JP2664167B2 - Fuel assembly - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial applications]   The present invention relates to a fuel assembly, and particularly to a boiling water reactor.
Fuel collection suitable for application to save nuclear fuel material consumption
It is about coalescence. [Conventional technology]   A conventional boiling water reactor is disclosed in JP-A-54-121389.
To promote neutron deceleration as described
A pipe through which only cooling water flows (hereinafter referred to as a water rod)
The fuel assembly is loaded in the core. like this
The use of water rods depends on the operating conditions of conventional boiling water reactors.
In, the more hydrogen atoms to uranium atoms, the more reaction
The nuclear fuel material loaded in the reactor core
Wear.   However, in order to further utilize nuclear fuel materials effectively
Changes the number of hydrogen atoms in the core as the nuclear fuel material burns.
You should get it.   JP-A-57-125390 and JP-A-57-125391
Shows one such method. That is, these
The gazette discloses a low-speed neutron-absorbing push rod and a more
Medium-speed neutral made of stainless steel with high reactivity value
A water absorbing push rod is installed, and these push rods are inserted into the reactor core.
State that the amount of cooling water in the core is controlled by controlling the amount
I have. A water push rod is a means of changing the number of hydrogen atoms in the core.
You. Increasing the insertion amount of the water push rod into the core increases the cooling water in the core.
The amount of cooling water in the core increases when this amount is reduced.
Add. The method described above adds a new type of water push rod.
Provided, the water push rod must be operated by the driving means,
It becomes complicated in structure and production.   Using static means to solve such problems
A fuel assembly is disclosed in JP-A-61-38589.
This publication describes fuel assemblies as a means of changing the number of hydrogen atoms.
A fuel rod with low uranium 235 concentration is installed in the water rod of
In the water rod before and after uranium 235 disappears from this fuel rod
Is described as utilizing the change in the amount of voids.   Also, it is necessary to provide new operating means such as a water push rod.
There is no way to adjust the cooling water flow through the core
There is a law. Cooling water flow through the core at the beginning of the fuel cycle
And increase the cooling water flow during the fuel cycle.
It is easy. [Problems to be solved by the invention]   Changing the number of hydrogen atoms in the core as the fuel material burns
The advantages of this case will be described below.   Figure 9 shows typical fuels used in boiling water reactors.
One finger of the burn-up on the horizontal axis and the reactivity on the vertical axis
The characteristics are shown using an infinite multiplication factor, which is a standard.
Both lines are the same fuel assembly, but the dashed line is
Volume fraction of vapor bubbles in the coolant flow path in the fuel assembly (bore
(Constant void rate) (30% void rate)
, And the solid line is the first operation with a high void rate (void rate 50%)
This shows a case where the void ratio is lowered (void ratio 30%) on the way.
As is clear from FIG.
The higher the burnup, the lower the void fraction after baking.
Obtainable.   This is due to the high void fraction,
The smaller the atomic ratio, that is, the smaller the number of hydrogen atoms
However, the average velocity of neutrons is high and absorbed by uranium-238.
Because it is easy. Nuclear fuel used in boiling water reactors
The substance contains uranium 235 and uranium 238,
Uranium 235 makes up the majority of uranium 238, a few percent of all nuclear fuel material
Is occupied. Of these, neutrons are absorbed to generate fission.
It is mainly uranium 235 only, and uranium 238 is almost
No fission occurs. Therefore, uranium 235 burns
As a result, the reactivity decreases.   However, uranium 238 also has high energy due to fission.
Absorption of neutrons from giants turns them into plutonium-239. Step
Ruthenium 239, like Uranium 235, has a slowed enthusiasm
Absorb neutrons and cause fission. The higher the void fraction, the more
High neutron energy, uranium 238 to plutonium 239
Uranium 235 and plutonium
The fission of Um-239 is suppressed. Therefore, the void ratio is high.
Slowing down the reduction of the total amount of uranium 235 and plutonium 239
No.   However, when the void fraction is high, the absolute value of the reactivity is low.
Therefore, if the void ratio remains high,
Reactivity reaches the minimum level that can maintain criticality faster than
Will reach. Therefore, reduce the void fraction at that point
And the neutron moderating effect increases, burning at a constant high void fraction.
Fission of uranium 235 and plutonium 239 compared to
And the reactivity is higher. Therefore, the critical
Fissile nature of nuclear fuel material until minimum reactivity
The substance can be burned longer.   What has been described above is the result of the burning of each fissionable material
Effective use of nuclear fuel materials by changing the
This is called the spectrum shift operation.   Method of providing static means in a simple water rod of construction and
Changing the cooling water flow rate (core flow rate) flowing through the core
Therefore, there is no way to change the number of hydrogen atoms in the core.
However, the variation in core void rate cannot be so large
Therefore, it is difficult to apply to an actual nuclear reactor.   Figure 10 shows the dependence of core average void fraction on core flow rate
It shows. The core flow rate depends on the thermal limit
Limit, with an upper limit on recirculation pump capacity and flow oscillations
Is restricted by Therefore, boiling water reactors are rated
100% of the core flow rate
The void fraction can be changed only within a certain narrow range.
I can't do that. For example, the width in which the core flow rate can be changed
If it is up to 120%, the variation of void ratio is about 9%.
You.   Further, as disclosed in JP-A-61-38589,
Heating element (nuclear fuel
Quality), the void fraction in the water rod is at most 30
It only changes by about%. The water in the water rod contributes to cooling
Therefore, the cross-sectional area of the water rod in the fuel assembly is
Can't be too big. If the cooling water flow in the fuel assembly
Even if 30% of the road is devoted to the cross section of the water rod, 30%
The change in the void fraction of the fuel assembly is 9% (30%
× 0.3). Also, low concentration as a heating element
Since fuel rods are used, the structure is complicated and the structure is
It is upside down.   To achieve a larger change in void fraction, the water rod
If the flow rate in the inside of the
Does the calorific value of the nuclear fuel material in the gas drastically change?
Should be done, but such a large change in flow rate and heat value
It cannot be done without moving parts. Place with moving parts
Problems such as reliability problems and complicated mechanisms
There is a title.   An object of the present invention is to provide a simple structure and a stable state.
That can greatly change the average void fraction of the part
To provide an aggregate. [Means for solving the problem]   The purpose of the above is to provide multiple fuels with nuclear fuel material inside.
A fuel rod, a water rod disposed between the fuel rods, and the fuel rod.
Lower part having a rod and a fuel holding part for holding the water rod
A tie plate, above the fuel holding portion and the
A first portion formed between the fuel rods outside the water rod.
A coolant passage, and a second coolant formed in the water rod.
A fuel assembly comprising: a second coolant passage;
A passage opening to an area below the fuel holding portion;
Inlet opening and opening to the first coolant passage
An outlet located near the lower end of the fuel rod and the inlet
An ascending flow path for elevating coolant flowing from the
The coolant that has risen in the ascending channel is lowered toward the outlet.
A descending flow path for lowering the fuel,
The descending flow path provided so as to be distributed in the axial direction of the aggregate
Can be achieved by providing a plurality of discharge ports communicating with the
You. [Action]   When the flow rate of coolant through the core decreases, the water rod
Is filled with steam in the coolant descending channel, and the coolant flow rate
Increases the amount of steam in the coolant downflow channel
You. Therefore, it is possible to increase reactivity at the end of the fuel cycle.
Become.   In addition, the side wall of the coolant rising channel is
Multiple outlets communicating with the descending channel so that
The coolant flow rate supplied into the water rod
Even if the discharge increases, a part of the discharge
Since it flows into the descending channel from the outlet, it exists in the water rod
The change in the amount of steam that occurs is moderate, and the nuclear
The spectrum shift operation becomes possible. 〔Example〕   Before describing the embodiments of the present invention, the principle of the present invention will be described.
I do. FIG. 6 shows the structure. Basically,
A resistor (for example, a lower tie)
The coolant inlet 4 opens in a region below the plate 6)
Coolant ascending flow path 2 and the flow in the coolant ascending flow path
The coolant flow is reversed and guided downward, and the coolant is discharged
Coolant descending with opening 5 opening above resistor 6
A water rod 1 having a flow path 3 is provided in a fuel assembly.
It is. A plurality of coolant flow holes 7 are provided in the resistor 6.
Have been.   Coolant flowing through coolant flow holes 7 provided in resistor 6
When the flow rate of (cooling water) changes, the area below the resistor 6
ΔP changes between the pressure region and the region above the resistor 6.
You. The differential pressure due to the contraction resistance is almost proportional to the square of the cooling water flow rate.
Therefore, for example, the flow rate of cooling water passing through
% To 120%, the differential pressure ΔP is about 2.25 times
become.   On the other hand, the amount of cooling water in the water rod 1
Differential pressure between the inlet and outlet (between the coolant inlet 4 and the coolant outlet 5)
7) is as shown in FIG. Cooling water
When the flow rate is increased from zero, the differential pressure between the inlet and outlet of water rod 1
Reaches the maximum value, and when the cooling water flow rate is further increased,
The pressure difference between the entrance and exit of C1 is reduced to a minimum and then monotonously
To increase. This is due to the phenomenon shown in FIG.
You. FIG. 8 (a) shows the state inside the water rod 1 at the point S in FIG.
FIG. 8B shows the state at the point T in FIG.
FIG. 7C shows the state in the water rod 1 at the point U in FIG.
Each is shown.   The cooling water in the water rod 1 is also cooled by the fuel around the water rod 1.
0.5% depending on the neutrons and gamma rays
~ 2W / cm^TwoIt generates heat at about the same rate. Flowing in water rod 1
When the flow rate of the cooling water is very small (the state of point S in FIG. 7)
State) means that the cooling water in the water rod 1 is irradiated by neutrons or the like.
It generates heat and evaporates, and this vapor is shown in FIG.
As shown, above the coolant upflow channel 2 and the coolant downflow channel 3
Charge department. Liquid level L in coolant rise channel 2₁Can be
The pressure difference between the inlet and outlet of water rod 1₁And water rod 1
Level L of the coolant discharge port 5 (outlet of the coolant descending flow path 3)_Two
This is caused by the difference in the hydrostatic head. Flow into the coolant rise channel 2
The flow rate of the incoming coolant is changed from the coolant outlet 5 to steam.
Balance with outflow.   As the cooling water flow rate is increased from point S in FIG.
The amount of cooling water flowing into the material ascending passage 2 determines the amount of cooling water vapor.
Surpass. In such a case (for example, point T in FIG. 7)
As shown in FIG. 8 (b), the cooling water is
Flow down. At this time, the hydrostatic head in the coolant rising channel 2
Partly depends on the weight of the cooling water flowing in the coolant descending channel 3.
Pressure difference between the entrance and exit of water rod 1
Large value S₀Less than. However, the cooling water flow rate was further increased.
When added, the unsaturated water flowing from the coolant inlet 4 is cooled.
Boiling is suppressed in the material ascending passage 2 and the coolant descending passage 3
Coolant discharge (with void fraction significantly reduced)
It flows out from the outlet 5 (the state at the point U in FIG. 7, FIG. 8).
(C)). Therefore, the coolant ascending flow path 2 and the coolant descending flow
The passage 3 has almost a single-phase flow. Therefore, FIG. 8 (a)
In the coolant upflow channel 2 and the coolant downflow channel 3
Each hydrostatic head at the level of the reject material discharge port 5 cancels out
Their hydrostatic head difference is very small. But water lots
The flow rate of the cooling water flowing through
The pressure loss due to the reversal of the flow increases, and the water rod 1 enters and exits.
The pressure difference between the mouths rises again.   Due to the phenomenon described above, the difference between the entrance and exit of the water rod 1
Even if the pressure change amount is small, the cooling water flow rate in the water rod 1
The change width is very large, and the change width of the void fraction is also remarkable.
Increase.   Therefore, for example, the water rod when the core flow rate is 80%
The differential pressure between the entrance and exit is the minimum value T in Fig. 7.₀Water ro corresponding to
When the core flow rate is 120% or less and the pressure difference between
The pressure difference between the inlet and outlet of the water rod 1 is the maximum value S in FIG.₀To
The resistance to exceed the differential pressure between the corresponding inlet and outlet of water rod 1
If the resistance of the antibody 6 is adjusted, it will flow in the fuel assembly
Significant void fraction due to changes in cooling water flow rate (core flow rate)
Change can be realized. Core flow 80% in the above example
Is the local maximum S₀To the left, preferably the point Q in FIG.
Value T₀Pressure difference between the entrance and exit) and the core flow
120% is the minimum value T₀To the right, preferably R in FIG.
Point (maximum value S₀Pressure difference between the same entrance and exit)
You.   In the configuration of the above principle, if the coolant flow rate increases
At some point, the amount of steam in the water rod suddenly decreases and nuclear
It was found that stability occurred. Eliminate this instability phenomenon
For this purpose, the side wall of the coolant upflow channel is connected to the coolant downflow channel.
That is, a discharge port is provided.   One preferred embodiment of the present invention utilizing the principles described above,
That is, the first fuel assembly applied to a boiling water reactor is
The description will be made based on FIG. 2, FIG. 2 and FIG.   The fuel assembly 10 of this embodiment includes a fuel rod 11, an upper type tray.
Plate 12, lower tie plate 13, fuel spacer 16, chang
It consists of a Nervox 17 and a water rod 18. fuel
The upper and lower ends of rod 11 are upper tie plate 12 and lower type
Held at rate 13. Water rod 19 also has upper ends at both ends
It is held in the tie plate 12 and the lower tie plate 13.
Several fuel spacers 16 are arranged in the axial direction of the fuel assembly 10.
And maintain the gap between the fuel rods 11 in an appropriate state.
You. The fuel spacer 16 is held by a water rod 19. H
Yannel box 17 is attached to upper tie plate 12
The outer circumference of the bundle of fuel rods 11 held by the fuel spacer 16 is
Surrounding. The lower tie plate 13 has a fuel rod at the upper end.
It has a support portion 14 and a space 15 below the fuel rod support portion 14.
have. The fuel rod support portion 14 is provided with the fuel rod 11 and the water rod.
The lower end of the door 19 is supported. The fuel rod 11 is shown in FIG.
Both ends are sealed by the upper end plug 31 and the lower end plug 32
A large number of fuel pellets 33 loaded in a cladding tube 30
is there. A gas plenum 34 is formed at the upper end inside the cladding tube 30.
It is. The diameter of the water rod 19 (the outer diameter of the outer tube 21 described later) is
The water rod 19 is larger than the diameter of the fuel rod 11 and the water rod 19 is
Is located at the center of the cross section.   The detailed structure of the water rod 19 will be described with reference to FIG. Water ro
The tongue 19 includes an inner tube 20, an outer tube 21, and a spacer 22.
You. The outer tube 21 and the inner tube 20 are arranged in a concentric circle, and the outer tube 21 is
It surrounds the outer circumference of the inner tube 20. The upper end of the outer tube 21 is a cover
Sealed at 23, the upper part of the cover part 23 is the upper type
Inserted and held in rate 12. The cover part 23
The upper end of the inner tube 20 so as to form a gap with the upper end of the tube 20
Have suffered. The upper end of the inner pipe 20 is located at the axis of the water rod 19.
Outer tube 21 via plate-like spacers 22 radially arranged from
It is fixed to the inner surface of. The lower end of the outer tube 21 is closed at the sealing section 24
Is done. The lower end of the inner tube 20 penetrates the sealing portion 24
Protruding downward. The lower end of the inner tube 20 is
The fuel rod support portion 14 of the plate 13 penetrates. Inner tube 20
The cooling water inlet 28 formed at the lower end is a lower tie plate
It is open to 13 spaces 15. The inside of the inner pipe 20 is above the cooling water
The ascending channel 25. A ring formed between the inner tube 20 and the outer tube 21
The passage is the cooling water descending passage 26. Of the lower end of the outer tube 21
A plurality of cooling water discharge ports 29 are formed in the pipe wall in the circumferential direction.
These cooling water discharge ports 29 are provided at equal intervals in the circumferential direction.
ing. The cooling water discharge port 29 is located above the fuel rod support portion 14.
Is open in the region of. Cooling water rises on the side wall of the inner pipe 20
Side discharge port through which the flow can flow to the cooling water descending channel 26
36 are provided. The side discharge port 36 is located above the inner pipe 20.
A plurality of tubes are arranged at equal intervals in the axial direction, and
A plurality are also arranged in the direction.   In the present embodiment, the fuel rod support portion 14 is formed of a resistor shown in FIG.
6 functions. Coolant rise channel 25 and cool down
The channel 26 is a reversing part 27 formed at the upper end of the water rod 19.
Has been contacted by In this way, the water rod 19
Cooling water ascending passage 25, cooling water descending passage 26 and reversing section 27
And an inverted U-shaped cooling water flow path.   The fuel assembly 1 of this embodiment is placed in the core of a boiling water reactor.
Loading (all fuel assemblies are fuel assembly 1) and boiling water atom
When operating the furnace, most of the cooling water is
13 penetration space 18 provided in the space 15 and the fuel rod support portion 14.
(Fig. 1) Fuel of fuel assembly 10 loaded in the core through
It is introduced directly between the bars 11. Lower tie plate 13
The remaining part of the cooling water that has flowed into the space is
Flows into the cooling water ascending passage 25 of the water rod 19 from the
Cooling water discharge port 29 via reversing section 27 and cooling water descending flow path 26
From the fuel rod support 14 to the region above the fuel rod support 14. cooling
The cooling water discharged from the water discharge port 29 is supplied to the cooling water inlet 28.
Depending on the flow rate of the cooling water flowing into the water rod 19
As described above, the liquid or gas (vapor) is used.   In this embodiment, the core flow rate is 100% (the water rod 19 has
Maximum S in the figure₀8 (a) below
Is generated in the water rod 19, and the core flow rate is 110% (in the water rod 19).
Then, the state of flow rate at point R in FIG. 7) and the state of FIG.
Pressure in the fuel rod support 14 so that
Loss, specifications of inner tube 20 and outer tube 21 are preset
I have.   Next, the effect of the discharge port 36 provided on the side wall of the inner pipe 20 will be described as a fifth example.
This will be described with reference to the drawings. FIG. 5 shows the water supply in the water rod 19.
The relationship between the cooling water flow rate and the average void fraction in the water rod 19.
doing. The average void fraction in the water rod 19 is shown in FIG.
In the structure of the rod, the cooling of the water rod 19 is performed as shown by the curve X.
When the cooling water flow rate exceeds the water flow rate V, the water rod 19
It decreases sharply due to the change in pressure difference between the inlets. Cooling water flow rate V
In this state, the steam layer in the inner pipe 20 disappears due to the increase of the cooling water flow rate.
The state at the time of losing (the state shown in (b) in FIG. 8)
State). That is, the cooling water in the liquid state in the inner pipe 20 is cooled.
It can be said that it is the state at the time when it starts flowing into the recirculating water descending channel 26.
You. In this embodiment, a plurality of discharge ports 36 are provided on the side wall of the inner pipe 20.
The transition point at which all of the cooling water changes to the vapor phase
The cooling water in the state flows from the cooling water ascending passage 25 to the cooling water descending passage 26.
(Flow point) is earlier. That is, the real
In the embodiment, the cooling water flow rate at the transition point shifts to W smaller than V.
Move. For this reason, in this embodiment, the cooling water flow rate W is exceeded.
And the average void fraction is higher than the curve X as shown by the curve Y in FIG.
Decreases slowly. As a result, compared to the structure of FIG.
In the embodiment, the spectrum shift operation is performed while maintaining the stability.
It becomes possible. The diameter of this outlet 36 is above the cooling water
Between the inlet of the ascending channel 25 and the tube outlet of the cooling water descending channel 26
It is determined by the value of the differential pressure.   FIG. 11 shows another embodiment of the present invention. Real truth
The water rod 19A in the embodiment is a side wall of the cooling water rising flow path 2.
The intervals in the axial direction of the plurality of discharge ports 37 provided in
The discharge ports 37 are densely formed between the lower discharge ports 37. What
In addition, the opening of the discharge port 37 in the part where the mutual interval is coarse
The area is the opening of the outlet 37 in the part where it is dense
It is larger than the area. Discharge port 37, coolant rises
The flow path 2 communicates with the coolant descending flow path 3. This embodiment
The gap between the discharge ports 37 is dense at the top and coarse at the bottom
By arranging, the above transition point is the position of cooling water flow rate W1
When the cooling water flow rate exceeds W1, the curve Z1 in FIG.
As a result, the average guide rate gradually decreases. Slope of curve Z1
Is steeper than curve Y but slower than curve X
You. This embodiment is slightly more stable than the previous embodiment.
Although inferior, stability is maintained while plutonium is effectively used.
The spectrum shift operation can be performed while maintaining.   FIG. 12 shows another embodiment of the present invention. This embodiment
The water rod 19B has a plurality of outlets 37 opposite to the water rod 19A.
The gap in the axial direction of the lower
The outlets 37 are densely arranged. Vomiting of rough part
The opening area of the outlet 37 is as large as the water rod 19A.
ing. As in this embodiment, the interval between the discharge ports 37 is increased.
By arranging it so that it is rough at the bottom compared to the part
Low differential pressure between coolant inlet and outlet of water rod 19B
When the discharge ports are arranged at equal intervals (water rod 19)
The average void fraction change in the water rod compared to
As shown by line Z2 (the cooling water flow rate at the transition point is W2),
You. Therefore, the present embodiment is more effective than the embodiment of FIG.
The coolant flow rate can be changed while maintaining stability,
Enables spectrum shift operation with improved core stability
You.   The fuel assembly 10 occupies a water rod 19 as shown in FIG.
Is the cross-sectional area of the cooling water passage of the fuel assembly 1.
It is about 10%. However, two or more water
By providing rod 19, the average volume of the fuel assembly
The width of change in the id ratio can be increased. Improve fuel economy
A fuel assembly with nine water rods is proposed
Have been. In this case, traverse the cooling water flow path of the fuel assembly.
The ratio of the cross-sectional area of all water rods to the area is 30%
Also. FIG. 13 shows the fuel assembly 35 of this embodiment.
The fuel assembly 35 is described in JP-A-61-167972, p.
Lines 11 to 11, line 5 and the water lot of the fuel assembly shown in FIG.
In this case, all the rods are replaced with the aforementioned water rod 19. Real truth
The fuel assembly 35 of the embodiment is disclosed in the specification of Japanese Patent Application No. 61-176792.
The effect of the fuel assembly 1 as shown in FIG.
Response gain effect) can also be obtained.   A fuel assembly according to another embodiment of the present invention will be described below.
You.   Before we discuss this example, we need
The following describes the background.   This embodiment is different from the water rod shown in FIG.
Is done to discover what is happening and to solve that challenge.
It is a thing. The challenge is the cooling water flow supplied to the core.
If the amount suddenly increases for some reason, the water rod 1
The flow rate of the cooling water flowing into the
The average void fraction decreases sharply. like this
The phenomenon is a rapid injection of positive reactivity into the core.
The fuel rods may be adversely affected such as breakage. above
Various studies were conducted to resolve the issues.
In the coolant rising channel 2 of the ridge 1 (for example, the coolant rising channel 2
At the inlet), a hand that can suppress a sudden increase in coolant flow rate
The inventors notice that a step, for example, a resistor may be provided.
With. An example of this structure is shown in FIG. Water rod 19
In the lower portion of the coolant rising channel 2 of C,
Means for suppressing the increase (coolant flow rate rapid increase suppression means)
Then, the movable resistance element 38, the upper bracket 39 and the lower bracket 40
Is provided. The upper bracket 39 is located at the center
It is a disk provided with an opening,
Installed. The opening at the center is
It is smaller than the size (diameter). The lower bracket 40 is above the coolant
It is attached to the side wall of the rising channel 2 and extends toward the center of the channel.
These are four round bars. The lower bracket 40 is below the upper bracket 39
So that the movable resistance element 38 does not fall downward.
I support it. The movable resistance element 38 is a sphere made of metal.
is there.   The state in the water rod 19C is shown in FIG. 8 (a) or (b).
When the reactor is operating to
When the amount increases rapidly, the coolant flows into the coolant upflow channel 2.
The inflow of water rapidly increased and immediately changed to the state shown in FIG.
Try to move. At this time, a sudden increase in the cooling water flow rate
The movable resistance element 38 is pushed up, and the upper bracket 37 is opened.
Close the mouth and flow into the coolant rise channel 2 of the water rod 19C
The flow rate of the cooling water to be injected is restricted, and the state shown in FIG.
Prevent sudden transitions. The rate of increase in cooling water flow decreases
The movable resistance element 38 gradually descends as
It is held by the metal fitting 40. The movable resistance element 38 gradually
By descending, the area of the opening of the upper bracket 39 gradually increases.
And the flow rate of the cooling water supplied into the coolant rising flow path 2 increases.
Increase gradually. As a result, as described above, the coolant
It is possible to prevent a rapid increase in the amount of cooling water flowing into the ascending passage 2.
Wear.   The means for suppressing a sudden increase in the coolant flow rate shown in FIG.
FIG. 15 shows a specific structure of this embodiment applied to a fuel assembly.
It is described below based on The same configuration as that of FIG.
The same reference numerals are used. Parts that differ from the configuration in FIG.
Will be described only.   In the cooling water rise channel 25 of the water rod 19, a ball
Shaped metal movable resistance element 38, above which a movable resistance element
It has an opening 39A (Fig. 15 (c)) smaller than the diameter of the element 38.
A disk-shaped upper bracket 39 and a movable resistor element 38
A lower metal fitting 40 for holding the dynamic resistance element 38 is provided.
You. The upper bracket 39 and the lower bracket 40 are attached to the inner tube 20
Have been. The movable resistance element 38 is used for a predetermined change in the flow rate of the cooling water.
Weight so that it can be pushed up by a sudden increase
Has been adjusted.   Next, the device of the movable resistance element 38 provided in the cooling water rising flow path 25
The function will be described below. Core flow is within the specified range (120% or less)
In (bottom), the cooling water passes through the gap between the lower
It flows into the ascending channel 25. However, the cooling water flow rate changes
If the ratio suddenly increases beyond the ratio, the moving resistance element 38 flows in
Is pushed upward by the cooling water
Pressed. At this time, the movable resistance element 38 is
The opening 39A of the tool 39 is closed, and the water rod 19 enters.
Of the cooling water is temporarily blocked.   Therefore, the vehicle is operating in the state of FIG. 8 (a) or (b).
In the case where there is a sudden change in the flow rate, the state shown in FIG.
In the fuel assembly.
Prevent reduction.   The movable resistance element 38 is determined from the differential pressure generated in the water rod 19.
Is determined.   In the present embodiment, an example of a spherical rotation resistance element has been described.
The same effect can be obtained with pillars, cones, truncated cones, plates, etc.
Can be obtained.   Although not described in detail, the effect of the discharge port 36 is not described.
And cooling water descending passage 25 and cooling water descending passage 26
Can be obtained in the same manner as in the embodiment of FIG.
Wear.   If you want only the functions shown in Fig. 14,
The fuel assembly is constructed by removing the discharge port 36 from the structure
It should be done. 〔The invention's effect〕   According to the present invention, while maintaining stability with a simple structure,
The average void fraction in the fuel
It is possible to carry out a vector shift operation.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 (a) is a detailed structural view of the vicinity of a fuel assembly and a water rod according to a preferred embodiment of the present invention, and FIG. 1 (b) is a view of FIG. 1 (a). IV-IV sectional view, FIG. 2 is a longitudinal sectional view of the fuel assembly having the water rod of FIG. 1, and FIG. 3 is III-III of FIG.
FIG. 4 is a partial sectional view of the fuel rod shown in FIG. 2, and FIG. 5 is a characteristic showing the relationship between the flow rate of cooling water supplied into the water rod and the average void fraction in the water rod. FIG. 6 is an explanatory view showing the principle of the water rod used in the present invention. FIG. 7 is a conceptual diagram of a differential pressure characteristic generated between the inlet and the outlet of the water rod in FIG. 6, and FIGS. (B) and (c) of FIG.
Explanatory diagram showing the flow state in the water rod at ST and U points,
FIG. 9 is a characteristic diagram showing a change in the infinite multiplication factor with respect to the burnup when the spectrum shift operation is not performed and when the spectrum shift operation is performed. FIG. 10 is a characteristic diagram showing a relationship between the core flow rate and the core average void fraction. FIGS. 11, 12, and 13 are structural diagrams of a fuel assembly according to another embodiment of the present invention, and FIG. 14 is a structural diagram of a water rod of a fuel assembly according to another embodiment of the present invention. FIG. 15 is a structural view of a fuel assembly according to another embodiment of the present invention using the principle of the water rod shown in FIG. 10,35… fuel assembly, 11… fuel rod, 12… upper tie plate, 13… lower tie plate, 14… fuel rod support, 19, 19A to 19C… water rod, 20… Inner pipe, 21 ... Outer pipe, 25 ... Cooling water ascending flow path, 26 ... Cooling water descending flow path, 28
… Cooling water inlet, 29… Cooling water discharge port, 36, 37… Discharge port, 38… Movable resistance element, 39… Upper bracket, 40…
Lower bracket.

   ────────────────────────────────────────────────── ─── Continuation of front page    (72) Inventor Yuichiro Yoshimoto               3-1-1, Sachimachi, Hitachi City, Ibaraki Prefecture Shares               Hitachi, Ltd. Hitachi factory (72) Inventor Kazuo Wataniki               3-1-1, Sachimachi, Hitachi City, Ibaraki Prefecture Shares               Hitachi, Ltd. Hitachi factory                (56) References JP-A-62-250392 (JP, A)                 Patent 2533499 (JP, B2)

Claims (1)

(57) [Claims] A plurality of fuel rods filled with nuclear fuel material therein, a water rod disposed between the fuel rods, a lower tie plate having the fuel rods and a fuel holding unit for holding the water rods, and the fuel holding unit A fuel assembly including a first coolant passage formed between the fuel rods above the water rod and outside the water rod, and a second coolant passage formed in the water rod. (2) an inlet portion opening to a region below the fuel holding portion, and an outlet portion opening to the first coolant passage and located near a lower end portion of the fuel rod. A rising channel for raising the coolant flowing from the inlet portion, a descending channel for lowering the coolant rising in the rising channel toward the outlet portion, So that it is distributed in the axial direction of the fuel assembly on the side wall. A fuel assembly, comprising: a plurality of discharge ports provided to communicate with the descending flow path. 2. 2. The fuel assembly according to claim 1, wherein an interval between the plurality of discharge ports in the axial direction is smaller in an upper portion than in a lower portion. 3. 2. The fuel assembly according to claim 1, wherein an interval between the plurality of discharge ports in the axial direction is larger at an upper side than at a lower side. 4. 2. The fuel assembly according to claim 1, wherein an opening area of the plurality of discharge ports is larger at an upper side than at a lower side. 5. 2. The fuel assembly according to claim 1, wherein an opening area of the plurality of discharge ports is smaller at an upper side than at a lower side. 6. 2. The fuel assembly according to claim 1, wherein a resistor for a flow of a coolant is provided in the upflow channel.