JPS6111392B2 - - Google Patents

Description

[Detailed description of the invention]

FIELD OF APPLICATION OF THE INVENTION The present invention relates to a fuel assembly. BACKGROUND OF THE INVENTION Boiling water reactors have a void distribution in the axial direction, which causes skewing in the power distribution that is not seen in other reactor types. The output distribution is shown in Figure 1. FIG. 1 shows an axial void distribution 21 and a power distribution 22 of a reactor core loaded with fuel assemblies having a constant enrichment distribution in the axial direction. At the bottom of the core, which is the coolant inlet, approximately
It is in a subcooled state of 10Kcal/Kg, and as the coolant rises in the core, subcooled boiling,
In the upper part of the core, where the reactor enters the saturated boiling region and the coolant exits, the void volume fraction reaches around 70%. As a result, the thermalization of neutrons in the lower part of the reactor core progresses more than in the upper part of the reactor core, and although other reactor types have an axial power distribution close to a cosine distribution, in boiling water reactors the position of the power peak is skiwing at the bottom of the reactor core. Therefore, the position of the power peak moves from the lower end of the fuel assembly (lower end of the reactor core) to a position about 1/4 of the height of the reactor core, and the value of the power peak is also larger than that of other reactor types. In order to solve this problem, in current boiling water reactors, control rods are inserted shallowly (these control rods are called shallow control rods) at the power peak position, about 1/4 from the bottom of the reactor core. Countermeasures are taken such as using fuel rods containing Gatlinia (Gd ₂ O ₃ ) at the position where the output peak occurs. In order to suppress the power peak that occurs at a position approximately 1/4 from the bottom of the core, a shallow control rod is inserted at that position. The output distribution is as shown in FIG. In the axial power distribution control using the shallow control rod, as shown in FIG. 2, an output peak and a sudden change in the output distribution occur near the tip of the shallow control rod. When pulling out the shallow control rod,
Sudden power fluctuations are applied to the fuel rods, and there is a risk of damage to the fuel rods. Inserted into the reactor core are shallow control rods that control the power distribution in the axial direction and control rods (these control rods are referred to as deep control rods) that control the power distribution in the radial direction of the reactor core. Deep control rods are deeply inserted into the reactor core, and the degree of insertion is greater than that of shallow control rods. Reactor power control is achieved by a combination of deep control rods and shallow control rods, and in order to simultaneously satisfy three functions, including the deep/shallow control rod drive system and reaction control, it is necessary to formulate a control rod operation plan. It has drawbacks such as requiring a lot of calculations. Control rod operations would also be extremely complex. Figure 3 shows a fuel assembly having fuel rods with gadolinia added to about 1/4 of the lower end, and the power distribution characteristics of the fuel assembly when loaded into the core of a boiling water reactor. Gadolinia is added at position 23 in the figure. When using fuel assemblies with such fuel rods containing gadolinia, the output distribution changes greatly depending on the position where the gadolinia is placed, so a small design error may make it difficult to control the output distribution or reduce the output due to combustion. This causes operational difficulties such as large changes in distribution. Further, as the amount of gadolinia decreases with the passage of operating time of the nuclear reactor, the power distribution changes, and when gadolinia in the region 23 is completely eliminated, the power distribution becomes as shown in FIG. 1. Therefore, adjustment by control rod operation becomes complicated. It is an object of the present invention to provide a fuel assembly that eliminates the above-described drawbacks of the prior art and can achieve effective utilization of nuclear fuel in the upper region and flattening of the power distribution in the axial direction with a simple structure. Summary of the Invention The present invention is characterized in that a plurality of first fuel rods having a uniform axial enrichment are arranged in the center in a plane perpendicular to the axis of the fuel assembly, and the first fuel rods are arranged in the upper region in the axial direction. a plurality of second fuel rods divided into a lower region, in which the enrichment in the upper region is higher than that in the lower region; a third fuel rod smaller than the largest first fuel rod is disposed in the plane at a peripheral portion adjacent to and surrounding the central portion, and the upper portion of the second fuel rod The difference between the average enrichment of the central part in the upper region and the average enrichment of the peripheral part in the lower region is such that the enrichment of the region does not exceed that of the first fuel rod having the highest enrichment. The reason is that the difference is smaller than the difference between the average concentration of the central portion and the average concentration of the peripheral portion. According to the present invention, a simple fuel assembly can be constructed using a small number of types of fuel materials,
The nuclear fuel in the upper region and the lower region can be used effectively, and the power distribution in the axial direction can be maintained flat during the operation period of the nuclear reactor. The present invention is basically to change the infinite multiplication factor between the upper and lower parts of the fuel assembly, that is, to make the infinite multiplication factor in the upper part larger than that in the lower part. As a way to change the infinite multiplication factor in this way,
There are three types: The first method is to change the average enrichment in the plane perpendicular to the axis of the fuel assembly between the upper and lower parts of the fuel assembly. Note that in the following, the enrichment level refers to at least one of the enrichment level of uranium and the enrichment level of plutonium. In the second method, the average enrichment in the plane perpendicular to the axis is almost the same, but the enrichment distribution in the plane perpendicular to the axis is changed at the upper and lower parts of the fuel assembly, and the concentration is infinite. This is a method of changing the multiplication factor. Generally, the enrichment distribution in a plane perpendicular to the axis of the fuel assembly is high at the center and relatively low at the periphery. This is because there is gap water near the saturation temperature around the fuel assembly, and if the enrichment distribution is made uniform, the output at the periphery will be much higher than at the center due to the thermalization of neutrons by the gap water. This is to prevent so-called local output peaking from becoming excessive.
However, the importance in the fuel assembly is higher in the periphery than in the center, so within the range where local power peaking is allowed, enrichment between the center and the periphery should be adjusted so that the power peak occurs at the periphery. Designed to minimize differences in degrees. Therefore, if we further increase the difference between the enrichment in the center and the enrichment in the periphery so that output peaking occurs in the center, the average enrichment of the plane is the same, and the infinite multiplication factor is higher than that of the conventional one. It becomes possible to manufacture slightly smaller fuel assemblies. The desired fuel assembly can be obtained by making the lower part of the fuel assembly have such an enrichment distribution and the upper part having an enrichment distribution such that output peaking occurs in the peripheral area as in the conventional case. Here, the central portion refers to the inner region when the cross section of the fuel assembly is divided into two regions, and the peripheral portion refers to the remaining outer region. The periphery is an annular region adjacent to the center. The third method is to add more gadolinia to the lower part of the fuel rod to be incorporated into the fuel assembly than to the upper part. Generally, these three methods may be combined. EXAMPLES OF THE INVENTION The present invention will now be explained in detail by way of examples. Example 1 This example describes the first method (the average enrichment in the plane perpendicular to the axis is changed between the upper and lower regions of the fuel assembly) and the second method (the average enrichment in the plane perpendicular to the axis is changed between the upper and lower regions of the fuel assembly). An example in which the enrichment distribution in the plane is changed between the upper region and the lower region of the fuel assembly will be described. FIG. 4 is a cross section of the 8×8 fuel assembly of this example. 11 is a fuel pellet, 12 is a cladding tube, 13 is a coolant region, 14 is a channel box, 15 is a water rod, and 16 is a control rod. The numbers 1, 2, and 2 shown on the fuel pellets
3, 4, 5, and 6 indicate fuel rods with different enrichments, and Table 1 shows the enrichments of fuel rods 1 to 6.

[Table] Four types of fuel rods 1, 2, 4, and 6 each have 1
In terms of enrichment, 1 is 2.5% by weight, 2 is 2.0% by weight, 4 is 1.5% by weight, and 6 is a fuel rod in which 5% by weight of gadolinia is added to uranium enriched to 1.5% by weight. Fuel rods 3 and 5 are divided into 24 equal parts in the axial direction,
It is divided into two areas, an upper area and a lower area, between the 11th and 12th from the bottom, and in 3, the upper area is
2.0% by weight, 1.5% by weight in the lower region, and 1.5% by weight in the upper region and 1.3% by weight in the lower region. The enrichment in the upper and lower regions of each fuel rod is uniformly distributed along their axis. Therefore,
The production of fuel rods is also easy. The fourth fuel rod in Table 1
Table 2 shows the uranium enrichment distribution of the fuel assembly arranged and formed as shown in the figure.

[Table] (Unit of uranium enrichment: weight %)
In Table 2, the "center" indicates the inside of the dashed-dotted line A in FIG. 4, and the "periphery" indicates the outside of the dashed-dotted line A in FIG. When the void volume fraction of the coolant is 40%, the infinite multiplication factors in the upper region and lower region of the fuel assembly with the enrichment distribution shown in Table 2 are 1.126 and 1.092, respectively, and when the temperature and void fraction are the same, The difference in infinite multiplication factor is 3.4%, and the upper region of the core is higher. When this fuel assembly is loaded into a boiling water reactor, due to voids generated in the axial direction,
The difference in infinite multiplication factors between the upper region and the lower region is canceled out, and a relatively flat axial power distribution can be achieved in the axial direction. FIG. 5 shows the axial power distribution of this example. Characteristic A is the initial power distribution of the fuel cycle,
Characteristic B shows the power distribution in the middle of the fuel cycle, and characteristic C shows the power distribution in the final stage of the fuel cycle.
Output peaks are formed in the upper and lower regions, respectively, and the axial output peaking coefficient is approximately 1.3 or less. Therefore, without using shallow control rods or special gadolinia distribution, only deep control rods are used as control rods,
By simple control rod operations, the power distribution in the axial direction can be maintained almost flat throughout the operating period of the reactor, as shown in FIG. In this embodiment, the infinite multiplication factor in the upper region is increased by changing the enrichment in the upper and lower regions of the fuel rods in the peripheral region, so the infinite multiplication factor in the upper region and the lower region of the fuel rods in the peripheral region is increased. The difference does not change throughout the operating life of the reactor. Therefore, the axial power distribution can always be maintained flat throughout the operating period of the nuclear reactor without any axial power distribution flattening operation by the control rods. In this example, since the second method described above is also used,
Compared to the case where the difference in the infinite multiplication factor between the upper and lower regions is achieved only by the difference in the average enrichment between the upper and lower regions, as in the first method described above, it is possible to effectively use the fissile material in the fuel assembly. can be used with the added benefit of improving fuel economy. That is, if the average enrichment in the upper region is made significantly greater than that in the lower region as in the first method, the content of fissile material in the upper region will be much larger than that in the lower region. In the lower region, where the amount of fissile material is small, the fissile material disappears much faster than in the upper region over the course of operation time. Even if the fissile material in the lower region has disappeared and a significant amount of fissile material in the upper region remains, it must be replaced with a new fuel assembly. For this reason, the fissile material in the upper region cannot be used effectively. In this example, since the above-mentioned second method is used together with the above-mentioned first method, the average enrichment in the upper region and the average enrichment in the lower region are higher than when only the first method is used. (difference in infinite multiplication factor) becomes smaller. Therefore,
In this embodiment, the fissile material in the upper region can be used more effectively than when only the first method is used. In particular, in this embodiment, fuel rods 3 with two enrichment regions, upper and lower,
and 5 are arranged near the water gap, the deceleration effect of water can be effectively utilized, and the infinite multiplication factor in the upper region of the fuel assembly can be efficiently increased more than that in the lower region. Therefore, the above-described difference in infinite multiplication factors can be generated without increasing the difference in enrichment between the upper and lower portions of the fuel rods 3 and 5. From this point of view as well, fissile materials can be used effectively. In this embodiment, the enrichment in the upper region of the fuel rods 3 and 5 does not exceed that of the fuel rod 1, and the enrichment of the fuel rod 4 is smaller than that of the fuel rod 1. The average enrichment of the fuel assembly is smaller than that of the central part, and the power distribution in the cross section of the fuel assembly is flattened. Furthermore, since this embodiment uses fuel rods 1 with uniform enrichment in the axial direction, a fuel assembly is constructed. As is clear from Table 1, there are four types of fuel substances, that is, types of fuel pellets filled in each fuel rod. The fuel assemblies of this example have enrichments of 2.5% by weight, 2.0% by weight, and 1.5% by weight.
and 1.3% by weight of four types of fuel pellets. This example uses fewer types of fuel pellets than the fuel assembly (composed of nine types of fuel pellets with different enrichments) shown in Table 3 of JP-A No. 53-40188. The structure is significantly simplified. As mentioned above, the fuel assembly of this example is
A fuel rod with uniform axial enrichment is placed in the center in a plane perpendicular to the axis of the fuel assembly, and in the periphery of the plane, the enrichment in the upper region in the axial direction is higher than that in the lower region. Large fuel rods are arranged, and the enrichment in the upper and lower regions of the latter fuel rods is lower than that of the former fuel rods, resulting in a simple structure composed of fewer kinds of fuel materials, and the upper region is less enriched than the former fuel rods. It is possible to effectively utilize the fissile material in the axial direction and to flatten the power distribution in the axial direction. 1/3 of the total length from the axial lower end of the fuel assembly
By dividing the fuel assembly into two regions, an upper region and a lower region, between and 7/12 and increasing the infinite multiplication factor in the upper region, flattening of the output can be achieved most effectively. That is, as shown in FIG. 3, the output peaking coefficient can be made smaller than when gadolinia is added. Example 2 Similar to Example 1, this example is a combination of the first method and the second method described above. FIG. 6 shows a cross section of the fuel assembly of this example. Six types of fuel rods, designated 31 to 36, are used. The enrichment of each fuel rod is shown in Table 3. 15 is a water rod.

[Table] 31 and 35 are fuel rods in one area, and 32, 33, and 34 are fuel rods that are divided into two areas, upper and lower, at the 11/24th position from the bottom end. In the latter fuel rods, the enrichment in the upper and lower regions is uniformly distributed in the respective axial directions. As in Example 1, this embodiment includes a plurality of fuel rods with uniform enrichment in the axial direction at the center, and a plurality of fuel rods in the periphery where the enrichment in the upper region is higher than that in the lower region in the axial direction. fuel rods are placed in each location. Table 4 shows the uranium enrichment distribution of a fuel assembly formed by arranging the fuel rods shown in Table 3 as shown in FIG.

[Table] (Unit of uranium enrichment: weight %)
In this example, the central part is the fuel rods 31 and 36.
The peripheral portion is an annular region in which fuel rods 32 to 35 are arranged adjacent to and surrounding the center. As is clear from Table 4, in this example as well, the average enrichment in the upper region was made larger than that in the lower region, and the difference in enrichment between the center and the periphery in the cross section was such that the upper region was higher than the lower region. It is also becoming smaller. It has such an enrichment distribution.
The fuel assembly has the same infinite multiplication factor difference in the upper and lower regions as in the first embodiment, and the axial power distribution is also almost the same as in the first embodiment. Compared to Example 1, this example has a large difference in enrichment in one fuel rod, relatively easy inspection during manufacturing, and a very small local power peaking of 1.12. ,
This is a feature of the second embodiment. The second embodiment also provides the same effects as the first embodiment. It is desirable that the boundary between the upper region and the lower region be located in the range of 1/3 to 7/12 as in the first embodiment described above. Effects of the Invention According to the present invention, since the fuel rods having a uniform infinite multiplication factor in the axial direction are arranged in the center, the fuel assembly can be made into a simple structure composed of a small number of types of fuel materials. The nuclear fuel in the upper region can be used effectively and the power distribution in the axial direction can be made flat.

[Brief explanation of drawings]

Fig. 1 is a characteristic diagram showing the void fraction distribution and relative power distribution in the axial direction of the reactor core when the control rods of the reactor core loaded with fuel assemblies having a constant infinite multiplication factor in the axial direction are fully withdrawn; Figure 2 is a characteristic diagram showing the axial power distribution of the fuel assembly near the control rod when the control rod is inserted shallowly from the bottom of the reactor core with the characteristics shown in Figure 1. Figure 3 shows the gadolinia power distribution at the position of the power peak. FIG. 4 is a horizontal sectional view of a fuel assembly according to a preferred embodiment of the present invention; FIG. This figure is a characteristic diagram showing the axial power distribution of the reactor core loaded with the fuel assembly of the embodiment shown in FIG. 4, and FIG. 6 is a horizontal sectional view of another embodiment of the present invention. 1, 2, 3... fuel rod, 4, 5, 6... fuel rod, 11... fuel pellet, 12... cladding tube, 1
2...Channel Boxes, 15...Water Rods.

Claims (1)

[Claims] 1. In a fuel assembly having a plurality of fuel rods loaded in the core of a boiling water reactor in which coolant flows from the bottom to the top and voids occur in the top, A plurality of first fuel rods having a uniform enrichment are arranged centrally in a plane perpendicular to the axis of the fuel assembly, and are divided into an upper region and a lower region in the axial direction, and the first fuel rods have a uniform enrichment. a plurality of second fuel rods having a higher enrichment than that of the lower region; and a plurality of second fuel rods having a uniform axial enrichment and whose enrichment is greater than that of the first fuel rod having the highest enrichment. small third fuel rods are disposed in the plane at a periphery adjacent to and surrounding the central portion;
The enrichment of the upper region of the second fuel rod does not exceed that of the first fuel rod having the highest enrichment, and the average enrichment of the central portion and the average enrichment of the peripheral portion in the upper region The fuel assembly is characterized in that the difference is smaller than the difference between the average enrichment of the central part and the average enrichment of the peripheral part in the lower region. 2. The fuel according to claim 1, wherein the boundary between the upper region and the lower region is located in a range between 1/3 and 7/12 of the total length of the second fuel rod from the lower end thereof. Aggregation.