JP2003075575A - Boiling water reactor and operation method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize an operation method of a boiling water reactor having a super long life reactor core slow in response to a transient event through an operation period while suppressing the degree of excessive reaction. SOLUTION: In the case of 100% heavy water, the absolute value of void reactivity in an initial state of combustion is as small as about 0.55 $, but the void reactivity is transferred to positive side by the accumulation of<239> Pu generated by neutron capture reaction of<238> U or nuclear fission product according to combustion. Therefore, light water is mixed to the heavy water according to the increase in number of operation years, and its mixing ratio is gradually increased, whereby the neutron spectrum is softened to suppress the increase in positive void reactivity. The light water mixing ratio in coolant is set to 4% in the number of operation years ranging from 0 to 4, 12.5% from 4 to 8, 24% from 8 to 12, 40% from 12 to 16, and 70% from 16 to 20. Accordingly, the void reactivity can be kept at 1 $ or less throughout the operation period.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a boiling water nuclear reactor, and more particularly to a boiling water nuclear reactor capable of operating for a very long life and its operating method.

[0002]

2. Description of the Related Art In commercial power reactors such as boiling water type light water reactors that use enriched uranium oxide fuel with light water as a moderator and a coolant, (Reference 1) "Nuclear Fuel Management Method and Analysis", Etch W. Graves Author, Mikami, Sekimoto Translation, Hyundai Engineering Co.
(1983), pp.275-280, a batch exchange system is adopted in which a part of the fuel assembly is exchanged with fresh fuel in each operation cycle. And the continuous operation period of the current LWR is about one year.

Further, the idea of a water-cooled breeder reactor using the concept of a seed blanket in which fuel (seed) is moved in the axial direction during operation (Reference 2) "A Heavy Water."
Breeder Conceptual Core Design ”
References EPRI NP-2176, Project 712-1, Final Report, 198
Dec. 0 (A.Radkowsky et. Al.,: A Heavy Water Breed
er Conceptual Core Design).

In this document 2, the volume ratio VM / VF of moderator to fuel is set to 0.5 to 0.6, and heavy water is used as a moderator (and a coolant) to enable fuel multiplication. . Further, in this document 2, 20% of light water is mixed in the moderator in order to keep the temperature coefficient at an appropriate value.

[0005]

By the way, as described above, the continuous operation period of the existing light water reactor is about one year,
The continuous operation period is 10 years or more, and if the core that does not require refueling or can be completed once or twice during the life of the plant is realized, in addition to improving the operating rate, simplifying fuel operation, Non-diffusivity can be improved. Therefore, in order to significantly extend the operation period by using light water coolant (H ₂ O) and enriched uranium oxide fuel (UO ₂ ) that have been practically used in light water reactor systems and have the most experience in use, uranium enrichment is required. It is necessary to increase the degree. However, if the enrichment of uranium is increased, as a result, the initial excess reactivity increases, so that the number of rods mixed with burnable poison (gadolinia) increases,
Design measures such as gadolinia concentration increase and control rod value / number increase are required, which may impair fuel economy and increase plant construction costs. In addition, the operating period that can be reached is also limited due to the limit or limitation of the uranium enrichment.

Further, in the technique described in the above-mentioned document 2, the cooling medium temperature coefficient is made negative by mixing 20% of light water into the moderator, but since the mixing ratio of light water is kept constant at 20%, The absolute value of the void reactivity coefficient (coolant temperature coefficient) decreases due to the generation of plutonium and the accumulation of fission products (FP). For this reason, the behavior of the core during transition changes with combustion, which is not desirable.

The object of the present invention is to obtain water (heavy water) with a proven track record.
It is intended to realize a boiling water reactor having an ultra-long-life core having a slow response to a transient event throughout the operation period while suppressing an increase in surplus reactivity by using enriched UO ₂ , and an operating method thereof.

[0008]

In order to achieve the above object, the present invention is configured as follows. (1) In a method of operating a boiling water reactor in which a fuel assembly in which a dense lattice fuel is arranged is loaded, heavy water is used as a coolant in the initial operation of the reactor, and light water is mixed with the heavy water as the operating time elapses. However, the mixing ratio of light water to heavy water gradually increases as the operating time increases.

(2) Preferably, in the above (1),
The mixing ratio of light water is determined so that the void reactivity is 1 $ or less.

(3) Further, preferably, in the above (1) or (2), the fuel material of the dense lattice fuel is enriched uranium.

(4) Preferably, the above (1),
In (2) or (3), the fuel assembly to be loaded has a fuel rod for enclosing the fuel substance and a guide tube for inserting and removing the control rods bundled in a cluster from the upper core, The bundled control rods have a neutron absorbing substance region in which a neutron absorbing substance is enclosed, and a water exclusion region having a smaller neutron absorbing cross-sectional area than the neutron absorbing substance, which is on the tip side of the neutron absorbing substance region, The water exclusion region of the control rod is inserted into the core fuel region during normal operation of the reactor, and the neutron absorbing substance region is inserted into the core fuel region during reactor shutdown.

(5) Further, preferably, the above (1),
In (2), (3) or (4), a partition plate for separating the coolant flowing upward from below in each fuel assembly is installed in the gap between the fuel assembly and the fuel assembly,
Combustible poisons are added to this partition.

(6) In a boiling water nuclear reactor loaded with a fuel assembly in which a dense lattice fuel is arranged, heavy water replenishing means for replenishing heavy water to the core and heavy water and light water are mixed and supplied to the core. In the early stage of operation of the nuclear reactor, heavy water is used as a coolant, and light water is mixed with the above-mentioned heavy water as the operating time elapses. Gradually increase.

(7) Preferably, in the above (6),
The mixing ratio of light water is determined so that the void reactivity is 1 $ or less.

(8) Further, preferably, in the above (6) or (7), the fuel substance of the dense lattice fuel is enriched uranium.

(9) Further, preferably, the above (6),
In (7) or (8), the fuel assembly to be loaded has a fuel rod for enclosing the fuel substance, and a guide tube for putting in and out the control rods bundled in a cluster shape from the upper part of the core. The bundled control rods have a neutron absorbing substance region in which a neutron absorbing substance is enclosed, and a water exclusion region having a smaller neutron absorbing cross-sectional area than the neutron absorbing substance, which is on the tip side of the neutron absorbing substance region, The water exclusion region of the control rod is inserted into the core fuel region during normal operation of the reactor, and the neutron absorbing substance region is inserted into the core fuel region during reactor shutdown.

(10) Further, preferably, the above (6),
In (7), (8) or (9), a partition plate for separating a coolant flowing upward from below in each fuel assembly is provided in a gap between the fuel assembly and the fuel assembly,
Combustible poisons are added to this partition. In the case of 100% heavy water, the absolute value of the void reactivity at the initial stage of combustion is small, but the void reactivity shifts to the positive side due to the accumulation of fission products and the like produced by the neutron capture reaction of the fuel during combustion. Therefore, as the number of years of operation increases, light water is mixed with heavy water, and the mixing ratio is gradually increased to soften the neutron spectrum and suppress an increase in positive void reactivity. This makes it possible to keep the void reactivity below 1 $ throughout the operation period.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the accompanying drawings. In the boiling water reactor which is the first embodiment of the present invention, the fuel assembly 40 shown in FIG.
Two types of fuel assemblies including the fuel assembly 50 shown in FIG. 5 are loaded. FIG. 4 is a vertical (a) and horizontal sectional (b) view of the first fuel assembly 40. In FIG. 4, the fuel assembly 4
0 is a fuel rod 45 (391 pieces) having an outer diameter of 11.7 mm,
Six tie rods 4 for fixing and supporting the fuel rod 45
6, an upper tie plate 48 and a lower tie plate 49 for fixing the tie rods 46, and a seven-stage grid spacer 47 for maintaining a space between the fuel rods 45.

The fuel rods 45 are densely arranged in the shape of a triangular lattice (dense lattice fuel), and the interior of the fuel rods 45 is filled with pellets of concentrated UO ₂ . Above the pellets of the concentrated UO ₂ , a spring 42 for absorbing the fixation of the fuel pellets and the expansion of the fuel pellets during transportation is provided, and a gaseous fission product generated by the nuclear fission in the fuel pellets is provided. A gas plenum 44 is provided for storing the gas. The length of the area containing the fuel pellets is about 120 cm
Is. The coolant is below the fuel assembly 40 (below in FIG. 4).
More inflow, and most of it flows out between the fuel rods 45 and 45 from the upper part. Unlike the conventional boiling water reactor fuel assemblies, since there is no channel box, some of the coolant flows laterally as well as into adjacent fuel assemblies.

The distance between the fuel rods 45 and the fuel rods 45 is 1.3.
mm. When the fuel assembly 40 having the above-mentioned structure is loaded in the reactor, the assembly pitch is about 26 cm. Figure 5
FIG. 3A is a vertical (a) and horizontal sectional view (b) of the second fuel assembly 50. This fuel assembly 50 corresponds to 55 of the fuel rods 45 in the first fuel assembly 40 shown in FIG.
Guide tube 51 for inserting a cluster type control rod into a book
It is replaced with.

As will be described later, when heavy water is used as a coolant, the reactivity has a very small void ratio dependency.
The power distribution in the axial direction of the core has a substantially cosine shape.

Therefore, unlike the existing boiling water type light water reactor in which the control rod is inserted from below, the heavy water reactor according to the present invention adopts the upper core insertion method. The dimensions of the fuel rods 45 and the assembly pitch are the same as those of the first fuel assembly 40. Here, as shown in FIG. 6, the cluster-type control rod 63 has a connecting portion 6 that is connected to a control rod drive mechanism (not shown).
It is connected to the tip of the arm 62 extending from 1.

The cluster-type control rod 63 includes a neutron absorbing substance region 64 in which the B ₄ C pellet is contained, a water exclusion region 65 provided at the tip portion thereof and having a smaller absorption cross section than the neutron absorbing substance, and a neutron absorbing region. He generated by the (n, α) reaction of B-10 with the spring 66 for fixing the absorbing substance
And a gas plenum 67 for accommodating the gas.

During normal operation of the reactor according to the first embodiment of the present invention, the water exclusion area 6 of the cluster type control rod 63 is
5 is inserted into the core region, and at the time of reactor shutdown or abnormality, the entire neutron absorbing material region 64 of the control rod 63 is inserted into the core region.

As a result, the core-average coolant-fuel volume ratio can be reduced, and the internal conversion ratio can be improved.

In order to control the excess reactivity, the neutron absorbing material region 64 of some of the cluster type control rods 63 is inserted into the axial core fuel region even during normal operation.

FIG. 7 is a horizontal sectional view of the core of the boiling water reactor according to the first embodiment of the present invention. In FIG.
The core 70 includes 91 inner uranium enrichment 13.1 wt% inner core fuel assemblies and 60 uranium enrichment 18.4 wt% outer core fuel assemblies, totaling 151 uranium enrichment cores. Is 15.2 wt%. The core 7
The equivalent diameter of 0 is about 3.4 m. Of the inner core fuel assemblies, the first fuel assembly 71 without the control rod guide tube 51.
Is the second fuel assembly 72 having the control rod guide tube 51 and the remaining 76. Further, of the outer core fuel assemblies, the first fuel assembly 73 without the control rod guide tube 51
Is the second fuel assembly 74 having 45 bodies and the remaining 15 bodies having the control rod guide tube 51.

The average coolant-to-fuel volume ratio of the entire core is about 0.5, which is as small as about a quarter of the current light water reactor, and the electric output of the nuclear reactor is 150,000 kW.

FIG. 2 shows the combustion change of infinite neutron multiplication factor when the reactor of the first embodiment of the present invention is operated at 100% heavy water and the combustion change of infinite neutron multiplication factor when operated at light water cooling. It is a graph which compares with a change. The average void fraction of the coolant is about 42%. In FIG. 2, the case of operating with 100% heavy water is shown by the solid line 22 and the light water 10
A case of operating at 0% is shown by a broken line 21. 21 when operating with 100% heavy water, 21 when operating with 100% light water
Compared with the above, the internal conversion ratio becomes large, so that the initial excess reactivity is significantly small, and the operation period is extended, so that the fuel-free operation can be performed for 20 years as compared with 14 years in the case of light water.

In order to maintain the criticality for 20 years with 100% light water, it is necessary to increase the degree of enrichment and to use burnable poisons in order to secure a margin for reactor shutdown. This is because the deceleration capacity of heavy water is smaller than that of light water.

[0031]

[Table 1] However, in Table 1, ξ is the amount of reduction in energy (resergy) per neutron collision, and Σs is a macroscopic scattering cross section. Further, Σa is a macroscopic absorption cross section. As shown in Table 1, since the neutron moderating ability of heavy water (index of moderating ability) is as small as about 1/9 of light water, the neutron energy of the core average increases and the internal conversion ratio increases.

Further, the neutron absorption cross section of heavy water is as small as 1/1000 or less as compared with light water, and the neutron economy is also improved.

Here, the difference in the decelerating ability is also noticeable in the void reactivity coefficient. Figure 3 shows the dependence of the infinite neutron multiplication factor on the coolant void fraction in the early stage of combustion
It is a graph comparing the case of 00% (shown by a solid line 32) and the case of 100% light water (shown by a broken line 31).

In FIG. 3, in the case of light water cooling indicated by the broken line 31, the void ratio dependency of the infinite neutron multiplication factor is very large, whereas in the case of 100% heavy water indicated by the solid line 32, the void ratio dependency. It turns out that is very small.

As a result, in the case of heavy water cooling, the change in reactivity due to the injection of coolant at the time of an accident or the increase in the flow rate due to the failure of the pump or the like is very small, and the behavior with respect to transient events is very slow. Something is expected.

Similarly, the reactivity change during cold shutdown (20.degree. C.) and during normal operation is small, so a furnace shutdown margin can be secured even if the number of control rods is reduced.

As a result, for example, even in the case of 20 years no fuel exchange operation, as shown in FIG. 7, there are seven control rod drive mechanisms (in FIG. 7, there are seven control rod drive mechanism positions 75). Existing).

By the way, in the case of 100% heavy water, the absolute value of the void reactivity at the early stage of combustion is as small as about 0.55 $, but ²³⁹ Pu and fission product FP produced by the neutron capture reaction of ²³⁸ U accompanying combustion are produced. (Fission Prod
The void reactivity shifts to the positive side by the accumulation of (uct).

Therefore, as the number of years of operation increases, light water is mixed with heavy water, and the mixing ratio is gradually increased to soften the neutron spectrum and suppress an increase in positive void reactivity.

That is, as shown in FIG. 1, the mixing ratio of light water in the coolant is 4% from 0 to 4 years of operation, 12.5% from 4 years to 8 years, and from 8 years to 12 years. 24% a year,
The void reactivity (broken line 2) can be kept below 1 $ throughout the operating period by changing it to 400% from 12 to 16 years and 70% from 16 to 20 years.

In the above example, it was shown that the light water mixing rate increases stepwise with the elapsed time of operation, but the same effect can be obtained by continuously changing it.

Here, the mixing of the light water into the heavy water is performed by newly installing a device for injecting the light from the light water tank into the coolant purification system piping or the like using a dedicated pump.

That is, as shown in FIG.
The light water injection system 101 includes a heavy water replenishment tank 102, a light water injection tank 103, a pipe 108 for supplying a coolant to the core in the pressure vessel 100, a circulation pump 106 arranged on the path of the pipe 108, and heat exchange. And a valve 10 arranged between the heavy water supply tank 102 and the pipe 104.
7 and a valve 108 arranged between the light water injection tank 103 and the pipe 104.

Then, the control device (not shown) controls the valves 107 and 108, and the light water is mixed into the heavy water in accordance with the operating time and the light water mixing ratio shown in FIG. 1, for example.

When light water is mixed with heavy water, the effective neutron multiplication factor changes. FIG. 9 is a graph showing changes in the effective neutron multiplication factor, where the thick line 200 shows the case of 100% heavy water, and the thin line 300 shows the case of heavy water diluted with light water.

As can be seen from FIG. 9, even when diluted with light water, the criticality can be maintained for almost 20 years (when the neutron effective multiplication factor is 1.00 or more), which is not a problem.

As described above, according to the first embodiment of the present invention, light water is mixed with heavy water according to the elapsed operating time of the boiling water reactor, and the mixing ratio is changed so as to increase. Therefore, the void reactivity can be maintained below a certain value, and the use of water (heavy water) -concentrated UO ₂ suppresses the increase of the excess reactivity and suppresses the increase of the excess reactivity, and the response to the transient event is very slow. A boiling water reactor having a life core and an operating method thereof can be realized.

Further, the reactor according to the first embodiment of the present invention is located above the pressure vessel in the containment vessel, like the next-generation nuclear reactor disclosed in Japanese Patent Laid-Open No. 6-34783. Even if it is combined with a gravity drop reactor cooling system in which a cooling water pool is installed in the pressure vessel and the cooling water is poured into the pressure vessel by its own weight, the reactivity change due to heavy water injection at the time of the accident is small. , Even if the number of control rods is small, it is possible to secure a margin for shutting down the furnace.

Further, in the technique described in the above-mentioned prior art document 2, it is necessary to take a part of the core fuel into and out of the core region during operation, which complicates the structure and operation of the nuclear reactor, and When the drawn-out core fuel falls, a positive reactivity is inserted, which is not preferable in terms of core safety. However, according to the first embodiment of the present invention, it is not necessary to take in and out the fuel, and therefore the drawn-out fuel drops. Is avoided.

FIG. 10 is an explanatory diagram of the second embodiment of the present invention. In the second embodiment, only the structure of the core is different from that of the first embodiment, and the description of the other configurations will be omitted.

In FIG. 10, in the core 80, a partition plate 82 is provided between adjacent fuel assemblies 81 to prevent the coolant from flowing in the lateral direction of the assemblies.

With this partition plate 82, the fuel assembly 8
The thermal margin can be improved by distributing the flow rate for each one.

In addition, by mixing a burnable poison such as gadolinia into the partition plate 82, the method disclosed in Japanese Patent Laid-Open No.
In combination with the gravity drop reactor cooling system disclosed in Japanese Patent Publication No. 34783, the reactor shutdown margin can be secured even when light water is injected in the event of an accident.

In this case, since heavy water for an accident is not required, the economical efficiency of plant equipment is improved.

As described above, in the second embodiment of the present invention, the same effect as in the first embodiment can be obtained, and in addition, the flow rate is distributed for each fuel assembly 81 to provide a thermal margin. Can be improved.

In the embodiment described above, concentrated UO
_{Although two} fuels are used, the present invention is not limited to this, and the effects of the present invention can be obtained even if the fuel is replaced with a mixed oxide fuel or a fuel in the form of metal or nitride.

[0057]

EFFECTS OF THE INVENTION According to the present invention, light water is mixed with heavy water according to the elapsed time of operation of the boiling water reactor, and the mixing ratio is changed so as to increase the mixing ratio. A boiling water reactor having an ultra long life core (for example, 20 years without refueling) and its operation that can be kept below and that has a slow response to transient events throughout the operation period while suppressing an increase in excess reactivity The method can be realized.

Further, it is possible to realize a boiling water reactor which does not require refueling for a long period of time and can greatly reduce the number of control rod drive mechanisms, which simplifies operation, reduces equipment, and has excellent nuclear non-diffusion property.

[Brief description of drawings]

FIG. 1 is a graph showing changes in a light water mixing ratio and a void reactivity with an operating period in a first embodiment of the present invention.

FIG. 2 is a graph comparing changes in infinite neutron multiplication factor with operation in the case of 100% heavy water and the case of 100% light water.

FIG. 3 is a graph comparing the void ratio dependence of the infinite neutron multiplication factor with 100% heavy water and 100% light water.

FIG. 4 is a diagram showing a first core fuel assembly of a boiling water reactor which is a first embodiment of the present invention.

FIG. 5 is a diagram showing a second core fuel assembly of the boiling water reactor according to the first embodiment of the present invention.

FIG. 6 is a diagram showing a cluster-type control rod in the boiling water reactor which is the first embodiment of the present invention.

FIG. 7 is a horizontal sectional view of the core of the boiling water reactor according to the first embodiment of the present invention.

FIG. 8: Coolant purification / in the first embodiment of the present invention
It is a figure which shows a light water injection system.

FIG. 9 is a graph showing a change in effective neutron multiplication factor when light water is mixed with heavy water as time passes.

FIG. 10 is a horizontal sectional view of a core of a boiling water reactor according to a second embodiment of the present invention.

[Explanation of symbols]

1 Light water mixing ratio 2 Void reactivity 21 Infinite neutron multiplication factor when operating at 100% light water 22 Infinite neutron multiplication factor when operating at 100% heavy water 31 Infinite multiplication factor for light water cooling 32 At 100% heavy water Neutron infinite multiplication factor 40 First fuel assembly 41 Concentrated UO ₂ pellet 42 Spring 43 Fuel rod cladding tube 44 Gas plenum 45 Fuel rod 46 Tie rod 47 Grid spacer 48 Upper tie plate 49 Lower tie plate 50 Second fuel assembly 51 Control rod guide pipe 61 connecting portion 62 cluster type control rod supporting arm 63 cluster type control rod 64 neutron absorbing material 65 water exclusion region 66 spring 67 gas plenum 70 core of boiling water reactor 71 first inner core fuel assembly 72 Second inner core fuel assembly 73 First outer core fuel assembly 74 Second outer core fuel assembly Assembly 75 Control rod drive mechanism position 76 Cluster type control rod support arm 80 Boiling water reactor core 81 Boiling water reactor core fuel assembly 82 Partition plate 100 Pressure vessel 101 Coolant purification / light water injection system 102 Heavy water Replenishment tank 103 Light water injection tank 104 Pipe 105 Fuel exchanger 106 Circulation pump 107, 108 Valve

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ identification code FI theme code (reference) G21C 15/28 G21C 7/26 S (72) Inventor Junichi Yamashita 3-1-1, Saiwaicho, Hitachi City, Ibaraki Prefecture No. Stock Company Hitachi Ltd. Nuclear Business Division

Claims (10)

[Claims] 1. A method of operating a boiling water reactor in which a fuel assembly in which a dense lattice fuel is arranged is loaded, wherein heavy water is used as a coolant at the initial stage of operation of the reactor, and light water is added to the heavy water as the operating time elapses. And a ratio of light water to heavy water is gradually increased as the operating time increases. 2. The boiling water nuclear reactor according to claim 1, wherein the mixing ratio of the light water is determined so that the void reactivity is 1 $ or less. How to operate the furnace. 3. The method for operating a boiling water reactor according to claim 1, wherein the fuel material of the dense lattice fuel is enriched uranium. 4. The method of operating a boiling water reactor according to claim 1, 2 or 3, wherein the fuel assembly to be loaded is a fuel rod for enclosing a fuel substance.
The control rod bundled in a cluster has a guide tube for putting in and out from the core upper part, and the control rod bundled in a cluster has a neutron absorbing substance region in which a neutron absorbing substance is enclosed, and a tip from this neutron absorbing substance region. And a water exclusion area with a smaller neutron absorption cross section than the neutron absorbing material, and inserts the water exclusion area of the control rod into the core fuel area during normal operation of the reactor, and above when the reactor is shut down. A method of operating a boiling water reactor, characterized in that a neutron absorbing material region is inserted into a core fuel region. 5. The method of operating a boiling water reactor according to claim 1, 2, 3 or 4, wherein each fuel assembly is provided in a gap between the fuel assembly and the fuel assembly. A method for operating a boiling water nuclear reactor, characterized in that a partition plate for separating a coolant flowing from the lower part to the upper part in the body is installed, and a combustible poison is added to the partition plate. 6. A boiling water reactor for loading a fuel assembly in which a dense lattice fuel is arranged, for supplying heavy water to the core, and for supplying heavy water and light water to the core. In the initial stage of operation of the nuclear reactor, heavy water is used as a coolant, and light water is mixed with the above-mentioned heavy water as the operating time elapses, and the mixing ratio of light water to heavy water is increased as the operating time increases. A boiling water reactor characterized by a gradual increase. 7. The boiling water reactor according to claim 6,
The boiling water reactor characterized in that the mixing ratio of the light water is determined so that the void reactivity is 1 $ or less. 8. A boiling water reactor according to claim 6 or 7, wherein the fuel material of the dense lattice fuel is enriched uranium. 9. The boiling water reactor according to claim 6, wherein the fuel assembly to be loaded is a fuel rod for enclosing a fuel substance,
The control rod bundled in a cluster has a guide tube for putting in and out from the core upper part, and the control rod bundled in a cluster has a neutron absorbing substance region in which a neutron absorbing substance is enclosed, and a tip from this neutron absorbing substance region. And a water exclusion area with a smaller neutron absorption cross section than the neutron absorbing material, and inserts the water exclusion area of the control rod into the core fuel area during normal operation of the reactor, and above when the reactor is shut down. A boiling water reactor characterized by inserting a neutron absorbing material region into a core fuel region. 10. A boiling water reactor according to claim 6, 7, 8 or 9, wherein each fuel assembly is lowered in a gap between the fuel assembly. A boiling water reactor characterized in that a partition plate is installed to separate the coolant flowing upward from the partition, and a combustible poison is added to the partition plate.