JP3350199B2 - Boiling water reactor and fuel assemblies - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a boiling water reactor (BW).
R) and a fuel assembly, and more particularly, to a boiling water reactor and a fuel assembly which limit the amount of change in the assembly interval due to vibration such as an earthquake.

[0002]

2. Description of the Related Art FIG. 19 is a schematic view showing a structure inside a reactor pressure vessel of a boiling water reactor.
The reactor core 2 is accommodated in the reactor, and the reactor 2 is controlled by a control rod driving device 3 so that the control rod is moved up and down to control the fission reaction.
Water as reaction coolant is boiled. The boiled water is separated into steam and water by a steam separator 4, and the steam is further dried by a steam dryer 5 and sent to a turbine (not shown) through a main steam pipe 6. The turbine drives a generator to rotate. .

As shown in FIGS. 20 and 21, a core 2 has a fuel assembly 8 whose upper part is supported by an upper grid plate 7 formed in a cross-girder shape, and the fuel assembly 8 has an integral ratio of four fuel assemblies. Control rod 9 having a cruciform cross-sectional structure, and fuel support fitting 10 and core support plate 1 installed at the lower part of the core.
And 1. Four fuel assemblies 8 are placed in each lattice of the upper lattice plate 7, and a control rod 9 is inserted in the center thereof.
The core 2 has a C lattice core in which the intervals of the fuel assemblies 8 are uniform, and a narrow gap (narrow gap) on the upper lattice plate 7 side (not shown) as shown in FIG. There are two D lattice cores in which the installation intervals of the aggregates 8 are not uniform.

In general, in the case of a BWR, a fuel assembly 8 includes a channel box 12 having a rectangular cross section including a plurality of fuel rods and an upper tie plate 13 as shown in FIG.
, Lower tie plate 14 and channel fastener 15
The channel box 12 surrounds fuel rods, water rods, spacers, and the like. An upper tie plate 13 is provided above the channel box 12,
The lower tie plate 14 is fixed to the lower part, respectively.

[0005] As shown in FIG. 20, the fuel assemblies 8 are arranged such that the lower tie plate 14 is fitted into holes at four upper corners of the fuel support fitting 10 held by the core support plate 11 in the reactor every four units. Supported. The upper end of the fuel assembly 8 is held by the upper lattice plate 7. Also, a cross-shaped control rod 9
Is a mechanism for moving up and down between the fuel assemblies 8 through a central cross portion of the fuel support fitting 10. An instrumentation tube containing a neutron flux detector is inserted from the lower part of the core below the lattice intersection of the upper lattice plate 7 along the fuel assembly 8. The interval between the fuel assemblies is determined by the nuclear design and the thermal hydraulic design.

Water, which is a reaction coolant, flows into the channel box 12 from the lower tie plate 14 and flows out from the upper tie plate 13 as shown in FIG. On the outside of the channel box 12, a channel fastener 15 is provided.
And a fuel pad (also referred to as a channel spacer) 1
6 are provided on two sides of the channel box 12 corners.

[0007] The fuel pad 16 is provided with a control rod 9 inserted between the fuel assemblies 8 so as not to interfere with the adjacent fuel assemblies 8 even when the fuel assemblies 8 loaded in the core 2 are bent by their own weight. In addition, the fuel assembly 8 is designed so as not to interfere with the operation when the fuel assembly 8 is moved in and out of the reactor core 2.

The channel fastener 15 has a leaf spring structure, and presses the fuel assembly 8 loaded in the reactor core 2 against an adjacent fuel assembly, thereby preventing flow vibration caused by cooling water during operation of the reactor. Also, the space between the fuel assemblies 8 is held so as not to interfere with the control rods 9 inserted between the fuel assemblies 8. Further, the fuel assembly 8 is designed so as not to hinder the work when the fuel assembly 8 is taken in and out of the core 2.

[0009]

However, in the case of a D-lattice core, as shown in FIG. 23, the direction in which the installation intervals of the fuel assemblies 8 are uniformed over the entire core 2 due to the sway of the earthquake, that is, the upper part, Even if the gap on the grid plate 7 side is increased and the gap on the control rod 9 side is displaced by only about 1.6 mm, the reactivity is applied only for a very short time due to nuclear characteristics, and the "neutron flux" An “altitude signal” is issued and a reactor emergency shutdown called Scrum is automatically activated.

[0010] The scrum itself is a function provided to ensure the safety of the nuclear power plant. However, due to a very small earthquake that does not affect the structure of the nuclear power plant at all, the nuclear power plant is Scrambling is not preferable from the viewpoint of stable operation of a nuclear power plant because it causes a large fluctuation in a power grid.

The reason why the interval between the fuel assemblies becomes uniform due to such an extremely small earthquake is as shown in FIGS. This is because it is supported by. When a load acts in the horizontal direction due to the roll of the earthquake, there are a fuel assembly pressed against the upper lattice plate 7 and a fuel assembly which falls on the fuel assembly.

At the upper part of the fuel assembly 8, as shown in FIG. 22, a leaf fastener 1 is attached to a channel fastener 15 which is a fixture.
The leaf spring 17 restrains the displacement of the fuel assembly 8, but when the spring force is weak, the fuel assembly 8
The horizontal load acting on the upper lattice plate 7 cannot be received, and the interval on the upper lattice plate 7 side increases.

FIGS. 25A and 25B show the upper lattice plate and the horizontal displacement direction of the earthquake. FIG. 25A shows a case where the displacement is parallel to the upper lattice plate 7. In this case, in addition to the neutron flux increase, the “neutron flux high / low signal” generated by the sum of the apparent neutron flux increase due to the increase in water near the neutron flux detector (expansion of the fuel assembly interval) causes to reach as upper relative displacement 2.delta. ^max between fuel assemblies 8, it is necessary relative displacement of approximately 3.2 mm.

On the other hand, FIG. 25B shows that the displacement is
Is 45 ° with respect to In this case, similarly, in order to reach the scrum by the “neutron flux height signal”, the upper relative displacement 2δ ^max between the fuel assemblies 8 is about 1.6 mm.
Is required. That is, in order to improve the seismic performance of the reactor core, measures must be taken assuming FIG. 25 (B). The manufacturing tolerance of the upper lattice plate 7 is ±
0.9 mm, and it is necessary to take this into consideration. Unless this measure is taken, it can be seen that the apparent "neutron flux high and high signal" makes the plant unnecessarily scrum.

Further, the C lattice core having a uniform spacing of the fuel assemblies used in recent boiling water reactors has a stable state with the highest reactivity as shown in FIG. Even if the interval between the aggregates is slightly displaced, the reactivity is only slightly reduced and no scrum is caused.

Although the problem can be solved by changing the D-lattice core to the C-lattice core, a major modification such as changing the core structure of the operating plant or replacing the upper lattice plate 7 is tentative. It is not as simple as using the period during inspection, and shutting down the plant for a long period of time has a large economic loss and poses a major problem for the power industry.

Further, in the fuel assembly 8 described above, even when the seismometer vibrates due to a relatively small earthquake below the seismic scram set point of the reactor, the channel fastener 15 is pushed by the horizontal component of the vibration. The distance between the clusters may change due to contraction or extension.

As a result, the neutron flux fluctuates at the place where the assembly interval changes, reaches the reactor scram set point called “neutron flux high”, the reactor is shut down, and the operability and economy of the reactor are reduced. It may be disadvantageous in terms of performance and stable supply of power.

The present invention has been made in consideration of the above circumstances, and significantly reduces the relative displacement of the fuel assembly, prevents the application of the reactivity for a short time, and prevents the application of the reactivity even if the displacement occurs. It is an object of the present invention to provide a boiling water reactor and a fuel assembly capable of achieving a C-lattice core.

[0020]

In order to solve the above-mentioned problems, a boiling water reactor according to the present invention comprises a horizontal grid-like upper grid plate and a plurality of grids in the upper grid plate. And a fuel assembly having a rectangular tubular channel box, the upper part of which is horizontally supported by the upper lattice plate, and a fuel assembly inserted between the plurality of fuel assemblies from below the upper lattice plate. Control rod, means for detecting the neutron flux near the fuel assembly, and means for inserting the control rod when the neutron flux exceeds a predetermined value, in a boiling water reactor comprising: The neutron flux detected by the neutron flux detection means during an earthquake has a different spring constant that restricts relative displacement between a plurality of fuel assemblies in the lattice of the upper lattice plate so that the neutron flux does not exceed the predetermined value. Plate springs facing each other Is attached to each of the two channels fastener is attached to each of the two channels fastener to the opposite, different leaf springs having spring constants is the overall spring constant of the different above plate spring having a spring constant K, High neutron flux scram
The minimum value of the displacement of the fuel assembly is L, and the seismic scram set point
Earthquake acceleration G, the mass of fuel assemblies m of different of the above plate spring having a spring constant, when the spring constant of each of the leaf springs and K _1, K _2, the whole spring constant K
To

K> mG / (2L) where K = (K ₁ · K ₂ ) / (K ₁ + K ₂ ).

According to a second aspect of the present invention, a plurality of fuel rods are arranged in a grid in a channel box having a rectangular cross section, and an upper tie plate and a lower tie plate are fixed to upper and lower portions of the channel box, respectively. In this fuel assembly, a leaf spring having a different spring constant is fixed to each of the two channel fasteners mounted opposite to the upper outer surface of the channel box, and the leaf springs having different spring constants are different from each other. The overall spring constant of the above leaf spring having the spring constant
Minimum value of displacement of fuel assembly resulting in high neutron flux scram
L, seismic acceleration at seismic scrum set point G, fuel set
Assuming that the mass of the body is m and the spring constants of the individual leaf springs among the above leaf springs having different spring constants are K ₁ and K ₂ , the total spring constant K is

K> mG / (2L) where K = (K ₁ · K ₂ ) / (K ₁ + K ₂ ).

[0022]

According to the first aspect of the present invention having the above structure, a plurality of neutron fluxes in the lattice of the upper lattice plate are so set that the neutron flux detected by the neutron flux detection means during an earthquake does not exceed the predetermined value. Simplifies the structure by using leaf springs that combine leaf springs with different spring constants to restrict the relative displacement between the fuel assemblies, while the spacing between the bundles changes during vibrations due to relatively small earthquakes, etc. However, the relative displacement can be reduced and the short-time application of the reactivity can be prevented.

According to the second aspect of the present invention, a leaf spring having a combination of leaf springs having different spring constants is fixed to a channel fastener attached to an upper outer surface of the channel box, and leaf springs having different spring constants are combined. For the leaf spring, the overall constant of the combined leaf spring is K, the neutron flux height
The minimum value of the displacement of the fuel assembly that becomes a scrum is L, earthquake
The seismic acceleration at the scrum set point is G, and the mass of the fuel assembly is
m, when the spring constants of the individual leaf springs are K ₁ and K ₂ among the overall constants K of the combined leaf springs, the overall constant K of the combined leaf springs is

K> mG / (2L) where K = (K ₁ · K ₂ ) / (K ₁ + K ₂ ), thereby simplifying the structure as in the first aspect. Even when the fuel assembly interval changes during vibration due to a relatively small earthquake or the like, the relative displacement can be reduced, and short-time application of reactivity can be prevented.

[0024]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a perspective view showing a main part of a first embodiment of a fuel assembly of a boiling water reactor according to the present invention. In addition,
The same parts as those of the conventional configuration are described using the same reference numerals. The overall configuration of the boiling water reactor is shown in FIGS.
Therefore, the description of the fuel assembly 8 is omitted, and a plurality of fuel rods are arranged in a lattice shape in a channel box 12 having a square cross section as in the conventional example. An upper tie plate 13 and a lower tie plate 14 are fixed to the upper and lower portions of 12, respectively.

As shown in FIG. 1, the channel box 1
A channel fastener 20 is attached to the upper outer surface of the second 2, and a highly rigid leaf spring 21 is attached to the channel fastener 20 as a roll prevention metal fitting. That is, the channel fastener 20 and the highly rigid leaf spring 21 are attached to the triangular plate 22 fixed to the upper corner of the channel box 12 by the fixing screw 23. The channel fastener 20 and the thick fuel pad 24 are both attached to the outer surface of the channel box 12,
The central portions in the height direction of the highly rigid leaf spring 21 and the thick fuel pad 24 coincide with the installation height of the upper tie plate 13.

A displacement limiting projection 25 is formed integrally with the channel fastener 20.
By giving the upper limit of the deformation of the highly rigid leaf spring 21, the structure has sufficient rigidity against a large horizontal external force.

Next, the operation of this embodiment will be described.

FIG. 2 shows a state in which the fuel assembly 8 shown in FIG. 1 is loaded in a core. As shown in FIG. 2, a rigid plate spring 21 and a thick fuel pad 24 as anti-rolling brackets are provided.
The upper portion of the fuel assembly 8 is strongly pressed against the upper lattice plate 7 by the action of the above, and the loading structure has sufficient rigidity against a large horizontal external force.

The displacement limiting projection 2 of the channel fastener 20
By sufficiently increasing the maximum thickness t including 5, the gap g between the fuel assemblies 8 can be limited to a predetermined value described below. That is, the leaf spring 21 having high rigidity does not deform any more due to the contact between the displacement limiting protrusions 25 of the two fuel assemblies. Since the manufacturing tolerance of the upper lattice plate 7 is about ± 0.9 mm, the average gap g is set to 0.9 mm.
The following cannot be set. This is because the loading of the fuel assemblies 8 into the core may be hindered.

On the other hand, in the upper lattice plate 7 having the widest manufacturing tolerance, the maximum gap 2e is 1.8 mm from Equation 1 described later, and the maximum allowable gap is 1.6 mm only by increasing the thickness of the channel fastener 20. It is difficult to determine the thickness t to be limited to the following. Therefore, as shown below, it is necessary to set the spring constant of the highly rigid leaf spring 21 to a predetermined value or less. There is an upper limit of the spring constant for smooth loading of the fuel assembly 8 into the core containing water, and the actual spring constant needs to be set to this upper limit. In addition, since the channel fastener 20 is attached to each of the fuel assemblies 8, a description will be given below using a combination spring constant K in which two highly rigid leaf springs 21 are combined.

FIG. 3 shows the combination spring constant K of the highly rigid leaf spring 21 and the maximum upper horizontal load 600 in the direction of the upper lattice plate.
N: shows the relationship between the maximum displacement 2.delta. ^Max between the horizontal direction of the fuel assembly of the fuel assembly top when (Newton SI units). As shown in FIG. 3, the maximum displacement 2δ ^max between the fuel assemblies must be set to 1.6 mm or less from the core characteristics of the core. It is necessary to set the material and the thickness of the leaf spring so that K is 375 N / mm or more.

FIG. 4 shows the maximum displacement 2δ ^max of the highly rigid leaf spring 21 when a maximum upper horizontal load 600N in the direction of the upper lattice plate is applied. The maximum horizontal displacement of the upper part is 2 × δ ^max = 1.6 mm as a total value of two fuel assemblies.
Can be suppressed. The formula for calculating such a combination spring constant K is expressed by Equation 3 described below. As a specific example, the spring constant K
₁ = 1200 N / mm, spring constant K ₂ = 540 N / mm
It can be realized as This is equivalent to increasing the thickness of the conventional leaf spring by 1.3 times.

Accordingly, the leaf spring 21 having high rigidity according to the present embodiment uses the spring constant calculated from the maximum allowable displacement of earthquake scram and the horizontal load as the lower limit, and allows the fuel assembly 8 to be loaded by its own weight in water. The constant is set as the upper limit, and the spring constant of the highly rigid leaf spring 21 is set between the lower limit and the upper limit.

As described above, according to the present embodiment, the lower limit of the spring constant of the leaf spring 21 having high rigidity is set to the spring constant calculated from the maximum allowable displacement of seismic scram generation and the horizontal load. 8 to reduce the relative displacement,
It is possible to prevent short-time application of reactivity.

Further, by setting the upper limit of the spring constant of the highly rigid leaf spring 21 to a spring constant at which the fuel assembly 8 can be loaded by its own weight in water, there is no problem in taking the fuel assembly 8 into and out of the core. It can be loaded smoothly without coming, and can be easily pulled out.

FIG. 5 is a plan view showing a state where the fuel assembly of the second embodiment is loaded on the core. The same parts as those in the first embodiment will be described with the same reference numerals. In this embodiment, in order to fix the upper lattice plate 7 and the upper part of the fuel assembly 8, an upper lattice plate fixing bracket 26 shown in FIG.

FIG. 6 shows a specific structure of the fuel assembly according to the second embodiment. The upper grid plate 7 is sandwiched between the upper grid plate fixing bracket 26 and the thick fuel pad 24, and the fuel assembly 8 is moved in the horizontal direction. Displacement is constrained. The upper lattice plate fixing bracket 26 fixed to the channel box 12 on the opposite side is structured to be inserted into a gap between the upper lattice plate 7 generated by the thick fuel pad 24.

Therefore, according to the second embodiment, the upper portion of the fuel assembly 8 can be firmly fixed to the upper grid plate 7 by the upper grid plate fixing bracket 26 and the thick fuel pad 24.

FIG. 7 is a plan view showing a state where the fuel assembly of the third embodiment is loaded on the core. The same parts as those in the first embodiment will be described with the same reference numerals. In this embodiment, a plurality of anti-rolling brackets 3 are used as anti-rolling brackets for fixing the upper portions of the fuel assemblies on the upper lattice plate side to each other.
0 is fixed to the upper part of every four fuel assemblies 8.

FIG. 8 shows details of the anti-rolling fixture 30. The anti-rolling fixture 30 has four fixing legs 31 formed in a tapered shape having a thick upper portion, and has a structure for sandwiching the upper portion of the channel box 12. Therefore, since the four fixing legs 31 of the anti-rolling fixing bracket 30 are formed in a tapered shape with a thickened base, the refueling machine grip handle 32 is grasped by the refueling machine, and the fixing bracket 30 is moved to the upper part of the fuel assembly 8. When placed, the fixture 3
Numeral 0 causes the upper portions of the fuel assemblies 8 to be pulled toward each other in the horizontal direction by their own weight, pressed against the upper lattice plate 7, and strongly restrained. The fixing bracket 30 has a weight that does not rise due to the flow of the coolant or the like.

Therefore, according to the third embodiment, the existing fuel assembly 8 can be used without processing the fuel assembly 8 itself, and the four fuel assemblies 8 can be fixed at one time. can do. Further, since the four fixing legs 31 are formed in a tapered shape in which the upper part is thickened, the upper part of the fuel assembly on the upper lattice plate side can be easily fixed even if the position of the fixing bracket 30 is slightly shifted.

FIG. 9 is a perspective view showing the appearance of the fuel assembly of the fourth embodiment. The same parts as those in the first embodiment will be described with the same reference numerals. This embodiment uses an eccentric lower tie plate 41 and an upper fuel pad fitting 42 to make the spacing between the channel boxes 12 uniform and change the D-lattice core to the C-lattice core without replacing the furnace internals such as the upper lattice plate. Is what you do.

That is, the eccentric lower tie plate 41 is
As shown in FIG. 10, the tip is inserted into the fuel support hole for the D lattice core, and the center axis in the longitudinal direction is set to the center O _{1 of the} fuel support hole in order to arrange the channel box 12 in the C lattice core. From the center O ₂ of the channel box 12. A loading direction display 43 of the fuel assembly 8 such as an arrow is provided above the triangular plate 22 so that the fuel loading direction is not mistaken, and the fuel loading direction is automatically determined by a pattern recognition function using a remote television camera and a computer. To make sure.

Therefore, according to the fourth embodiment, it is possible to realize a C-lattice core in which no reactivity is applied even if the fuel assembly 8 is displaced, and a loading direction designation mark is provided on the upper part of the triangular plate 22. Since the fuel assembly 8 is provided, the fuel assembly 8 can be loaded in an accurate direction.

Next, FIG. 11 is a schematic view showing a state in which a fuel assembly in a boiling water reactor according to the present invention is loaded on a core of a first embodiment. In FIG. 11, channel fasteners 15 and channel spacers 16 are attached to two orthogonal surfaces at the upper end of the channel box 12, and face each of the adjacent channel boxes 12. The upper ends of the fuel assemblies are held in the meshes of the upper lattice plate 7 every four units. A neutron flux detector 51 is inserted from below the core below the lattice intersection of the upper lattice plate 7 along the fuel assembly.

Next, an example of how to determine the thickness of the channel spacer 16 of the first embodiment will be described. In the first place, when the fuel assembly receives a horizontal force due to an earthquake or the like,
As shown in FIG. 12, the fuel assemblies sway, the intervals between the fuel assemblies change, and the reactivity enters the reactor according to the displacement of the fuel assemblies as shown in the graph of FIG.

The reactivity can be calculated based on the design of the fuel assembly and the reactor core, and the intensity and cycle of the vibration. The design of the fuel assembly and the reactor core is a known value given for each target reactor. Quantity. The vibration is given by using typical seismic data already obtained. At this time, it is only necessary to consider the case of vibration below the seismic scrum set point by the seismometer, and the magnitude of the vibration is relatively small.

FIGS. 14 (A), (B) and (C) show that, for a certain fuel assembly, the maximum reactivity at the time of the earthquake below the seismic scram set point which occurred in the past was calculated for each maximum displacement of the fuel assembly. Things. The maximum displacement of the fuel assembly is defined as the amount of displacement at the upper end of the fuel assembly, and this definition is used hereinafter. Further, FIG. 14 also shows the maximum reactivity in consideration of the deflection of the fuel assembly. In the figure, "¢
(Cents) "represents the degree of reactivity input and 1
When a reactivity of 00 ° is input, only the prompt neutrons generated by the fission reaction become critical or more, and the neutron flux rapidly increases.

Next, FIGS. 15 to 18 show calculation results of changes in the neutron flux and the average surface heat flux based on the maximum reactivity and the like shown in FIG. 14, and show the peak values of the neutron flux, respectively. I have. Since the neutron flux high scram set point is generally set at 118% of the steady state neutron flux, FIG.
7 and the graph shown in FIG.
At about 20 ° or more, it is understood that the neutron flux may exceed the neutron flux high scram set point. The change in the average surface heat flux is small and does not matter.

From these calculations, according to FIGS. 14, 17 and 18, in the case of the fuel assembly shown here, if the displacement amount due to vibration is about 1.6 mm or more, in the case of the earthquake mentioned as an example, It turns out that the neutron flux may be high scrum. In the same manner, the minimum value of the displacement of the fuel assembly that becomes a high neutron flux scram due to vibration such as an earthquake below the seismic scram set point is obtained. Here, the minimum value is set to L.

When the fuel assembly shown in FIG. 11 receives a force due to an earthquake or the like in the horizontal direction and there is no influence of the channel fastener 15, the relative maximum displacement Δ of the fuel assembly faces. This is considered to be the distance w between the channel spacers 16. For example, when the swing is applied in the direction of the arrow shown in FIG. 12, the fuel assemblies 8a to 8d are considered to swing in the same direction. The interval between the fuel assemblies at a certain position of the neutron flux detector 51, that is, the interval between the fuel assemblies 8b and 8c starts to fluctuate while maintaining substantially the same width.

However, the fuel assembly 8b
Before the fuel assemblies 8a and 8c contact the fuel assembly 8a, the fuel assemblies 8a and 8c abut on the upper grid plate 7, so that after the abutment, the neutron flux detectors remain until the fuel assembly 8b abuts on the fuel assembly 8a. It is considered that the interval between the fuel assemblies at the location 51 is widened. Therefore, the relative maximum displacement Δ of the fuel assembly is the distance w between the opposed channel spacers 16.

When the manufacturing tolerance of the upper lattice plate 7 is ± e, the interval w between the opposed channel spacers 16 is the designed fuel assembly interval B and the thickness t of the channel spacer 16.
From w = B-2t + 2e. If the distance w is smaller than the minimum value L of the displacement of the fuel assembly that becomes the neutron flux high scram, the neutron flux high scram does not occur structurally.

Therefore, the thickness t of the channel spacer 16 may be determined so as to satisfy the following relationship. However, a value larger than the thickness of the channel spacer obtained by the conventional design method regarding the fuel assembly interval is adopted.

[0056]

Δ = w = B−2t + 2e <L

T> (B−L) / 2−e> 0 Further, since the relative maximum displacement Δ of the fuel assembly can be obtained with many structural analysis calculation codes, analysis is performed instead of the above equation. Using the result is more accurate.

Next, the channel fastener 2 of the first embodiment
A method for determining a spring constant of 0 will be described. As already mentioned,
In order to avoid the scram due to the neutron flux height, the relative maximum displacement amount Δ of the fuel assembly should be smaller than the minimum value L of the displacement amount of the fuel assembly having the neutron flux high scram.
Further, in the present embodiment, it is only necessary to consider the time of vibration such as an earthquake below the seismic scrum set point, so that the maximum sway applied in the horizontal direction can be the seismic acceleration G of the seismic scrum set point.

Each of the high-rigidity leaf springs 21 mounted on each of the channel fasteners 20 disposed so as to face each other generally has different spring constants, which are designated as K ₁ and K ₂ . Since the leaf springs are connected in series, the following relational expression holds if K is the overall spring constant of a combination of leaf springs having different spring constants from K ₁ and K ₂ .

[0059]

K = (K ₁ · K ₂ ) / (K ₁ + K ₂ ) A spring having this spring constant K becomes a neutron flux high scram.
Of the displacement of the fuel assembly at the minimum value L,
If it is greater than the seismic acceleration G at the seismic scram set point, the fuel bundle spacing of the neutron flux detector 51 will not be wide enough to cause a high neutron flux scram. Therefore, the spring constant of the channel fastener may be determined so as to have the following relationship. Note that the fuel assembly is assumed to be a rigid body having a mass of m, and the fuel assembly is assumed to tilt about the lower tie plate.

[0060]

KL> mG / 2 That is,

K> mG / (2L) Further, since the maximum displacement amount of the channel fastener 15 given the spring constant can be obtained with many structural analysis calculation codes, the analysis result is used instead of the above equation, The accuracy is higher when the required spring constant is determined.

As described above, according to the present embodiment, even when the interval between the assemblies changes during vibration caused by a relatively small earthquake below the seismic scram set point of the reactor by the seismometer, the neutron flux is changed by the change amount. Fluctuates and no longer reaches the reactor scram set point called neutron flux height,
The reactor can be operated without unnecessary shutdown.

In each of the above embodiments, it is needless to say that a necessary reactor scram is generated by a signal from a seismometer or the like in order to ensure the safety of the reactor during an earthquake.

[0063]

As described above, according to the boiling water reactor and the fuel assembly according to the present invention, the neutron flux detected by the detection means during an earthquake does not exceed the predetermined value. The structure is simplified by combining leaf springs with different spring constants that restrict the relative displacement between multiple fuel assemblies in the lattice of the upper lattice plate, while simplifying the structure during vibrations due to relatively small earthquakes. Even when the assembly interval changes, the relative displacement can be reduced, and short-time application of reactivity can be prevented. Thus, the reactor can be operated without being stopped unnecessarily.

[Brief description of the drawings]

FIG. 1 is a perspective view showing a main part of a first embodiment of a fuel assembly of a boiling water reactor according to the present invention.

FIG. 2 is a partial cross-sectional view showing a state where the fuel assembly shown in FIG. 1 is loaded on a reactor core.

FIG. 3 is a graph showing the relationship between the spring constant of a highly rigid leaf spring and the maximum horizontal displacement of the upper part of the fuel assembly.

FIG. 4 is a sectional view showing a maximum displacement of a highly rigid leaf spring when a maximum upper horizontal load is applied.

FIG. 5 is a plan view showing a state where a fuel assembly according to a second embodiment is loaded on a reactor core.

FIG. 6 is an essential part perspective view showing a specific structure of a fuel assembly according to a second embodiment.

FIG. 7 is a plan view showing a state where a fuel assembly according to a third embodiment is loaded on a reactor core.

FIG. 8 is a perspective view showing details of an anti-rolling fixture according to a third embodiment.

FIG. 9 is a perspective view showing an appearance of a fuel assembly according to a fourth embodiment.

FIG. 10 is a plan view showing a fuel assembly according to a fourth embodiment.

FIG. 11 is a schematic view showing a state in which a fuel assembly according to the present invention is loaded on a core of the first embodiment.

FIG. 12 is an explanatory diagram showing displacement caused by swinging of a fuel assembly in the first embodiment.

FIG. 13 is a graph showing a displacement caused by the swing of the fuel assembly and the reactivity due to the displacement in the first embodiment.

FIGS. 14A, 14B, and 14C are explanatory diagrams showing a relationship between a maximum reactivity and a maximum displacement of a fuel assembly.

FIG. 15 is a graph showing changes in the neutron flux and the average surface heat flow rate when the maximum reactivity is about 5 °.

FIG. 16 is a graph showing changes in neutron flux and average surface heat flux at a maximum reactivity of about 10 °.

FIG. 17 is a graph showing changes in the neutron flux and the average surface heat flux at a maximum reactivity of about 15 °.

FIG. 18 is a graph showing changes in neutron flux and average surface heat flux at a maximum reactivity of about 20 °.

FIG. 19 is a schematic view showing a structure of a reactor pressure vessel of a conventional boiling water reactor.

FIG. 20 is an enlarged view showing an upper part and a lower part of a conventional core.

FIG. 21 is a plan view showing the arrangement of a conventional D lattice core.

FIG. 22 is a perspective view showing an upper part and a lower part of a conventional fuel assembly.

FIG. 23 is a graph showing characteristics of relative displacement and reactivity application of fuel assemblies of a conventional D lattice core and a C lattice core.

FIGS. 24A and 24B are a schematic side view and a schematic plan view showing displacement of a fuel assembly when a conventional D lattice core receives horizontal displacement due to an earthquake.

FIGS. 25A and 25B are explanatory diagrams showing horizontal displacement directions of an earthquake with respect to an upper lattice plate.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Reactor pressure vessel 2 Reactor core 7 Upper lattice plate 8 Fuel assembly 9 Control rod 10 Fuel support bracket 11 Core support plate 12 Channel box 13 Upper tie plate 14 Lower tie plate 15 Channel fastener 16 Fuel pad 20 Channel fastener 21 Leaf spring 22 Triangular plate 24 Thick fuel pad 26 Upper grid plate fixing bracket 30 Anti-rolling fixing bracket 31 Fixed leg 41 Eccentric lower tie plate 42 Upper fuel pad bracket 43 Loading direction display

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Kazuyoshi Kataoka 1 Koga Toshiba-cho, Saiwai-ku, Kawasaki-shi, Kanagawa Pref. Toshiba R & D Center (72) Inventor Wataru Mizumachi 8 Shin-Sugita-cho, Isogo-ku, Yokohama, Kanagawa Address Toshiba Corporation Yokohama Office (72) Inventor Yu Sumita 8 Shinsugita-machi, Isogo-ku, Yokohama-shi, Kanagawa Prefecture Toshiba Corporation Yokohama Office (56) References JP-A-61-264289 (JP, A) JP-A-59 -5991 (JP, A) Japanese Utility Model Showa 50-13098 (JP, U) JP-B-44-9556 (JP, B1) (58) Fields investigated (Int. Cl. ⁷ , DB name) G21C 3/33

Claims (2)

(57) [Claims] 1. An upper lattice plate having a horizontal lattice shape, and a square tubular channel box which is provided upright in a plurality of lattices of the upper lattice plate and whose upper part is horizontally supported by the upper lattice plate. A fuel assembly, a control rod inserted between the plurality of fuel assemblies from below the upper lattice plate, a means for detecting a neutron flux near the fuel assembly, and the neutron flux having a predetermined value. Means for inserting the control rod when it exceeds
The neutron flux detected by the neutron flux detection means during an earthquake has a different spring constant that restricts the relative displacement between a plurality of fuel assemblies in the lattice of the upper lattice plate so that the neutron flux does not exceed the predetermined value. The leaf springs attached to each of the two opposed channel fasteners and attached to each of the two opposed channel fasteners, and the leaf springs having different spring constants correspond to the leaf springs having different spring constants. The overall spring constant is K, medium
The minimum value of the displacement of the fuel assembly that results in a high neutron flux scram
L, G is the seismic acceleration of the seismic scrum set point, and fuel assembly
Let m be the mass of m, and among the leaf springs having different spring constants, let K ₁ and K ₂ be the spring constants of the individual leaf springs.
Boiling water to the spring constant K of the whole, characterized in that it is set in the range of Equation 1] K> mG / (2L) _{where, K = (K 1 · K} 2) / (K 1 + K 2) Reactor. 2. A fuel assembly in which a plurality of fuel rods are arranged in a grid in a channel box having a rectangular cross section, and an upper tie plate and a lower tie plate are fixed to the upper and lower portions of the channel box, respectively.
A leaf spring having a different spring constant is fixed to each of two channel fasteners mounted opposite to the upper outer surface of the channel box, and the leaf spring having a different spring constant is a plate spring having a different spring constant. The overall spring constant of the spring is K, and the neutron flux high scram
The minimum value of the displacement of the fuel assembly is L, and the seismic scram set point
Earthquake acceleration G, the mass of fuel assemblies m of different of the above plate spring having a spring constant, when the spring constant of each of the leaf springs and K _1, K _2, the whole spring constant K
K> mG / (2L) where K = (K ₁ · K ₂ ) / (K ₁ + K ₂ ).