JPH09230079A - Core of boiling water reactor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a core of a boiling water reactor which can be also applied to a high converter promoting conversion to plutonium, and can restrain sufficiently the deformation quantity of a channel box in the vicinity of fuel assembly center part of large neutron flux. SOLUTION: A part of coolant flows out through a through hole 45 →a control rod guide tube 43 inner part → a through hole 51A → a core bypath 24a → a through hole 26. The remaining flows out through a through hole 25 → a reactor core bypath 24b → a through hole 26. By this coolant flow behavior, pressure loss at the through hole 25 of a core lower supporting plate 21 and the through hole 45 of the control rod guide tube 43 is nearly equal to pressure loss at the inlet orifice 52 of a high output fuel assembly 31a. When height of the minimun flow passage part of the core bypaths 24a, 24b is H, the flow area is AB, hydraulic equivalent diameter of the core bypaths 24a, 24b is d, flow passage area of the through holes 25, 45 is At, and flow passage area of the through hole 26 is Au, respectively set, the following relation is realized: At<2> Au<2> /(At<2> +Au<2> )>10(d/H) AB<2> .

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a core of a boiling water reactor, and more particularly to reducing the ratio of a neutron moderator coolant to a fuel and utilizing fast neutrons to convert plutonium to plutonium. The present invention relates to a boiling water reactor core with improved conversion.

[0002]

2. Description of the Related Art A large number of fuel assemblies surrounded by a channel box are arranged in the core of a boiling water reactor. In recent years, a boiling water reactor (hereinafter referred to as high conversion reactor) of a type has been proposed in which the volume ratio of the coolant to the fuel in the core is reduced and the conversion to plutonium is promoted by utilizing fast neutrons. For example, in the boiling water nuclear reactors described in G2 and G3 of the proceedings of the "1994 Autumn Meeting" of the Atomic Energy Society of Japan, a breeding ratio of 1.0 or more is aimed at.

A major advantage of the high conversion reactor is that uranium resources can be effectively used as compared with a conventional general boiling water reactor (hereinafter appropriately referred to as a conventional reactor). However, on the other hand, the creep deformation of the channel box is almost equal to the elastic deformation due to the pressure difference inside and outside the channel box and the product of the fast neutron flux and the residence time in the core (that is, the time integral of fast neutrons in the core). Since they are proportional to each other, in the high conversion reactor, the creep deformation amount of the channel box due to fast neutrons is larger than that in the conventional reactor.

Known techniques for reducing the amount of creep deformation of a channel box in a boiling water reactor include:
For example: Japanese Patent Publication No. 13075/1990 This known technique suppresses the bulging and bending of the channel box by forming a thick portion in the corner portion of the channel box in the conventional furnace.

Japanese Patent Laid-Open No. 63-121788 This known technique suppresses the swelling of the channel box by joining a beam to the inner surface of the channel box in a high conversion furnace.

[0006] In this known technique, in a conventional furnace, the spring between the lower tie plate and the channel box is bent in a concave-convex shape to weaken the spring force to reduce the amount of deformation of the channel box. It reduces and suppresses the increase in leakage flow between the lower tie plate and the channel box.

This Japanese Patent Publication No. 52-15759 discloses that in the conventional reactor, a window is provided in the upper portion of the channel box so that the coolant above the core can easily flow in the event of an accident, thereby cooling the fuel. Is to promote.

[0008]

However, the above-mentioned known technology has the following problems. That is, in the known art, in the fuel assembly arranged in the limited space, the channel box is thinned except for the corner portion, and the area of the core bypass outside the channel box and the amount of the coolant are increased. Will be done. Therefore, it cannot be applied to a high conversion reactor that reduces the volume of the coolant and promotes conversion to plutonium.

In the known technique, the beam also undergoes creep deformation, so that the effect of suppressing the creep deformation of the channel box is small. In addition to this, since the beam and the channel box are fixed, it becomes impossible to inspect the fuel by pulling out the channel box.

In the known technology, only the lower part of the fuel assembly is targeted, and there is no effect in suppressing the deformation amount of the channel box near the central part of the fuel assembly where the fast neutron flux is large.

In the known technique, the deformation amount is suppressed by eliminating the internal / external pressure difference at the upper part of the channel box, but the deformation amount of the channel box near the center of the fuel assembly where the fast neutron flux is large is suppressed. Is not considered, and the prevention of horizontal flow in the entire core due to the provision of windows is not considered.

The object of the present invention can be applied to a high conversion reactor which promotes conversion to plutonium, and to sufficiently suppress the amount of deformation of the channel box near the central portion of the fuel assembly where the fast neutron flux is large. To provide a core of a boiling water reactor.

[0013]

In order to achieve the above object, according to the present invention, a plurality of fuel rods are arranged in a reactor pressure vessel and the outer periphery of a plurality of fuel rods is surrounded by a channel box. A plurality of first fuel assemblies arranged on the radial center side of the pressure vessel and having a relatively high output, and a plurality of second fuel assemblies arranged on the radial outer side of the pressure vessel and having a relatively low output. A plurality of fuel assemblies including assemblies,
From the upper core support plate and the lower core support plate that respectively support the upper end vicinity and the lower end vicinity of the first and second fuel assemblies, and the first lower throttle means provided below each first fuel assembly. A plurality of first coolant channels for leading out the introduced coolant from a first upper opening provided in the core upper support plate through the inside of the channel box of the first fuel assembly; The coolant introduced from the second lower throttle means provided below the second fuel assembly is passed through the inside of the channel box of the second fuel assembly to form the second coolant provided on the core upper support plate. A plurality of second coolant passages led out from the upper opening and a channel outside the channel boxes of the first and second fuel assemblies, a bypass lower throttle means near the lower end, and the core near the upper end. Provided on the upper support plate In a core of a boiling water nuclear reactor having a coolant bypass passage having a bypass upper opening, the flow rate of the coolant flowing through the coolant bypass passage is reduced in place of the bypass upper opening. A bypass upper throttle means having a function is provided in the core upper support plate, and an average value in a flow direction of a pressure gradient in a portion of the coolant bypass flow path excluding the bypass lower throttle means and the bypass upper throttle means. Is substantially equal to the average value in the flow direction of the pressure gradient in a portion of each of the first coolant passages excluding the first lower throttle means and the first upper opening, and the coolant bypass flow is A boiling water reactor characterized in that the pressure loss in the bypass lower throttle means of the passage is made substantially equal to the pressure loss in the first lower throttle means of each first coolant passage. The reactor core is provided. That is, most of the coolant that has flowed into the core from below is introduced into the first and second coolant passages from the first and second lower throttle means, and the first and second fuel assemblies are introduced. After passing the inside of the channel box, the first and second
From the upper opening of the core to above the core upper support plate.
The remaining coolant is introduced from the bypass lower throttle means into the bypass coolant passage, passes through the outside of the channel boxes of the first and second fuel assemblies, and then passes from the bypass upper throttle means to above the core upper support plate. And is merged with the previous flow. At this time, the pressure distribution in the first coolant flow channel drops sharply by the first lower throttle means, which is the inlet, and then gradually decreases in the vicinity of the heat generating portion of the fuel assembly along the flow direction. However, as the boiling starts, the resistance increases and the pressure loss increases accordingly, so that the pressure gradient gradually increases in the flow direction. Also the second
Similarly, in the pressure distribution in the coolant passage, the pressure gradually decreases by the second lower throttle means, and then gradually decreases while increasing the pressure gradient along the flow direction in the vicinity of the heat generating portion. However, since the second fuel assembly has a relatively lower output than the first fuel assembly, the pressure drop by the second lower throttle means is larger than that by the first lower throttle means, and the pressure after that is lower. The value is always smaller than the pressure value of the first coolant passage.
In addition, the pressure distribution in the bypass coolant flow path drops sharply in the bypass lower throttle means, which is the inlet, and then flows except for the bypass lower throttle means and the bypass upper throttle means, which almost correspond to the heat generating portion of the fuel assembly. Although it gradually decreases along the flow direction in the minimum flow passage portion with the minimum passage area, boiling does not occur and pressure loss occurs only due to friction loss, so the pressure gradient in the minimum flow passage portion is almost constant. In contrast to the pressure distribution behavior as described above, in the present invention, by adjusting the opening area of the bypass lower throttle means, for example, the pressure loss in the bypass lower throttle means is reduced to the first lower portion of the first coolant passage. It is approximately equal to the pressure loss in the throttle means. In addition to this, the average value in the flow direction of the pressure gradient in the portion of the bypass flow path excluding the bypass lower / upper throttle means is set to the first lower throttle means / first upper portion of the first coolant flow path. It is made approximately equal to the average value of the pressure gradient in the flow direction in the portion excluding the opening. Due to these two constituent requirements, in the height direction region where the channel box is mounted in the core, that is, in the heat generating portion of each fuel assembly and the minimum flow passage portion of the coolant bypass passage, the first coolant passage is formed. Since the pressure distribution curve and the pressure distribution curve of the coolant bypass passage are relatively close to each other, (the pressure value in the first coolant passage-the pressure value in the coolant bypass passage) becomes relatively small. . Therefore, the pressure difference between the inside and the outside of the channel box can be reduced in the vicinity of the central portion in the flow direction of the fuel assembly where the fast neutron flux is large, so that elastic deformation and creep deformation of the channel box due to the pressure difference can be sufficiently suppressed. Therefore, it can be applied to a high conversion reactor that promotes conversion to plutonium.

Further, in the present invention, for example, the coolant flow velocity in the minimum flow passage portion is made higher than the coolant flow velocity in the first and second coolant flow passages, or the first and second coolant flow passages are provided. At least the flow direction average value of the pressure gradient in the portion upstream of the boiling start position of the coolant and excluding the first and second lower throttle means corresponds to the boiling start position of the coolant bypass passage. It is smaller than the average value of the pressure gradient in the flow direction in the upstream side of the position and excluding the bypass lower throttle means, and the pressure loss in the part of the coolant bypass passage excluding the bypass lower and upper throttle means is reduced. , By making the pressure loss in the bypass lower / upper throttle means larger than the pressure loss in the second coolant passage.
It is also made substantially equal to the average value of the pressure gradient in the flow direction in the portion excluding the lower throttle means and the second upper opening. In this case, the pressure distribution curve of the coolant bypass passage is relatively close to the pressure distribution curve of the second coolant passage in the heat generating portion of each fuel assembly and the minimum passage portion of the coolant bypass passage. Therefore, (pressure value in the first coolant channel-pressure value in the coolant bypass channel) and (pressure value in the coolant bypass channel-pressure value in the second coolant channel) Both are relatively small. Therefore, elastic deformation and creep deformation of the channel box due to the differential pressure can be suppressed more reliably.

[0015] Preferably, in the core of the boiling water reactor, the pressure loss in the bypass lower throttle means of the coolant bypass passage is controlled by the second lower throttle means of each second coolant passage. There is provided a core of a boiling water reactor characterized in that the average value of the pressure gradient in the flow direction in the portion excluding the second upper opening is also substantially equal.

More preferably, in the core of the boiling water nuclear reactor, the minimum flow passage portion of the coolant bypass flow passage having the smallest flow passage area except for the bypass lower throttle means and the bypass upper throttle means. From the coolant flow rate in a portion of the first and second coolant flow paths excluding the first and second lower throttle means and the first and second upper openings. A core of a boiling water reactor is also provided which is characterized in that

More preferably, in the core of the boiling water reactor, the core is arranged in the reactor pressure vessel so as to surround the outermost periphery of the core, and the lower end is supported by the lower core support plate and the upper end is There is provided a core of a boiling water reactor characterized in that an isolation wall supported by the upper core support plate is provided. This makes it possible to reduce the flow passage area of the coolant bypass flow passage by using only the inside of the isolation wall as the coolant bypass flow passage, and therefore it is necessary to supply an extra coolant to the coolant bypass flow passage. In addition, the flow velocity in the coolant bypass passage can be increased without increasing the capacity of the coolant recirculation pump.

Further preferably, in the core of the boiling water reactor, at least upstream of the boiling start position of the coolant in the first and second coolant passages, and the first coolant channel.
And the average value in the flow direction of the pressure gradient in the portion excluding the second lower throttle means is upstream of the position corresponding to the boiling start position in the coolant bypass flow path and excluding the bypass lower throttle means. A core of a boiling water reactor is provided, which is characterized in that the pressure gradient in the open portion is smaller than the average value in the flow direction.

Further, preferably, in the core of the boiling water reactor, the pressure loss in a portion of the coolant bypass passage excluding the bypass lower throttle means and the bypass upper throttle means is reduced to the bypass lower throttle means and the bypass lower throttle means. A core of a boiling water nuclear reactor is provided which is characterized in that the pressure loss in the bypass upper throttle means is made larger.

More preferably, in the core of the boiling water nuclear reactor, the minimum flow passage portion of the coolant bypass flow passage having the smallest flow passage area except for the bypass lower throttle means and the bypass upper throttle means. , H and A _B , respectively, the hydraulic equivalent diameter of the coolant bypass flow passage is d, the flow passage area of the bypass lower throttle means is A _L , and the flow passage area of the bypass upper throttle means is Assuming A _U , A _L ² A _U ² / (A _L ² + A _U ² )> 10 (d / H) A
A boiling water reactor core is provided which is configured to satisfy _B ² .

In order to achieve the above object, according to the present invention, the fuel container is arranged in the reactor pressure vessel, the outer circumference of a plurality of fuel rods is surrounded by a channel box, and the pressure vessel is arranged in a radial direction. A plurality including a plurality of first fuel assemblies arranged on the center side and having a relatively high output, and a plurality of second fuel assemblies arranged on the radially outer side of the pressure vessel and having a relatively low output. Fuel assembly of
And an upper core support plate and a lower core support plate respectively supporting near the upper end and near the lower end of the second fuel assembly, and the first lower throttle means provided below each first fuel assembly. A plurality of first coolant passages for leading the coolant from the first upper opening provided in the core upper support plate through the inside of the channel box of the first fuel assembly; The coolant introduced from the second lower throttle means provided below the fuel assembly is passed through the inside of the channel box of the second fuel assembly to form the second upper opening provided in the core upper support plate. A plurality of second coolant flow paths that are derived from the outer side of the channel box of the first and second fuel assemblies, and a bypass lower throttle means near the lower end and the upper core support plate near the upper end. Bye provided in In a core of a boiling water nuclear reactor having a coolant bypass passage having a cooling upper bypass opening, instead of the bypass upper opening, a function of reducing the flow rate of the coolant flowing through the coolant bypass passage is provided. At least 1 / of the flow direction region corresponding to the heat generating portions of the first and second fuel assemblies in the coolant bypass passage is provided with the bypass upper throttle means provided in the core upper support plate.
The pressure value in the two or more regions is smaller than the pressure value at the position in each corresponding first coolant channel, and each corresponding second value
There is provided a core of a boiling water reactor characterized by having a pressure value higher than a pressure value at a position in the coolant passage.
That is, in the heat generating portion of the fuel assembly and the portion corresponding to 1/2 or more of the flow direction region, the magnitude relation of the pressure values is
The first coolant channel> the bypass coolant channel> the second coolant channel. As a result, the pressure distribution curve of the first coolant passage, the pressure distribution curve of the coolant bypass passage, and the pressure distribution curve of the second coolant passage are relatively close to each other. Pressure value in coolant flow path-pressure value in coolant bypass flow path) and (pressure value in coolant bypass flow path)-
The pressure value in the second coolant passage is relatively small. Therefore, the pressure difference between the inside and the outside of the channel box can be reduced in the vicinity of the central portion in the flow direction of the fuel assembly where the fast neutron flux is large, so that elastic deformation and creep deformation of the channel box due to the pressure difference can be sufficiently suppressed. Therefore, it can be applied to a high conversion reactor that promotes conversion to plutonium.

Preferably, in the core of the boiling water reactor, the channel boxes of the first and second fuel assemblies are downstream including the upper end positions of the heat generating portions of the first and second fuel assemblies. Provided is a core of a boiling water reactor characterized in that a plurality of first openings communicating with the inside and outside of the channel box are formed on the side. This allows
On the downstream side of the heat generating portion, the pressure difference between the inside and outside of the channel box becomes substantially the same through the first opening, and the differential pressure can be extremely reduced, so that elastic deformation and creep deformation at the upper part of the channel box can be suppressed.

Further, preferably, in the core of the boiling water reactor, at least a part of the flow passage area of the plurality of first lower throttle means provided in the plurality of first fuel assemblies is A core of a boiling water nuclear reactor is provided which is 0.3 times or more of a flow passage area in a first fuel assembly. That is, at least a part of the plurality of first lower throttle means is relaxed to reduce the pressure drop by the first lower throttle means, and accordingly the pressure drop by the second lower throttle means. Will also be smaller,
As a result, the pressure distribution curve of the first coolant flow path and the pressure distribution curve of the second coolant flow path are closer together. Therefore, the pressure difference inside and outside the channel box can be further reduced, and elastic deformation and creep deformation of the channel box can be further suppressed. In addition to this, in order to suppress elastic deformation and creep deformation in the upper part of the channel box, for example, a plurality of first and second fuel assemblies including the heat generating parts are provided on the downstream side so as to communicate the inside and outside of the channel box. Even if the first hole is provided, the first coolant passage and the second coolant passage
Due to the small pressure difference with the coolant passage of, the horizontal flow that occurs across the entire core in the radial direction of the pressure vessel is suppressed based on this pressure difference, and the flow vibration of the fuel rods in the fuel assembly is suppressed. The increase can be prevented.

More preferably, in the core of the boiling water reactor, it is arranged in the reactor pressure vessel so as to surround the outermost periphery of the core, and the lower end is supported by the lower core support plate and the upper end is The partition wall supported by the core upper support plate communicates with the inside and outside of the partition wall, which is formed on the downstream side including the upper end positions of the heat generating portions of the first and second fuel assemblies of the partition wall. A core of a boiling water nuclear reactor is provided, further comprising a plurality of second openings. As a result, even if an attempt is made to generate a radial horizontal flow from the center of the pressure vessel to the outer peripheral side due to the pressure difference between the first coolant channel and the second coolant channel, the second opening from the outside of the isolation wall. The coolant is supplied to the second coolant flow path in the outer peripheral portion of the core through the holes. Therefore, the generation of this radial horizontal flow can be suppressed more reliably.

Preferably, in the core of the boiling water reactor, a plurality of control rods arranged in the coolant bypass passage and driven up and down to control the core output, and above the control rods, respectively. A core of a boiling water nuclear reactor is provided, further comprising a plurality of followers connected to each other and having a flow direction length substantially equal to that of the control rod. Thereby, even if the control rod arranged in the coolant bypass passage is operated to move up and down, at least in the vicinity of the heat generating portion of the fuel assembly where the fast neutron flux is large,
The flow passage area and pressure gradient in the coolant bypass flow passage do not change. Therefore, it is possible to prevent changes in the differential pressure between the inside and outside of the channel box and an increase in elastic deformation and creep deformation due to the control rod operation.

[0026]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a first embodiment of the present invention.
This will be described with reference to FIG. FIG. 2 is a vertical cross-sectional view showing the overall structure of a boiling water reactor provided with the core according to this embodiment. In FIG. 2, a reactor pressure vessel 1 holding a coolant
A shroud 11 is provided inside the shroud 0, and a core 12 is housed inside the shroud 11. The coolant is heated by the fuel in the core 12 and boils, and flows into the shroud head 13 as a mixed two-phase flow of water and steam. A large number of steam separators 14 are connected above the shroud head 13,
Further, a steam dryer 15 is installed above the steam separator 4.

The mixed two-phase flow of the coolant and the steam generated in the core 12 is separated into the coolant and the steam by the steam separator 14. The steam separated by passing through the steam separator 14 is steam dryer 1
After being dried at 5, it flows out from the main steam nozzle 16 and is supplied to a turbine (not shown). On the other hand, the main steam nozzle 1
Water supply for replenishing the coolant that has flowed out as steam from 6 is sent to a water supply nozzle 17 by a water supply pump (not shown). The feed water supplied from the feed water nozzle 17 is mixed with the coolant separated by the steam separator 14 in the reactor pressure vessel 10 and is recirculated to the core 12 by the internal pump 18.

FIG. 3 is a cross-sectional view showing the overall structure of the core 12.
Shown in As shown in FIG. 3, a core 12 has a plurality of fuel assemblies 31 arranged in a substantially hexagonal crystal structure in a shroud 11 having a substantially circular cross section, and is arranged between the fuel assemblies 31 to control the core output. And a plurality of control rods 41 for Note that these fuel assemblies 31 are high-power fuel assemblies 31a that are arranged on the radial center side and have a relatively high output.
And a low-power fuel assembly 31b which is arranged on the radially outer side (for example, the outermost side) and has a relatively low output.

FIG. 1 is a partial vertical sectional view showing a further detailed structure of the core 12. In FIG. 1, the fuel assembly 31 includes a lower tie plate 32 and an upper tie plate 3.
3, the outer periphery of a plurality of fuel rods 34 arranged between the fuel cells 3 and 3 is surrounded by a channel box 37. Fuel rod 3
A plurality of spacers 36 are installed to prevent vibration of No. 4, and a gas plenum 35 is provided above the fuel rods 34.

The control rod 41 is provided inside the control rod guide tube 43,
The through hole 51A of the fuel assembly support fitting 51 and the core bypass 24 outside the channel box 37 are vertically driven by a drive shaft 44 driven by a control rod drive device (not shown). At this time, a follower 42 is connected and fixed above the control rod 41 in order to reduce the volume of the coolant in the core bypass 24.
Even when 1 is driven up and down in the core bypass 24, the flow passage area and pressure gradient in the core bypass 24 do not change at least in the heat generating portion of the fuel assembly 31 where the fast neutron flux is large, and It is designed to prevent changes in pressure difference and increase in elastic deformation and creep deformation.

The upper end of the control rod guide tube 43 is connected to the core lower support plate 21, and the fuel assembly support fitting 51 is inserted into the control rod guide tube 43 from above. FIG. 4 is a top view showing the support structure of the fuel assembly 31 by the fuel assembly support fitting 51. In FIG. 4, each fuel assembly support fitting 51 supports three fuel assemblies 31. Further, a through hole 51A through which the control rod 41 moves up and down is provided at the center of each of the through holes 51A.
A plurality of independent flow paths 51B are formed around the area. An inlet orifice 52 is installed at the lower end of each flow path 51B (see FIG. 1), and the fuel assembly 31 is mounted at the upper end of the flow path.

The three fuel assemblies 31 supported at this time
5 is a top view showing the support structure of FIG. As shown in FIG. 5, the distance between the adjacent fuel assemblies 31 and the fuel assemblies 31 is maintained by the positioning members 39 and 38 provided in the channel box 37, and the fuel assembly 3 is maintained.
The upper end of 1 is supported by the core upper support plate 22 via the upper tie plate 33.

FIG. 6A is a partial longitudinal sectional view showing the positioning structure of the fuel assembly 31 by the positioning members 38 and 39.
A partial side view is shown in FIG. First, the control rod 41
In the core bypass 24a (see FIGS. 1 and 5) into which the follower 42 is inserted, the fuel assembly 31 and the fuel assembly 31 have a wide space, so that the space between the fuel assembly 31 and the conventional assembly is used as the positioning member 39 for maintaining the space. A leaf spring or the like provided outside the same channel box 37 is used. on the other hand,
In the core bypass 24b (see FIGS. 1 and 5) in which the control rod 41 and the follower 42 are not inserted, the gap between the fuel assembly 31 and the fuel assembly 31 is narrow, and therefore the plate is formed on the concave surface 37A provided on the side surface of the channel box 37. The spring-like positioning member 38 is arranged, and the convex portion of the leaf spring is brought out to the outside through the opening of the concave surface 37A and abutted against each other to maintain the gap.

The control rod guide tube 43 is provided with a through hole 45 as a narrowed portion at the lower end of the core bypass 24, and the lower core support plate 21 is also provided with a through hole as a narrowed portion at the lower end of the core bypass 24. 25 are provided.

Returning to FIG. 1, the core 12 surrounds the outermost periphery of the core 12 and contacts the lower core support plate 21 at the lower end so as to reduce the flow passage area of the core bypass 24, and the upper core support plate 22 at the upper end. A contact wall 23 is provided for contact. By providing the isolation wall 23, the core bypass 24
It is possible to reduce the flow passage area, and it is not necessary to supply an extra coolant to the core bypass 24, and the capacity of the internal pump 18 can be reduced.

A top view showing the detailed structure of the upper core support plate 22 is shown in FIG. In FIG. 7, the upper core support plate 22
Upper tie plate 3 to ensure its structural strength
The insertion port 22A of No. 3 is smaller than the cross-sectional area of the fuel assembly 31. The upper core support plate 22 is used for the fuel assembly 3
In this case, each fuel assembly 31 has the fuel assembly support fitting 51 and the positioning members 38, 3 removed.
Since it is positioned at 9, the upper core support plate 22 can be easily reset. At this time, in order to facilitate resetting of the upper core support plate 22, the upper end of the upper tie plate 33 has a small cross-sectional area and is tapered. A through hole 26 having a throttling function is provided in the core upper support plate 22 as an outlet for the coolant flowing into the core bypass 24.

In the above structure, during the normal operation of the boiling water reactor, the coolant introduced from below into the channel box 37 of the fuel assembly 31 is the opening of the control rod guide tube 43. 46 → inlet orifice 52 → flow passage 51B of fuel assembly support fitting 51 → lower tie plate 32 of fuel assembly 31 → channel box 3
7 Inside → Upper tie plate 33 → Core upper support plate 22A
And then flows upwards. And at this time, fuel rod 3
A part of the coolant is boiled by the heat generation of No. 4 and becomes a mixed two-phase flow of the steam and the coolant. On the other hand, a part of the coolant guided to the outside of the channel box 37 of the fuel assembly 31, that is, the core bypass 24, is partially through the through hole 45 of the control rod guide tube 43.
→ Inside the control rod guide tube 43 → Through hole 51A of the fuel assembly support fitting 51 → Core bypass 24 where the control rod 41 moves up and down
a-> Flows upward through the through hole 26 of the core upper support plate 22. The rest is the through hole 2 of the lower core support plate 21.
5 → core bypass 24b where the control rod 41 does not exist → flows upward through the through hole 26 of the core upper support plate 22.

Here, with respect to the coolant flow behavior as described above, the core 12 of the present embodiment also has the following two characteristic constitutional requirements. Through hole 25 in lower core support plate 21 and control rod guide tube 4
3, the pressure loss in the through hole 25 of the lower core support plate 21 and the through hole 45 of the control rod guide tube 43, which is the narrowed portion at the lower end of the core bypass 24, is equal to the high power fuel assembly 31a.
The flow passage areas of the through holes 25 and 45 are adjusted so that the pressure loss at the inlet orifice 52 is substantially equal.

Requirements concerning the through hole 25 of the lower core support plate 21, the through hole 45 of the control rod guide tube 43, and the through hole 26 of the upper core support plate 22, that is, the through holes 25, 45 of the core bypass 24,
H the height of the smallest flow path section flow area at the intermediate portion excluding the 26 is the smallest, the hydraulic equivalent diameter of the core bypass 24 d, the flow passage area of the through-hole 25 and the through hole 45 A _L, through Assuming that the flow passage area of the hole 26 is A _U and the flow passage area of the minimum flow passage portion of the core bypass 24 is A _B , A _L ² A _U ² / (A _L ² + A _U ² )> 10 (d / H) The flow passage area A _L and the flow passage area A _U are determined so as to be A _B ² .

The operation and effect of this embodiment will be described in detail below step by step. (1) Necessity to reduce the pressure difference between the inside and the outside of the channel box 37 The deformation amount ε of the channel box 37 provided in the fuel assemblies 31a and 31b is mainly the elastic deformation amount ε ₁ due to the pressure difference ΔP _io between the inside and the outside of the channel box 37. And the amount of creep deformation ε _2, which is approximately given by the following equation.

Ε = ε ₁ + ε ₂ = a ₁ · ΔP _io + a ₂ · ε ₁ · φ · t = a ₁ (1 + a ₂ · φ · t) ΔP _io (a) However, in the formula (a) A ₁ and a ₂ are proportional coefficients, φ is fast neutron flux, and t is the core 122 of the channel box 37.
It is the stay time in. As is clear from the equation (a), if the pressure difference ΔP _io between the inside and outside of the channel box 37 is reduced, the elastic deformation amount ε ₁ and the creep deformation amount ε ₂ can be reduced.

Then, such a channel box 3
In order to prevent the contact between the channel box 37 and the control rod 41 or the follower 42 due to the deformation of No. 7, the gap g between the channel box 37 and the control rod 41 or the follower 42 is set.
Should be larger than the deformation amount ε of the channel box 37 expressed by the equation (a). That is, when the condition therefor is expressed by an equation, g> a ₁ (1 + a ₂ · φ · t) ΔP _io .

Here, in the high conversion reactor which is provided with the core 12 of the present embodiment and promotes conversion to plutonium, the fast neutron flux φ is higher than that of the conventional reactor (that is, the creep deformation amount ε ₂ of the channel box 37). Increased compared to conventional furnaces). And in addition to this, in the core 12,
Amount of coolant in fuel assemblies 31a, 31b and core bypass 24
In order to reduce the amount of coolant in the fuel assemblies 31a and 31b, the flow passage area of the core bypass 24 is narrower than that of the conventional reactor. As a result, the channel box 37 and the control rod 4 are
1 and the gap g with the follower 42 are extremely narrow. Therefore, since the demand for suppressing the deformation amount of the channel box 37 is more severe, the channel box 3
The pressure difference ΔP _io between 7 and 7 needs to be set to, for example, ⅕ of the conventional technique.

(2) Principle of reducing the pressure difference ΔP _io between the inside and outside of the channel box 37 The core bypass 24 and the fuel assembly 31 already described above.
FIG. 8 shows the pressure distribution in the height direction in the coolant flow behavior in a and b. In FIG. 8, the horizontal axis represents the fuel assemblies 31a and 31b.
The relative value of the position in the height direction is represented as 0 to 1.0 for the heat generation portion of the above, and the relative value of the represented pressure is taken on the vertical axis with the pressure below the lower core support plate 21 as the reference. .
For comparison, the pressure distribution in the core bypass 24 of the conventional reactor is also shown. Generally, in the core of a boiling water reactor including the core 12 of the present embodiment, the pressure is uniform in the space below the core lower support plate 21 (lower plenum) and the space above the core upper support plate 22 (upper plenum). However, as shown in FIG. 8, the fuel assemblies 31a, 31b
In the inside, a large pressure loss occurs in the inlet orifice 52, the lower tie plate 32, the spacer 36, and the upper tie plate 33, which have a narrow flow passage area, and the distribution is generally downward rightward except for both end portions. At this time, the absolute values of the pressure loss and the pressure gradient increase as the temperature rises (downstream) due to steam generation due to heat generation of the fuel rods 34. Further, since the fuel assemblies 31a and 31b are separated from each other by the channel box 37, the internal pressure distribution differs due to the difference in the output (heat generation amount), and the inside of the output fuel assembly 31a except for both ends. The pressure value of the coolant is higher. Further, in the high-power fuel assembly 31a, since the amount of steam generated is large and the flow rate of the coolant is small, in the inlet orifice 52 and the lower tie plate 32, which are the coolant single-phase flow parts up to the position (point A) where steam is generated. Although the pressure loss is smaller than that of the low-power fuel assembly 31b, the absolute value of the pressure loss and the pressure gradient becomes large when boiling starts.

On the other hand, regarding the core bypass 24, in the core of the conventional reactor, the through hole of the control rod guide pipe 43 is taken into consideration from the viewpoint of hydraulic stability at the time of natural circulation operation when the recirculation pump is stopped. 45 and the through hole 2 in the lower core support plate 21
The flow passage area of No. 5 was narrowed, the opening of the core upper support plate 22 was extremely large, and the flow passage was wide. As a result, the pressure loss in the through hole 45 and the through hole 25 becomes 90% or more of the total pressure loss, and the absolute value of the pressure gradient in the minimum flow path portion of the core bypass 24 becomes small, which is shown in the lower part of FIG. The pressure distribution is as shown by the broken line, and there is a large difference from the pressure distribution of the high / low output fuel assemblies 31a, 31b shown by the solid line. And by doing this, the channel box 37
The maximum pressure difference ΔP _io between the inside and outside was as large as about 65% of the total pressure loss. At this time, the coolant flow velocity in the core bypass 24 was also smaller than the coolant flow velocity in the fuel assemblies 31a and 31b.

Therefore, in order to reduce the pressure difference ΔP _io between the inside and the outside of the channel box 37 in the core 12 of this embodiment, the pressure distribution in the core bypass 24 is set as shown in FIG.
The pressure distributions of the high and low power fuel assemblies 31a and 31b indicated by the solid line must be as close as possible. For that purpose, it is necessary to increase the average value in the flow direction of the intermediate portion of the core bypass 24 excluding the through holes 45, 25, 26 which are the narrowed portions at both ends. As one means for this purpose, the pressure loss of the intermediate portion of the core bypass 24 excluding the through holes 45, 25, 26 can be reduced by the through holes 45,
It is conceivable to make it larger than the pressure loss at 25 and 26, and if it is made simpler, the pressure loss ΔP _M in the minimum flow path portion of the core bypass 24 and the pressure loss ΔP _L at the through holes 45 and 25 penetrate Sum of pressure loss ΔP _H at hole 26 (=
It may be considered to be larger than ΔP _L + ΔP _H ).

This can be expressed by an equation, where the coolant density is ρ, the friction loss coefficient in the minimum flow passage portion of the core bypass 24 is f, the height of the minimum flow passage portion of the core bypass 24 is H, and the core bypass 24 is , D is the hydraulic equivalent diameter, v is the coolant velocity in the core bypass 24, and the through hole 25 and the through hole 45.
Pressure at A _L, zeta _U through hole 26 the pressure loss coefficient of zeta _L, the flow path area of a minimum flow path portion of the core bypass 24 A _B, the flow passage area of the through-hole 25 and the through hole 45 in loss factor, the flow passage area of the through-hole 26 as _{a U, 0.5ρ (f · H} / d) v 2> 0.5ρ (ζ L (a B 2 / a L 2) + ζ U (a B 2 / a U ² ) v ² (c) Here, for simplification, if f = 0.05 and ζ _L = ζ _U = 0.5 are substituted as typical representative values in the equation (c), the following transformation is made. A _L ² A _U ² / (A _L ² + A _U ² )> 10 (d / H) A _B ² (d) That is, the flow passage area A _L and the flow passage area are satisfied so as to satisfy the expression (d). the be determined a _U, of the pressure loss in the core bypass 24, 50% or more becomes a pressure loss in the minimum flow path portion, the flow direction mean value of the pressure gradient at the minimum flow path portion of the core bypass 24 size It will be made.

The core 12 of the present embodiment satisfies the above equation (d) as a constituent factor as already described. Therefore, the average value of the pressure gradient in the flow direction in the minimum flow path portion of the core bypass 24 is made larger than that in the conventional reactor, and as a result, the inlet orifices 52 and the insertion ports 22A at both ends of the fuel assemblies 31a and 31b are excluded. It can be approximately equal to the mean value of the pressure gradient in the part in the flow direction.
In addition to this, as a constituent requirement, the through holes 25, 4
The pressure loss at No. 5 is adjusted to be substantially equal to the pressure loss at the inlet orifice 52 of the high power fuel assembly 31a. Therefore, due to these two, in the core bypass 24 of the core 12 of the present embodiment, as indicated by the upper broken line in FIG. 8, the pressure of the high / low output fuel assemblies 31a, 31b shown by the solid line in FIG. The pressure distribution is very close to the distribution. Therefore, (the high-power fuel assembly 31
pressure value in a-pressure value in core bypass 24) and (pressure value in core bypass 24-low power fuel assembly 31
The pressure value in b) becomes relatively small. Therefore, the pressure difference ΔPio between the inside and the outside of the channel box 37 is reduced in the vicinity of the central portion in the flow direction of the fuel assemblies 31a, b having a large fast neutron flux (for example, about 13% of the total pressure loss,
Approximately one-fifth of the conventional technique), and elastic deformation ε ₁ and creep deformation ε ₂ due to the pressure difference between the inside and outside of the channel box 37
And can be significantly suppressed. Therefore, it can be applied to a high conversion reactor that promotes conversion to plutonium.

In the above embodiment, as a constituent requirement, A _L ² A _U ² / (A _L ² + A _U ² )> 10 (d / H) A
_From a different point of view, the configuration of satisfying _B ² is such that the coolant flow velocity in the minimum flow path portion of the core bypass 24 is obtained by excluding the inlet orifice 52 and the insertion port 22A in the fuel assemblies 31a and 31b. The flow velocity of the coolant is larger than that in the portion, or at least the position where the coolant in the fuel assemblies 31a and 31b starts boiling (high-power fuel assembly 31).
The average value in the flow direction of the pressure gradient in the corresponding region of the core bypass 24 in the portion below the point A in a and the point B in the low power fuel assembly 31b (downstream side in the flow direction) and excluding the inlet orifice 52. The fuel assembly 31
This is almost equivalent to making the pressure gradient in a and b larger than the average value in the flow direction. Further, in the above embodiment, depending on the constituent requirements and the pressure distribution of the high / low output fuel assemblies 31a and 31b indicated by the solid line in FIG. Aggregate 31
In the area of the core bypass 24 corresponding to the heat generating parts of a and b, the pressure value in the core bypass 24 is more than half (for example, a large part) from the pressure value in the high power fuel assembly 31a. Lower and lower power fuel assembly 31b
It is almost equivalent to making it higher than the internal pressure value. In addition,
The cross section of the fuel assembly 31 in the first embodiment is hexagonal, but may be square.

A second embodiment of the present invention is shown in FIGS.
This will be described below. Members equivalent to those in the first embodiment are denoted by the same reference numerals. FIG. 9 is a partial vertical cross-sectional view showing the detailed structure of the core 212 according to the present embodiment, and FIG. 10 shows the pressure distribution in the height direction in the coolant flow behavior in the core bypass 24 of the core 212 and the fuel assemblies 31a and 31b. Show. These are views corresponding to FIGS. 1 and 8 of the first embodiment, respectively. Note that FIG. 10 also shows the pressure distribution outside the isolation wall 23 in order to clarify the action and effect.

The core 212 of this embodiment differs from the core 12 of the first embodiment in that the channel boxes of the first and second fuel assemblies are the fuel assemblies 31a, 31b.
A plurality of holes 240 communicating with the inside and outside of the channel box 37 are formed on the downstream side in the flow direction including the upper end position of the heat generating portion of the fuel assembly 31a, b and the core bypass 2
4 is in communication; at least the flow passage area of the inlet orifice 52 provided in the high power fuel assembly 31a is 0.3 times or more the flow passage area in the high power fuel assembly 31a (by the conventional furnace). Inlet orifice 52 and fuel assembly 31
1.3 times or more of the flow passage area ratio to the inside); a plurality of openings communicating the inside and outside of the partition wall 23 on the downstream side including the upper end positions of the heat generating portions of the fuel assemblies 31a and 31b of the partition wall 23. 227 is formed. Other configurations and the flow behavior of the coolant are almost the same as those of the core 12 of the first embodiment.

Also according to this embodiment, the same effect as that of the first embodiment can be obtained. In addition to this, since the inside and outside of the channel box 37 are communicated with each other through a plurality of openings 240, as shown in FIG. 10, the downstream side of the heat generating portion (right side from the 1.0 position in FIG. 10) is opened. Since the inside and outside of the channel box 37 become substantially the same pressure through the hole 240 and the differential pressure can be extremely reduced, elastic deformation and creep deformation at the upper part of the channel box 37 can also be suppressed. Further, since the pressure drop here is reduced by reducing the rate at which the inlet orifice 52 is throttled, and the pressure drop through the insertion port 22A is also reduced accordingly, as shown in FIG. The pressure distribution curve in 31a and the pressure distribution curve in the low-power fuel assembly 31b are closer to each other. Therefore, the pressure difference between the inside and outside of the channel box 37 can be further reduced, and the elastic deformation and creep deformation of the channel box 37 can be further suppressed. In addition to this, since the pressure difference between the high and low fuel assemblies 31a and 31b is small, even if the opening 240 is provided, the entire core 12 can be traversed in the radial direction of the pressure vessel based on this pressure difference. It is possible to suppress the horizontal flow generated and prevent an increase in flow vibration of the fuel rods 34 in the fuel assemblies 31a and 31b.

Further, downstream of the hole 240, a flow from the central portion of the core 12 toward the peripheral portion of the core 12 may occur due to the pressure loss difference based on the difference in the steam generation amount of the high / low power fuel assemblies 31a and 31b. By communicating the inside and outside of the isolation wall 23 with the opening 227, the coolant is supplied from the outside of the isolation wall 23 to the peripheral portion of the core 12 (see the external pressure distribution of the isolation wall 23 at the uppermost dashed line in FIG. 10). The generation of this flow can be prevented.

A third embodiment of the present invention will be described with reference to FIG. 11, the core 312 according to the present embodiment
Is, for example, Study of Design Improvement of Key Reac
tor Components for BWR of the Next Century, The 3r
d JSME / ASME Joint International Conference on Nucl
ear Engineering, Volume 2, April 23-27, 1995 (T.Sai
to, S. Sumida, RCChallberg, CWRelf, S. Kinoshit
a, T. Nakao, T. Fukuda, K. Murano), a coolant guide tube 319 is provided instead of the shroud 11, and the upper core support plate 22 and the lower core support plate 21 are provided in the reactor pressure vessel. 10 is a structure in which the structure of the core 212 of the second embodiment is applied to a core of a type in which the coolant guide pipe 319 is directly supported by the upper core support plate 22 and the lower core support plate 21. In this case, in FIG. 2, the coolant separated by the steam separator 14 is mixed with the water supply supplied from the water supply nozzle 17 in the reactor pressure vessel 10, and then passes through the coolant guide pipe 319 to form an interface. It is recirculated to the core 12 by the null pump 18.

According to this embodiment, the same effect as that of the second embodiment can be obtained. In addition to this, since it has a structure in which it is divided into a plurality of coolant guide tubes 319 and downsized as compared to the shroud 11, it is easier to replace than the shroud 11 when material deterioration due to high fast neutron flux occurs. The advantage is that

In addition to the core 312 of the third embodiment, the core of the boiling water reactor according to the present invention can be applied to any boiling water reactor regardless of the structure other than the core. it can.

[0057]

According to the present invention, the first cooling is performed in the height direction region where the channel box is mounted in the core, that is, in the heat generating portion of each fuel assembly and the minimum flow passage portion of the coolant bypass passage. Since the pressure distribution curve of the material flow path, the pressure distribution curve of the coolant bypass flow path, and the pressure distribution curve of the second coolant flow path are relatively close to each other, (pressure value in the first coolant flow path-cooling The pressure value in the material bypass passage) and (the pressure value in the coolant bypass passage-the pressure value in the second coolant passage) are relatively small. Therefore, the pressure difference between the inside and the outside of the channel box can be reduced in the vicinity of the central portion in the flow direction of the fuel assembly where the fast neutron flux is large, so that elastic deformation and creep deformation of the channel box due to the pressure difference can be sufficiently suppressed. Therefore, it can be applied to a high conversion reactor that promotes conversion to plutonium.

[Brief description of drawings]

FIG. 1 is a partial longitudinal sectional view showing a detailed structure of a core of a boiling water reactor according to a first embodiment of the present invention.

FIG. 2 is a vertical cross-sectional view showing the entire structure of a boiling water nuclear reactor provided with the core shown in FIG.

3 is a cross-sectional view showing the overall structure of the core shown in FIG.

FIG. 4 is a top view showing a structure for supporting a fuel assembly by the fuel assembly support fitting shown in FIG.

5 is a top view showing a support structure for the fuel assembly shown in FIG. 1. FIG.

6A and 6B are a partial vertical cross-sectional view and a partial side view showing the positioning structure of the fuel assembly by the positioning member shown in FIG.

FIG. 7 is a top view showing a detailed structure of the upper core support plate shown in FIG.

FIG. 8 is a diagram showing the pressure distribution in the height direction in the coolant flow behavior in the core bypass and the fuel assembly.

FIG. 9 is a partial vertical sectional view showing a detailed structure of a core of a boiling water reactor according to a second embodiment of the present invention.

FIG. 10 is a diagram showing the pressure distribution in the height direction in the coolant flow behavior in the core bypass and the fuel assembly.

FIG. 11 is a partial vertical sectional view showing a detailed structure of a core of a boiling water reactor according to a third embodiment of the present invention.

[Explanation of symbols]

10 Reactor Pressure Vessel 12 Core 21 Lower Core Support Plate 22 Core Upper Support Plate 22A Insertion Port (First and Second Upper Openings) 23 Isolation Walls 24a, b Core Bypass (Coolant Bypass Channel) 25 Through Hole ( Bypass lower throttle means) 26 Through hole (bypass upper throttle means) 31a High output fuel assembly 31b Low output fuel assembly 34 Fuel rod 37 Channel box 41 Control rod 42 Follower 43 Control rod guide tube 45 Through hole (bypass lower throttle means) ) 51 fuel assembly support metal fitting 52 inlet orifice (first and second lower throttle means) 212 core 227 open hole (second open hole) 240 open hole (first open hole) 312 core

Claims (12)

[Claims] 1. A plurality of fuel rods, which are arranged in a reactor pressure vessel, are formed by surrounding a plurality of fuel rods with a channel box around the outer periphery thereof, and are arranged on the radial center side of the pressure vessel and have a relatively high output. A plurality of fuel assemblies including a first fuel assembly and a plurality of second fuel assemblies that are arranged on the outer peripheral side in the radial direction of the pressure vessel and have a relatively low output, and the first and second fuel assemblies. A core upper support plate and a core lower support plate that respectively support the vicinity of the upper end and the vicinity of the lower end of the fuel assembly;
The coolant introduced from the first lower throttle means provided below each first fuel assembly is passed through the inside of the channel box of the first fuel assembly to form the first coolant provided on the core upper support plate. The plurality of first coolant passages leading out from the upper openings of the second fuel assembly and the coolant introduced from the second lower throttle means provided below each second fuel assembly. A plurality of second lead-outs extending from a second upper opening provided in the core upper support plate through the inside of the channel box of the body
Of the coolant passage and the channel boxes of the first and second fuel assemblies, a bypass lower throttle means is provided near the lower end, and a bypass upper opening is provided in the core upper support plate near the upper end. In a core of a boiling water reactor having a coolant bypass flow path including a portion, a bypass having a function of reducing the flow rate of the coolant flowing through the coolant bypass flow path instead of the bypass upper opening An upper throttle means is provided on the core upper support plate, and an average value in a flow direction of a pressure gradient in a portion of the coolant bypass flow path excluding the bypass lower throttle means and the bypass upper throttle means is set to a first value. Of the cooling medium flow path except the first lower throttle means and the first upper opening, and substantially equal to the flow direction average value of the pressure gradient, The core of a boiling water reactor characterized in that the pressure loss in the bypass lower throttle means of the coolant bypass passage is made substantially equal to the pressure loss in the first lower throttle means of each first coolant passage. . 2. The core of the boiling water reactor according to claim 1, wherein the pressure loss in the bypass lower part throttle means of the coolant bypass passage is set to the second lower part of each second coolant passage. A core of a boiling water reactor characterized in that a flow direction average value of a pressure gradient in a portion excluding the throttle means and the second upper opening is made substantially equal. 3. The core of a boiling water reactor according to claim 2, wherein the coolant bypass passage has a minimum flow passage area except for the bypass lower throttle means and the bypass upper throttle means. The coolant flow velocity in the road
Of the first and second coolant flow paths, the first and second coolant flow paths
The cooling water flow velocity in the portion excluding the lower throttle means and the first and second upper openings is set to a boiling water reactor core. 4. The core of a boiling water reactor according to claim 3, wherein the core is arranged in the reactor pressure vessel and so as to surround the outermost periphery of the core, and the lower end is supported by the lower core support plate and the upper end. A core of a boiling water reactor characterized in that an isolation wall is provided in the vicinity of which is supported by the core upper support plate. 5. The core of a boiling water nuclear reactor according to claim 2, wherein at least the upstream side of the boiling start position of the coolant in the first and second coolant channels and the first and second coolant channels. The average value in the flow direction of the pressure gradient in the portion excluding the lower throttling means is upstream of the position corresponding to the boiling start position in the coolant bypass passage and in the portion excluding the bypass lower throttling means. Of the boiling water reactor characterized by being smaller than the mean value of the pressure gradient in the flow direction. 6. The core of a boiling water reactor according to claim 1, wherein a pressure loss in a portion of the coolant bypass passage except the bypass lower throttle means and the bypass upper throttle means is reduced by the bypass lower throttle. A core of a boiling water reactor characterized in that the pressure loss in the means and bypass upper throttle means is made larger. 7. The core of a boiling water nuclear reactor according to claim 6, wherein a minimum flow area of the coolant bypass passage is the smallest except for the bypass lower throttle means and the bypass upper throttle means. The height and flow passage area of the passage portion are H and A _B respectively, the hydraulic equivalent diameter of the coolant bypass passage is d, the flow passage area of the bypass lower throttle means is A _L , the flow passage of the bypass upper throttle means is when the area was ^{_{a U, a L 2 a U}} 2 / (a L 2 + a U 2)> 10 (d / H) of the boiling water nuclear reactor, characterized by being configured to satisfy the a _B ² Core. 8. A plurality of fuel rods, which are arranged in a reactor pressure vessel, are formed by surrounding the outer circumference of a plurality of fuel rods with a channel box, and are arranged on the radial center side of the pressure vessel and have a relatively high output. A plurality of fuel assemblies including a first fuel assembly and a plurality of second fuel assemblies that are arranged on the outer peripheral side in the radial direction of the pressure vessel and have a relatively low output, and the first and second fuel assemblies. A core upper support plate and a core lower support plate that respectively support the vicinity of the upper end and the vicinity of the lower end of the fuel assembly;
The coolant introduced from the first lower throttle means provided below each first fuel assembly is passed through the inside of the channel box of the first fuel assembly to form the first coolant provided on the core upper support plate. The plurality of first coolant passages leading out from the upper openings of the second fuel assembly and the coolant introduced from the second lower throttle means provided below each second fuel assembly. A plurality of second lead-outs extending from a second upper opening provided in the core upper support plate through the inside of the channel box of the body
Of the coolant passage and the channel boxes of the first and second fuel assemblies, a bypass lower throttle means is provided near the lower end, and a bypass upper opening is provided in the core upper support plate near the upper end. In a core of a boiling water reactor having a coolant bypass flow path including a portion, a bypass having a function of reducing the flow rate of the coolant flowing through the coolant bypass flow path instead of the bypass upper opening An upper throttle means is provided on the core upper support plate, and at least a half or more of a flow direction region of the coolant bypass passage corresponding to a heat generating portion of the first and second fuel assemblies. Therefore, the pressure value is smaller than the pressure value at the corresponding position in each first coolant channel and is larger than the pressure value at the corresponding position in each second coolant channel. Feature Boiling water reactor core of that. 9. The core of a boiling water reactor according to claim 1, wherein the channel boxes of the first and second fuel assemblies have upper ends of heat generating portions of the first and second fuel assemblies. A core of a boiling water reactor characterized in that a plurality of first openings communicating the inside and outside of the channel box are formed on the downstream side including the position. 10. The core of the boiling water reactor according to claim 1, wherein at least a part of the flow paths of at least a part of the plurality of first lower throttle means provided in the plurality of first fuel assemblies. The area is 0.3 of the flow passage area in the first fuel assembly.
Boiling water reactor core characterized by being more than doubled. 11. The core of a boiling water reactor according to claim 10, wherein the core is arranged in the reactor pressure vessel so as to surround the outermost periphery of the core, and the lower end is supported by the lower core support plate and the upper end. An isolation wall whose vicinity is supported by the core upper support plate, and the first and second isolation walls of the isolation wall
And a plurality of second openings that are formed on the downstream side including the upper end position of the heat generating portion of the fuel assembly and communicate with the inside and outside of the isolation wall. 12. A core of a boiling water reactor according to claim 1, wherein a plurality of control rods are arranged in the coolant bypass passage and are driven up and down to control core output.
A core of a boiling water nuclear reactor, further comprising a plurality of followers each connected above the control rods and having a length in a flow direction substantially equal to the control rods.