JPH10111375A - Fuel assembly - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce stress acting on the welded portion of the bottom end plug of MOX fuel rod to be equivalent to an less than that for uranium fuel rod. SOLUTION: Especially for MOX fuel rod whose power at its bottom is high, an insulated object 21 is installed between a fuel pellet 10 at the bottom of effective fuel length and the bottom end plug 12 of the fuel rod. The insulating object 21 is made from Al2 O3 whose composition variation at high temperature is small like aluminum dioxide, and has almost same cylindrical shape as that of the fuel pellet or has pedestal construction. It is useful to create plural slits upper or lower surface of the insulating object 2 and to reduce contacting area between the fuel pellet 10 and the bottom end plug 12.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel assembly loaded in a nuclear reactor, and more particularly to an improved fuel for reducing stress acting on a lower end plug weld of a uranium-plutonium mixed oxide fuel rod. Regarding the aggregate.

[0002]

_2. Description of the Related Art Until now, uranium dioxide (UO ₂ ) has been mainly used as a reactor fuel, but recently, in order to improve fuel economy, it has been obtained by reprocessing spent uranium fuel. A plan for Plutonium thermal utilization using plutonium in light water reactors is underway. This is to produce and use mixed-oxide fuel (hereinafter referred to as MOX fuel) by mixing uranium dioxide (UO ₂ ) with plutonium dioxide (PuO ₂ ) recovered by reprocessing. It is. That is, plutonium newly generated by the combustion of uranium dioxide is recovered, and pellets made of MOX fuel mixed with this and natural uranium or depleted uranium are manufactured and used as fuel together with uranium fuel pellets.

FIG. 9 is a cutaway perspective view showing the structure of a conventional fuel assembly 1. The fuel assembly 1 has a plurality of fuel rods 3 and at least one water rod 4 housed in a rectangular cylindrical channel box 2. The fuel rods 3 and the water rods 4 are arranged in a grid pattern with a plurality of spacers 5 maintaining an interval therebetween, and further supported by an upper tie plate 6 and a lower tie plate 7. Further, a finger spring 8 provided at a lower portion in the channel box 2 is provided for suppressing a coolant flowing in the fuel assembly from flowing out.

FIG. 10 is a longitudinal sectional view of the fuel rod 3.
The fuel rod 3 has a plurality of sintered pellets 10 loaded in a cladding tube 9, and the upper and lower ends are sealed with an upper end plug 11 and a lower end plug 12. The loading portion of the sintered pellet 10 is referred to as a fuel effective portion 13. Further, a plenum portion 14 serving as a gas reservoir when the FP gas is discharged into the fuel rod 3 is provided above the fuel effective portion 13, and the plenum portion 14 is provided in the axial direction of the sintered pellet 10. A spring 15 for suppressing vibration is provided. A getter 16 for absorbing moisture in the fuel rod is incorporated in the spring 15.

FIG. 11A is a sectional view of a conventional uranium fuel rod, and FIG. 11B is a sectional view of a conventional MOX fuel rod. As shown here, there is a great difference between the fuel enrichment distribution of the uranium fuel rod and the fuel enrichment distribution of the MOX fuel rod. In other words, the upper and lower ends of the fuel rods of the uranium fuel rods 3a where the utilization rate of the uranium 235 is small are the natural uranium regions 17a where the natural uranium pellets 17 are arranged.
It has become. The area excluding the upper and lower ends of the fuel effective portion 13a is an enriched uranium area 18a in which enriched uranium pellets 18 are arranged.

However, the MOX fuel rod 3b has MOX pellets 19 having the same plutonium enrichment throughout the entire area of the fuel effective portion 13b. This is because the manufacturing process of MOX fuel from powder processing to pellet molding and fuel rod loading is currently performed as a series of operations. The reason is that it is difficult to produce pellets of different enrichment from the viewpoint of production.

[0007]

FIG. 12 is a graph schematically showing the axial output of the above-mentioned uranium fuel and MOX fuel. In the figure, the positions of the enriched uranium region 18a and the natural uranium region 17a in the uranium fuel rod 3a shown in the above-mentioned conventional technique are also shown. As can be seen from this figure, the output of the MOX fuel in the lower part of the fuel is higher than that of the uranium fuel. This is because the natural uranium region is not arranged below the MOX fuel rod unlike the uranium fuel rod, and MOX pellets 19 having the same enrichment as the central part of the fuel rod are arranged.

FIG. 13 is an enlarged sectional view of the lower part of the fuel rod.
Although there is a gap between the fuel pellet 10 and the cladding tube 9 in the fuel effective portion and they are not in contact with each other, as shown in the figure, the fuel pellet 10 located at the bottom of the fuel effective portion is located at the lower end of the fuel rod. A heat flux indicated by an arrow in the figure is generated from the fuel pellet 10 to the lower end plug 12 of the fuel rod. Due to this heat flux, the temperature of the lower end plug 12 in contact with the fuel pellet 10 becomes relatively higher than that of the cladding tube 9 not in contact with the fuel pellet 10. Further, as shown in FIG. 12, the MOX fuel having a higher output at the lower part of the fuel has a larger calorific value of the fuel pellet at the lower part of the fuel rod, and the temperature of the lower end plug 12 is higher than that of the uranium fuel.

Therefore, particularly in the case of MOX fuel, the lower end plug 12
And the temperature difference between the lower end of the cladding tube 9 tends to be larger. Therefore, compared with the uranium fuel rod, in the MOX fuel rod, a large difference in thermal expansion occurs at the joint between the lower end plug 12 and the cladding tube 9 due to the temperature difference between the two, and the stress increases accordingly, MOX fuel rods may exceed the elastic range.

The present invention has been made in view of the above points, and provides a fuel assembly including a MOX fuel rod in which the stress acting on the lower end plug weld 20 is reduced to the same level as a uranium fuel rod. The purpose is to do.

[0011]

According to the present invention, a fuel pellet comprising a fissile material is loaded into a cladding tube and a plurality of fuel pellets having upper and lower end plugs at upper and lower ends. A fuel assembly comprising a rod, an upper tie plate and a lower tie plate that hold the upper and lower ends of the fuel rod by bundling the fuel rods in a grid, and a plurality of spacers that maintain the distance between the fuel rods between the two tie plates At
A fuel assembly is provided wherein at least a part of the fuel rod is provided with a heat insulator between a fuel pellet and a lower end plug of the fuel rod.

Thus, the stress at the welded portion of the lower end plug generated by the difference in thermal expansion between the lower end portion of the fuel cladding tube and the lower end plug of the fuel rod can be reduced. In addition, by setting the fuel pellets of the fuel rods provided with the heat insulator to be formed of a mixed oxide formed by mixing uranium dioxide and plutonium dioxide, the MOX fuel having a high calorific value of the fuel pellets particularly at the lower part of the fuel rods is provided. By applying the present invention to a rod, MOX
And the stress at the lower end plug welded portion caused by the difference in thermal expansion described above can be reduced.

Further, by using a substance having a small change in physical properties even at a high temperature, such as aluminum dioxide (Al 2 O 3), the heat insulating effect is further improved. Further, the heat insulator is formed into a cylindrical shape having substantially the same diameter as the fuel pellet. This facilitates handling during fuel rod production by loading the heat insulator in the same manner as the pellets.

Alternatively, the heat insulator is a pedestal structure provided on the upper surface of the lower end plug, and the pedestal structure includes a contact portion with the fuel pellet and a connection portion with the lower end plug. Is set to be smaller than the diameter of the contact portion. This reduces the contact area between the fuel rod lower end plug and the heat insulator, thereby reducing the stress of the lower end plug weld generated due to the above-described difference in thermal expansion.

Further, a plurality of slits are provided on at least one of the upper surface and the lower surface of the heat insulator. Thus, by reducing the contact area between the fuel rod lower end plug and the heat insulator or the contact area between the fuel pellet and the heat insulator, it is possible to reduce the stress of the lower end plug weld generated due to the above-described difference in thermal expansion. .

Further, a plurality of fuel rods each having an upper end plug and a lower end plug at the upper and lower ends and a fuel pellet made of fissile material loaded in a cladding tube, In a fuel assembly having an upper tie plate and a lower tie plate that hold the ends, and a plurality of spacers that maintain the distance between the fuel rods between the two tie plates, at least a part of the fuel rod has a fuel rod lower end plug. A plurality of slits are provided on the upper surface of the fuel assembly. This reduces the contact area between the fuel rod lower end plug and the heat insulator, thereby reducing the stress of the lower end plug weld generated due to the above-described difference in thermal expansion.

Alternatively, a fuel pellet made of fissile material is loaded into a cladding tube, a plurality of fuel rods having upper and lower ends provided with an upper end plug and a lower end plug, and these fuel rods are bundled in a grid to form upper and lower fuel rods. In a fuel assembly having an upper tie plate and a lower tie plate that hold the ends, and a plurality of spacers that keep the distance between the fuel rods between these two tie plates, the lower surface of the fuel pellet located at the bottom of the fuel rod A fuel assembly provided with a plurality of slits is provided. Thereby, the stress of the lower end plug welding part generated by the above-mentioned difference in thermal expansion can be reduced by reducing the contact area between the fuel pellet and the heat insulator.

[0018]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is an enlarged sectional view of a lower portion of a fuel rod of a fuel assembly according to a first embodiment of the present invention. In this embodiment, a non-heat generating body (heat insulating body) 21 formed between a fuel pellet 10 and a lower end plug 12 is formed into a cylindrical shape having a diameter substantially equal to that of the pellet. The thermal insulator, as in the example 300 to 1500 ° C. Physical properties are small aluminum dioxide changes also under conditions of use of (Al ₂ O _3), to use those capable of withstanding the high temperature conditions. Thereby, the fuel pellet 10 as the heating element and the lower end plug 12 do not come into direct contact with each other, so that the temperature rise of the lower end plug 12 is suppressed, and the difference in thermal expansion due to the temperature difference between the lower end plug 12 and the cladding tube 9 is reduced. Can be suppressed.

The operation and effect of the present embodiment will be described below. Here, Al ₂ O ₃ is used as the heat insulator 21. FIG. 2 is a graph showing an analysis result of the lower end plug temperature in the vicinity of the lower end plug weld 20 in the present embodiment. The horizontal axis shows the height of the heat insulator 21, the vertical axis shows the corresponding lower end plug temperature, and the relative maximum value of the lower end plug when no conventional heat insulator is placed, as 1 as a relative value. According to this figure, when the height of the heat insulator 21 is 5 mm and 10 mm, the lower end plug maximum temperature is reduced to 80% or less and 70% or less, respectively, as compared with the conventional case. When the height of the heat insulator 21 is 10 mm or more, the maximum temperature of the lower end plug hardly changes, that is, the heat insulating effect tends to be saturated regardless of the height of the heat insulator 21. Therefore, a sufficient heat insulating effect can be achieved with the heat insulator 21 having the same height as the fuel pellet 10 and having a height of 10 mm.

FIG. 3 shows the lower end plug 12 and the cladding tube 9 in this case.
6 is a graph showing an analysis result of a temperature difference between the graph and FIG. On the vertical axis of the graph, the maximum temperature difference between the lower end plug 12 and the coating tube 9 is shown as a relative value, assuming that the temperature difference when no conventional heat insulator is provided is 1. It can be seen that, when the height of the heat insulator 21 is set to 5 mm and 10 mm, the temperature difference is reduced to 60% or less and 40% or less as compared with the conventional case, respectively, due to the heat insulating effect of the heat insulator 21 described above. Therefore, the lower end plug 12 and the cladding tube 9
Can be greatly suppressed.

FIG. 4 is a graph showing the results of analysis of the stress acting on the lower end plug weld 20. On the vertical axis of the graph, the stress generated in the lower end plug welded portion 20 is shown as a relative value with the stress when no conventional heat insulator is provided as 1. Due to the effect of suppressing the difference in thermal expansion between the lower end plug 12 and the cladding tube 9 described above, when the height of the heat insulator 21 is set to 5 mm and 10 mm, the stress generated in the lower end plug weld 20 is smaller than that of the conventional case. 50%, reduced to about 30%.

On the other hand, the stress generated in the lower end plug welding portion 20 in the conventional case shown in FIG. 13 is approximately 0.6 for the uranium fuel rod, assuming that the MOX fuel rod is 1. In consideration of this, by providing the heat insulator 21, especially when the height of the heat insulator 21 is set to 5 mm or more, the stress generated in the lower end plug weld 20 is almost the same as that of the conventional uranium fuel rod. Alternatively, it can be reduced to less than that.

Hereinafter, a second embodiment of the present invention will be described. FIG. 5 is an enlarged sectional view of a lower part of a fuel rod of the fuel assembly according to the present embodiment. In the present embodiment, the fuel pellet 1
A pedestal structure 22 serving as a heat insulator is provided between the lower end plug 12 and the lower end plug 12. This pedestal structure 22
Comprises a connecting portion 22b between the contact portion 22a of the fuel pellet 10 and the lower end plug 12, and the connecting portion 22b is
The diameter was set to be smaller than 2a.

By thus reducing the diameter of the connecting portion 22b, the pedestal structure 22 and the lower end plug welding portion 20 are formed.
A fuel pellet 1 which is a heating element by providing a gap between
By increasing the distance between 0 and the lower end plug 12 as compared with the related art, heat transfer to the lower end plug 12 is suppressed. According to this configuration, the height of the pedestal structure 22 can be made lower than the height of the heat insulator 21 provided below the effective fuel length of the fuel rod in the first embodiment described above, and at the same time, the first As in the embodiment, the stress generated in the lower end plug weld 20 can be suppressed.

Hereinafter, a third embodiment of the present invention will be described. In the present embodiment, a plurality of slits 23 are formed on the base portion above the lower end plug 12 of the fuel rod shown in FIG. 13 as a conventional case, that is, the contact surface of the lower end plug 12 with the fuel pellet 10 which is a heating element. Are provided in one direction.
FIG. 6A is an enlarged sectional view of the fuel rod lower end plug 12 of the fuel assembly according to the present embodiment, and FIG. 6B is a top view of the fuel rod lower end plug 12 of FIG. 6A. In FIG. 6A, the hatched portion is the surface that directly contacts the fuel pellet 10.

By providing a plurality of slits 23 in one direction in the base portion of the lower end plug 12 as described above, the contact area between the fuel pellet 10 as a heating element and the lower end plug 12 is reduced, so that the fuel pellet 10 The heat conduction to the lower end plug 12 can be suppressed, and the stress caused by the difference in thermal expansion due to the temperature difference between the lower end plug 12 and the cladding tube 9 can be reduced.

As a modification of the present embodiment, there is a lower end plug 12 provided with a lattice-shaped slit in the base portion. FIG. 6C is a top view of the lower end plug 12 in this case. With this configuration, the same operation and effect as the embodiment can be obtained.

Hereinafter, a fourth embodiment of the present invention will be described. In this embodiment, a plurality of slits 23 are provided in one direction on the contact surface of the lower surface of the lowermost fuel pellet 24 of the fuel effective length of the fuel rod shown in FIG. Things. FIG. 7A shows the lowermost fuel pellet 24 of the active fuel length of the fuel assembly according to this embodiment.
7 (b) is a bottom view of FIG. 7 (a). In FIG. 7A, the hatched portion is the lower end plug 1.
2 is a surface that directly contacts.

Although a similar slit 23 may be provided on the upper surface of the fuel pellet 24 as shown in FIG. 7A, the essence of this embodiment is that the slit is provided on the lower surface of the fuel pellet 24. is there.

As described above, the lower end plug 1 of the fuel pellet 24
By providing a plurality of slits in one direction on the contact surface with the second end 2, the contact area between the fuel pellet 24, which is a heating element, and the lower end plug 12 is reduced, so that heat from the fuel pellet 10 to the lower end plug 12 is reduced. Conduction can be suppressed, and stress caused by a difference in thermal expansion due to a temperature difference between the lower end plug 12 and the cladding tube 9 can be reduced.

As a modification of this embodiment, there is a fuel pellet 24 in which a lattice-shaped slit is provided on the contact surface with the lower end plug 12. FIG. 7C is a bottom view of the fuel pellet in this case. With this configuration, the same operation and effect as the embodiment can be obtained.

Although several embodiments have been described above, by appropriately combining these embodiments, the contact area between the fuel pellet and the lower end plug is reduced, so that the fuel rod can be connected to the lower end plug and the cladding tube. The structure can reduce the stress caused by the difference in thermal expansion due to the temperature difference. One example is described below as a fifth embodiment. FIG. 8 is an enlarged sectional view of the pedestal structure 25 located above the fuel rod lower end plug according to the present embodiment.

The pedestal structure 25 shown in FIG. 8 is a contact surface of the pedestal structure 22 of the non-heating element located above the lower end plug 12 shown in FIG. In this structure, the slit 23 described in the third embodiment is provided. As shown and described in the third embodiment, this slit may be either one-way or lattice-shaped. This allows
The functions and effects of the second embodiment and the third embodiment can be combined.

The above embodiments are applicable not only to MOX fuel rods but also to conventional uranium fuel rods. In this case, the stress acting on the lower end plug welded portion can be further reduced by the above-described operation.

[0035]

According to the fuel assemblies according to the embodiments of the present invention, the stress acting on the lower end plug welding portion of the MOX fuel rod is reduced to a level equal to or less than that of the uranium fuel rod, thereby providing a safer fuel. It is possible to provide a simple fuel assembly.

[Brief description of the drawings]

FIG. 1 is an enlarged sectional view of a lower portion of a fuel rod of a fuel assembly according to a first embodiment of the present invention.

FIG. 2 is a graph showing an analysis result of a lower end plug temperature in the vicinity of a fuel rod lower end plug weld in the first embodiment of the present invention.

FIG. 3 is a graph showing an analysis result of a temperature difference between a fuel rod lower end plug and a cladding tube according to the first embodiment of the present invention.

FIG. 4 is a graph showing an analysis result of stress acting on a fuel rod lower end plug weld in the first embodiment of the present invention.

FIG. 5 is an enlarged sectional view of a lower portion of a fuel rod of a fuel assembly according to a second embodiment of the present invention.

FIG. 6A is an enlarged sectional view of a lower end plug of a fuel rod of a fuel assembly according to a third embodiment of the present invention, and FIG.
(C) is a top view of a lower end plug of a fuel rod of a fuel assembly according to a modification of the present embodiment.

FIG. 7A is an enlarged sectional view of a fuel pellet located at the bottom of a fuel effective length of a fuel rod of a fuel assembly according to a fourth embodiment of the present invention, and FIG. FIG. 8C is a top view of the fuel pellets, and FIG. 9C is a top view of the fuel pellets located at the bottom of the effective fuel length of the fuel rods of the fuel assembly according to the modification of the embodiment.

FIG. 8 shows a pedestal structure 18 located above a lower end plug of a fuel rod of a fuel assembly according to a fifth embodiment of the present invention.
It is an expanded sectional view of.

FIG. 9 is a cutaway perspective view showing the configuration of a conventional fuel assembly.

FIG. 10 is a longitudinal sectional view of a fuel rod of a conventional fuel assembly.

11A is a cross-sectional view of a conventional uranium fuel rod, FIG.
(B) is a sectional view of a conventional MOX fuel rod.

FIG. 12 is a graph schematically showing the axial output of a conventional uranium fuel and MOX fuel.

FIG. 13 is an enlarged sectional view of a lower part of a fuel rod of a conventional fuel assembly.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Fuel assembly 2 Channel box 3 Fuel rod 4 Water rod 5 Spacer 6 Upper tie plate 7 Lower tie plate 8 Finger spring 9 Fuel cladding tube 10, 24 Fuel pellet 11 Upper end plug 12 Lower end plug 13 Effective fuel section 14 Fuel rod Plenum part 15 Spring 16 Getter 17 Natural uranium pellet 18 Concentrated uranium pellet 19 MOX pellet 20 Lower end plug welding part 21, 22, 25 Heat insulator

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Seiichi Komura 8 Shinsugita-cho, Isogo-ku, Yokohama-shi, Kanagawa Inside the Toshiba Yokohama office

Claims (8)

[Claims] 1. A fuel pellet comprising fissile material loaded in a cladding tube, a plurality of fuel rods having upper and lower end plugs at upper and lower ends, and a plurality of fuel rods bundled in a lattice shape. In a fuel assembly having an upper tie plate holding an upper end and a lower tie plate, and a plurality of spacers holding an interval between the fuel rods between the two tie plates, at least a part of the fuel rod has the fuel A fuel assembly comprising a heat insulator provided between the pellet and the fuel rod lower end plug. 2. The fuel assembly according to claim 1, wherein the fuel pellet of the fuel rod provided with the heat insulator is made of a mixed oxide formed by mixing uranium dioxide and plutonium dioxide. 3. The heat insulator is made of aluminum dioxide (Al).
_2. The fuel assembly according to claim 1, comprising ₂ O ₃ ). ₃ . 4. The fuel assembly according to claim 1, wherein the heat insulator is formed into a cylindrical shape having substantially the same diameter as the fuel pellet. 5. The heat insulator is a pedestal structure provided on an upper surface of the lower end plug, and the pedestal structure includes a contact portion with the fuel pellet and a connection portion with the lower end plug. 2. The fuel assembly according to claim 1, wherein the diameter of the connecting portion is smaller than the diameter of the contact portion. 6. The fuel assembly according to claim 1, wherein a plurality of slits are provided on at least one of an upper surface and a lower surface of the heat insulator. 7. A plurality of fuel rods, each of which is provided with an upper end plug and a lower end plug at upper and lower ends by loading a fuel pellet made of fissile material into a cladding tube; In a fuel assembly having an upper tie plate holding an upper end and a lower tie plate, and a plurality of spacers holding an interval between the fuel rods between the two tie plates, at least a part of the fuel rod has the fuel A fuel assembly comprising a plurality of slits provided on an upper surface of a rod lower end plug. 8. A plurality of fuel rods each having a fuel pellet made of fissile material loaded in a cladding tube and having upper and lower end plugs at upper and lower ends, and a plurality of fuel rods bundled in a lattice shape. In a fuel assembly having an upper tie plate holding an upper end and a lower tie plate and a plurality of spacers holding an interval between the fuel rods between the two tie plates, a fuel pellet located at the bottom of the fuel rod A fuel assembly comprising a plurality of slits provided on a lower surface of a fuel assembly.