JP2002006073A - Fuel assembly - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve a thermal margin and heat removal performance to a fuel rod. SOLUTION: A plurality of flow tabs 19 projecting from the upper end part of a support band 18 for composing a nearly square frame body are formed in parallel with the support band 18. A cutout 20 is provided at the central tip part of each of the flow tabs 19 for dividing into two, and the tip part of the flow tabs 19 being divided into two is bent toward the fuel rod 3 for projecting a bent part 21. A liquid drip during a bi-phase flow is distributed to the surface of the fuel rod 3 by the tip part of the projected flow tabs 19, thus improving the thermal margin effect and heat removal effect.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel assembly incorporating a fuel spacer incorporated in a fuel assembly of a boiling water reactor to maintain fuel rods at appropriate intervals.

[0002]

2. Description of the Related Art In a light water reactor, a fuel assembly having a non-homogeneous shape in the radial direction is required to reduce the operating cost of a nuclear power plant, to operate the fuel cell for a long period of time, and to economically burn the fuel for realizing these. Has been introduced.

FIG. 11 shows the structure of a fuel assembly used in a boiling water reactor as an example of such a fuel assembly. That is, in the fuel assembly denoted by reference numeral 1, two large-diameter water rods 4 corresponding to four fuel rods 3 are arranged in the center of a channel box 2 which is an annular flow path having a substantially rectangular cross section. 74 fuel rods 3 are arranged in a square lattice shape as a whole.

[0004] At the upper and lower ends of the water rod 4, a cooling water inflow hole 5a and a cooling water ejection hole 5b are formed. A plurality of fuel spacers 7 are arranged in the axial direction in order to keep the horizontal spacing between the fuel rods 3, water rods 4, and short fuel rods 6 arranged in a square lattice.

The upper ends of the plurality of fuel rods 3 and water rods 4 are supported by an upper tie plate 8, and the lower ends of the plurality of fuel rods 3, water rod 4 and short fuel rods 6 are supported by a lower tie plate 9. I have.

Further, a bundle of the fuel rods 3, the water rods 4 and the short fuel rods 6 bundled by the fuel spacer 7, that is, the fuel channel is surrounded by the channel box 2. The channel box 2 is attached to the upper tie plate 8 and covers the outer surface of the lower tie plate 9. Reference numeral 10 in the figure denotes an external spring inserted into the upper end plug of the fuel rod 3 which is in contact with the lower surface of the upper tie plate 8.

FIG. 12 is an enlarged side view of the fuel spacer 7 shown in FIG. 11, showing a state in which many fuel rods 3 are inserted into the fuel spacer 7. The fuel spacer 7 has a rectangular frame-shaped support band 11 that supports a large number of fuel rods 3 from the outer peripheral side, and a fuel rod insertion passage that is provided in the support band 11 and passes the fuel rods 3 and the short fuel rods 6. For example, an annular ferrule 14 shown in FIG.

The same number of the annular ferrules 14 as the fuel rods 3 and the short fuel rods 6 are bound and arranged in a lattice pattern, and the outer periphery of the bundle of annular ferrules 14 is surrounded by the support band 11, whereby the fuel spacer 7 is formed. It is configured.

A projection 11a called a bathtub is formed on the outer surface of the support band 11 so that the bundle of the fuel rods 3 does not significantly shift in a specific direction of the channel box 2. A flow tab 12 protrudes from an upper end edge on the downstream side of the flow.

The coolant flowing in the channel box 2 rises in the channel box 2 from below and is heated by the fuel rods 3 to boil to form a gas-liquid two-phase flow consisting of a liquid phase and a gaseous phase. Pass through. At this time, the gas phase mainly flows in a relatively wide flow passage between the fuel rods 3, and the liquid phase partially flows along with the gas phase, and a part thereof flows on the surface of the fuel rod 3 and the inner surface of the channel box 2. Flows as a liquid film flow.

When the liquid phase flowing along the surface of the fuel rod 3 decreases, the surface heat transfer coefficient of the fuel rod 3 decreases (referred to as the start of boiling transition), and overheating (referred to as burnout) may occur. The thermal limit of the fuel assembly 1 is a state in which the boiling curve L shifts from the nucleate boiling region H1 to the transition boiling region H2 as shown in FIG. 13, and the surface heat flux at that time is reduced to the critical heat flux Q _{C HF.}
Is defined.

The boiling mode during normal operation is the nucleate boiling region H1.
This region is in a stable state, and the temperature of the fuel rod surface (cladding tube surface) is kept constant at a temperature several degrees higher than the saturation temperature of the coolant.

On the other hand, when the temperature exceeds the burnout point A, the difference (superheat) between the fuel rod surface (cladding tube surface) temperature and the coolant saturation temperature gradually increases, resulting in a boiling state in which heat transfer is unstable. Although this burnout point A is not a critical point that actually leads to thermal damage to the cladding tube, it is a boiling region that is unacceptable for the fuel rod even during normal operation and single failure transients.

It is experimentally known that the burnout point A depends on parameters such as pressure, coolant flow rate, fuel assembly shape, axial power distribution, and nuclear fuel rod power distribution. The axial position where burnout occurs is
It is known that the fuel spacer is located within several centimeters upstream from the lower end of the fuel spacer disposed in the region having a high void ratio.

As a cause of this, as shown in FIG. 11, the coolant which has risen in the pipe group flow path between the fuel rods 3 causes the flow resistance of the fuel spacer 7 to be disturbed on the fuel rod 3 side. A part of the liquid film flow adhered to the surface of the fuel rod 3 was separated due to the turbulence, and the surface of the fuel rod 3 was dried (dry out).
There is a theory that the heat transfer rate of a dry part decreases when the state becomes a state.

Further, the liquid film flow rate increases downstream of the fuel spacer 7 due to the effect that the droplets in the vapor adhere to the fuel rods by the fuel spacer 7, but the further downstream, the evaporation and the generation of the droplets cause the liquid film flow rate to increase. The liquid film becomes the thinnest immediately upstream of the fuel spacer 7. When the output exceeds a certain output, the liquid film on the surface of the fuel rod 3 becomes dry (dry-out), and the heat of the dry portion is reduced. There is a theory that the transmission rate decreases.

The index currently used as an index relating to the thermal margin of the core includes a minimum critical power ratio (MCPR) as shown in the following equation.
There is. MCPR = Q _CP / Q _BUNDLE (1) where, Q _{BUNDLE is a} fuel assembly operation output, and Q _{CP is a} thermal limit output.

The minimum power limit is determined by the core combustion, as shown in FIG.
The trajectory is as shown in the figure. Because of the need to increase the concentration of nuclear separated nuclides in the fuel rods due to long-term cycle operation,
As shown in (1), the thermal margin of the core tends to decrease after refueling.

Therefore, in order to reduce the operating cost of a nuclear power plant, a fuel assembly design having a high thermal limit output is required, and a hardware design (fuel spacer,
This is one of the objectives of fuel rods and software design (fuel enrichment distribution, combustion management, etc.).

In order to design a fuel assembly having a high thermal limit power, a method of increasing the limit power has been conventionally proposed from the following viewpoints. (A) Supply of liquid phase to fuel rod surface by mixing coolant (b) Reduction of heat flux per unit fuel rod surface

Based on these findings, in the design of the fuel assembly 1 corresponding to (a), the cooling efficiency of the fuel rods 3 is increased by increasing the proportion of the coolant used for cooling the fuel rods 3. Means have been proposed for directing the liquid phase flowing over the non-contributing channel box surface to the fuel rods.

One of them is a flow tripper (for example, Japanese Patent Application Laid-Open No. 2-44289) that separates a liquid film by providing a groove in a channel box and making a step, and a liquid phase flowing in the channel box surface is applied in a horizontal direction. The effect of increasing the speed is to increase the proportion of coolant used for cooling the fuel rods. However, the step in the channel box increases the pressure loss of the fuel assembly and decreases the two-phase flow velocity near the channel box.
The amount of droplet generation due to the step is small, and the effect of improving the limit output is small.

The mixing effect of the coolant of the fuel assembly (mixing effect) has the following two points. (1) Lateral transport by average flow (forced lateral flow to coolant) (2) Lateral transport by turbulence mechanism

Although the above-mentioned flow tripper is a method corresponding to (1), it does not aim for a positive effect on the operation of (2). Further, in the fuel assembly design corresponding to (b), it is proposed to reduce the diameter of the fuel rod to increase the surface area of the fuel rod and reduce the heat flux per unit surface area. A design to increase the number of fuel rods on a × 9 grid has been proposed.

On the other hand, a component of the fuel assembly that affects the thermal limit output of the fuel assembly includes the fuel spacer 7. A plurality of fuel spacers 7 are provided in the height direction of the fuel assembly 1 to maintain a gap between the fuel rods 3 and the water rods 4 and a gap between the channel box 2 and the fuel rods 3 and the water rods 4. The shape of the aggregate 1 is maintained.

The following points (1) to (10) are generally considered in designing a fuel spacer. (1) Seismic resistance of fuel assembly (2) Maintaining fuel rod spacing (3) Suppressing fuel rod vibration (4) Relaxing fuel rod expansion (5) Ease of fuel assembly assembly (6) Fuel rod and (7) Maximization of thermal limit power (8) Minimization of fuel assembly pressure drop (9) Minimization of parasitic neutron absorption (10) Minimization of number of parts

The effect of the fuel spacer 7 on the dryout is mainly considered as an effect (mixing effect) of supplying liquid droplets to the fuel rod surface liquid film by mixing the coolant.

From this knowledge, as a design element of the fuel spacer 7, the thickness of the fuel spacer is increased, or as shown in FIG. 12, a number of inwardly provided projections (flow tabs 12) are provided above the fuel spacer 7. There are things.

The flow tab 12 above the fuel spacer 7
Is originally provided as an introduction portion when the fuel rod 3 is inserted into the channel box 2. However, the mixing effect of the fuel spacer 7 is promoted, and the droplets in the two-phase flow are effectively reduced. It is known that it is directed to the surface and contributes to an increase in the thermal limit power.

That is, the flow tab 12 is connected to the fuel spacer 7.
The coolant flowing from the upstream side (lower part) of the fuel spacer 7 is directed to the fuel side, and thereby the heat of the fuel rods 3 located at the outermost periphery is provided. It also has a role to improve the margin.

[0031]

In the conventional ferrule type fuel spacer 7, the upper end of the flow tab 12 is bent inside the fuel flow path as shown in FIG. The coolant flowing in the gap between the fuel rod 3 and the support band 11 is deflected to the fuel rod 3 side as shown by the arrow in FIG. 16, and the coolant in the gap between the fuel rods 3 and 3 is directed to the fuel rod 3 side. More and more effective drift improves the thermal margin of the fuel.

However, in order to further secure a thermal margin, in consideration of the aforementioned (b) reduction of the heat flux per unit surface of the fuel rod, in the ferrule type spacer shown in FIG. As the number of parts increases, the number of parts increases, which goes against the minimization of the number of parts in item (10) of the ten points considered in the fuel spacer design.

In order to minimize the number of parts, a grid-type spacer in which strip-shaped flat plates are combined in a grid-like shape is more advantageous than a ferrule type. However, in the case of the grid type spacer, if the flow tab 12 is provided on the support band 11 as in the conventional case, the region 16 where the two-phase flow is slow as shown by a broken line in FIG. Located downstream.

Therefore, the influence of the flow tub 12 on the two-phase flow is small, and the effect of directing the droplets in the two-phase flow to the surface of the fuel rod 3 is also small. There is a problem that becomes smaller.
FIG. 3 (a) shows a region 15 where the two-phase flow is fast in the grid fuel spacer where no flow tab is provided.

The present invention has been made in order to solve the above-mentioned problem, and the droplets in the gas-liquid two-phase flow are directed to the fuel rod to enhance the heat removal effect, thereby improving the heat removal performance. An object of the present invention is to improve the safety margin of a power plant and to provide a fuel assembly suitable for long-term cycle operation.

[0036]

The invention according to claim 1 is
A plurality of fuel rods and water rods are arranged and the upper end is supported by an upper tie plate, the lower end is supported by a lower tie plate, and a flow of coolant is provided to hold the fuel rods and the water rods at intervals. In a fuel assembly provided with a plurality of fuel spacers in a direction, the fuel spacers are constituted by fuel spacer constituent members and a support band surrounding them, and the support band has a flow of the coolant upstream and downstream thereof. A plurality of flow tabs extending in the direction are provided in parallel, and the front end of the flow tab is cut and divided, and each of the divided flow tabs is provided with a bent portion bent in the fuel rod direction. It is characterized by the following.

According to the first aspect of the present invention, the center end of the flow tab provided integrally with the support band is cut and divided, and each of the divided flow tabs is bent in the fuel rod direction. The flow tab can be effectively extended in the flow path surrounded by the lattice plate.

The flow tab projecting into the flow path has a large effect on the gas-liquid two-phase flow, and the effect of directing the droplets in the two-phase flow to the fuel rod surface is increased, so that the thermal margin improving effect is also increased. Can be.

According to a second aspect of the present invention, a plurality of fuel rods and water rods are arranged, the upper end is supported by an upper tie plate, the lower end is supported by a lower tie plate, and the fuel rods and the water rods are spaced from each other. In a fuel assembly provided with a plurality of fuel spacers in the flow direction of the coolant for holding the fuel cell, the fuel spacer has a lattice structure in which a plurality of flat plates are combined in a lattice in the support band. A divided flow tab is formed by extending in the downstream direction of the band of the joint portion between the flat plate and the support band and cutting the leading end thereof, and is located at a position closest to each of the divided flow tabs. A bent portion is provided in a central axis direction of the fuel rod.

According to the second aspect of the present invention, the tip of the flow tab is bent to a position closest to the fuel rod,
The effect of promoting adhesion of droplets to the fuel rods by the flow tab is increased, and the heat removal effect is further improved.

According to a third aspect of the present invention, a plurality of fuel rods and water rods are arranged, the upper end is supported by an upper tie plate, the lower end is supported by a lower tie plate, and the fuel rods and the water rods are spaced from each other. In a fuel assembly provided with a plurality of fuel spacers in the flow direction of the coolant for holding the fuel spacers, the fuel spacers have a lattice structure in which a plurality of flat plates are combined in a lattice shape,
Extending in the downstream direction of the band at the joining portion of the flat plate and the band, a flow tab having a divided distal end portion is formed, and each of the divided flow tabs has a bent portion in the fuel rod direction. It is characterized in that a twist is formed in the direction of the central axis of the fuel rod at the closest position.

According to the third aspect of the present invention, a large amount of liquid droplets can be attached to the fuel rod by forming a twist at the tip of the flow tub, the liquid film flow rate on the fuel rod surface can be increased, and the fuel rod surface can be increased. Becomes difficult to dry, and the heat removal performance further increases.

The invention according to claim 4 is characterized in that the fuel spacer is provided with swirling vanes downstream of a position where the plurality of support bands arranged vertically and horizontally intersect at a substantially right angle. According to the fourth aspect of the present invention, the pressure loss coefficient can be increased by providing the swirling blade downstream of the position where the support bands intersect, and the coolant can be directed to the outer peripheral flow path having the flow tab. This allows
The thermal margin of both the outer fuel rod and the inner fuel rod can be improved, and the thermal margin of the fuel assembly can be improved.

The invention according to claim 5 is characterized in that the fuel spacer is provided with a torsion plate on the downstream side of the position where the support bands intersect. According to the fifth aspect of the invention, the same operation and effect as the fourth aspect of the invention can be obtained by providing the torsion plate in place of the turning blade.

According to a sixth aspect of the present invention, a plurality of fuel rods and water rods are arranged, the upper end is supported by an upper tie plate, the lower end is supported by a lower tie plate, and the fuel rods and the water rods are spaced from each other. In a fuel assembly provided with a plurality of fuel spacers in the flow direction for holding the fuel spacers, the fuel spacers are constituted by fuel spacer constituent members and a support band surrounding them, and A flow tab protruding in the downstream direction is provided, and the tip of the flow tab has a bent portion bent in the fuel rod direction, and closes the flow path outside the support band, avoiding the upstream side of the flow tab. A mechanism for performing the operation.

According to the sixth aspect of the present invention, the number of protrusions called bathtubs provided on the support band is increased, and the liquid film is collected upstream of the flow tub. As a result, when the liquid film thickness and the liquid film flow rate increase, the turbulence of the liquid film increases, and together with the effect of the flow tub, the generation of liquid droplets is promoted, the adhesion of the liquid droplets to the fuel rod surface is promoted, The thermal margin of the fuel rod can be improved.

According to a seventh aspect of the present invention, there is provided a fuel rod comprising a plurality of fuel rods provided in an axial position from 2/7 to 6/7 of the effective heat generation length from the upstream to the effective heat generation portion of the fuel rod. Wherein at least one fuel spacer selected from the fuel spacers according to the first to sixth aspects is provided. According to the seventh aspect of the present invention, the fuel spacer according to the first to sixth aspects is replaced with the heat generating portion.
The heat removal performance of the fuel assembly can be improved at positions / 7 to 6/7.

The invention according to claim 8 is that the fuel rod has an effective heating length of 2/7 from the upstream to the downstream of the effective heating portion.
The fuel spacers selected from the fuel spacers according to any one of claims 1 to 6 are provided to be shorter than the fuel spacer intervals of the other heat generating portions in the range of the axial position from (6) to (6/7). And According to the invention of claim 8,
The heat removal performance of the fuel assembly can be improved by narrowing the interval of the number of the fuel spacers according to claims 1 to 6.

[0049]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of a fuel assembly according to the present invention will be described with reference to FIGS. 1 (a) and 1 (b) to FIG. FIG. 1A is a cross-sectional view showing a state in which a fuel rod and a water rod of a fuel assembly according to the present embodiment are held by a fuel spacer 17, and FIG. FIG. FIG. 2 is an enlarged view of a main part showing the support band 18 of the fuel spacer 17 in FIGS. 1A and 1B, and FIGS. 3A to 3C are flow areas of the coolant of the fuel spacer of the present embodiment. FIG. 4 is a top view showing a comparison with the conventional example, and FIG. 4 is a curve diagram showing a comparison of the droplet attachment effect between the present embodiment and the fuel spacer of the conventional example.

The present embodiment is different from the conventional example in that the fuel spacer is improved. Therefore, the structure of the fuel assembly is almost the same as that of the fuel assembly shown in FIG. Therefore, the description of the configuration of the fuel assembly is omitted, and the configuration of the fuel spacer according to the present embodiment and the operation and effect thereof will be described. The number of fuel rods 3 and water rods 4
The arrangement, shape, and the like are not limited to those in FIG.

In FIGS. 1 to 3, the same parts as those in FIGS. 11 and 12 are denoted by the same reference numerals, and the description of the overlapping parts will be omitted. In FIGS. 1A and 1B, a plurality of fuel spacers 17 according to the present embodiment are provided integrally with the support band 18 on the downstream side of the support band 18 formed of a square frame having a slightly curved surface at a corner. Flow tabs 19 are formed in parallel. 2, a notch 20 is formed at the center of the flow tab 19 formed on the support band 18, and the flow tab 19 is divided into two parts.
A bent portion 21 is formed by bending the tip of the fuel rod 19 in the direction of the fuel rod 3. On the outside of the support band 18, a protrusion 22 called a bathtub is formed.

In the case of a grid type fuel spacer in which the grid plate 13 is combined in a grid shape in the support band 18, the flow tab 12 attached to the conventional support band 11 has the shape of the grid plate 13 as shown in FIG. As described above, the effect of the flow tub 12 on the two-phase flow is small as described above, and the effect of directing the droplets in the two-phase flow to the surface of the fuel rod 3 is also small. small.

On the other hand, as in the present embodiment, a cut 20 is made at the center tip of the flow tab 19 to divide it into two.
By folding the flow tab 19 toward the fuel rod 3 closest to the divided flow tab 19, the divided flow tab 19 can be effectively extended to the flow path surrounded by the fuel rod 3, the support band 18, and the lattice plate 13. .

That is, FIG.
As shown in (a), the portion surrounded by the support band 11, the grid plate 13 and the fuel rods 3 becomes a region 15 where the flow is fast, and the flow velocity is accelerated by the reduction of the flow path area by the spacer. FIG. 3B shows a conventional example, in which the flow tub 12 has a small effect on the two-phase flow since the flow tub 12 is located downstream of the grid plate 13 and is located in the region 16 where the flow is slow.

On the other hand, in the present embodiment, FIG.
As shown in (c), the flow tab 19 that protrudes from the support band 18 to the flow path on the fuel rod 3 side has a large effect on the two-phase flow, and the droplets in the two-phase flow are directed to the surface of the fuel rod 3. Since the effect is increased, the thermal margin improving effect can also be increased.

FIG. 4 shows the flow tab 19 of this embodiment and FIG.
7 is a curve diagram comparing the effect of the flow tab 12 according to the conventional example shown in FIG. 5. In the present embodiment (the present invention), the effect (curve a) of promoting adhesion of the droplet to the fuel rod 3 by the flow tab 19 is the effect of the conventional flow tab ( It increases compared to curve b). If a large amount of liquid droplets can be attached to the fuel rod 3, the flow rate of the liquid film on the surface of the fuel rod 3 can be increased.

Next, a second embodiment of the fuel assembly according to the present invention will be described with reference to FIG. FIG. 5 shows only an essential part of the present embodiment in an enlarged manner. In FIG. 5, the same parts as those in FIG. 2 are denoted by the same reference numerals, and the description of the overlapping parts will be omitted.

This embodiment is a fuel spacer in which a support band 18 having a bent portion 21 formed by directly bending a flow tab 19 from an upper end surface of a support band 18 and the flow tab 19 are integrated. That is, a notch 20 is provided in a flow tab 19 formed on the upper surface of the support band 18, and the flow tab 19 is divided into two.
Is directly bent along the upper end surface of the support band 18 to form a bent portion 21 to form an integrated shape.

According to the present embodiment, the divided flow tab 19 receives a strong force due to the two-phase flow, but by integrating the divided flow tab 19 and the support band 18,
The strength of the divided flow tab 19 can be increased.

Next, a third embodiment of the fuel assembly according to the present invention will be described with reference to FIGS. 6 (a) and 6 (b). FIG.
FIG. 6A is a perspective view showing a main part of the support band 18 and the flow tab 19 of the fuel spacer in the present embodiment, and FIG.
6A shows a front view of FIG. 6A, and the same parts as those in FIG.

In this embodiment, a notch 20 is provided in the flow tab 19 formed at the upper end of the support band 18 to divide the flow tab 19 into two, and a bent portion 21 is formed at the tip of the divided flow tab 19. This is because the twisted portion 23 is formed from 21 to the tip.

According to the present embodiment, the bent portion 21 and the twisted portion 23 are continuously formed on the flow tab 19 divided into two parts, whereby the same effects as those of the first embodiment can be obtained, and the fuel rods can be obtained. More droplets can be attached to the substrate, and the heat removal performance can be further increased.

Next, a fourth embodiment of the fuel assembly according to the present invention will be described with reference to FIG. This embodiment is different from the first to third embodiments in that notches 20 are formed as shown in FIG. 7 in order to improve the thermal margin of the inner fuel rods.
The swirl vanes 24 are provided on the downstream side of the crossing positions of the lattice plates 13 arranged side by side in the support band 18 having the divided flow tabs 19. That is, the turning blade 24 having the twist portion 25 is protruded at the upper end near the intersection of the lattice plates 13.

The swirl vanes 24 have the purpose of improving the thermal margin in the center of the fuel assembly and the purpose of adjusting the pressure loss distribution in the sectional direction of the fuel spacer 17. In the case of the grid type fuel spacer, the pressure loss coefficient of the fuel spacer in the fuel assembly central flow path is small, but the pressure loss coefficient of the outer flow path is large due to the influence of the flow tab 19.

Therefore, by providing a swirling blade 24 at the upper end near the intersection of the lattice plates 13 as in the present embodiment, the pressure loss coefficient is increased, and the flow tab 19 divided into two parts is provided.
The coolant can be further diverted to the outer peripheral flow path where there is. As a result, the thermal margin of both the outer fuel rod and the inner fuel rod can be improved, and the thermal margin of the fuel assembly as a whole can be improved.

Next, a fifth embodiment of the fuel assembly according to the present invention will be described with reference to FIG. The present embodiment is different from the fourth embodiment in that a torsion plate 26 is provided on the downstream side of the position where the lattice plate 13 intersects with the vertical bar, instead of the swirling blade 24 in the fourth embodiment. Since the other parts are the same as those of the fourth embodiment, in FIG. 8, the same parts as those of FIG. 7 are denoted by the same reference numerals, and the description of the overlapping parts will be omitted. The operation and effect of this embodiment are almost the same as those of the swirling blade 24, and therefore, the description thereof is omitted.

Next, referring to FIGS. 9 and 10A, a sixth embodiment of the fuel assembly according to the present invention will be described with reference to FIGS.
The seventh embodiment will be described with reference to FIG. FIG. 9 schematically shows the relationship between the fuel rod 3 and the fuel spacer 7 for explaining the thermal action of the conventional fuel assembly for a boiling water reactor. As shown in FIG. 9, in the conventional fuel assembly, seven fuel spacers 7 are provided in the heat generating portion of the fuel rod 3 at substantially equal intervals along the axial direction.

When numbers are assigned in order from the upstream fuel spacer 7, the thermal stiffness is usually lower than the lowermost seventh fuel spacer 7.
Immediately upstream of the sixth spacer and immediately upstream of the sixth spacer. When the margin is improved thermally, the temperature becomes more severe immediately upstream of the fifth and fourth spacers.

The fuel spacer 7 has the effect of directing the droplets in the two-phase flow to the surface of the fuel rod 3 and preventing the surface of the fuel rod 3 from drying and heat removal from deteriorating. This has the effect of improving the thermal margin immediately upstream of the fourth spacer.

Therefore, when the heat-generating portion is divided into seven equal parts, the sixth embodiment according to the present invention, as shown in FIG. 7
At least one fuel spacer 17 selected from the fuel spacers described in the first to fifth embodiments is provided in a range of the axial position from to 6/7.

In the seventh embodiment according to the present invention, as shown in FIG. 10B, the first to fifth embodiments fall within the range of the axial position from 2/7 to 6/7. The fuel spacers selected from the fuel spacers described in the above are provided, and the number of the fuel spacers 17 is increased to narrow the interval between the fuel spacers 17.

According to the sixth and seventh embodiments, the heat removal performance of the fuel assembly can be improved, the safety margin of the nuclear power plant can be improved, and the fuel suitable for long-term cycle operation can be improved. Aggregates can be provided.

[0073]

According to the present invention, the droplets in the gas-liquid two-phase flow are directed to the fuel rod surface by the flow tab projecting to the fuel rod side, the liquid film flow rate is increased, and the heat removal effect can be increased.
Therefore, the safety margin of the nuclear power plant is improved by improving the heat removal performance, and a fuel assembly suitable for long-term cycle operation can be provided.

[Brief description of the drawings]

FIG. 1A is a cross-sectional view showing a fuel spacer portion holding a fuel rod and a water rod in a first embodiment of the fuel assembly according to the present invention, and FIG. 1B is a fuel spacer in FIG. FIG.

FIG. 2 is an enlarged perspective view showing a main part of a support band in the fuel spacer in FIG. 1;

3A is a top view showing a flow region in a grid type fuel spacer, FIG. 3B is a top view showing a flow region in a conventional fuel spacer, and FIG. 3C is a fuel in an embodiment of the present invention. FIG. 4 is a top view showing a flow region in the spacer.

FIG. 4 is a curve diagram for explaining the effect of droplet adhesion of a fuel spacer according to the present invention and a conventional example.

FIG. 5 is an enlarged perspective view showing a main part of a second embodiment according to the present invention.

FIG. 6A is an enlarged perspective view showing a main part of a third embodiment according to the present invention, and FIG. 6B is a front view of FIG.

FIG. 7 is an enlarged perspective view showing a main part of a fourth embodiment according to the present invention.

FIG. 8 is an enlarged perspective view showing a main part of a fifth embodiment according to the present invention.

FIG. 9 is a schematic elevation view for explaining the thermal action of the fuel assembly for a boiling water reactor.

FIG. 10A is a schematic elevation view for explaining a sixth embodiment according to the present invention, and FIG. 10B is a seventh elevational view according to the present invention.
FIG. 2 is a schematic elevation view for explaining the embodiment.

FIG. 11 is an elevational view showing a part of a conventional fuel assembly in a longitudinal section.

FIG. 12 is a side view showing a state where a fuel rod is inserted into the fuel spacer in FIG. 11;

FIG. 13 is a curve diagram for explaining the thermal limit of the fuel assembly in FIG. 11;

FIG. 14 is a comparison diagram for explaining an output ratio of fuel.

FIG. 15 is a perspective view showing a main part of a conventional fuel spacer.

FIG. 16 is a side view schematically showing a partial cross section for explaining the operation of a flow tab of a conventional fuel spacer.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Fuel assembly, 2 ... Channel box, 3 ... Fuel rod, 4 ... Water rod, 5a ... Cooling water inlet, 5b ...
Cooling water jet, 6 ... short fuel rod, 7 ... fuel spacer (conventional example), 8 ... upper tie plate, 9 ... lower tie plate, 10 ... external spring, 11 ... support band, 11a ... protrusion (bathtub), 12: Flow tab, 13: Grid plate, 14: Ferrule, 15: Fast flow area, 16: Slow flow area,
17 ... fuel spacer (invention), 18 ... support band, 19 ... flow tab, 20 ... cut, 21 ... bent part, 22 ... projecting part, 23 ... twisted part, 24 ... swirl blade, 25 ... twist part, 26 ... twist Board.

Claims (8)

[Claims] 1. An arrangement of a plurality of fuel rods and water rods, wherein an upper end is supported by an upper tie plate, and a lower end is supported by a lower tie plate, and the fuel rods and the water rods are held at intervals. In a fuel assembly in which a plurality of fuel spacers are provided in the flow direction of the coolant, the fuel spacer is constituted by fuel spacer constituent members and a support band surrounding them, and the support band is provided on the upper downstream side thereof. A plurality of flow tabs extending in the flow direction of the coolant are provided in parallel, and the front end of the flow tab is cut and divided, and each of the divided flow tabs is bent in the fuel rod direction. A fuel assembly comprising: a fuel cell; 2. A method for arranging a plurality of fuel rods and water rods, supporting an upper end portion with an upper tie plate, supporting a lower end portion with a lower tie plate, and holding the fuel rods and the water rods at intervals. In a fuel assembly in which a plurality of fuel spacers are provided in the flow direction of the coolant, the fuel spacer has a grid-type structure in which a plurality of flat plates are combined in a grid in the support band, and A split flow tab is formed by extending the joint portion of the support band in the downstream direction of the band and cutting a tip portion thereof, and the central axis of the fuel rod located closest to each of the split flow tabs is formed. A fuel assembly, wherein a bent portion is provided in a direction. 3. Arrangement of a plurality of fuel rods and water rods, supporting an upper end portion with an upper tie plate, supporting a lower end portion with a lower tie plate, and holding the fuel rods and the water rods at intervals. In a fuel assembly provided with a plurality of fuel spacers in the flow direction of the coolant, the fuel spacer has a lattice structure in which a plurality of flat plates are combined in a grid pattern, and A flow tab extending in the downstream direction of the band and having a divided distal end portion is formed, each of the divided flow tabs has a bent portion in the fuel rod direction, and the fuel rod located closest to the bent portion is provided. A fuel assembly characterized by forming a twist in the direction of the central axis of the fuel assembly. 4. The fuel spacer according to claim 1, wherein the fuel spacer is provided with swirling vanes downstream of a position where the plurality of support bands arranged vertically and horizontally intersect at a substantially right angle. Fuel assembly. 5. The fuel assembly according to claim 1, wherein the fuel spacer is provided with a torsion plate on a downstream side of a position where the support bands intersect. 6. An arrangement of a plurality of fuel rods and water rods, wherein the upper end is supported by an upper tie plate, the lower end is supported by a lower tie plate, and the fuel rods and the water rods are held at intervals. In the fuel assembly provided with a plurality of fuel spacers in the flow direction, the fuel spacer is constituted by a fuel spacer constituent member and a support band surrounding them, and protrudes from the support band in a downstream direction of the coolant. A flow tab is provided,
A tip portion of the flow tab has a bent portion bent in the fuel rod direction, and a mechanism is provided outside the support band to close a flow path avoiding an upstream side of the flow tab. Fuel assembly. 7. A plurality of fuel spacers provided in an axial position from 2/7 to 6/7 of an effective heat generation length from an upstream to an effective heat generation portion of the fuel rod.
A fuel assembly comprising at least one fuel spacer selected from the fuel spacers according to claim 1. 8. The fuel spacer according to claim 1, wherein the fuel rod has an axial position from 2/7 to 6/7 with respect to the effective heat generation length from the upstream to the effective heat generation portion of the fuel rod. 7. The fuel assembly according to claim 1, wherein a fuel spacer interval selected from the following is provided shorter than a fuel spacer interval of another heat generating portion.