JPS6359116B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a nuclear reactor, and in particular to a nuclear reactor in which control rods can be easily operated. 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 1 and a power distribution 2 of a reactor core loaded with fuel assemblies having a constant enrichment distribution in the axial direction. Approximately 10Kcal/Kg at the bottom of the core, which is the inlet of the coolant.
As the coolant rises in the core, it enters the region of subcool boiling and saturated boiling, and at the top of the core, where the coolant exits, the void volume fraction reaches around 70%. There is. 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 output peak is at the lower end of the fuel assembly (lower end of the core)
It moves to a position approximately 1/4 of the height of the reactor core, and the peak output value is also larger than that of other reactor types. To solve this problem, current boiling water reactors
A control rod is shallowly inserted at the position of the output peak, about 1/4 from the bottom of the reactor core (this control rod is called a shallow control rod), or gadolinia (Gd ₂ O ₃ ) is inserted at the position where the output peak occurs. Countermeasures such as using fuel rods are being taken. In order to suppress the power peak that occurs at about 1/4 of the way from the bottom of the core, shallow control rods are inserted at that location. 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 that control the power distribution in the radial direction of the reactor core (these control rods are referred to as deep control rods). 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 operation,
It becomes extremely complex. FIG. 3 shows the characteristics of the power distribution of a fuel assembly when a fuel assembly having fuel rods doped with gadolinia in about 1/4 of the lower end is loaded into the core of a boiling water reactor. Gadolinia is added at position 3 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.
Furthermore, as the amount of gadolinia decreases as the reactor operating time passes, the power distribution changes and
When gadolinia is completely eliminated, the output distribution will be as shown in Figure 1. Therefore, adjustment by control rod operation becomes complicated. An object of the present invention is to provide a nuclear reactor that can flatten the power distribution in the axial direction, eliminate the need for shuffling of fuel assemblies during fuel exchange, and simplify the structure of the replacement fuel assembly. . The present invention is characterized by a plurality of first fuel assemblies in which the infinite multiplication factor in the upper region is larger than that in the lower region, and a plurality of second fuel assemblies in which the infinite multiplication factor is uniform in the axial direction. are arranged in the reactor core where coolant flows from the bottom to the top and voids occur at the top, and a plurality of deep insertion control rods are arranged in the radial direction of the reactor core as control rods for power distribution adjustment. The reason is that only the rod is provided. According to the present invention, the power distribution in the axial direction of the reactor core can be significantly flattened, and control rod operations during reactor operation can be significantly simplified. Furthermore, since only the deeply inserted control rods are used as control rods for power distribution adjustment, the fuel assemblies burn uniformly in the radial direction of the core, and there is no need to shuffle the fuel assemblies during fuel exchange. Furthermore, the second fuel assembly, which has a uniform infinite multiplication factor in the axial direction and has a simple structure, can be used as a replacement fuel assembly, which facilitates the construction of the replacement fuel assembly. The second fuel assembly functions as a fuel assembly in which the infinite multiplication factor in the upper region is higher than that in the lower region as the operating time of the nuclear reactor progresses. The present invention is basically to change the infinite multiplication factor between the upper and lower parts of the reactor core, that is, to make the infinite multiplication factor in the upper part larger than that in the lower part.
There are three ways to change the infinite multiplication factor in this way: 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 is now possible to manufacture a slightly smaller fuel assembly, and by making the lower part of the fuel assembly have such an enrichment distribution and the upper part having an enrichment distribution where output peaks appear in the periphery, as before, it is possible to create the desired fuel assembly. You get a body. 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. The present invention will be explained in detail below with reference to Examples. Example 1 A boiling water reactor with a thermal output of 2400 MW is shown in FIG. The boiling water reactor has a reactor core 10 housed within a reactor pressure vessel 9 . The reactor core 10 has a large number of fuel assemblies 1 shown in FIG.
6 are arranged parallel to each other and spaced apart. In this example, the height of the core section 10 is 146 inches, and the number of fuel assemblies 16 is 560. A control rod 11 having a cross-shaped cross section is arranged so as to be inserted between fuel assemblies 16 in the reactor core 10 . The reactor pressure vessel 9 is filled with coolant, and a pump 12 circulates the coolant. The coolant receives heat from the fuel assembly 16, and a portion of the coolant turns into steam, which drives the turbine 13, is condensed by the condenser 14, and is sent back to the reactor pressure vessel 9 by the pump 15.
Go back inside. The fuel assembly 16 includes a channel box 17 having a rectangular cross section, and a large number of fuel rods 20 are disposed within the channel box 17 between upper and lower tie plates 18 and 19 at both upper and lower ends. In this embodiment, one type of fuel assembly A is
Loaded with 0. Figure 6 shows 1/4 of the reactor core 10.
The arrangement of fuel assemblies A in a cross section is shown. The fuel assembly A is 8×8 as shown in FIG.
6 types of fuel rods 31, 32, 33, 34, 35, 36, including fuel rods arranged in a square grid pattern.
(62 rods in total) and two water rods 24. 21 is a fuel pellet, 22 is a cladding tube, and 23 is a coolant region. The six types of fuel rods have different uranium enrichment levels as shown in Table 1. Fuel assembly A was prepared using the first method described above (changing the average enrichment in the plane perpendicular to the axis of the fuel assembly in the upper and lower regions) and the second method (changing the average enrichment in the plane perpendicular to the axis). The enrichment is almost the same, but the enrichment distribution in the plane perpendicular to the axis is changed between the upper and lower regions of the fuel assembly.

[Table] The four types of fuel rods 31, 32, 34, and 36 each have one type of enrichment, with 31 being 2.5% by weight and 3
2 is a fuel rod with uranium enrichment of 2.0% by weight, 34 with 1.5% by weight, and 36 with 1.5% by weight of uranium to which 5% by weight of gadolinia is added. The fuel rods Nos. 33 and 35 are divided into 24 equal parts in the axial direction and divided into two regions, an upper region and a lower region, between the 11th and 12th from the bottom end. area is 1.5
% by weight, 35 is 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, manufacturing of fuel rods is also easy. Table 2 shows the uranium enrichment distribution of a fuel assembly formed by arranging the fuel rods shown in Table 1 as shown in FIG.

[Table] When the void volume fraction of the coolant is 40%, the infinite multiplication factors in the upper region and lower region of a fuel assembly with the enrichment distribution shown in Table 2 are 1.126 and 1.092, respectively, and the temperature and void fraction are The difference in infinite multiplication factor in the same case is 3.4%. Naturally, the infinite multiplication factor in the upper region of the core 10 is higher than in the lower region. When this fuel assembly is loaded into the core 10 of a boiling water reactor, the difference in infinite multiplication factors between the upper and lower regions of the core 10 is canceled out due to the void distribution that occurs in the axial direction. , a relatively flat axial power distribution in the axial direction can be achieved. FIG. 8 shows the axial power distribution of this embodiment. Characteristic L shows the power distribution at the beginning of the fuel cycle, characteristic M shows the power distribution at the middle stage of the fuel cycle, and characteristic N shows the power distribution at the end 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, and the power distribution in the axial direction can be controlled by simple control rod operations, as shown in Figure 8. It can be maintained substantially flat throughout the operating period. FIG. 9 shows an example of the control rod pattern of this embodiment. FIG. 9 is a horizontal sectional view of the reactor core 10, in which four fuel assemblies are arranged in one square, and one control rod is arranged at the center of these fuel assemblies. The numerical values shown are the insertion rate of the control rods into the reactor core 10, which divides the axial direction of the reactor core 10 into 24 equal parts, and indicate that the control rods are inserted from the lower end of the reactor core 10 to the position indicated by the number. The larger the number, the deeper the control rod is inserted into the reactor core 10. Also, the control rods in the blank spaces are fully withdrawn. During operation of a boiling water reactor, the maximum linear power density in the reactor core 10 is determined by a simple control rod pattern in which deep control rods are inserted at the same depth.
The power output is extremely low at 11.3kw/ft. In this embodiment, it is not necessary to use the control rods to flatten the power distribution in the axial direction of the reactor core, and the power distribution in the radial direction in the reactor core can be flattened only by the deep control rods. That's fine. For reference, FIG. 10 shows a conventional control rod pattern for a reactor core composed of fuel assemblies in one region in the axial direction (uranium enrichment is constant in the axial direction). As is clear from FIG. 10, shallow and deep control rods are inserted into the reactor core, and the control rod pattern is complicated. The maximum linear power density at this time is 11.9kw/ft. The maximum power density of the reactor core 10 of this embodiment is 0.6 kw/ft lower than that of the conventional reactor core, and the control rod pattern of this embodiment is also simpler than that of the conventional one. This makes it easier to operate the control rods during reactor operation. In this embodiment, the fuel assembly A
The axial power distribution of 0 is flattened and the core 10
The power distribution in the radial direction of the reactor core 10 is flattened by the plurality of deep control rods arranged in the radial direction of the reactor, and since shallow control rods are not used, the fuel assembly At the time of replacement, there is no need to perform a shuffling operation of the fuel assemblies (an operation of moving the fuel assemblies loaded in the reactor core to another position in the reactor core). Therefore, the time required to replace the fuel assembly is significantly reduced. In this example, the output distribution in the axial direction is flattened based on the first and second methods described above. Compared to the case achieved only by the first method of making a difference, the fissile material in the fuel assembly can be used effectively, resulting in a new effect of improving fuel economy. That is, when only the first method is used, the content of fissile material in the upper region is significantly higher 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. Therefore, even if the fissile material in the lower region disappears and a large amount of fissile material remains in the upper region, the fuel assembly A 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 this example, the average enrichment of the upper and lower regions of the reactor core is changed to increase the infinite multiplication factor in the upper region, so the difference in the infinite multiplication factor between the upper region and the lower region is does not change throughout the period. Therefore, the power distribution in the axial direction can always be maintained flat throughout the operating period of the nuclear reactor, as described above. 1/1/2 of the total length of fuel assembly A from the lower end in the axial direction
If the fuel assembly is divided into two regions, an upper region and a lower region, between 3 and 7/12, and the infinite multiplication factor is increased in the upper region, flattening of the output can be achieved most effectively. That is, the output peaking coefficient can be made smaller than when gadolinia is added as shown in FIG. Example 2 In this example, instead of the fuel assembly A described above,
Fuel rods 51, 52, 53, 54, 55 shown in Table 3
and 56 are arranged as shown in FIG. 1, and a fuel assembly A1 is arranged in the reactor core.
The reactor core of this embodiment is composed of a fuel assembly A1. The fuel assembly A1, like the fuel assembly A, uses both the first and second methods.

[Table] 51 and 55 are fuel rods in one area. 52, 53,
54 is divided into two areas, upper and lower, at the 11/24th position from the bottom end. The enrichment levels in the upper and lower regions are uniformly distributed along their respective axes. 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] As is clear from Table 4, even in fuel assembly A1, the average enrichment in the upper region is larger than that in the lower region, and the difference in enrichment between the center and peripheral regions in the cross section is smaller than that in the upper region. is smaller than the lower area. The core section in which the fuel assembly A1 having such an enrichment distribution is arranged has a difference in infinite multiplication factor in the upper and lower regions that is similar to that of the core section 10 of Example 1, and the axial output The distribution is also the 8th
It is almost the same as the figure. In this example, compared to Example 1, the difference in enrichment in one fuel rod is large;
The characteristics of Example 2 are that inspection during manufacturing is relatively easy and that the local output peaking is 1.12, which is very small. In the second embodiment, the same effects as in the first embodiment can be obtained. The control rods inserted into the reactor core during operation are only deep control rods, as shown in FIG. The boundary between the upper region and the lower region of fuel assembly A1 is 1/3 to 7/1 as in fuel assembly A described above.
It is desirable to locate it in the range of 2. Example 3 In this example, instead of the fuel assembly A of Example 1, a fuel assembly A2 is constructed in which six fuel rods having the enrichments shown in Table 5 are arranged as shown in FIG.
is placed in the reactor core. Fuel assembly A2
The above-mentioned second method (the average enrichment distribution in the plane perpendicular to the axis is almost the same in the upper and lower regions, and (the enrichment distribution is changed).

[Table] Fuel rods 61, 62, 63, 64, 65 and 6
No. 6 is divided into two regions at the 11/24th position from the bottom of the fuel assembly, and the enrichment is different between the upper and lower regions. The enrichment of the upper and lower regions is uniformly distributed in the axial direction. The concentration distribution in this example is as shown in Table 6.

[Table] In fuel assembly A2, the average enrichment is 2.37% by weight in the upper region and 2.33% by weight in the lower region, which are almost the same, but the enrichment distribution in the cross section is different between the upper region and the lower region. There is. That is, in the upper region, the difference in enrichment between the center and the periphery is small, and in the lower region, this difference is large, and the enrichment in the periphery is much larger in the upper region than in the lower region. Since the peripheral portion has higher importance than the central portion, the infinite effective multiplication factor is approximately 2.2% higher in the upper region than in the lower region of the fuel assembly A2 even if the average enrichment of the plane is the same. Such a fuel assembly A
The infinite multiplication factor of the upper region of the core where 2 is placed is:
It is naturally larger than that in the lower region, and the axial power distribution is flattened as shown in FIG. Example 3 was able to achieve the same effects as Example 1, and was characterized in that the reactivity deterioration due to combustion showed a relatively even tendency in the upper and lower regions compared to Examples 1 and 2. be. Fuel assembly A2
is in the range of 1/3 to 7/12, similar to fuel assembly A.
It is desirable to locate the boundaries of the upper and lower regions. Embodiment 4 The reactor core 25 shown in FIG. 13 is configured by loading two types of fuel assemblies, the above-mentioned fuel assembly A and the below-mentioned fuel assembly B. As shown in Table 7, fuel assembly B includes fuel rods 41, 42, 43, 44, which have uniform enrichment in the axial direction.
45 and 46 and two water rods 24 to the 14th
They are arranged as shown in the figure. Therefore, the fuel assembly B has a constant infinite multiplication factor in the axial direction.

[Table] The infinite multiplication factor in the upper region of the core 25 is naturally larger than that in the lower region because the fuel assembly A is loaded therein. This embodiment can obtain the same effects as the first embodiment. As shown in FIG. 9, the control rods inserted into the reactor core 25 are only deep control rods, which simplifies control rod operation. As the operating time of the nuclear reactor progresses, the infinite multiplication factor in the upper region of fuel assembly B becomes larger than the infinite multiplication factor in its lower region. In other words, in a boiling water reactor, the amount of plutonium produced in the upper part of the reactor core is greater than in the lower part due to neutron energy and spectrum hardening due to voids in the coolant.
For this reason, even if a fuel assembly with uniform enrichment in the axial direction, such as fuel assembly B, is used, the amount of fissile material in the upper region is relatively greater than that in the lower region, and the upper region naturally The infinite multiplication factor of becomes larger than the lower region. It is also possible to use only fuel assembly B, which has a simple structure, as a replacement fuel assembly. In this example, fuel assemblies B with a simple structure are loaded in the reactor core together with fuel assemblies A with a complex structure, so the core configuration is simpler than when the core is made up of only fuel assemblies A. become. For this reason,
The number of fuel assemblies A with a complicated structure is reduced compared to the case where the core part is composed of only fuel assemblies A,
Manufacture of fuel assemblies becomes easier. Even in a nuclear reactor having a reactor core in which the fuel assemblies A1 and A2 described above are combined with the fuel assembly B, the same effects as in the fourth embodiment can be obtained.

[Brief explanation of the drawing]

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. Figure 4 is a system diagram of a boiling water reactor, and Figure 5 is a diagram showing the power distribution in the axial direction of a reactor core loaded with a fuel assembly having fuel rods containing fuel rods. FIG. 6 is a partial plan view of the core of a nuclear reactor according to a preferred embodiment of the present invention, and FIG. 7 shows the core of FIG. 6. Fig. 8 is a characteristic diagram showing the axial power distribution of the reactor core shown in Fig. 6, and Fig. 9 is a cross-sectional view of the fuel assembly loaded in the reactor core shown in Fig. 6 during reactor operation. An explanatory diagram showing a control rod pattern, FIG. 10 is an explanatory diagram showing a control rod pattern during reactor operation in a conventional reactor core, and FIGS. 11 and 12 are other implementations of the fuel assembly shown in FIG. 7. FIG. 13 is a local plan view of the core of a nuclear reactor according to another embodiment of the present invention, and FIG. 14 is a cross-sectional view of a fuel assembly loaded in the core of FIG. 13. It is a diagram. 9...Reactor pressure vessel, 10...Reactor core, 12
... Control rod, 16, A, A1, A2, B ... Fuel assembly, 21 ... Pellet, 22 ... Cladding tube.

Claims (1)

[Scope of Claims] 1. A plurality of first fuel assemblies that are divided in the axial direction into an upper region and a lower region, and the infinite multiplication factor of the upper region is larger than that of the lower region; A plurality of second fuel assemblies, which are uniform in the axial direction, are arranged in the reactor core where coolant flows from the bottom to the top and voids occur at the top, and are used as control rods for power distribution adjustment. , A nuclear reactor comprising only a plurality of deep insertion control rods arranged in the radial direction of the reactor core. 2. The nuclear reactor according to claim 1, wherein the average enrichment in the upper region of the first fuel assembly is higher than that in the lower region.