JP2931573B2 - Fuel assembly - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [0001] TECHNICAL FIELD The present invention relates to a fuel assembly,
In particular, a boiling water type with a long operation cycle and a high stop margin
The present invention relates to a fuel assembly suitable for a nuclear reactor. [0002] 2. Description of the Related Art A fuel assembly of a boiling water reactor is made of metal.
Numerous fuel rods filled with nuclear fuel material inside cladding
What is correctly arranged is inside the rectangular channel box
It is housed in a unit. And the boiling water atom
In the core of the furnace, there is usually one cruciform control rod and it surrounds it.
Cells composed of four fuel assemblies
Are located. In other words, each core of the boiling water reactor
The fuel assemblies and control rods are perpendicular to each other and
It is arranged so that it becomes parallel and has a function as a moderator
Coolant is configured to flow upward from below the core.
Has been established. Then, the lower end of the core effective portion, that is, below the heating portion
No bubbles are generated near the end, but from the center to the upper end of the core.
A large amount of air bubbles are generated in the area, and the generated air bubbles
It flows above the core. Volume ratio occupied by bubbles, that is, void splitting
The neutron moderation characteristics decrease when the
The neutron flux decreases and the output decreases. To avoid this,
At sites with a high void fraction, the fission nuclide concentration or enrichment
Or suppress output rise in areas with low void ratio.
We have taken measures such as adding burnable poisons as much as possible. Therefore, in a boiling water reactor, the upper part of the core
Combustion is easily delayed, which causes relative U-235 concentration
Is higher than other parts, and voids cause Pu-239
Fissile nuclides such as
It is well known that the shutdown margin of a reactor is likely to be severe.
You. In addition, with the primary objective of improving
Efforts to extend fuel consumption and improve fuel burnup continue
I have. In this case too, the enrichment of the fuel is necessarily increased
As a result, the shutdown margin of the reactor becomes one step higher. Next, boil
Fuel assemblies used in rising water reactors and their use in the near future
A typical example of a fuel assembly expected to be
Will be explained. Figures 14 (a) and (b) show the conventional
Perspective view of a fuel assembly and fuel constituting the fuel assembly
It is an outline longitudinal section of a stick. In FIG. 1A, a fuel assembly is a water rod (see FIG.
(Not shown) and fuel rod 2 are connected to upper tie plate 4 and spacer
5.Fix with lower tie plate 6
It is configured to be surrounded by a channel box 1. fuel
The rod 2 is inserted into the cladding tube 7 as shown in FIG.
And a spring 9 in the gas plenum above it.
With an upper end plug 10 at the upper end and a lower end plug 11 at the lower end.
I have. FIG. 15 shows the conventional fuel assembly shown in FIG.
FIG. Channel box 1 contains 62 fuels.
The fuel rod 2 and the two water rods 3 are arranged to form a fuel assembly.
ing. Water rod 3 lacks water as moderator inside the assembly
This water rod 3 is uniform in the axial direction.
Water in the lower part of the core and water shortage in the upper part
There is a problem. [0006] The fuel assembly shown in FIG.
It was developed to improve the characteristics of
Place one large-diameter water rod (thick water rod) 12 inside and use non-boiling water
Has been introduced. However, even in this example,
There is a problem that water is excessive and water shortage occurs above. [0007] The fuel assembly shown in FIG.
This is an improvement of coalescence, and four small channel boxes 13 are installed.
Boiled cooling water in the small channel box 13
Non-boiling in the cross-shaped gap 14 between the small channel boxes 13
The rise in the cooling water area allows the horizontal power distribution to be flat.
This type of fuel assembly is
The problem of excessive water below the core and water shortage above the core
There is a point. [0008] The fuel assembly shown in FIG.
It was developed as an improved version of the assembly. This fuel
The assembly is composed of nine subassemblies 15
Each sub-assembly 15 is composed of nine fuel rods 2.
ing. A slightly wider gap 16 is provided between the subassemblies 15.
Have been killed. In the case of this fuel assembly as well, the water
The problem of overs and shorts has not been solved. [0009] As mentioned above, boiling
The bottom of the fuel assembly, which is the heating section of a water reactor (BWR)
No air bubbles are generated at the end, but
Even here, bubbles are generated, and the generated bubbles are upward (below).
Flow). Therefore, the bubble ratio of the BWR (void
Ratio) becomes higher toward the upper part of the core. As a result, neutron reduction
And the fission rate decreases.
You. That is, combustion proceeds below the core and lags above the core
Will be. Therefore, a reduction in power above the core is suppressed.
To increase the fission nuclide concentration above the core
Proposed. [0010] However, an increase in the void ratio above the reactor core
Increasing the concentration of fission nuclides in the reactor
The subcriticality in the upper part of the heart will be shallow. Meanwhile, driving
In order to extend the cycle and improve economics,
The enrichment must be further increased, but this is the core
The subcriticality at the top will become increasingly shallow and eventually
May be unable to shut down the reactor. Sand
This point is a bottleneck, and the conventional reactor core
There was a problem that the cycle could not be lengthened. The present invention has been made to solve the above problems.
The purpose is to increase the fuel enrichment
Enables reactor shutdown and improves axial power distribution
To provide a fuel assembly that constitutes the core of a boiling water reactor
It is to be. [0012] [MEANS FOR SOLVING THE PROBLEMS] To achieve the above object
In the present invention, the nuclear fuel material was filled inside the metal cladding tube.
Multiple fuel rods and at least two fuel rods next to each other
With a moderator rod of a size that contains
In the fuel assembly, the fuel rod includes a long fuel rod and the fuel rod.
Effective fuel length of long fuel rod(H)The long fuel
Fuel effective part of fuel rodFrom the bottom2 / 3H position to 5 / 6H
In an area that includes at least part of the location rangeIsNuclear fuel material
Is filledNot filled with this nuclear fuel material
The area above the area is not filled with nuclear fuel material
A short fuel rod; and
A position adjacent to the moderator rod or a plurality of the moderators
Characterized by being arranged at a position sandwiched between rods.
You. Alternatively, the fuel rod is placed in a cross-shaped non-boiling cooling water region.
Therefore, the fuel rod is divided into a plurality of small units, and the short fuel rod is
That it is located adjacent to the non-boiling cooling water area
It is a feature. Hereinafter, the short fuel rod is referred to as P
Abbreviated as fuel rod. As described above, according to the fuel assembly of the present invention,
Fissile material above the effective length of the short fuel rod
And the moderator rod or cross-shaped non-boiling cooling water area
Of the intervening region, with the non-heat generating portion consisting of
Neutron interaction (coupling effect) weakens during cold
A phenomenon that increases during warm operation, particularly when voids occur, occurs.
This phenomenon is mainly due to the action of thermal neutrons with a short diffusion distance.
Can be explained. That is, when cold, the water
Degree (about 1.0g / cm ^Three ), The diffusion distance of thermal neutrons
The separation is shortened, and neutron interaction on both sides of the intervening region
The neutron multiplication properties are reduced as a result. high temperature
During operation, boiling water atoms even in the absence of voids
In the furnace, the water temperature (reference value) is about 286 ° C and the water density is about 0.74
g / cm^ThreeBecomes Thermal neutron migration distance in water is cold
 It increases 1 / 0.74 (= 1.35) times. Furthermore, void generation
The density of the air-water mixture at that time drops to about 0.3,
As a result, the thermal neutron diffusion distance in the air-water mixture is 1 / 0.3 (≒
3) It increases twice. As a result, both sides of the intervening region
Neutron interaction increases, and neutron multiplication properties increase. By utilizing the above-described operation, the introduction of the intervening region
Reduces the multiplication factor during cold conditions, that is, the reactor shutdown margin
(Subcriticality) to increase the amount of fuel during high-temperature operation
Even if it is reduced by introducing an area,
If it is prevented or a suitable design is made,
It is even possible to increase the multiplication factor over the case without one. Next, the operation of the present invention will be described with reference to FIG.
I will tell. As shown in FIG.
There are two fuel zones I and II, between which a water gap of width w
Shall exist. In addition, water gaps in fuel areas I and II
Width w in the same direction as_fIs larger than the water gap width w.
It shall be wide enough. Water gap width w and neutrons at this time
The relationship of the change in the multiplication factor is as shown in Fig.
Part (c) of FIG. (B) is enlarged and shown in FIG. here,
"Changes in neutron multiplication factor" are for high temperature (dashed line) and cold
(Solid line), neutron multiplication factor when water gap width is 0
It indicates that it is a change from Axial direction in fuel assembly
At right angles (usually horizontal in light water reactors)
It is difficult to take a gap area. That is,
Widening the water gap over a given range is a fuel area
And the heat generation area becomes narrower.
You. In the present invention, in the direction perpendicular to the axis of the fuel assembly,
Since the intervening area is inserted, the characteristics of the narrower intervening area are clarified.
Need to be clear. Fig. 3 (c) shows the purpose of this
This is an enlarged view of a portion c in FIG. 3 at best
Theoretical calculation for the case where a water gap of about 5cm is provided
The value gives a curve almost similar to that of FIG. That is, high temperature luck
When turning (when a void occurs), the multiplication factor increases with the width of the water gap.
The change increases in the positive direction (effective multiplication factor k_effIncreases),
In the cold state, when the water gap width exceeds about 1 cm,
_effDecreased due to the increase in the width of the water gap.
It can be seen from this figure that it helps to increase the criticality. In the above description of the operation,
Of neutron interaction between two fuel regions sandwiching a gap
The infinite multiplication factor k of the fuel assembly_∞The old
To be explained in the well-known four-factor method
Can also. In this method, the curve in FIG.
Changes in the properties of thermal neutron utilization and the probability of escape from resonance
Is also explained. Reduced number of fuel rods inside fuel assembly
When widening the water gap without
Must shrink, this is the fuel rod in resonance absorption
Increase the shielding effect of resonant neutrons between each other, so that
This has the effect of increasing the probability of escaping resonance, while
The thermal neutron flux ratio between the feed region and the water gap decreases,
The effect of reducing the thermal neutron utilization rate occurs. Fig. 3 (c)
Is the water density dependence and water gap width dependence of the above two effects
Is largely determined by the offset effect of The gap between fuel rods is fixed and the water gap is widened.
To remove fuel material from the fuel rods
No. In that case, the probability of escaping the above resonance absorption
Is not a shielding effect of resonant neutrons, but a deceleration effect.
The increase will increase the probability of escaping resonance.
In other words, the reactor is operating at high temperature,
If there is no moderator,
This has been alleviated by the introduction of
The probability of escaping increases. Thermal neutron utilization changes above
This is almost the same as the above example. In the present invention, the effective multiplication factor k during high-temperature operation
_effTo reduce excessive negative void coefficient,
When the state is stopped k_effAnd thus the subcriticality
That is, the intervening region having the characteristic of increasing the furnace stop margin,
Eliminate fissile material above the effective length of the short fuel rod
Part and large diameter moderator rod (water rod or high density hydrogen
Zirconium hydride with small neutral absorption effect _Two What
Or the synergy of the cross-shaped non-boiling cooling water region
You. That is, at least two fuels adjacent to each other
When a large-diameter moderator rod large enough to contain the rod is placed,
The bar is k at high temperature operation._effAnd k during cold shutdown
_effPositive operation during high-temperature operation.
Interference effect occurs, and a negative interference effect occurs during cold shutdown
You. Such interference effects are more concentrated in the core than in the surroundings.
Positive interference when two fuels of different shrinkage are placed close to each other
The effect occurs, and conversely, two control rods are placed close to each other
This is essentially the same interference that produces a negative interference effect
The effect is. In the present invention, such a phenomenon is considered as a large-diameter moderator.
Generated effectively with rods and above the effective length of short fuel rods
By placing a part excluding fissile material,
The interference effect is further enhanced. Take advantage of these characteristics
Then do not reduce the fuel inventory too much,
The same characteristics as when a thick moderator rod is introduced can be obtained.
You. If the inventory does not decrease, much use of pyrogens
This contributes to the improvement of reactor power. When the output is constant
The power density of the fuel rods
The soundness of the vehicle is improved. Further, such a large synergistic effect is generated.
In particular, place the part that increases the reactor shutdown margin in the furnace above the core.
The inventory is limited to the areas where
The reduction is suppressed. From the bottom of the core to about 2/3 of the total length
Has a large-diameter moderator rod disposed between the moderator rods.
The fuel rods are arranged around the moderator rods as before.
Have been. The thermal neutron flux is rising around the moderator rod
However, for large diameter moderator rods,
Is smaller than a small moderator rod equivalent to one fuel rod.
wide. In particular, wrap at least two fuel rods next to each other
For large diameter moderator rods of a size that includes
Is also replaced by non-boiling water, so it is reduced by moderator rods
A wide range of moderator rods can be used effectively for fuel inventory
The thermal neutron flux can be increased. In the above description, a large-diameter moderator rod is used instead of a large-diameter moderator rod.
The same effect can be achieved by providing a cross-shaped non-boiling cooling water area
Can be achieved. That is, in the conventional example of FIG.
Of the cross-shaped gap 14 between the small channel boxes 13
The boiling cooling water region has the same action as the large diameter water rod. Obedience
The short fuel rod adjacent to the cross-shaped non-boiling cooling water area
By arranging, the effective length of the short fuel rod is
The above-mentioned interference effect is eliminated by the part excluding fissile material.
Can be raised step by step. In the thermal neutron flux rising region, the reactivity of the fuel increases.
Ascending, k_effCan be increased. Also highly concentrated fuel
Use more fuel rods with burnable poisons
Multiplication factor must be suppressed, but thermal neutron flux rise
If the area is large, the location within the assembly will be widened and the reactivity will be effective
The number of fruits can be reduced by increasing the number of fruits. Further, a feature of the present invention is that a large-diameter moderator rod is provided.
Or short fuel rods located adjacent to the cross-shaped non-boiling water area
At the same time as improving the furnace shutdown margin described above.
The pressure loss of the coolant can be reduced. Sand
By using short fuel rods, high void fraction
Coolant that causes a particularly high flow rate and a large pressure loss
Coolant flow path downstream (upper of fuel assembly)
Therefore, the pressure loss is effectively reduced. This results in cold
Pump power for driving waste material is reduced, and the efficiency of nuclear power generation is improved.
improves. That is, according to the present invention, the fuel vent
Pressure loss of the coolant while
Effectively, during high-temperature output operation_effIncrease the coldness
While stopped_effCan be reduced. [0026] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described with reference to the drawings.
explain. FIG. 1 is a plan view of one embodiment of the present invention.
The fuel assembly of the present embodiment uses four fuels as moderator rods having a large diameter.
Two thick water sticks 44 (A, B) of the size equivalent to a stick cell
And five P fuel rods 45 between the heavy water rods A and B.
(See FIG. 2). With these thick water rods A and B
Normally in the area excluding the area where five P fuel rods 45 are arranged
Are regularly arranged in 9 rows and 9 columns. But
Therefore, this fuel assembly has two fat rods 44 and five P fuel rods 45.
The normal fuel rod 33 is composed of 68 rods. Fuel collection
As a whole, the united structure is symmetrical with respect to the diagonal.
I have. And, this fuel assembly is a furnace with a tight shutdown margin
The non-fuel portion of the P fuel rod (vani
shing rod), and at this height,
As shown, from non-fuel rod A to non-fuel rod B
Since an area is formed, the reactor shutdown margin (SDM: shut down ma
rgin) is improved. The thick water rod 44 and the P fuel rod 45
The fuel rods H and h surrounding the fuel rods are ordinary fuel rods
However, the part where the fuel rod H is in contact with the fat rod 44 more than the fuel rod h
, The contribution to the reactivity is large. In addition, P
In the non-fuel part height of the fuel rod 45, the fuel rod H and the fuel rod h
The thermal neutron flux increases, the reactivity effect of the fuel rod increases,
Except for the height of the non-fuel portion of the P fuel rod 45, the fuel rods H and P
The thermal neutron flux rises at the fuel rod 45. Therefore, these
Site is k_effSuitable for intentionally raising or lowering
It is. FIG. 2 (a) is a side view of the fuel assembly of the present invention.
Figures (b) and (e) show the fuel used in the fuel assembly of the present invention.
It is a schematic sectional drawing of a material rod. In FIG. 2 (b), the fuel rod
33 is a normal fuel rod and a fuel pellet 35 in a fuel cladding tube 34.
Is enclosed, and a gas plenum 36 is formed on the upper part
I have. FIG. 2 (e)45Is a P fuel rod according to the present invention.
P fuel rod45Is a short fuel rod as shown in Fig. 2 (e).
The total length is almost 3 / 4H of a normal fuel rod,
Gas plenum 36 and output spike suppressor 38 at the upper end
An auxiliary gas plenum 40 is provided via
Non-fuel part (vanishing rod) 39 on top of gas plenum 40
Is located. Normally downstream of the coolant flow, ie the fuel assembly
The flow rate is very high at the top of
Pressure loss, which increases almost in proportion to the square of the flow velocity,
growing. However, the P fuel rod 43 shown in FIG. 2 was used.
In this case, the area downstream of the coolant flow,
And the flow path of the coolant expands, reducing the flow rate of the coolant
I do. Therefore, the pressure loss at this part is greatly reduced.
Can be. These effects and effects are provided unless otherwise specified.
The same applies to the other embodiments described below.
You. The feature of the P fuel rod is that the furnace can be stopped.
Fissile at a location that contains part of the 2 / 3H to 5 / 6H section
The point is that nuclides were eliminated. This is from the lower fuel effective length
This is because the subcriticality becomes particularly shallow near 3 / 4H. Ma
The output spike suppressor 38 is used to prevent output spikes.
It is inserted adjacent to the upper end. FIG. 4 is a plan view of a second embodiment of the present invention.
is there. The same parts as those in the first embodiment described above have the same reference numerals.
A description will be given with numbers. The same applies to the following embodiments.
is there. This embodiment is different from the first embodiment in that a thick water rod is used.
A was moved down one row, and thick water stick B was moved down one column.
Because of this configuration, this fuel assembly
The structure is such that two P fuel rods 45 are arranged. did
Therefore, in this fuel assembly, two fat rods 44 and two P fuel rods 45 are used.
Is composed of two rods, and the normal fuel rod 33 is composed of 71 rods. Furnace
At the position where the SDM in the upper part of the heart becomes severe,
The non-fuel area surrounding the water rod 44 and the P fuel rod 45 is indicated by a dotted line.
So small. In this embodiment, the water gap width around the fuel assembly
Is suitable for a different core (BWR-D lattice core). FIG. 5 is a plan view of a third embodiment of the present invention.
is there. The fuel assembly of the present embodiment is a 3 × 3
It consists of 9 bundles. And each subband
A slightly wide water gap 50 is formed between the holes. Gya
Two thick water rods 44 (A,
B) is inserted, and P fuel is inserted between the thick water rods A and B.
Five rods 45 are arranged. In this way, a little bit of the gap
Since the thick water rod 44 is inserted in the wide part, the vent
You can use a thick water stick to reduce the amount of water
Also, it is difficult to collect the cooling water that is
I can do it. Therefore, in the upper part of the core, the thick water rods A and B are P
A large non-fuel area is formed through the fuel rod 45. further,
At the intersection of the wide sections of the water rods A and B and the gap at right angles
Add thin FC (flow control material) 49 and cool water
And the thick water rods A and B also cool the gap.
It has the function of controlling the flow of waste material. FC
49 is usually composed of a thin water rod, as shown by the arrow (upper half of the core)
It is recommended that the flow rate be sufficient to discharge the cooling water.
No. The fuel assembly is composed of two thick water rods 44 and two P fuel rods 45.
Consists of 5 pieces, normal fuel rod 33 consists of 68 pieces, FC49 consists of 2 pieces
Have been. FIG. 6 is a plan view of a fourth embodiment of the present invention.
is there. The fuel assembly of the present embodiment has the same overall structure as in FIG.
Is composed of nine 3 × 3 sub-bundles. So
And a slightly wide water gap 50 is formed between each sub-bundle.
However, the thick water rods A and B are closer than in FIG.
You. Therefore, the thick water rods A and B are water rod binding material and flow control.
The roll fins 51 are joined at several places in the axial direction. this
Fins 51 tend to collect cooling water in the center of the assembly
It has the effect of pushing things back. In the upper part of the core,
It is the fuel section. P fuel rod 45 extends along the longitudinal direction of fin 51
Are arranged three by one. Therefore, the fuel assembly
Represents two thick water rods 44, six P fuel rods 45, and normal fuel rods 33.
Consists of 68 lines. FIG. 7 is a plan view of a fifth embodiment of the present invention.
is there. In the assembly of this embodiment, the arrangement of the fuel rods is 4-1-4 type.
It is a lattice aggregate. Of the diagonal 4 × 4 subbundle
At the side, insert four thick water rods 44 (A, B) of normal fuel rods 33.
And put five P fuel rods 45 between these thick water rods A and B.
It was done. Similar to the embodiment of FIG. 1, but with P fuel rods
The non-fuel area surrounded by the dotted line at the top of the core
Is also widely formed. This fuel assembly has two thick water rods 44, P
Five fuel rods 45 and 68 normal fuel rods 33
I have. FIG. 8 is a plan view of a sixth embodiment of the present invention.
is there. In the assembly of this embodiment, the arrangement of fuel rods is 5-4 type.
It is a child aggregate. Thick rods A and B are asymmetric lattice assemblies
Inserted into the inner corner. P fuel rod 45 is a symmetric grid
They are arranged one by one at the inner corners of the union. This fuel assembly
The body is two thick water rods 44, two P fuel rods 45, normal fuel rods.
33 is composed of 71 lines. In this embodiment, the BWR
-Suitable for D lattice core. FIG. 9 is a plan view of a seventh embodiment of the present invention.
is there. In the assembly of the present embodiment, the arrangement of the fuel rods is 4-2-3 type.
It is a lattice aggregate. This embodiment is similar to FIG.
FC (flow control material) 49 is provided as shown in Fig. 5.
Have been. This fuel assembly consists of two thick water rods 44 and a P fuel rod 45
Consists of two, normal fuel rod 33 consists of 71 and FC49 consists of two
Have been. This embodiment is suitable for a BWR-D lattice core.
Suitable. FIG. 10 is a plan view of an eighth embodiment of the present invention.
is there. In the assembly of this embodiment, the arrangement of the fuel rods is 4-3-2 type.
It is a lattice aggregate. This embodiment is similar to FIG.
Due to the shape of the child, the thickness of the thick water sticks A and B is slightly
Is different. This fuel assembly consists of two thick water rods 44 and P fuel
2 rods 45, 71 normal fuel rods 33, 2 FC49
It is configured. In this embodiment, a BWR-D lattice core is used.
It is suitable for. FIG. 11 is a plan view of a ninth embodiment of the present invention.
is there. The assembly of the present embodiment is a 10 × 10 type and the arrangement of the fuel rods is 4-
It is a 2-4 type dense and dense lattice aggregate. Therefore, the thick water stick 44
(A, B) can be large, but otherwise it is similar to FIG.
Configuration. This fuel assembly consists of two thick water rods 44 and a P fuel rod.
45 consists of two, normal fuel rod 33 consists of 90, and FC49 consists of two.
Has been established. FIG. 12 is a plan view of a tenth embodiment of the present invention.
is there. The assembly of the present embodiment is a kind of 11 × 11 type and has a fuel rod arrangement.
This is a modification of the (5 × 5) × 4 type close-packed lattice assembly. Fig. 8
Of the fuel bundles
A total of eight fuel rods are arranged, and 5-5 type is changed to 5-5 type.
Increase fuel inventory by adding more fuel rods
ing. One feature of this embodiment is that the channel
Between the channel material and the fuel rods in the center of the
It has a wide water area. This configuration allows
The water gap between the fuel assembly and the fuel
This contributes to improving the furnace shutdown margin. Heavy water
The rod 44 (A, B) is large. This fuel assembly is a thick water rod
44 are two, P fuel rods 45 are two, normal fuel rods 33 are 98
It is composed of FIG. 13 is a plan view of an eleventh embodiment of the present invention.
is there. In this embodiment, four sub-bundles 51 are provided,
The cross-shaped gap 52 between the bundles is defined as the non-boiling cooling water region.
And the corner part located at the center of the aggregate in each sub-bundle 51
3 P fuel rods 45 and 1 water rod 46 per minute
Are arranged in a way. In this fuel assembly, the P fuel rod 45 is 12
The water rod 46 consists of four rods, and the fuel rod 33 consists of 84 rods.
You. In this embodiment, a large non-combustible
A material region is formed. In the embodiment described above, the P fuel rod
At the top of 45, there is no nuclear fuel material above it,
High neutron flux. Therefore, in the nuclear fuel material at the upper end, 2
output peak (spy
H), which is disadvantageous in terms of fuel integrity.
2 pellets (approximately 2 cm) containing burnable poisons
Have been. These pellets contain toxic substances on the outer periphery
Not output, so output is compared over the entire operating cycle
Little fluctuation. As the end of the cycle approaches
Absorption characteristic disappears, output of this part gradually rises
To be designed. Fuel with smaller Gd pellets
Gd may be mixed into the entire pellet instead of the pellet.
No. [0042] As described above, according to the present invention,
The following effects are provided. (1) When the reactor is shut down, the water temperature is low and the water density is high.
Although the neutron diffusion distance is small, the fuel assembly of the present invention
In the non-fuel region where fissile material does not exist,
Neutron interaction in the horizontal direction of the fuel region across the
Effect is reduced, and as a result the subcriticality of the
Can be larger. (2) During high temperature operation, the average density of water is significantly
The thermal neutron diffusion distance is greatly increased (2 to 3 times)
Extend. As a result, the coupling effect across the intervening region is improved
However, the effective multiplication factor has a region where no fissile material exists.
Nevertheless, it increases, though slightly
You can even do it. No disadvantage due to the introduction of intervening areas
No. (3) In the present invention, the intermediate region is adjacent to the intervening region in the axial direction.
Burnable poisons are effectively placed in a limited area of
Causes local output peaks (output spikes)
No fuel integrity is therefore maintained. (4) Large diameter moderator rod or cruciform non-boiling cooling
Reduction of fuel inventory due to positive interference in water area
With low diameter, moderator rod or cruciform non-boiling cold
Wide area around the water reject area and between the large moderator rods
The thermal neutron flux can be raised,
The fuel reactivity effect in the part increases. As a result, the effective increase
Magnification can be improved or burnable poison insertion
The number can be reduced. (5) Furthermore, according to the present invention, a short-range fuel
The large amount of water generated downstream of the coolant
Coolant pressure loss can be greatly reduced,
Reduce the power of dynamic pumps and increase the efficiency of nuclear power generation
be able to.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a plan view of one embodiment of the present invention. FIG. 2 (a) is a side view of the fuel assembly of the present invention, and (b) and (e).
1 is a schematic sectional view of a fuel rod used in a fuel assembly of the present invention. FIGS. 3A to 3C are diagrams for explaining the operation of the present invention. FIG. 4 is a plan view of each different embodiment of the present invention. FIG. 5 is a plan view of each different embodiment of the present invention. FIG. 6 is a plan view of each different embodiment of the present invention. FIG. 7 is a plan view of different embodiments of the present invention. FIG. 8 is a plan view of each different embodiment of the present invention. FIG. 9 is a plan view of each different embodiment of the present invention. FIG. 10 is a plan view of each different embodiment of the present invention. FIG. 11 is a plan view of different embodiments of the present invention. FIG. 12 is a plan view of each different embodiment of the present invention. FIG. 13 is a plan view of each different embodiment of the present invention. 14 (a) and (b) are a perspective view of a conventional fuel assembly and a schematic longitudinal sectional view of a fuel rod constituting the fuel assembly, respectively. FIG. 15 is a cross-sectional view of the fuel assembly of FIG. FIG. 16 is a cross-sectional view of a conventional fuel assembly. FIG. 17 is a cross-sectional view of a conventional fuel assembly. FIG. 18 is a cross-sectional view of a conventional fuel assembly. [Description of Signs] 31: fuel assembly, 33: fuel rod, 34: fuel cladding tube, 35: fuel pellet, 36: gas plenum, 38: output spike suppressor, 39: burnishing rod, 40: auxiliary gas plenum,
45: P fuel rod, 44: thick water rod, 46: thin water rod, 49: flow control material, 50: gap.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) G21C 3/328 G21C 3/28 G21C 3/326

Claims (1)

(57) [Claims] In a fuel assembly in which a number of fuel rods filled with nuclear fuel material inside a metal cladding tube and a moderator rod having a size including at least two fuel rods adjacent to each other are regularly arranged, the fuel rods have a long length. A region including at least a part of a range of a 2 / 3H position to a 5 / 6H position from a lower end of a fuel effective portion of the long fuel rod in a full length (H) of a fuel effective portion of the long fuel rod; A short fuel rod not filled with nuclear fuel material, and a region above the region not filled with nuclear fuel material, which is not filled with nuclear fuel material, wherein the short fuel rod is at least the moderator rod. A fuel assembly, wherein the fuel assembly is arranged at a position adjacent to the fuel assembly. 2. In the short fuel rod, the nuclear fuel material is filled.
Fuel pellets adjacent to the upper end
Inclusion of burnable poison in the range of cm to 5 cm
The fuel assembly according to claim 1, wherein: 3. 2. The moderator rod according to claim 1, wherein the moderator rod is a water rod.
The fuel assembly of the mounting. 4. A large number of fuel rods and a part with a narrow distance
Are arranged regularly so as to have
The fuel assembly according to claim 1, wherein a moderator rod is arranged . 5. Place the flow control material in the wide area
The fuel assembly according to claim 4, wherein: 6. A plurality of moderator rods are connected to at least one pair of fuel assemblies.
Arrange them so that they are symmetrical with respect to the corners, and
Provision of flow control fins between fast rods
The fuel assembly according to claim 1, wherein: 7. Near the center between adjacent channel box corners
The inner surface of the channel box and the fuel
It is characterized by providing a widened area with the material rod
The fuel assembly according to claim 1 . 8. Wide and narrow spaces between fuel rods are formed
Many fuel rods are densely arranged as shown in the
Arrange the moderator rod so as to partially include the wide part, and
At least one short fuel rod is located adjacent to the fast rod
2. The fuel assembly according to claim 1, wherein: 9. Numerous fuels filled with nuclear fuel material inside a metal cladding tube
Includes a fuel rod and at least two fuel rods adjacent to each other
With multiple moderator rods of different sizes arranged regularly
In the assembly, the plurality of moderator rods are small in the fuel assembly.
At least one diagonal line is placed so as to be symmetrical.
The fuel rod is composed of a long fuel rod and the entire fuel effective portion of the long fuel rod.
From the lower end of the effective fuel section of the long fuel rod,
At least one of the range from the 2 / 3H position to the 5 / 6H position
The area containing the part is not filled with nuclear fuel material and
The nucleus is located above the area not filled with fuel material.
A short fuel rod, which is not filled with fuel material,
A position where the short fuel rod is sandwiched at least by the moderator rod
A fuel assembly, comprising: a fuel assembly; 10. The short fuel rod is filled with nuclear fuel material.
Fuel pellets adjacent to the upper end
Including combustible poisons in the range of 2cm to 5cm
10. The fuel assembly according to claim 9, wherein: 11. 10. The moderator rod is a water rod.
The fuel assembly as described. 12. Where many fuel rods are wide and narrow
The parts are arranged regularly so as to have minutes
The moderator rod according to claim 9, wherein a moderator rod is disposed on the vehicle.
Fuel assembly. 13. Place flow control material in the wide area
The fuel assembly according to claim 12, wherein the fuel assembly is disposed. 14． Nearly the center between adjacent channel box corners
The inner surface of the channel box from the part other than the central part
It is characterized by providing an area where the distance from the fuel rod is widened.
The fuel assembly according to claim 9, wherein 15. Wide and narrow spaces between fuel rods
As many fuel rods as shown in FIG.
The moderator rod is arranged so as to partially include the wide part,
At least one short fuel rod is located adjacent to the moderator rod
The fuel assembly according to claim 9, wherein: 16. Numerous nuclear fuel materials filled inside metal cladding
In a fuel assembly in which fuel rods are regularly arranged,
Fuel rods have multiple small units by cross-shaped non-boiling cooling water area
The fuel rods are divided into long fuel rods and the fuel rods.
Of the total fuel effective length (H) of the long fuel rod,
2 / 3H position to 5 / 6H from the lower end of fuel effective part of fuel rod
Nuclear fuel material is included in the area including at least a part of the location range.
Is not filled and this nuclear fuel material is not filled
The area above the area is not filled with nuclear fuel material
A short fuel rod; and
Located at a position adjacent to the cross-shaped non-boiling cooling water area
A fuel assembly, characterized in that: 17． The short fuel rod is filled with nuclear fuel material.
Fuel pellets adjacent to the upper end
Including combustible poisons in the range of 2cm to 5cm
17. The fuel assembly according to claim 16, wherein: