JP3141452B2 - Fuel assembly - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel assembly, and more particularly to a fuel assembly suitable for use in a boiling water reactor.

[0002]

2. Description of the Related Art Conventional fuel assemblies loaded in the core of a boiling water reactor include upper and lower tie plates, fuel rods,
It has a water rod and a fuel spacer. The upper and lower ends of the fuel rod and the water rod are held by the upper and lower tie plates, respectively. The fuel rod is obtained by filling fuel pellets in a cladding tube whose upper and lower ends are sealed. A gas plenum is formed in the cladding tube. The fuel spacer keeps a predetermined distance between the fuel rods.

[0003] Normally, a gas plenum in a fuel rod is formed at the upper end of the fuel rod. However, there is known a fuel rod in which a gas plenum is formed at the lower end of the fuel rod in order to reduce the internal pressure of the fuel rod. Among them, a fuel rod used for a light water reactor is described in JP-A-61-114184. In this fuel rod, the length of the gas plenum (lower plenum) at the lower end is longer than that of the gas plenum (upper plenum) at the upper end. In this way, by making the length of the gas plenum at the lower end longer than that of the upper end, the rise in the pressure in the fuel rod is suppressed. JP 2-9
No. 1597 also shows a fuel rod having a gas plenum at the upper end and the lower end. In this fuel rod, the outer diameter of the upper half is smaller than that of the lower half. The decrease in fuel mass due to the smaller outer diameter of the upper half can be compensated for by a larger outer diameter in the lower half.
The fuel rod disclosed in Japanese Unexamined Patent Publication No. 2-91597 achieves low pressure loss for two-phase flow.

[0004]

The fuel rod disclosed in Japanese Patent Application Laid-Open No. Hei 2-91597 has an increased outer diameter at the fuel pellet filling portion in the lower half in the axial direction. This is not always desirable in terms of increasing the seismic strength of the entire fuel assembly. This is for the following reason. The amplitude due to the flow vibration of the fuel rod is inversely proportional to the natural frequency of the fuel rod. This natural frequency is inversely proportional to the mass per unit length of the fuel rod and proportional to the second moment of area of the fuel rod. Therefore, when the outer diameter of the fuel rod at the fuel pellet filling portion is increased, the second moment of area increases, but the mass per unit length of the fuel rod further increases due to the increase in the diameter of the fuel pellet. It was found that the natural frequency of the fuel rod decreased and the amplitude due to the flow oscillation of the fuel rod increased.

In recent years, studies have been made to increase the burnup of fuel assemblies by increasing the enrichment. In order to suppress an increase in the linear power density of the fuel rods due to an increase in the enrichment, an increase in the number of fuel rods per fuel assembly is considered. This results in increased fuel assembly pressure drop.

SUMMARY OF THE INVENTION It is an object of the present invention to suppress a decrease in neutron utilization, to suppress flow oscillation of a fuel rod, and to reduce a pressure drop.
An object of the present invention is to provide a fuel assembly that can reduce loss .

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to reduce the bending stiffness per unit length of a cladding tube at a lower plenum portion of a fuel rod by a cladding at a portion corresponding to the entire axial length of a fuel pellet filling region. with larger than that of the tube, the bottom
The projected area for the cross section of the fuel spacer located at
By making it smaller than that of other fuel spacers
Can be achieved. Also, six fuel spacers are arranged in the axial direction.
Can also be achieved.

[0008]

According to the present invention, since the bending stiffness per unit length of the cladding in the lower plenum is greater than that of the cladding in the fuel pellet filling region, the flow vibration of the fuel rods is reduced. Can be suppressed. Further, since the increase in bending stiffness is limited to the lower plenum, the neutron absorption amount of the cladding tube in the fuel pellet filling region does not increase, and a decrease in neutron utilization can be suppressed. Furthermore, the fuel spacer located at the bottom
The projected area for the cross section is that of other fuel spacers.
Smaller and six fuel spacers are arranged in the axial direction
This contributes to a reduction in pressure loss.

[0009]

BRIEF DESCRIPTION fuel assemblies is a preferred embodiment of the present invention applied to a boiling water reactor below with reference to FIGS.

The fuel assembly has a fuel rod 1, a lower tie plate 2, an upper tie plate 3, and a fuel spacer 4. The upper end of the fuel rod 1 is held by the upper tie plate 3, and the lower end is held by the lower tie plate 2. The fuel spacer 4 keeps the distance between the fuel rods 1 at a predetermined width. The fuel rod 1 is formed by connecting the upper end and the lower end of the cladding tube 9 to the upper end plug 2.
2 and the lower end plug 23, and the cladding tube 9 is filled with fuel pellets 5. The lower plenum 6
The fuel pellet 5 is formed in the cladding tube 9 below a region filled with the fuel pellet 5 (hereinafter, referred to as a fuel effective length). A spring 25 supporting the fuel pellets 5 is arranged in the lower plenum 6. FIG. 3 is an enlarged view of the inside of the lower plenum 6. The spring 25 supports the pellet support plate 10 disposed below the lowermost fuel pellet 5. The pellet support plate 10 is provided with a through-hole so that the fission product gas reaches the lower plenum 6. The support structure for the fuel pellet 5 may be as shown in FIG. In this structure, a cylindrical member 12 having a hole in a side wall is provided instead of the spring 25. This cylindrical member 12 supports the pellet support plate 10. Upper plenum 8
Are formed in the cladding tube 9 above the active fuel length. The axial length of the lower plenum 6 is longer than that of the upper plenum 8 and accounts for most of the total axial length of all plenums. Spring 26 pressing fuel pellet 5
Are provided in the upper plenum 8. The cladding tube 9 has a portion corresponding to the axial length of the lower plenum 6 (hereinafter referred to as a lower plenum portion LG) and a portion corresponding to the axial length of the active fuel length portion (hereinafter referred to as a pellet filling portion). PC). The thickness of the lower plenum portion LG is equal to that of the pellet filling region portion PC. For this reason, in the cladding tube 9, the inner diameter of the lower plenum portion LG is larger than that of the pellet filling portion PC. In the conventional fuel assembly, the expansion spring 7 is attached to the upper end plug of the fuel rod, but in this embodiment, it is attached to the lower end plug 23.

When the fuel assembly of this embodiment is loaded into the core of a boiling water reactor and the operation of the reactor is started, cooling water is supplied from the lower tie plate 2 into the fuel assembly. This cooling water is heated by heat generated by nuclear fission of the nuclear fuel when ascending between the fuel rods 1 and turns into steam. A two-phase flow of cooling water and steam flows in the upper part of the fuel assembly. This two-phase flow exits the fuel assembly through the upper tie plate 3. The upstream (lower) side of the boiling point of the fuel assembly is a single-phase flow of the cooling water.

When increasing the diameter of the cladding tube, if the fuel pellets 5 are filled in the region where the cladding tube diameter changes, the fuel pellets will expand in volume and come into contact with the cladding tube to cause mechanical interaction. Large stress is generated in the cladding tube. In the present embodiment, such a problem does not occur because the diameter of the lower plenum portion not filled with the fuel pellet is increased. In addition, increasing the diameter of the cladding tube not only in the lower plenum portion but also in the upper portion of the fuel rod leads to an increase in pressure loss of the fuel assembly.

In this embodiment, the following functions and effects are produced by providing the lower plenum 6 and increasing the outer diameter of the lower plenum portion LG of the cladding tube 9.

First, the operation and effect of the lower plenum 6 will be described. The friction pressure loss of the cooling water generated on the fuel rod surface is larger in the two-phase flow region than in the single-phase flow region. For the same surface area, the friction pressure loss in the two-phase flow region is about three times that of the single-phase flow region. Therefore, in the present embodiment, the lower plenum 6 not filled with the fuel pellets 5 is located upstream of the boiling point of the cooling water flowing between the fuel rods 1, that is, in the single-phase flow region. As compared with a conventional fuel assembly including a fuel rod having the following, the boiling start point moves to the downstream side, and the axial length of the two-phase flow region is reduced. This leads to a reduction in friction pressure loss in the two-phase flow region, and a reduction in pressure loss in the fuel assembly. In particular, the attachment of the expansion spring 7 to the lower end line 23 also increases the axial length of the single-phase flow region of the fuel assembly,
This contributes to a reduction in pressure loss.

The above is specifically described with reference to FIGS. FIG. 5 shows the axial pressure drop distribution of a conventional fuel assembly with fuel rods having only the upper plenum. FIG. 6 shows an axial friction pressure loss distribution of a fuel assembly including a fuel rod having a lower plenum 6 as in the present embodiment. In the conventional fuel assembly of FIG. 5, since the plenum is located in the two-phase flow region, the friction pressure loss at the same portion is large, and occupies about 16% of the friction pressure loss of the entire fuel assembly. In the fuel assembly of this embodiment, since the plenum is located in the single-phase flow region, the length of the single-phase flow region increases, and the length of the two-phase region is reduced by about 12% as compared with FIG. The plenum is located in the single-phase flow region, and the friction pressure loss in this region is shown in FIG.
34% of the friction pressure loss in the upper plenum of Therefore, the friction pressure loss of the present embodiment is 16% smaller than that of the conventional example of FIG.

Further, in the fuel assembly of this embodiment, since the boiling start point moves to the downstream side, when the installation position of the fuel spacer in the axial direction is the same as that of the conventional example of FIG. The void ratio at the position is decreased. The pressure loss generated in the fuel spacer is smaller in the single-phase flow region than in the two-phase flow region, and is smaller when the void fraction is smaller in the same two-phase flow region. Therefore, when most of the plenum is arranged in the single-phase flow region as in the present embodiment, the void ratio at each fuel spacer position is lower than in the prior art, and the pressure loss generated in each fuel spacer is also lower. When the expansion spring 7 is attached to the lower end plug 23, their effects are further increased. FIG. 7 shows the characteristics of the pressure loss and the void ratio generated at each fuel spacer in the fuel assembly of the present embodiment and the fuel assembly of FIG. 5 described above. The characteristics of the solid line (this embodiment) and the characteristics of the broken line (conventional example) indicate changes in the void ratio. Due to the above effects, the present embodiment can reduce the pressure loss generated in the fuel spacer by 14% as compared with the related art.

According to this embodiment, even if the number of fuel rods per fuel assembly is increased in order to reduce the linear output density of high burnup fuel, the axial length can be increased by providing the lower plenum for each fuel rod. The pressure loss of the fuel assembly can be reduced to the same level as before without increasing the number of short fuel rods having a short length. As a result, the amount of fuel loaded per fuel assembly is increased by 10% as compared with the conventional case, and fuel economy is improved. In addition, the number of replacement fuel assemblies is reduced, and the amount of radioactive waste is also reduced. As described above, the reduction in the pressure loss of the fuel assembly improves the hydraulic stability of the fuel assembly and also improves the stability of the entire core.

Next, two different effects produced by increasing the outer diameter of the lower plenum portion LG of the cladding tube 9 will be described.

One of the functions and effects is that the decrease in the neutron utilization rate of the fuel assembly can be significantly suppressed. Generally, an increase in cladding diameter results in an increase in the amount of cladding structural material. This leads to an increase in the amount of neutron absorbed by the cladding tube, and the neutron utilization rate decreases. However, in the present embodiment, the diameter of the lower plenum portion LG below the pellet filling portion PC is increased so that the pellet filling portion P
Since the diameter of C is not increased, neutron utilization hardly decreases. That is, in the present embodiment, the lower plenum L
The increase in bending stiffness per unit length at G hardly reduces neutron utilization.

The second effect is to suppress the flow vibration of the fuel rod by increasing the bending rigidity per unit length in the lower plenum portion LG. In the fuel assembly of this embodiment, since the cladding tube 9 has a larger outer diameter at the lower plenum portion LG than that at the pellet filling portion PC, the lower plenum portion LG
Is greater than the bending stiffness per unit length of the pellet filling portion PC. By increasing the bending rigidity at the lower end of the fuel rod 1, that is, at the lower plenum LG, the mechanical strength of the entire fuel rod is increased, and the characteristics against flow vibration are improved. Further, by increasing the bending rigidity in the lower plenum portion LG, the mechanical stress applied to the fuel spacer is also reduced. Therefore, the thickness of the fuel spacer 4 can be reduced. Further, the characteristics against the flow vibration are improved, and the vibration amplitude is reduced, so that the space between the fuel spacers in the axial direction can be increased. In this case, the number of fuel spacers 4 in the axial direction can be reduced.

The bending rigidity D of a hollow cylinder such as a cladding tube is
It is given by Equations 1 and 2.

[0022]

(Equation 1)

[0023]

(Equation 2)

Where E is Young's modulus, I is the second moment of area, d ₁ is the outer diameter of the pipe, and d ₂ is the inner diameter of the pipe (d ₂ = d ₁ −
2t) and t are the tube thickness.

The natural frequency of the fuel rod is the bending rigidity E
XI is obtained by the following equation.

[0026]

(Equation 3)

Here, λ is the vibration system number, l is the span length, g
Is the gravitational acceleration, m ₁ is the mass per unit length of the fuel rod,
And m ₂ are the added weights.

Further, the maximum displacement δ of the flow vibration of the fuel rods
According to the Quinn's empirical formula, the (amplitude) is given by the following formula using the natural frequency f.

[0029]

(Equation 4)

When the bending stiffness D obtained by Equation 1 increases,
The natural frequency f represented by Equation 3 increases. For this reason, the maximum displacement δ (amplitude) of the flow vibration of the cladding tube determined by Equation 4 is reduced, and the flow vibration of the cladding tube is suppressed. Furthermore,
Since the bending stiffness per fuel rod increases, the rigidity and vibration intensity of the entire fuel assembly increase.

The vibration amplitude of only the lower part of the fuel rod, that is, the vibration amplitude of the lower plenum portion LG of this embodiment in which the bending stiffness D (= EI) is increased, is compared with the conventional one using the above equations. I do. When the bending stiffness D increases by 10%,
The natural frequency f increases by 4.9%. At this time, according to Equation 4, the vibration amplitude is reduced by about 8%. In this embodiment, since the fuel pellet 5 is not provided in the portion where the diameter of the cladding tube 9 is increased, the natural frequency does not decrease due to the increase in the diameter of the fuel pellet.

As a method for increasing the bending stiffness of the fuel rod, specifically, the lower plenum LG of the cladding tube, from the equations ( ₁₎ and (2), the increase of the outer diameter of the lower plenum LG of the cladding tube (d1 besides increasing), (1) increasing the thickness of the lower plenum section LG of the cladding (increased t, reduction of d _2), (2) changing the material of the lower plenum section LG of the cladding (increased E) such Can be considered.

A fuel assembly according to another embodiment of the present invention will be described with reference to FIG. This embodiment is an example in which the thickness of the lower plenum LG of the cladding tube is increased. The fuel assembly of this embodiment has a plurality of fuel rods 1A. In the fuel rod 1A, the wall thickness of the lower plenum portion LG is the pellet filling portion P.
C has a cladding 9A that is thicker than that of C. In this example, the lower plenum LG and the pellet filling PC are:
The outer diameter is the same, but the inner diameter is different. The inner diameter of the lower plenum LG is smaller than that of the pellet filling region PC.

This embodiment can provide the same effects as the embodiment of FIG. Since the thickness of the cladding tube is increased inward only in the lower plenum portion LG, the bending stiffness is reduced without reducing the fuel filling amount and without decreasing the cross-sectional area of the cooling water passage outside the fuel rod. Can be increased. A decrease in fuel charge leads to poor fuel economy. Further, in the present embodiment, similarly to the embodiment of FIG. 1, even if the bending stiffness per unit length in the lower plenum portion LG is increased, the decrease in the neutron utilization rate of the fuel assembly due to this can be significantly suppressed. is there. In general, an increase in the cladding wall thickness also results in an increase in the amount of cladding structural material, leading to a decrease in neutron utilization. However, in this embodiment, the thickness of the lower plenum portion LG below the pellet filling portion PC is increased and the thickness of the pellet filling portion PC is not increased, so that the neutron utilization rate hardly decreases.

FIG. 9 shows the fuel pellet supporting structure of this embodiment.
To 11 may be used. In FIG. 9, the lower end of the cladding tube 9 extends to the lower end of the pellet filling portion PC, and a lower plenum cladding tube 13 thicker than the cladding tube 9 is joined to the lower end of the cladding tube 9. The fuel pellets 5 are supported as shown in FIG. In FIG. 10, the fuel pellet 5 is supported by a connecting member 14 joined to the cladding tube 9 or the non-plenum cladding tube 13A. The upper end of the above-mentioned non-plenum cladding tube 13 and the connecting portion position 14 are the pellet supporting plate 1.
It corresponds to 0. FIG. 11 shows the application of the technical idea of increasing the outer diameter of the lower plenum portion of FIG. 1 to the structure of the lower plenum portion of the embodiment of FIG. 8, and the fuel pellet 5 is a thick portion of the cladding tube 9B. Supported.

In the embodiment of FIG. 8, the lower plenum L
It is also possible to make the inner diameters of G and the pellet filling portion PC equal and increase the outer diameter of the lower plenum portion LG to increase the thickness of the lower plenum portion LG.

An embodiment in which the material of the lower plenum portion LG of the cladding tube is replaced with the material of the pellet filling portion PC will be described below. The fuel rod cladding tube used in the fuel assembly of the present embodiment is:
The pellet filling area PC is made of a zirconium alloy (for example,
The lower plenum portion LG is made of a different zirconium alloy (for example, a zirconium alloy containing niobium) having a larger bending rigidity per unit length than that of the zircaloy-2). When the cladding tube is made of a zirconium alloy of a different material as described above, structural members of different materials are joined by welding. Generally, in order to increase the bending stiffness of a zirconium alloy, it is necessary to add an element having a large neutron absorption cross section to it, and the neutron absorption cross section of the obtained zirconium alloy increases. When the pellet filling portion PC of the cladding tube is made of such a zirconium alloy, the neutron utilization rate decreases. In the present embodiment, the zirconium alloy having such an increased neutron absorption cross-section is used not for the pellet filling region PC but for the lower plenum L.
Since it is used for G, the neutron utilization rate hardly decreases even though the bending stiffness per unit length in the lower plenum LG increases. This embodiment can also achieve the same effects as the embodiment of FIG.

FIG. 12 shows a fuel assembly according to another embodiment of the present invention. In the present embodiment, the fuel spacer 4 located at the bottom of the fuel assembly in FIG. 2 is replaced with a fuel spacer 4A. The fuel spacer 4A is structurally the same as the fuel spacer 4, but has a smaller projected area.
This is because the thickness of the structural material of the fuel spacer 4A is smaller than that of the fuel spacer 4. Since the lowermost fuel spacer has a large bending rigidity in the lower plenum portion LG of the cladding tube 9, the force applied thereto is small and the generated mechanical stress is small, so that the fuel spacer can be thinned. This embodiment also has the same effect as the fuel assembly of FIG. Further, in the present embodiment, the pressure loss becomes smaller as the projected area of the fuel spacer 4A is smaller than that of the fuel assembly.

FIG. 13 also shows another embodiment of the fuel assembly of the present invention. This embodiment is different from the fuel assembly of FIG. 12 only in that the fuel spacer 4B is used instead of the fuel spacer 4A. The height of the fuel spacer 4B is
It is higher than that of the fuel spacer 4A. Therefore, the thickness of the structural material of the fuel spacer 4B is smaller than that of the fuel spacer 4A, and the pressure loss of the fuel spacer 4B is smaller than that of the fuel spacer 4. Therefore, this embodiment
The pressure loss is further reduced as compared with the fuel assembly of FIG. This embodiment also achieves the same effect as the fuel assembly of FIG.

A fuel assembly according to another embodiment of the present invention includes:
This will be described with reference to FIG. This embodiment is different from the above-described embodiments in that each embodiment has seven fuel spacers.
It has fewer fuel spacers. Since the bending stiffness of the lower plenum portion LG of the cladding tube 9 is large, the interval between the lower fuel spacers can be increased. That is, the distance between the lowermost fuel spacer 4 and the lower tie plate 2, and the lowermost fuel spacer 4 and the second lowermost fuel spacer 4
Is wider than the distance between the fuel spacers 4 located above the second stage from the bottom. In the present embodiment, FIG.
The same effect as that of the fuel assembly can be obtained. Furthermore, the pressure loss of the fuel assembly is smaller than that of the fuel assembly of FIG. 2 by one less fuel spacer.

FIG. 15 also shows a fuel assembly according to another embodiment of the present invention. A channel box 11 is attached to the upper tie plate 3. This embodiment also has a fuel rod 1. The thickness of the channel box 11 is greater than the other portions from the upper end of the lower plenum portion LG to a position one third of the axial length of the active fuel length portion based on the lower end of the active fuel length portion. . This contributes to reducing the deformation of the channel box in the fuel assembly having the fuel rod 1 in which the bending stiffness of the lower plenum portion LG is increased. The deformation of the channel box is caused by the neutron irradiation during the core loading period and the pressure difference between the inside and outside of the channel box. This deformation is greatest at a position 1/3 of the axial length of the active fuel length from the lower end of the active fuel length. In the present embodiment, similarly to obtaining the effect of the fuel assembly of FIG. 2, it is possible to efficiently suppress the channel box deformation at the time of loading the fuel assembly inside the core without waste.

The core of the present boiling water reactor is configured by loading only a fuel assembly C having a fuel rod having only an upper plenum shown in FIG. 5 as a fuel assembly. One cell is composed of four fuel assemblies arranged around the control rod,
Be composed. The core has a plurality of such cells. Loading of the fuel assembly of FIG. 2 into the core is performed at the time of refueling after the operation of the reactor in one operation cycle is completed. The spent fuel assembly C is removed from the core, and the fuel assembly A (fuel assembly in FIG. 2), which is a new fuel, is loaded in the core instead. FIG. 16 shows a state of the core loaded with the fuel assembly A. Some cells 28A in the core have four fuel assemblies C, and the remaining cells 28B have at least one fuel assembly A among the four fuel assemblies. The control rod 26 is inserted into the cell 28A, and the control rod 27 is inserted into the cell 28B. In the control rod 27, the axial length of the neutron absorber is longer than the length L of the control rod 26 by Lp. This length Lp is equal to the axial length of the lower plenum 6 of the fuel rod 1. By using the control rod 27, the neutron flux of the fuel assembly A whose active fuel length is higher than the fuel assembly C can also be effectively controlled. The control rod 27 is replaced with the control rod 26 which has been used at the time of refueling.

FIG. 18 shows another embodiment of the reactor core loaded with the fuel assembly A of FIG. A fuel rod 1 having a lower plenum 6 having an axial length Lp is provided in a core loaded with a fuel assembly C having a low active fuel portion at a position where the active fuel length is low. When A is loaded, the positions of the effective fuel lengths of the fuel cells greatly differ from each other, so that there is a possibility that the mismatch in the power distribution in the axial direction becomes large. For this reason, the loading of the fuel assembly A directly into the core composed of only the fuel assembly C as the fuel assembly is stopped. First, the fuel rod 1 having the lower plenum whose axial length is Lp / 2 is used.
The fuel assembly B provided with the fuel assembly B is loaded into the core constituted only by the fuel assembly C at the time of refueling. The operation in the next operation cycle is performed in the core constituted by the fuel assemblies B and C thus obtained. At the time of refueling after the operation of this operation cycle is completed, the spent fuel assembly C
Is taken out of the core, and the fuel assembly A is loaded. By such fuel exchange, the core shown in FIG. 18 is obtained. FIG.
Since the fuel assemblies A and B are loaded,
The output distribution mismatch in the axial direction is reduced. Refueling as described above can provide a core with a small power distribution mismatch. After the operation of the subsequent operation cycle, a core having only the fuel assemblies A and B is configured as a fuel assembly, and a core using only the fuel assembly A as the fuel assembly can be eventually obtained.

[0044]

According to the present invention, since the bending rigidity of the lower plenum portion is large, the flow vibration of the fuel rod can be suppressed. In addition, since the portion with large flexural rigidity is limited to the lower plenum, the neutron absorption at the pellet filling section does not increase,
A decrease in neutron utilization can be suppressed. Furthermore, low pressure loss
Can be reduced.

[Brief description of the drawings]

1A and 1B show the structure of a fuel rod used in a fuel assembly according to a preferred embodiment of the present invention. FIG. 1A is a structural diagram of an upper part of a fuel rod, and FIG.

FIG. 2 is a side view of a fuel assembly according to a preferred embodiment of the present invention.

FIG. 3 is an enlarged view of the vicinity of a lower tie plate in FIG. 1;

FIG. 4 is a structural view of another embodiment of a support structure for a fuel pellet.

FIG. 5 is a characteristic diagram showing an axial distribution of friction pressure loss of a conventional fuel assembly including a fuel rod having only an upper plenum.

FIG. 6 is a characteristic diagram showing an axial distribution of friction pressure loss of the fuel assembly of FIG. 2;

FIG. 7 is a characteristic diagram of a local pressure loss caused by a fuel spacer.

FIG. 8 is a bottom side view of another embodiment of the fuel rod applied to the present invention.

FIG. 9 is a structural view of another embodiment of a support structure for a fuel pellet.

FIG. 10 is a structural diagram of another embodiment of a support structure for a fuel pellet.

FIG. 11 is a structural view of another embodiment of a fuel pellet support structure.

FIG. 12 is a structural view of a fuel assembly according to another embodiment of the present invention.

FIG. 13 is a structural view of a fuel assembly according to another embodiment of the present invention.

FIG. 14 is a structural view 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.

FIG. 16 is a local plan view of a reactor core loaded with the fuel assembly of FIG. 2;

FIG. 17 is a side view of each control rod loaded on the core of FIG. 16;

FIG. 18 is a longitudinal sectional view of another embodiment of the reactor core loaded with the fuel assembly of FIG. 2;

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Fuel rod, 2 ... Lower tie plate, 3 ... Upper tie plate, 4 ... Fuel spacer, 5 ... Fuel pellet, 6 ... Lower plenum, 9 ... Cladding tube, 10 ... Pellet support plate, 11 ...
Channel box.

──────────────────────────────────────────────────続 き Continuing from the front page (72) Kunihara Kurihara, 1168 Moriyama-cho, Hitachi, Ibaraki, Japan Energy Research Laboratory, Hitachi, Ltd. Hitachi, Ltd. Hitachi Plant (72) Inventor Hajime Umehara 3-1-1, Sakaimachi, Hitachi City, Ibaraki Prefecture Hitachi, Ltd. Hitachi Plant (56) References JP-A-62-291593 (JP, A) (58) ) Surveyed field (Int.Cl. ⁷ , DB name) G21C 3/30

Claims (5)

(57) [Claims] 1. Filling a sealed cladding tube with fuel pellets
Inside the cladding tube below the fuel pellet filling area.
Fuels having a lower plenum formed at a lower end of a fuel cell
Rods and a plurality of fuel spaces for maintaining the distance between the fuel rods.
A fuel assembly including a fuel cell and a unit length of the cladding tube at the lower plenum.
Flexural rigidity of the fuel pellet filling area
The length of the fuel assembly of the fuel spacer , which is larger than that of the cladding at the portion corresponding to the length and located at the lowest level,
The projected area for a cross section perpendicular to the axial direction is
Of the fuel spacer located above the pacer
A fuel assembly characterized in that the fuel assembly is small. 2. A fuel cladding is filled in a sealed cladding tube.
Inside the cladding tube below the fuel pellet filling area.
Fuels having a lower plenum formed at a lower end of a fuel cell
Rods and a plurality of fuel spaces for maintaining the distance between the fuel rods.
A fuel assembly including a fuel cell and a unit length of the cladding tube at the lower plenum.
Flexural rigidity of the fuel pellet filling area
Greater than that of the cladding at the portion corresponding to the length,
Six fuel spacers are arranged in the axial direction.
Fuel assembly. 3. The method according to claim 2, wherein
A fuel spacer and the fuel space located at the second level from the bottom
The distance between the two is located above the second row from the bottom
The fuel spacers are larger than each other.
Fuel assembly. 4. The method according to claim 2, wherein
Is the distance between the fuel spacer and the lower tie plate lower?
Between the fuel spacers located above the second stage
A fuel assembly characterized by being larger than the interval. 5. A fuel clad is filled in a sealed cladding tube.
Inside the cladding tube below the fuel pellet filling area.
Fuels having a lower plenum formed at a lower end of a fuel cell
A rod and a channel box surrounding the bundle of fuel rods
In the fuel assembly provided with, the unit length of the cladding at the lower plenum portion
Flexural rigidity of the fuel pellet filling area
Greater than that of the cladding at the portion corresponding to the length,
The wall thickness of the channel box is
Fuel collection characterized by being thicker above the upper end
Coalescing.