JPH03267795A - Boiling water reactor - Google Patents

Abstract

PURPOSE:To constitute a reactor core for a next cycle which is proper even in the case of deterioration in reactor core characteristics due to variation in cycle length by preparing two types of fuel assemblies which are designed differently as two kinds of combustible poison-containing fuel rods, and devising them specially. CONSTITUTION:The fuel rods G1 and G2 which contain gadolinium are an overall-length gadolinium containing fuel rod which has low-concentration gadolinium at its upper half part and high-concentration gadolinium at its lower half part and a normal fuel which contains gadolinium only at its lower half part and no gadolinium at its upper part part. Then a fuel assembly A of type 1 includes both the fuel rods G1 and g2 and a fuel assembly B of type 2 includes relatively many fuel rods G1 only. Those assemblies A and B are used at the same time as new replacing fuel and their ratio is selected properly according to an initial programmed operation cycle, a halfway stop in a last cycle, or stretch operation.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to boiling water nuclear reactors, and is concerned with the flexibility of fuel design, especially the reactor shutdown margin, surplus reactivity, and suppression of power peaking. It is related to the improvement of

[Prior art] In boiling water reactors (BWRs), fuel materials (uranium dioxide, UO,) and burnable poisons (generally, gatolinium oxide,
A burnable poison-containing fuel (gadolinia-containing fuel rod) mixed with gadolinia (Gd2O, ) is used in some of the fuel rods.

A certain amount of surplus reactivity is required for continuous operation of a nuclear reactor, but this surplus reactivity decreases as the fuel burns.
The fuel core configuration is performed so that the temperature is T degrees τ at the end of normal continuous operation. That is, by periodically replacing the fuel assembly in which combustion has progressed with new fuel, the surplus reactivity necessary for continued continuous operation is ensured. In a typical reactor operation plan, one unit cycle is 12 months of continuous operation and 3 months of regular inspection shutdown, and the number of new fuel assemblies replaced per cycle is the total number of core fuel assemblies (1100 MWe class nuclear reactor). Approximately 1/3 of the total (approximately 800 bodies produced in the furnace).

The surplus reactivity given at the beginning of the cycle by such fuel replacement will be approximately 10%△ in the case of continuous operation for 12 months, although it depends on the cycle length, and most of it will be due to neutrons from burnable poisons in the new fuel. Suppressed by absorption. The remaining surplus reactivity is suppressed by the control rods, and the insertion amount of the control rods is adjusted so that the surplus reactivity of the reactor core is exactly τ under rated power operating conditions. The operating margin of this control rod is 1~
2% ΔK is appropriate, and operation is possible even beyond this range, but it becomes difficult to properly suppress power peaking and suppress reactivity during reactor shutdown (ensuring shutdown margin). .

As mentioned above, burnable poison suppresses most of the reactor's surplus reactivity, and the control rod operating margin is 1 to 2% ΔK.
This is an important design element to appropriately control the excess reactivity in each cycle. Moreover, in this case, the surplus reactivity varies depending on the loading ratio of new fuel depending on the cycle length, and furthermore, fuel production takes more than two years from arranging the enriched uranium to completion, so it is possible that troubles with the reactor plant equipment etc. A fuel design must be prepared that flexibly responds to changes in operation plans.

Conventionally, the so-called two-stream method, which uses two types of new fuels with different designs of burnable poison (gadolinia), has been known as a method to provide flexibility in fuel design in response to planned changes in cycle length. ing. For example, JP-A-5
In the method disclosed in Publication No. 8-178286, focusing on the reactor shutdown margin, one new fuel assembly is controlled in a core where the fuel assembly replacement ratio is about 1/3 or more of the total core fuel. When two bodies are loaded around a rod, one body is ('a
When three new fuel assemblies are loaded around one control rod in a reactor core that uses fuel rods containing more burnable poison or one more fuel assembly, and has a fuel exchange ratio of 50% or more of the total core fuel. One uses a fuel assembly with one more fuel rod containing burnable poison than the others. Also, JP-A-62-25
In the method disclosed in Publication No. 0393, focusing on the operating cycle length of the reactor, two types of new fuel assemblies are loaded into the reactor, and burnable poisons in one of the types of fuel assemblies are used. The concentration is lower than that of the other type of fuel assembly.

Figure 3 shows the 8×
A cross-sectional view of the Type 8 fuel assembly loaded into the reactor core is shown.

In the figure, the 8×8 type fuel assembly, generally designated by the reference numeral 80 (hereinafter referred to as “conventional 8×8 fuel”), is No.
, No. 1 to No. 62, a total of 62 fuel rods 81, and two hollow Ota rods W that do not contain fuel material and allow cooling water to flow through them are arranged in a square grid of 8 rows and 8 columns, These are housed in a channel hook 10 made of Zircaloy, and as shown in the figure, it is a cross-shaped control rod! and loaded into the reactor adjacent to the instrumentation tube l. During power operation of the nuclear reactor, cooling water (coolant) flows inside and outside the fuel assembly from the bottom to the top, removing heat generated by the fuel rods 81.

At that time, the coolant flow path is divided into an inch channel region I and a bypass region B by the channel hox 10, but approximately 98% of the heat generated during the reactor's power operation is
occurs in the inch-jannel region ■ due to nuclear fission of the fuel. This amount of heat is removed by channeling approximately 90% of the core flow through the inch channel region I.

Additionally, the overall structure of this conventional 8×8 fuel 80 is
As shown in the figure.

In FIG. 4, the fuel rod 81 and the upper and lower end plugs (not shown) of the water port W are fixed by an upper tie plate 2 and a lower tie plate 3 made of stainless steel, respectively. Furthermore, support grids S are arranged at regular intervals along the assembly, so that the spacing between the fuel rods is kept constant.

FIG. 5 shows the internal structure of the fuel rod 81, and shows the fuel material 82.
is loaded into a cladding tube 83 made of Zircaloy, and both ends of the cladding tube 83 are sealed with an upper end plug 84 and a lower end plug 85 made of Zircaloy. Note that a free space (Blenheim region) is provided within the fuel rod 81 in which a spring 86 is arranged to prevent movement of the fuel substance. The volume of this Blenheim region has the role of accumulating nuclear fission product gas generated from the fuel and adjusting the internal pressure of the fuel rod 81 so that it does not become excessive as combustion progresses.

Figure 'jiS6 is a sectional view showing the arrangement of fuel rods containing gadolinia in a conventional fuel assembly. In this example, seven fuel rods G containing gadolinia are used.

Fig. 7 is a diagram showing the effect of suppressing the surplus reactivity of a burnable poison (gadolinia). It is shown that the excess reactivity decreases to the solid line and is suppressed by using it.

At the end of a nuclear reactor's operating cycle, if the effects of burnable poisons have not disappeared, cycle burnup will be lost, so to avoid this, it is necessary to appropriately select the concentration of burnable poisons. For example, the cycle burn-up is 10. OOOMW
A burnable poison concentration of up to about 5% per d/l is suitable.

On the other hand, the excess reactivity at the beginning of the cycle can be suppressed to a desired value by selecting the number of fuel rods containing burnable poison. In the case of 8x8 type fuel, the reactivity suppression effect per fuel rod containing burnable poison such as cadrinia is about 3%Δ, and as mentioned above, when using 7 wood fuel rods containing burnable poison. can suppress excess reactivity at the initial stage of the cycle to about 20%Δ. However, if the loading ratio of new fuel is approximately 1/3, the amount of suppression of excess reactivity of the reactor is approximately 7%.
It is in Δ.

[Problems to be Solved by the Invention] The techniques disclosed in the preceding Japanese Patent Application Laid-Open Nos. 58-178286 and 62-250393 all focus on reactor shutdown margin and excess reactivity. It is not intended to improve power peaking characteristics related to thermal performance, which is important for satisfying core design flexibility with respect to cycle length fluctuations. However, in actual continuous operation of a BWR, variations in individual cycle lengths cause variations in the output distribution, resulting in output peaking, which is another problem.

Furthermore, in the methods disclosed in Japanese Patent Application Laid-open Nos. 58-178286 and 62-250393, two types of fuels having different burnable poison concentrations and different numbers of burnable poison-containing fuel rods are combined. However, these methods simply combine the gadolinia concentration and the number of gadolinia-loaded fuel rods, and the cycle length described below It is not effective in suppressing the occurrence of output peaking due to unplanned changes in power.

Normally, the periodic inspection starts from the end of the reactor periodic inspection (reactor startup).
The length of the period during which the reactor is shut down (reactor shutdown) is determined with priority given to the inspection of reactor equipment, and therefore, for safety reasons, it does not significantly exceed a certain standard operating period. On the other hand, in order to increase the operating rate of a nuclear reactor, the longer the continuous operation period, the better. Taking these two things into consideration, the planned operating cycle length of a nuclear reactor is automatically determined, and the operating period (cycle length) assumed in JP-A-58-178286 and JP-A-62-250393 is The planned changes in (a) are actually a closure. That is, by fixing the target operating period, the cycle burn-up is determined, and thereby the concentration of the burnable poison is also uniquely determined. Therefore, from a practical point of view,
In particular, there is little need to prepare two types of fuel using burnable poisons of different concentrations as described in Japanese Patent Application Laid-Open No. 62250393.

The present invention is designed to cope with fluctuations in the operating period due to unplanned reactor shutdowns, rather than planned changes in the operating cycle as envisaged in the prior art described above. Such unplanned fluctuations in the operating period may affect the characteristics of excess reactivity, reactor shutdown margin, and power peaking for the core configuration of the next cycle compared to the case where a certain planned operation continues smoothly. Significantly worsen.

In other words, the purpose of the present invention is to prevent unplanned operation, such as when a nuclear reactor that has been constructed according to a planned cycle length is forced to shut down in the middle of the cycle due to a failure of equipment in the reactor plant. The core configuration for the next cycle can be easily achieved with the normal operating length and the fuel combination that has already been prepared in response to period fluctuations, and if the previous cycle was configured in advance due to some unplanned reason. Even in the case of extended operation (stretch operation) exceeding the cycle burnup of the core, it is possible to similarly achieve the core configuration for the next cycle without difficulty.

[Means for Solving the Problems] According to the present invention, the cycle length can be reduced by specially devising two types of fuel assemblies with different designs of two types of fuel rods containing burnable poison that have been prepared in advance. Even if core characteristics deteriorate as a result of fluctuations in temperature, the core configuration for the next cycle can be achieved without difficulty.

That is, in the boiling water reactor according to the specified invention of the present application,
In a reactor core loaded with a fuel assembly formed by arranging a plurality of fuel rods in a square lattice, the fuel assembly is the first fuel assembly.
It consists of two types of fuel assemblies: type and second type. The first type of fuel assembly includes two types of fuel rods containing burnable poison, a first type and a second type, and most of the first type of fuel rod containing burnable poison is flammable except for the upper and lower ends. A poisonous oxide fuel is loaded, and the second type of burnable poisonous fuel rod is loaded with a burnable poisonous oxide fuel only in an area of about 1/2 the length of the lower part of the fuel rod, excluding the lower end. . Further, in the second type fuel assembly, one type of burnable poison fuel rod similar to the first type burnable poison fuel rod is included in the fuel rod than in the first type fuel assembly. There are multiple units installed, which is a relatively large number.

Furthermore, in the boiling water nuclear reactor according to the invention according to claim 2, the first type of burnable poison-containing fuel rods included in the first type and second type fuel assemblies have a length of about 1
/2 is divided into two regions, upper and lower, with the upper part containing a lower concentration of burnable poison than the lower part, and the second type of burnable poison contained in the first type of fuel assembly. The number of entering fuel rods is said to be multiple.

[Function] To simplify the explanation below, the reactor is composed of 764 fuel assemblies with an average enrichment of approximately 3.0 wt% at the initial stage of combustion, and the planned number of continuous operation months is 13 months. Let's take a case where this is the case as an example. At this time, the cycle burnup is approximately 9800
MWd/l, number of fuel assemblies replaced per cycle is 2
The target of 13 months of operation can be achieved with 40 units (approximately 30%).

Now, assuming that the operation of a nuclear reactor configured in this way is stopped two months before the planned value due to trouble in the reactor plant, the surplus reactivity will be approximately 1% ΔK (1500 MW
The reactor will be shut down in the remaining state (equivalent to cycle burnup of d/l). If the fuel is replaced in this state, the number of new fuels may be less than 240, assuming that the next cycle will run for 13 months. However, the reactivity of the fuel changes in a complicated manner with respect to the burnup, as shown in FIG. 8, for example. Figure 8 shows a typical example where the average enrichment of the fuel is 3.0%, the number of fuel rods in the Gadolinian is 7,
It shows the changes in burnup and infinite multiplication coefficient in the output operation state of fuel with a gadolinia concentration of 4wt% (moderator void volume fraction 40%).The reactivity of new fuel is small at the beginning of combustion, and as combustion progresses, it increases. It shows that the reactivity increases and then decreases. In this way, the reactivity of the fuel changes non-linearly with respect to the burnup, so if the loading ratio of new fuel changes, the value of the surplus reactivity at the beginning of combustion changes greatly,
The margin for reactor shutdown will also change. In general, in the case of a fuel design in which the number of fuel rods containing burnable poison is one type (e.g. 8 rods per assembly), if the next cycle is operated as planned under the conditions of early shutdown of the previous cycle as described above, the margin for reactor shutdown is It gets tough. To cope with this, the present invention uses a fuel assembly of the second type described above, in which the number of fuel rods containing burnable poison is relatively increased (for example, nine).

On the other hand, if the operation of a nuclear reactor configured for 13 months is extended by 2 months (stretch operation) due to some plan change, the excess reactivity will be approximately 1%Δ, contrary to the case of early shutdown described above. The reactor is shut down when there is a shortage of energy. This operation is so-called coast-down operation, which is performed under a partial load condition in which the rated output is not produced at the end of the cycle.

In this case, the number of new fuels loaded will be greater than 240 for the next cycle of 13 months of operation. However, ¥S
As shown in Figure 8, the reactivity of the new fuel is small at the beginning of combustion, so the surplus reactivity at the beginning of the cycle becomes too small, and if the surplus reactivity becomes smaller than τ, the rated output cannot be achieved at the beginning of the cycle. This will cause inconvenience. Therefore, the present invention addresses this problem by using a fuel assembly with a relatively small number of burnable poison-containing fuel rods (for example, 7) as the first type of fuel assembly.

In this way, two types of fuel assemblies with different numbers of gadolinia-filled fuel rods are prepared in response to fluctuations in cycle length from the planned value, and the core configuration for the next cycle is adjusted between the two types of fuel assemblies. Perhaps this is achieved reasonably by adjusting the composition ratio, but this method alone is effective against reactor shutdown margin and excess reactivity, but is insufficient as a countermeasure against power peaking. In other words, if the surplus reactivity changes, the control rod insertion depth changes, so if the surplus reactivity is particularly high, the control rod insertion depth must be increased, and at this time, a high power beak (bottom peaking) occurs at the bottom of the fuel. occurs.

In BWR, the void ratio increases in the height direction of the fuel, so
The power distribution tends to be higher in the lower part of the fuel where there are few voids and the neutron moderation effect is high. Furthermore, if the insertion depth of the control rods is increased too much, the output of the upper part of the fuel will be suppressed, and the output of the lower part will be relatively high, emphasizing bottom peaking.

Excessive peaking must be avoided because it may locally increase the fuel output and exceed the local output design limit for ensuring fuel integrity.

In the present invention, the above-mentioned two types of fuel are not used, and in order to prevent the above-mentioned peaking, two types of burnable poison-containing fuel rods, first and second, are installed in the first type fuel assembly. In this case, most of the fuel rods containing burnable poison of the first type are loaded with oxide fuel containing burnable poison except for the upper and lower ends, and the fuel rods containing burnable poison of the second type are loaded with oxide fuel excluding the lower ends. Oxide fuel containing burnable poison is loaded only in the lower half of the length of the fuel rod. On the other hand, the fuel rods containing only one type of burnable poison, similar to the fuel rods containing burnable poison of the first type, are inserted into the second type fuel assembly from the first type fuel assembly. A relatively large number of fuel rods are also arranged.

In particular, in the invention according to claim 2, in the above configuration, the fuel rod containing the first type of burnable poison is approximately 1/1/2 of its length.
It is divided into upper and lower areas with 2 as the boundary, and the upper part is closed to the lower part and contains a low concentration of burnable poison.
The first type of fuel assembly includes a plurality of fuel rods containing the second type of burnable poison.

In this way, the distribution of burnable poison in the first type of fuel assembly is different in the upper and lower three regions, and the amount of poison is relatively large in the lower half region, and moreover, compared to the first type, the distribution of burnable poison is different in the upper and lower three regions. In the second type, the number of burnable poisons or fuel rods is relatively large.

These two types of fuel assemblies are used simultaneously as replacement fuel for new fuel, and the loading ratio of both can be adjusted depending on the intended planned operation cycle, in the case of a stoppage in the middle of the previous cycle, or in the case of stretch operation. By appropriately selecting the
By arranging the partial length poison in the region, it becomes possible to effectively suppress the occurrence of the bottom peak at the initial stage of combustion.

In addition, in a design using the second type of fuel assembly in which toxic substances are placed along the entire length, it is possible to suppress the occurrence of the bottom peak in the middle stage of combustion by lowering the concentration of toxic substances in the upper part and increasing the concentration of toxic substances in the lower part. .

In addition, in the invention according to claim 2, the number of fuel rods containing burnable poison of the second type included in the fuel assembly of the first type is plural. This is because the difference in reactivity due to gadolinia between these types of fuel assemblies is too small to achieve the desired effect of the present invention. According to an example described later, a sufficient effect can be obtained when at least two Gadolinian fuel rods of the second type are arranged in a fuel assembly of the first type, and the poisonous substance is arranged by one tree in terms of the total length. It has been confirmed that this can be obtained, and from a practical point of view, it can be said that it is realistic to set the number to 2 or more and 4 or less.

To facilitate understanding of the features and advantages of the invention, preferred embodiments of the invention are described below.

[Example] % I The two types of fuel assemblies shown in Figures 1a and 1b are the basic components of the nuclear reactor of the present invention.

In the case of Kono, both use fuel rods containing 9 wood quadrilinear, or 2f! for the first type of fuel A in Figure 1a! i
Gadolinia-containing fuel rod G1. G2 is used, one of which has seven fuel rods G1 and the other fuel rod G2 has two. The fuel rod G1 has an upper half as shown in Fig. 2a.
is a full-length gadolinia-containing fuel rod containing gadolinia at a low concentration a (approximately 4 wt% in this example) and the lower half contains gadolinia at a high concentration b (approximately 5 wt% in this example), while fuel rod G2
As shown in Figure 2b, this is a partial-length gadolinia-filled fuel rod in which only about 1/2 of the length of the lower part of the fuel rod is a gadolinia-containing fuel region, and the upper 1/2 is a normal uranium dioxide fuel region that does not contain poisonous substances. It's a fuel rod.

Further, the second type fuel assembly B shown in FIG. 1b is different from the first type fuel assembly A in that all nine gadolinia-filled fuel rods are full-length gadolinia-filled fuel rods Gl shown in FIG. It's different.

In addition, these fuel rods G1. Approximately 15 each of the upper and lower ends of G2
The cm region is loaded with natural uranium fuel pellets and no cadrenia is used. Furthermore, the 15 cm upper and lower end regions of the normal fuel rod 81 that do not contain gadolinia are also natural uranium regions.

The two types of fuels A and B having the above configuration are used simultaneously as replacement fuels for the new fuel tank 4.

The loading ratio is approximately 1/3 of the total core fuel in a planned normal operation cycle, and the loading ratio of the two types of fuel is approximately half each.

If the previous cycle was stopped in the middle of the cycle due to trouble or due to regular inspection process, or if extended operation (stretch operation) was performed, the loading ratio of the two types of fuel should be adjusted appropriately for the core configuration of the next cycle. Respond by choosing. In other words, in the case of a premature shutdown of the previous cycle, the number of new fuel loads for the next cycle can be reduced, but the excess reactivity will be high and the margin for reactor shutdown will be tight.
The second type fuel B shown in FIG. 1b is loaded with priority.

On the other hand, in the case of the previous cycle stretch operation, it is necessary to increase the number of new fuels loaded in the next cycle, but at this time, the first type fuel A shown in FIG. 1a is loaded with priority.

In this way, the number of fuel rods equivalent to the number of fuel rods containing cadrenia differs between the first type and the second type fuel (8 for the 1st type fuel A and 9 for the 2nd type fuel B), so the plan of the previous cycle Flexibility of the core configuration is ensured in response to changes in the next cycle surplus reactivity due to changes. Furthermore, the first type fuel A uses partial length gadolinia in the design of the gadolinia-containing fuel rod, thereby effectively suppressing bottom peaking at the initial stage of combustion. Furthermore, even in the design of the full-length gadolinia (excluding the upper and lower ends) fuel rod G1, by making the upper part have a low concentration and the lower part a high concentration, it is possible to suppress the occurrence of bottom peaking in the middle stage of combustion.

Table 1 shows an example of reactor analysis based on fuel composition.

Table 1 As can be seen from Table 1, a nuclear reactor using the fuel/core configuration according to the present invention can easily adjust the fuel/core configuration of the current cycle in response to an unplanned change in the cycle length of the previous cycle.
easily achieved.

In addition, if we assume an unforeseen fluctuation in the cycle length of ±2 months for the standard number of continuous operation months (13 months in the tree analysis example), the second type fuel B will always have 128 units, and the first type fuel A has a maximum of 148 bodies, and as a reserve for the first type fuel A, the difference between the maximum number and the minimum number, that is, 148-112 = 3
It turns out that all you need to do is prepare 6 bodies.

In the past, unexpected cycle fluctuations made it difficult to configure the core configuration for the next cycle, which could result in the need to adjust the cycle length, but with the present invention, a small number of reserve fuel bodies is sufficient. can.

Analysis case 3: Continuous operation for 13 months in both the previous cycle and the current cycle [Effects of the invention] As described above, according to the fuel core configuration of the present invention,
Even if the cycle length changes due to trouble in the reactor plant, it is possible to provide a reactor that can easily operate the next cycle, and allows for planned operation of the reactor with a small number of reserve fuel bodies. This improves the economic efficiency and safety of nuclear reactor operation.

[Brief explanation of drawings]

Figures 1a and 1b are schematic cross-sectional views showing the configurations of two types of fuel assemblies formed by combining fuel rods containing gadolinia, which constitute the main parts of the nuclear reactor of the present invention, and Figure 2a is a schematic cross-sectional view showing the configuration of two types of fuel assemblies containing fuel rods containing gadolinia of the first type. Schematic diagram showing the axial configuration of fuel rods, 2nd
Figure b is a schematic diagram showing the axial configuration of the second type of fuel rod containing quadrilinear, Figure 3 is a cross-sectional view of a fuel assembly loaded in a general reactor, and Figure 4 shows the external appearance of the fuel assembly. 5 is a partially cutaway perspective view, FIG. 5 is a partial sectional view showing the internal structure of a fuel rod, FIG. 6 is a schematic sectional view showing an example of the arrangement of fuel rods containing gadolinia in a fuel assembly in a conventional nuclear reactor, and FIG. 7 is a partial sectional view showing the internal structure of a fuel rod. FIG. 8 is a characteristic diagram showing the effect of suppressing excess reactivity of burnable poison, and FIG. 8 is a diagram showing the combustion characteristics of a typical fuel assembly with an infinite multiplication coefficient. (Explanation of symbols of main parts) A first type fuel assembly, B second type fuel assembly, G1 first type gadolinia-containing fuel rod, G2.
The second type of quadrinia-filled combustion rod, control rod, and 10 channel hox.

Claims (1)

[Claims] 1. In a boiling water nuclear reactor loaded with a fuel assembly formed by arranging a plurality of fuel rods in a square lattice, the fuel assembly is of two types, a first type and a second type. The fuel assembly of the first type includes two types of fuel rods containing burnable poison, a first type and a second type, and the fuel rods containing burnable poison of the first type include the fuel rods containing burnable poison except for the upper and lower ends. Most of the fuel rods are filled with burnable poison-containing oxide fuel, and the second type of burnable poison-containing fuel rods are filled with burnable poison-containing oxide fuel only in the lower half of the length of the fuel rod, excluding the lower end. fuel is loaded,
The second type of fuel assembly has one type of burnable poison-containing fuel rods similar to the first type of burnable poison-containing fuel rods in an equivalent number of fuel rods compared to the first type of fuel assembly. A boiling water reactor characterized by a relatively large number of nuclear reactors. 2. The fuel rods containing the first type of burnable poison contained in the first type and second type fuel assemblies are divided into upper and lower regions with approximately 1/2 of the length as the boundary, and the upper region is divided into upper and lower regions. A claim characterized in that the fuel rod contains a burnable poison at a lower concentration than the lower part, and the number of fuel rods containing the second type of burnable poison contained in the first type of fuel assembly is plural. The boiling water reactor according to item 1.