a normal fuel active length and a short fuel rod having a fuel active length shorter than normal,
The plurality of MOX fuel rods are composed of a low-load MOX fuel rod having a relatively low plutonium load and a high-load MOX fuel rod having a relatively high plutonium load,
The uranium fuel rods are arranged on the outermost peripheral layer of the square lattice arrangement,
In the region including the second outermost layer from the outermost circumferential layer of the square lattice arrangement and the inner side thereof, the low load MOX fuel rods are at least four corner positions (2, 2), (2, n -1), (n-1, 2), (n-1, n-1) including the second layer including the normal fuel rods at the remaining positions. A MOX fuel assembly comprising fuel rods. The MOX fuel assembly according to claim 3, wherein
The MOX fuel assembly, wherein the plurality of uranium fuel rods containing the flammable poison are all arranged in the outermost peripheral layer of the square lattice array. The MOX fuel assembly according to claim 4, wherein
The MOX fuel assembly, wherein the plurality of uranium fuel rods containing the flammable poison are provided at positions further adjacent to positions adjacent to four corner positions in the outermost peripheral layer. The MOX fuel assembly according to claim 1 or 3,
Of the four sides formed by the second layer from the outermost peripheral layer of the square lattice arrangement, the fuel rods positioned at four corner positions and approximately the center of each side are respectively the short fuel rods. MOX fuel assembly. The MOX fuel assembly according to claim 1 or 3,
A fuel rod located at the approximate center of each of the four sides formed by the outermost peripheral layer of the square lattice array, and a fuel rod located near the water rod or water channel in the innermost layer, respectively. A MOX fuel assembly characterized by a short fuel rod.

Images