JP2012008083A - Set of mox fuel assemblies - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a set of MOX fuel assemblies which can be widely loaded on a reactor core and further suppress the required number of MOX fuel rods to improve productivity.SOLUTION: In a set of MOX fuel assemblies including first and second MOX fuel assemblies, the two assemblies are the same in types of plutonium enrichment and in the number of MOX fuel rods for each type of enrichment to each other, an uranium fuel rod containing inflammable poison is not contained in the fuel rods arranged in the outermost peripheral part in each assembly, and MOX fuel rods of the highest enrichment are arranged to be not in direct contact with a water pipe arrangement region. In the first MOX fuel assembly, mixed oxide fuel rods are arranged at all positions directly adjacent to the water pipe arrangement region, and in the second MOX fuel assembly, at least one uranium fuel rod containing inflammable poison is arranged at part of the positions directly adjacent to the water pipe arrangement region.

Description

  The present invention relates to a set of boiling water type MOX fuel assemblies, and more particularly to a set of MOX fuel assemblies capable of realizing two streams.

  Regarding the use of plutonium in Japan, it is important that the supply and demand of plutonium is balanced, based on the principle that there is no surplus plutonium. From the viewpoint of such plutonium balance, it is preferable that the amount of plutonium used in the uranium / plutonium mixed oxide fuel (hereinafter referred to as MOX fuel) core is large. Therefore, loading MOX fuel to the entire core is considered as a means of improving the utilization efficiency of plutonium.

  A fuel assembly loaded in a boiling water reactor is configured by bundling several types of fuel rods having different concentrations of nuclear fuel material in order to optimize the power distribution in the fuel assembly. The types of fuel rods vary depending on the type of fuel assembly, the type of core in which the fuel assembly is loaded, the operating method of the reactor, etc., but in the fuel manufacturing plant that manufactures fuel rods and fuel assemblies, The fewer types, the lower the cost of fuel production.

  Furthermore, in the manufacture of MOX (uranium / plutonium mixed oxide) fuel assemblies, it is necessary to reduce the number of fuels as much as possible from the viewpoint of reducing the exposure of workers engaged in the manufacture.

  In boiling water reactors, in order to suppress the reactivity at the beginning of the cycle, a method of adding a flammable poison to a part of the fuel rods that make up the fuel assembly has been conventionally adopted. Two types of fuel assemblies designed to change the number of fuel rods containing flammable poisons to have different reactivities at the beginning of combustion, and to make the reactivity equal after extinction of combustible poisons So-called two-stream fuel (for example, see Patent Document 1).

  Therefore, the same effect can be expected for the MOX fuel core by adopting the two-stream fuel by the MOX fuel assembly using the flammable poison. However, in the MOX fuel assembly, the fuel rods containing flammable poisons cannot be filled with plutonium, so if the number of fuel rods containing flammable poisons is changed as described above, the two types of fuel assemblies It is difficult to design with the same reactivity after the flammable poison disappears.

  Furthermore, unlike the uranium fuel, plutonium contained in the MOX fuel may change its isotope composition ratio for each fuel replacement batch. Prepare two or more types of fuel assemblies with different numbers of uranium fuel containing flammable poisons, and adjust the breakdown of the number of fuel assemblies when loading them into the core. There is a method of achieving operation of the core for a certain period of time (see, for example, Patent Document 2). However, even in this method, there is no means for making the reactivity the same after the combustible poison burns.

  As described above, in the MOX fuel manufacturing factory, it is necessary to reduce the types of fuel rods to be handled at the time of manufacturing the fuel assembly and to handle the 2-stream MOX fuel. In order to obtain the effect of the conventional two-stream fuel, the two-stream MOX fuel has two kinds of flammable poisons that are different from each other but have the same reactivity after the flammable poison burns and disappears. A technology for realizing a fuel assembly is required.

Japanese Patent No. 2958861 JP-A-10-260282 Japanese Patent Publication No. 6-36049 JP 2006-329867 A JP 2002-277580 A

  In conventional uranium fuel, the number of enrichment splits and the number of fuel rods containing flammable poisons and enrichment are optimized for each target core in order to optimize the power distribution. Has been.

  However, in the production of MOX fuel, it takes a lot of labor to adjust the enrichment by secondary mixing of powders and to clean the equipment at the time of enrichment switching. This increases the manufacturing cost over uranium fuel. In addition, an increase in the number of times of equipment cleaning increases the exposure amount of workers, which is not preferable from the viewpoint of reducing the exposure amount.

  For these reasons, it is necessary to reduce the type of enrichment as much as possible in the manufacture of MOX fuel rods. An effective method for this is to use a common MOX fuel rod among the fuels. For example, use of a common MOX fuel rod between two stream fuels, or a method using a common type of MOX fuel rod used in a MOX fuel assembly between a symmetric lattice and a symmetric lattice-asymmetric lattice (see Patent Document 5) .) For example, four types of MOX fuel rods are used in the MOX fuel assembly A, but only three types are used in the MOX fuel assembly B. It is not always used. Further, even though the types of MOX fuel rods used are the same, the number of MOX fuel rods used for each type is not necessarily the same, such as the number of MOX fuel rods used for each core being different.

  As described above, if the ratio of the types of MOX fuel rods differs between the two stream fuel assemblies or depending on the target core, the number of MOX fuel rods needs to be adjusted for each production project, and the productivity decreases. In addition, when the fuel produced in one project is diverted to another project for some reason, excess or deficiency occurs depending on the type of MOX fuel rods used, and it is necessary to perform additional production only for that fuel. There has been a problem that productivity is lowered due to the occurrence of a surplus in a specific fuel.

  Further, as a means for making the reactivity the same after the combustible poison is burned, the number of uranium fuel rods containing the combustible poison fuel between the two stream fuel assemblies is made the same, and the MOX constituting the fuel assembly A method is conceivable in which the plutonium enrichment types of fuel rods and the number of fuel rods are the same. According to this method, by making the number of uranium fuel rods the same, the number of the remaining MOX fuel rods can be made the same, and the average enrichment of the assemblies between the two stream fuel assemblies is equal to each other. can do. Of course, with the same number of uranium fuel rods and the same type and number of MOX fuel rods, there is a difference in the reactivity at the initial stage of combustion between the two-stream fuel assemblies. It is necessary to suppress local output peaking for the aggregate.

  Here, in order to realize the difference in reactivity at the initial stage of combustion between the two-stream fuel assemblies, attention is paid to the value of the flammable poison depending on the position inside the fuel assembly. For example, Patent Document 3 describes that the value of the flammable poison varies depending on the position inside the fuel assembly, and that the burning speed of the flammable poison varies depending on the position. However, here, the property is applied to the suppression of the output peaking in the assembly, and the problem relating to the reactivity adjustment between the fuel assemblies is not solved.

  Further, Patent Document 4 discloses a configuration in which a fuel rod containing a combustible poison is disposed around a water rod where the value of the combustible poison is high. This is intended to suppress peaking. The degree of reactivity at the early stage of combustion between the two-stream fuel assemblies is made different by changing the number of fuel rods containing flammable poisons. Therefore, another solution means focusing on the arrangement of the uranium fuel rod containing the combustible poison is required. It should be noted that the arrangement of the uranium fuel rods containing combustible poisons on the outermost peripheral portion of the fuel assembly is not preferable because it can reduce the value of control rods and affect the sensitivity characteristics of nuclear instrumentation.

  Regarding the suppression of local peaking, the MOX fuel rods are enriched to suppress local output peaking in the fuel assembly through combustion, based on the arrangement of uranium fuel rods containing flammable poisons for the above-mentioned reactivity adjustment. Degree and placement must be determined. At this time, it is necessary to consider the neutron spectrum distribution in the fuel assembly, the influence of the uranium fuel rod containing the flammable poison, and the change in the output peaking position accompanying combustion.

  In view of the above problems, the object of the present invention can be applied to two streams of MOX fuel while minimizing the enrichment type of the MOX fuel rod as much as possible. The object is to provide a set of MOX fuel assemblies capable of improving productivity.

To achieve the above object, each set of MOX fuel assemblies according to the invention described in claim 1 includes a predetermined number of uranium / plutonium mixed oxide fuel rods and at least one combustible poison. This is a set of MOX fuel assemblies for a boiling water reactor in which uranium fuel rods are arranged in a square lattice pattern, and water tubes are replaced and arranged in an area corresponding to a predetermined number of fuel rods in the square lattice array. And
The set of MOX fuel assemblies has two types of first and second MOX fuel assemblies,
The first and second MOX fuel assemblies have the same plutonium enrichment type and the same number of mixed oxide fuel rods for each enrichment type, and are disposed on the outermost periphery of the square lattice array, respectively. The uranium fuel rods containing flammable poisons are not included in the fuel rods, and the mixed oxide fuel rods with the highest plutonium enrichment are arranged at positions not directly adjacent to the water pipe arrangement region,
In the first MOX fuel assembly, mixed oxide fuel rods are arranged at all positions directly adjacent to the water pipe arrangement region,
The second MOX fuel assembly is characterized in that at least one uranium fuel rod containing a flammable poison is arranged at a part of a position directly adjacent to the water pipe arrangement region.

The set of MOX fuel assemblies according to claim 2 is the set of MOX fuel assemblies according to claim 1, wherein the first MOX fuel assembly is a flammable poison for all uranium fuel rods. And more than half of all the uranium fuel rods are disposed in a region that is obliquely adjacent to the water tube disposition region and a region that combines the second layer from the water tube disposition region,
The second MOX fuel assembly includes a flammable poison in all uranium fuel rods, and more than half of all uranium fuel rods are arranged at positions directly adjacent to the water pipe arrangement region. To do.

  The set of MOX fuel assemblies according to the invention described in claim 3 is the set of MOX fuel assemblies according to claim 1 or 2, wherein the mixed oxide fuel rods with the highest plutonium enrichment are the next highest. The enrichment ratio with the mixed oxide fuel rod of the plutonium enrichment is 1.1 or more and 1.3 or less.

  The set of MOX fuel assemblies according to the invention described in claim 4 is the MOX fuel assembly set according to any one of claims 1 to 3, wherein the first MOX fuel assembly and the second MOX are set. Of the uranium fuel rods with flammable poisons that the fuel assembly has, each flammable poison from the uranium fuel rod with the lowest flammable poison content to the uranium fuel rod with the highest flammable poison content For each level of content, the concentration ratio of the second MOX fuel assembly to the first MOX fuel assembly of the combustible poison at the same axial position between the uranium fuel rods of the same order is 1.4. The above is 2.0 or less.

  The set of MOX fuel assemblies according to claim 5 is the set of MOX fuel assemblies according to any one of claims 1 to 4, wherein the first MOX fuel assembly and the second MOX The fuel assembly is loaded into the same symmetrical lattice boiling water reactor core.

  A set of MOX fuel assemblies according to the invention described in claim 6 is the MOX fuel assembly set according to any one of claims 1 to 4, wherein the first MOX fuel assembly and the second MOX are set. The fuel assembly is loaded into different symmetrical lattice boiling water reactor cores.

The set of MOX fuel assemblies according to the invention of claim 7 is the set of MOX fuel assemblies according to claim 1, wherein the first MOX fuel assembly is a uranium fuel rod containing all combustible poisons. Half or more of them are arranged in a region where the water pipe arrangement region is diagonally adjacent to the water pipe arrangement region and the second layer from the water pipe arrangement region.
The second MOX fuel assembly is characterized in that half or more of all flammable poison-containing uranium fuel rods are arranged at positions directly adjacent to the water pipe arrangement region.

  The set of MOX fuel assemblies according to the invention described in claim 8 is the set of MOX fuel assemblies according to claim 7, wherein the mixed oxide fuel rod having the highest plutonium enrichment and the next highest plutonium enrichment are obtained. The enrichment ratio with the mixed oxide fuel rod of the degree of conversion is 1.1 or more and 1.3 or less.

  A set of MOX fuel assemblies according to the invention described in claim 9 is the MOX fuel assembly set according to claim 1, 7 or 8, wherein the first MOX fuel assembly is a symmetric lattice boiling water type. The reactor core is loaded, and the second MOX fuel assembly is loaded into an asymmetric lattice boiling water reactor core.

  In the set of MOX fuel assemblies according to the present invention, the use of various types of cores in the boiling water reactor core allows the use of common MOX fuel rods of various types between the cores. In addition to reducing the amount of exposure of a person compared to the prior art, the production cost of fuel rods can be reduced with high productivity.

  In addition, the first and second MOX fuel assemblies having the same plutonium enrichment type and the same number of MOX fuel rods are loaded while the reactivity at the initial stage of combustion is different from each other. Thus, it is possible to obtain an MOX core that realizes a two-stream MOX fuel assembly having the same reactivity after burning and extinguishing of the flammable poison.

FIG. 2 is a design diagram showing a fuel arrangement of a set of MOX fuel assemblies for a symmetric grid type core according to the first embodiment of the present invention, and having a water pipe arrangement area for nine fuel rods in the center portion; a) is a 9 × 9 arrangement of one first MOX fuel assembly constituting the set, and (b) is a 9 × 9 arrangement of the other second MOX fuel assembly constituting the set. FIG. 2 is a diagram showing a transition of an infinite multiplication factor of the first and second MOX fuel assemblies in FIG. 1. FIG. 2 is a diagram showing a transition of local output peaking of the first and second MOX fuel assemblies in FIG. 1. FIG. 2 is a diagram showing a local output peaking coefficient with respect to a ratio P1 / P2 between the highest enrichment P1 and the next highest enrichment P2, in each MOX fuel assembly of FIG. (B) is a diagram relating to the second fuel assembly. FIG. 6 is a design diagram showing a fuel arrangement of a set of MOX fuel assemblies for a symmetric lattice type core according to a second embodiment of the present invention, and having a water pipe arrangement area for four fuel rods in the center; a) is an 8 × 8 array of one first MOX fuel assembly constituting the set, and (b) is an 8 × 8 array of the other second MOX fuel assembly constituting the set. FIG. 6 is a diagram showing a local output peaking coefficient with respect to a ratio P1 / P2 between the highest enrichment P1 and the next highest enrichment P2, in each MOX fuel assembly of FIG. (B) is a diagram relating to the second fuel assembly. FIG. 10 is a design diagram showing a fuel arrangement of a set of MOX fuel assemblies for a symmetric lattice type core according to a third embodiment of the present invention, and having a water pipe arrangement area for seven fuel rods in the center, a) is a 9 × 9 arrangement of one first MOX fuel assembly constituting the set, and (b) is a 9 × 9 arrangement of the other second MOX fuel assembly constituting the set. FIG. 8 is a diagram showing a local output peaking coefficient with respect to a ratio P1 / P2 between the highest enrichment P1 and the next highest enrichment P2, in each MOX fuel assembly of FIG. (B) is a diagram relating to the second fuel assembly. FIG. 9 is a 9 × 9 fuel array of the second MOX fuel assembly for an asymmetrical lattice type core according to the fourth embodiment of the present invention, which is for nine fuel rods at a position shifted from the center by one row to the counter-control rod side. It is a design drawing which shows the case where it has a water pipe arrangement | positioning area | region.

  The MOX fuel assemblies for boiling water reactors of the present invention have the same plutonium enrichment type and the number of mixed oxide fuel rods (hereinafter referred to as MOX fuel rods) for each enrichment type, The types and positions of the fuel rods arranged on the outermost peripheral portion of the square lattice array are the same, and the fuel rods arranged on the outermost peripheral portion do not include the uranium fuel rods containing combustible poisons, and the highest plutonium-rich. The MOX fuel rods of the degree of conversion are composed of the first and second types of MOX fuel assemblies having a common configuration that they are arranged at positions not adjacent to the water pipe arrangement region, and the first MOX fuel assembly The body has MOX fuel rods placed at all positions directly adjacent to the water pipe placement area, and the second MOX fuel assembly contains at least one combustible poison at a part of the position directly adjacent to the water pipe placement area. Uranium fuel rod Those which are location.

  In the first and second types of MOX fuel assemblies according to the present invention, the plutonium enrichment type and the number of MOX fuel rods for each enrichment type are the same in the core manufacturing project. By not placing uranium fuel rods containing flammable poisons on the outer periphery, the effect of lowering the value of control rods and changing the sensitivity characteristics of nuclear instrumentation is eliminated, and the types and positions of fuel rods disposed on the outermost periphery are the same and The most enriched MOX fuel rods are arranged so as not to be directly adjacent to the water pipe arrangement region, so that the basic output peaking can be suppressed.

  Accordingly, if a set of the first and second MOX fuel assemblies having such a basic design is used as a manufacturing unit of the fuel assembly, the first and second MOX fuel assemblies are appropriately selected and predetermined It is possible to optimize for a wide variety of core loadings by adding a design, and it is also possible to load the first and second MOX fuel assemblies in the same core as a unit. The two MOX fuel assemblies can be loaded into different cores using their respective characteristics and used properly, but there is no need to adjust the number of MOX fuel rods for each production project. Can be used with high efficiency, realizing high productivity.

  In other words, the first and second MOX fuel assembly sets of the present invention can be employed in various core manufacturing projects, but in any MOX fuel assembly, the same plutonium enrichment type and Since it is composed of the number of MOX fuel rods, a common MOX fuel rod can be used for each core, and the production of fuel rods with limited types is sufficient. Therefore, the types of plutonium enrichment in various core productions can be suppressed, and the number of cleanings performed for each enrichment during production is reduced compared to the conventional method, reducing production costs while reducing the exposure of workers. Can be achieved.

  First, in the case where the MOX fuel assembly set according to the present invention is loaded on a symmetrical lattice type core in which the distance between the fuel assemblies is equal, the first MOX fuel assembly is added to the basic design described above. In addition, all uranium fuel rods contain flammable poisons, and more than half of all uranium fuel rods are diagonally adjacent to the water tube arrangement region, that is, diagonally adjacent to the square lattice arrangement and the second layer from the water tube arrangement region. And the second MOX fuel assembly contains flammable poisons in all uranium fuel rods, and more than half of all uranium fuel rods directly in the water tube arrangement region. It shall be arranged at an adjacent position.

  Accordingly, the first MOX fuel assembly has a characteristic of having a relatively high reactivity in the early stage of combustion, and the second MOX fuel assembly has a characteristic of having a relatively low reactivity in the early stage of combustion. Furthermore, in the set of these first and second MOX fuel assemblies, a two-stream core can be configured by providing a difference in reactivity at the initial stage of combustion between the two in the same core. Moreover, all but the necessary uranium fuel rods containing combustible poisons can be used as MOX fuel rods, and the utilization efficiency of plutonium can be improved accordingly.

  As shown in the examples described later, as a result of the study by the present inventors, in each MOX fuel assembly, the plutonium enrichment of the MOX fuel rod having the highest enrichment and the plutonium of the next highest MOX fuel rod are obtained. By setting the ratio with the enrichment to be 1.1 or more and 1.3 or less, local peaking particularly after the middle stage of combustion can be satisfactorily suppressed.

  Furthermore, among the uranium fuel rods containing flammable poisons, the flammable poison content from the uranium fuel rod with the lowest flammable poison content to the uranium fuel rod with the highest flammable poison content. In addition, the second MOX fuel assembly of the first MOX fuel assembly and the second MOX fuel assembly are in the same position in the axial direction between the uranium fuel rods of the same rank, and the second MOX fuel assembly By setting the concentration ratio of the MOX fuel assemblies to 1.4 or more and 2.0 or less, the extinction timing of the combustible poison can be made the same between the first and second MOX fuel assemblies.

  The first and second MOX fuel assembly sets according to the present invention can be loaded in various lattice-type cores. The symmetrical lattice-type core has a C-grid with equal intervals between the fuel assemblies, and an inner width of the C-grid. There are S lattices, C lattices, and N lattices with a large inter-fuel assembly pitch and control rod pitch, which are optimized for an N lattice core with high power density. By applying the enrichment type and fuel arrangement to the C-lattice core and the S-lattice core, the excess reactivity can be used effectively, and the cycle cost of the C-lattice core and the S-lattice core can be reduced. .

  In these various symmetrical lattice type cores, if the common first MOX fuel assembly or second fuel assembly or both MOX fuel assemblies are loaded, the same type and number of MOX fuel rods and the same uranium are used. Since it has the number of fuel rods, it will be loaded with a fuel assembly mainly composed of various common fuel rods between each core, and a large number of fixed types of fuel rods are manufactured together. Therefore, high productivity and reduction in manufacturing cost can be achieved.

  In addition, when the first MOX fuel assembly and the second MOX fuel assembly are loaded in different symmetric lattice cores, the first MOX fuel assembly has a low surplus reaction suppression effect due to its characteristics. The effect of increasing the reactivity of the core can be expected, and the second MOX fuel assembly can be expected to suppress the excessive reactivity of the core due to the high surplus reactivity suppression effect. Since the plutonium enrichment characteristic is the same, a significant efficiency improvement effect can be expected in this set of images.

  Further, in addition to the basic design in the present invention, the first MOX fuel assembly includes a position in which more than half of all flammable poison-containing uranium fuel rods are obliquely adjacent to the water pipe arrangement area and two from the water pipe arrangement area. It is assumed that the second MOX fuel assembly is disposed in a region adjacent to the water pipe disposition region, and the second MOX fuel assembly is disposed in a region directly adjacent to the water tube disposition region. By using the first MOX fuel assembly, the first MOX fuel assembly is used for the symmetrical lattice type core, while the kind and the number of the MOX fuel rods used in common between the first and second MOX fuel assemblies are the same. In addition, the second MOX fuel assembly can be used for an asymmetric lattice type core.

  Accordingly, if a set of combustion assemblies composed of the first MOX fuel assemblies for the symmetric lattice type core and the second MOX fuel assemblies for the asymmetric lattice type core are set as production units, the first and second It is possible to selectively use different MOX fuel assemblies in different cores utilizing their respective characteristics, and as a result, a common MOX fuel rod can be used with high efficiency.

  Even in such a set of the first MOX fuel assembly for the symmetric lattice core and the second MOX fuel assembly for the asymmetric lattice core, the plutonium enrichment of the MOX fuel rod with the highest enrichment and its By setting the ratio of the next highest MOX fuel rod to the plutonium enrichment to be 1.1 or more and 1.3 or less, particularly local peaking after the middle stage of combustion can be satisfactorily suppressed.

  As for the uranium fuel rods arranged in the fuel assembly of the present invention, if the uranium base material used is natural or deteriorated uranium, the amount of plutonium loaded can be increased as compared with the case of using enriched uranium, Costs are not required, which contributes to further reduction in manufacturing costs.

  As a first embodiment of the present invention, a case where the first and second MOX fuel assemblies are loaded in a symmetric lattice boiling water reactor core as a set of 9 × 9 arrangement MOX fuel assemblies is shown in FIG. It is shown in 1. FIGS. 1A and 1B are design diagrams showing the fuel arrangement of the first MOX fuel assembly and the second MOX fuel assembly, respectively, constituting the fuel assembly set. In the fuel assembly of the present embodiment, the nine fuel rods in the center are the water pipe arrangement region, and the arranged water pipe is a rectangular water channel.

  The first and second MOX fuel assemblies in this embodiment are each of four types 1 to 4 of the same type (p1, p2, p3, p4 from the highest enrichment order) with respect to the plutonium enrichment. This is a set of MOX fuel assemblies having the same number of MOX fuel rods and the same number of uranium fuel rods G1, G2 containing combustible poisons.

  Further, in both the first and second MOX fuel assemblies, the fuel rods arranged in the outermost peripheral portion do not include the uranium fuel rods (G1, G2) containing the combustible poison, and the MOX fuel rods having the highest plutonium enrichment degree. (Type 1) is arranged at a position not directly adjacent to the water pipe arrangement region.

  Further, the first MOX fuel assembly is arranged in a region where more than half of all uranium fuel rods are obliquely adjacent to the water channel and in a region where the second layer from the water channel is combined. In the MOX fuel assembly, more than half of all uranium fuel rods are arranged at positions directly adjacent to the water channel.

  In this set, the ratio P1 / P2 between the highest enrichment P1 and the next highest enrichment P2 is 1.21, and the flammable poisons of the uranium fuel rods containing the combustible poisons. Concentration ratio of flammable poison content (first MOX fuel assembly: C4, second MOX fuel assembly: C8) in the same axial position between the uranium fuel rods with the highest content (type G2) C8 / C4 is 1.67, and the flammable poison content (first MOX fuel assembly: 1st MOX fuel assembly) at the same axial position between the uranium fuel rods (type G1) having the second highest flammable poison content. The concentration ratio C6 / C2 of C2, the second MOX fuel assembly: C6) is set to 1.75. By setting this concentration ratio, the extinction timing of the combustible poison can be made substantially the same between the first and second MOX fuel assemblies.

  The transition of the infinite multiplication factor in the cross section of each fuel assembly regarding such a set of the first and second MOX fuel assemblies is shown in the diagram of FIG. From FIG. 2, it can be seen that the infinite doubling rates of the first and second MOX fuel assemblies are different at the initial stage of combustion and agree after the combustible poisons disappear. That is, it can be said that the first and second MOX fuel assemblies in this embodiment have the same tendency as the conventional uranium two-stream fuel assembly pair.

  Next, the transition of the local output peaking in the first and second MOX fuel assembly sets of the present embodiment is shown in the diagram of FIG. In this embodiment, P1 / P2 = 1.21. However, FIG. 3 shows the first and second MOX fuel assemblies, the local output peaking coefficient, and the average average wealth of the present embodiment. The transition of the local output peaking is also shown for the MOX fuel assembly set in which P1 / P2 = 1.41 without changing the degree of conversion. As shown in FIG. 3, in the first and second MOX fuel assembly pairs (P1 / P2 = 1.21) according to the present embodiment, the MOX fuel assembly pair with P1 / P2 = 1.41 is used. In comparison, it can be seen that local output peaking after the middle stage of combustion is well suppressed.

  Next, the local output peaking coefficients of the first and second MOX fuel assemblies with respect to P1 / P2 in the middle combustion period are shown in the diagrams of FIGS. 4 (a) and 4 (b), respectively. From FIG. 4, it can be seen that the local output peaking coefficient varies depending on the type and burnup of the fuel assembly. In any case, when P1 / P2 is in the range of 1.1 to 1.3, You can see that Therefore, it is clear that it is desirable to set P1 / P2 in the range of 1.1 to 1.3 in order to satisfactorily suppress local peaking after the middle stage of combustion in the MOX fuel assembly group. It became.

  As a second embodiment of the present invention, an 8 × 8 array MOX fuel assembly set loaded in a symmetrical lattice boiling water reactor core is shown in FIG. FIGS. 5A and 5B are design diagrams showing the fuel arrangement of the first MOX fuel assembly and the second MOX fuel assembly, respectively, constituting the fuel assembly set. In the fuel assembly of the present embodiment, the four fuel rods in the central portion are water tube arrangement regions, and the arranged water tubes are circular water rods.

  The first and second MOX fuel assemblies in this embodiment are each of four types of types 1 to 4 of the same type with respect to the plutonium enrichment (P1, P2, P3, P4 from the highest enrichment order). This is a two-stream MOX fuel assembly pair having the same number of MOX fuel rods and the same number of uranium fuel rods G1 containing combustible secular substances.

  Further, in both the first and second MOX fuel assemblies, the fuel rods arranged in the outermost peripheral portion do not include the uranium fuel rod (G1) containing the combustible poison, and the MOX fuel rod having the highest plutonium enrichment ( Type 1) is located at a position not directly adjacent to the water rod.

  Further, the first MOX fuel assembly is arranged in a region where more than half of all the uranium fuel rods are obliquely adjacent to the water rod and in the region where the second layer from the water rod is combined. In the MOX fuel assembly, more than half of all uranium fuel rods are arranged at positions directly adjacent to the water rod.

  In this group, the concentration ratio C2 / C1 of the flammable poison content (first MOX fuel assembly: C1, second MOX fuel assembly: C2) between uranium fuel rods (type G1) 1.4. By setting this concentration ratio, the extinction timing of the combustible poison can be made substantially the same between the first and second MOX fuel assemblies.

  Regarding the first and second MOX fuel assembly sets, the local output peaking coefficients of the first and second MOX fuel assemblies with respect to P1 / P2 in the middle of combustion are shown in FIGS. 6 (a) and 6 (b), respectively. Is shown in the diagram. From FIG. 6, it can be seen that the local output peaking coefficient varies depending on the type and burnup of the fuel assembly. In any case, when P1 / P2 is in the range of 1.1 to 1.3, You can see that Therefore, it is desirable to set P1 / P2 in the range of 1.1 to 1.3 in order to satisfactorily suppress local peaking after the middle stage of combustion in the MOX fuel assembly group. It became clear also in the Example.

  As a third embodiment of the present invention, FIG. 7 shows a set of 9 × 9 array MOX fuel assemblies loaded on a symmetrical lattice boiling water reactor core different from the first embodiment. FIGS. 7A and 7B are design diagrams showing the fuel arrangement of the first MOX fuel assembly and the second fuel assembly, respectively, constituting the fuel assembly set. The third embodiment differs from the first embodiment in the shape of the water tube arrangement region at the center of the assembly cross section, and two circular water rods are arranged in the region of seven fuel rods as water tubes. This is a second example of a 9 × 9 array.

  The first and second MOX fuel assemblies in this example are each of four types 1 to 4 of the same type with respect to the plutonium enrichment (P1, P2, P3, P4 from the highest enrichment order). 4 types of MOX fuel rods and 2 types of MOX fuel rods with partial length fuel rods with plutonium enrichment P1 and P2 as types 5 and 6, respectively, and uranium fuel rods G1 and G2 with flammable poisons Is a set of MOX fuel assemblies having the same number of fuel cells.

  Further, in both the first and second MOX fuel assemblies, the fuel rods arranged in the outermost peripheral portion do not include the uranium fuel rods (G1, G2) containing the combustible poison, and the MOX fuel rods having the highest plutonium enrichment degree. (Type 1) is arranged at a position not directly adjacent to the water rod.

  Furthermore, in the first MOX fuel assembly, more than half of all uranium fuel rods are disposed in a region where the second layer from the water rod arrangement region is combined with a position obliquely adjacent to the water rod arrangement region. In the second MOX fuel assembly, more than half of all the uranium fuel rods are arranged at positions directly adjacent to the water rod arrangement region.

  Also, in this set, the uranium fuel rods containing the flammable poisons of each other, the content of the flammable poisons in the same axial direction between the uranium fuel rods (type G2) having the highest flammable poison content (No. 1 MOX fuel assembly: C3, C4, second MOX fuel assembly: C7, C8) concentration ratio C7 / C3 is 1.5, C8 / C4 is 1.5, and the content of combustible poison is The content of combustible poisons in the same axial position of the second highest uranium fuel rods (type G1) (first MOX fuel assembly: C1, C2, second MOX fuel assembly: C5, C6) Concentration ratio C5 / C1 is 1.5 and C6 / C2 is 1.4. By setting this concentration ratio, the extinction timing of the combustible poison can be made substantially the same between the first and second MOX fuel assemblies.

  For the first and second MOX fuel assembly sets, the local output peaking coefficients of the lower and upper axial directions of the first and second MOX fuel assemblies with respect to P1 / P2 in the middle of combustion are shown in FIG. It is shown in the diagrams of a) and (b). It can be seen from FIG. 8 that the local output peaking coefficient varies depending on the type of fuel assembly and the burnup. In the transition of 30 GWd / t above the first MOX fuel assembly, P1 / P2 = 1.0 and 1.1 are almost the same minimum, but in either case, P1 / P2 is 1 It turns out that it is the minimum value when it is in the range of .1 to 1.3. Therefore, it is desirable to set P1 / P2 in the range of 1.1 to 1.3 in order to satisfactorily suppress local peaking after the middle stage of combustion in the MOX fuel assembly group. It became clear also in the Example.

  As a fourth embodiment of the present invention, a first MOX fuel assembly is loaded in a symmetrical lattice boiling water reactor core, and a second MOX fuel assembly is an asymmetric lattice boiling water reactor core. Shows a set of MOX fuel assemblies loaded. The first MOX fuel assembly according to this embodiment has the same fuel arrangement as the design shown in FIG. 1A. The fuel arrangement of the second MOX fuel assembly is shown in the design of FIG. Indicated.

  The first and second MOX fuel assemblies in this embodiment are each of four types of types 1 to 4 of the same type with respect to the plutonium enrichment (P1, P2, P3, P4 from the highest enrichment order). This is a set of MOX fuel assemblies having the same number of MOX fuel rods and the same number of uranium fuel rods containing flammable poisons.

  Further, in both the first and second MOX fuel assemblies, the fuel rods arranged in the outermost peripheral portion do not include the uranium fuel rods containing combustible poisons, and the MOX fuel rods having the highest plutonium enrichment (type 1) Is arranged at a position not directly adjacent to the water rod.

  Furthermore, in the first MOX fuel assembly, more than half of all the flammable poison-containing uranium fuel rods are arranged in a region where the position obliquely adjacent to the water rod and the second layer from the water rod are combined. In the second MOX fuel assembly, more than half of all the flammable poison-containing uranium fuel rods are arranged directly adjacent to the water rod.

Claims (9)

A predetermined number of uranium / plutonium mixed oxide fuel rods and at least one uranium fuel rod containing a flammable poison are arranged in a square lattice, and the predetermined fuel rods in the square lattice arrangement are arranged. A set of MOX fuel assemblies for a boiling water reactor in which water pipes are replaced in the number of regions,
The set of MOX fuel assemblies has two types of first and second MOX fuel assemblies,
The first and second MOX fuel assemblies have the same plutonium enrichment type and the same number of mixed oxide fuel rods for each enrichment type, and are disposed on the outermost periphery of the square lattice array, respectively. The uranium fuel rods containing flammable poisons are not included in the fuel rods, and the mixed oxide fuel rods with the highest plutonium enrichment are arranged at positions not directly adjacent to the water pipe arrangement region,
In the first MOX fuel assembly, mixed oxide fuel rods are arranged at all positions directly adjacent to the water pipe arrangement region,
In the MOX fuel assembly, the second MOX fuel assembly is characterized in that at least one uranium fuel rod containing a combustible poison is disposed in a part of a position directly adjacent to the water pipe arrangement region. set. The first MOX fuel assembly includes flammable poisons in all uranium fuel rods, and more than half of all uranium fuel rods are located adjacent to the water tube placement region diagonally and 2 from the water tube placement region. Placed in the area combined with the layer,
The second MOX fuel assembly includes a flammable poison in all uranium fuel rods, and more than half of all uranium fuel rods are arranged at positions directly adjacent to the water pipe arrangement region. The set of MOX fuel assemblies according to claim 1.   The enrichment ratio between the mixed oxide fuel rod with the highest plutonium enrichment and the mixed oxide fuel rod with the next highest plutonium enrichment is 1.1 to 1.3. The set of MOX fuel assemblies according to claim 1 or 2.   Of the uranium fuel rods with combustible poisons included in the first MOX fuel assembly and the second MOX fuel assembly, the most combustible poisons are contained in the uranium fuel rod having the lowest combustible poison content. The second to the first MOX fuel assembly of the flammable poison at the same axial position between the uranium fuel rods of the same rank for each rank of the flammable poison content up to the uranium fuel rod having a high amount. The MOX fuel assembly set according to any one of claims 1 to 3, wherein the concentration ratio of the MOX fuel assembly is 1.4 or more and 2.0 or less.   The first MOX fuel assembly and the second MOX fuel assembly are loaded on the same symmetrical lattice type boiling water reactor core, according to any one of claims 1 to 4. A set of MOX fuel assemblies as described.   The first MOX fuel assembly and the second MOX fuel assembly are loaded into different symmetric lattice boiling water reactor cores, respectively. A set of MOX fuel assemblies as described. The first MOX fuel assembly is located in a region where more than half of all flammable poison-filled uranium fuel rods are diagonally adjacent to the water tube placement region and the second layer from the water tube placement region. Arranged,
2. The MOX fuel assembly according to claim 1, wherein in the second MOX fuel assembly, half or more of all flammable poison-containing uranium fuel rods are arranged at positions adjacent to the water pipe arrangement region. A set of fuel assemblies.   The enrichment ratio between the mixed oxide fuel rod with the highest plutonium enrichment and the mixed oxide fuel rod with the next highest plutonium enrichment is 1.1 to 1.3. The set of MOX fuel assemblies according to claim 7.   The first MOX fuel assembly is loaded on a symmetric lattice boiling water reactor core, and the second MOX fuel assembly is loaded on an asymmetric lattice boiling water reactor core. The set of MOX fuel assemblies according to claim 1, 7 or 8.

Images