JP3501406B2 - 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 high burnup fuel assembly used in a boiling water reactor.

[0002]

2. Description of the Related Art In recent years, in order to improve the economical efficiency of nuclear power generation, the take-out burnup of fuel has been increased (increased burnup). An example of such a high burnup fuel assembly is shown in FIG. In the fuel assembly 1, a long fuel rod 2, a short fuel rod 3 and a large diameter water rod 6 are bound by a spacer 8 in a square lattice shape and fixed to an upper tie plate 4 and a lower tie plate 5 to form a fuel rod bundle. The fuel rod bundle is surrounded by a channel box 7. Reference numeral 9 in FIG. 5 is an external spring, which is composed of the long fuel rod 2 and the upper tie plate 4.
Is intervened between.

The high burnup fuel assembly thus constructed has the following features as compared with the conventionally known fuel assemblies. In order to achieve a high burnup, it is necessary to make the fuel highly enriched, but in the highly enriched fuel, the axial output peaking due to the void distribution is further increased. Further, since various kinds of fuels having different stay periods in the reactor are mixed in the core at the same time, radial power peaking also increases. As a result, thermal margins such as maximum line power density and minimum limit power ratio are reduced. In order to improve this, in the fuel assembly 1 of FIG. 5, the number of fuel rods is increased by changing the arrangement of the fuel rods 2 and 3 from 8 × 8 of the conventional fuel to 9 × 9.

However, as the number of fuel rods increases, the pressure loss increases, which impairs the stability of the nuclear reactor. Therefore, in the fuel assembly of FIG. 5, a short fuel rod 3 in which some of the fuel rods have a shorter length than the long fuel rod 2 is used, and the fuel with a large pressure loss is used because the coolant is a two-phase flow. The upper channel is enlarged to counteract the increase in pressure loss due to the increase in the number of fuel rods. The length of the short fuel rod 3 is about ⅔ of that of the long fuel rod 2, and the spacer 8 is set within a few cm from the upper end thereof so as not to vibrate due to the flow of the coolant.

Also, when the output is gradually increased,
The limit output is the output of the fuel assembly when the heat transfer from the fuel rods to the coolant makes a boiling transition from the efficient nucleate boiling to the inefficient film boiling. Since this boiling transition is likely to occur in the upper part of the fuel rod, the short fuel rod 3 can improve the limit output. For this purpose, short fuel rods 3
As for the position of, the location where the cooling efficiency of the fuel rod is the worst is selected, thereby increasing the minimum limit output ratio.

The short fuel rod 3 has a function of further improving the reactor shutdown margin. When the nuclear reactor is shut down, the neutron flux forms a peak at a position 1/4 to 1/3 below the upper end of the core. Since the coolant has a low temperature and a high density when the reactor is shut down, it acts as a neutron absorber, so the reactor shutdown margin can be improved by reducing the number of fuel rods in the axial upper part and increasing the amount of coolant. .

Further, in order to improve the economical efficiency and the thermal margin of the nuclear reactor, a technique for axially distributing the concentration and the concentration of burnable poisons has been developed, for example, Japanese Patent Publication No. 3-0789.
No. 54 publication. In this publication, the fuel is axially divided into five regions of a lower end portion, a lower portion, a central portion, an upper portion and an upper end portion. The enrichment are highest composed lowest central portion at upper and lower ends, gadolinia has not been loaded on the upper and lower ends is more burnable poison, high from the bottom <br/> order of internally Medium , Low concentration.

Each area plays the following roles.
By lowering the enrichment at the upper and lower ends, the leakage of neutrons to the outside of the reactor is reduced and the economic efficiency is improved. By making the enrichment in the central part higher than in the lower part, the output distribution is flattened throughout the operating cycle, and by making the gadolinia concentration in the central part lower than in the lower part, the output distribution is flattened especially from the middle to the end of the operating cycle. There is.

This is because in the boiling water reactor core, the void fraction of the coolant is distributed in the vertical direction, and an output peak is likely to occur in the lower part of the core where the void fraction is low and the moderator has a high average density. Is. Furthermore, in the upper part, by reducing the gadolinia concentration, the residual amount at the end of the operating cycle is reduced to improve economic efficiency, and at the same time, the enrichment is reduced to prevent deterioration of the reactor shutdown margin.

[0010]

The structures of the long fuel rods 2 and the short fuel rods 3 will be described with reference to FIG. That is, both the long fuel rods 2 and the short fuel rods 3 are filled with a plurality of fuel pellets 10 in a cylindrical cladding tube 11, and the cladding tube
The upper and lower ends of 11 are sealed with an upper end plug 12 and a lower end plug 13. A space called a plenum 14 having a length of about 40 cm is provided above the effective portion filled with the fuel pellets 10 in order to mitigate the increase in internal pressure due to the fission product gas.

A spring 15 is inserted in the plenum 14 for fixing the fuel pellet during transportation of the fuel assembly. In the long fuel rod 2, the fuel pellets are filled up to the lower end portion, but in the short fuel rod 3, the fuel pellet is also filled in the lower portion.
Is provided, and the position of the fuel pellet is fixed by a support material (not shown).

In a normal fuel rod, since the plenum 14 is located above the effective portion, it does not affect the core characteristics such as thermal margin and reactor shutdown margin. However, if the plenum is provided above the effective portion with a short fuel rod, it will be located in the effective portion for the long fuel rod, so the influence on the core characteristics cannot be ignored.

Originally, when the short fuel rod 3 is used, the fuel inventory per fuel assembly is reduced, so that the fuel cycle cost is increased and the economical efficiency is deteriorated. Further, at the plenum position of the short fuel rod, the amount of the coolant is smaller than that at the upper portion thereof, so that the reactor shutdown margin is deteriorated. In the example of FIG. 6, the deterioration of the reactor shutdown margin is minimized by distributing the plenum 14 to the lower end part.

Therefore, in order to extend the active fuel length in order to improve the fuel economy, the short fuel is reduced in terms of stability due to increased pressure loss and maintaining the relationship between the upper end of the short fuel rod 3 and the position of the spacer 8. It is necessary to shorten the plenum 14 because the entire length of the rod 3 cannot be extended any further.
Furthermore, it is desirable that the plenum 14 be as short as possible to improve the reactor shutdown margin. However, from the original purpose of the Plenum to suppress the internal pressure rise due to fission product gas,
Plenum 14 cannot be easily shortened.

Further, in order to realize the enrichment distribution in order to control the axial power distribution, in Japanese Patent Publication No. 3-078954, in some specific fuel rods, the enrichment of the fuel pellets filled therein is enriched. Are distributed in the axial direction. In the manufacturing process of the fuel rod, the fuel pellets are filled into the cladding tube one by one, but the manufacturing process becomes more complicated as the number of types of the fuel pellets filled increases.

Further, after the fuel pellets are filled into the cladding tube, an inspection is performed to see if the fuel pellets are definitely filled. This inspection also becomes more complicated as the number of types of fuel pellets increases. As a result, it may possibly lead to an increase in manufacturing cost.

Further, although the limiting output is improved by using the fuel rod having the worst cooling capacity as the short fuel rod, however, the boiling transition may occur in the long fuel rod. In order to improve this further, it is conceivable to lower the enrichment of the upper part of the long fuel rod, but this will give the enrichment distribution in the fuel rod. There is an unfavorable problem because the manufacturing process is complicated.

The present invention has been made to solve the above problems, and in a fuel assembly having short fuel rods, the thermal margin and the reactor shutdown margin are improved to improve fuel economy and have a simple structure. In order to provide a fuel assembly having a high burnup and excellent manufacturability.

According to the first aspect of the present invention, a plurality of fuel rods, which are formed by filling a plurality of fuel pellets in a cladding tube and sealing both ends thereof, are bundled in a lattice shape, and a water rod is provided at a central portion of the fuel rod bundle. In a fuel assembly provided and surrounding the fuel rod bundle with a channel box, the plurality of fuel rods include a first group of fuel rods and a first group of fuel rods having an effective portion shorter than that of the first group of fuel rods. The fuel pellets, which are composed of two groups of fuel rods and are filled in the first group of fuel rods and the second group of fuel rods, respectively, have a fissile material concentration in the vertical direction at least inside the upper and lower ends. And the first
The fuel rods of the group consist of at least two types of fuel rods having different fissile material concentrations in the fuel pellets filled inside except for the upper and lower ends, and are filled in the effective portion of the fuel rods of the second group. The fissionable material concentration of the fuel pellets is the fissionable material concentration of the fuel pellets filled inside except for the upper and lower ends of all the fuel rods of the first group arranged at positions other than the four corners of the fuel assembly. Lower than the lowest concentration and higher than the natural uranium concentration, and when the fuel rod bundle is divided into upper and lower parts with the upper end of the effective portion of the second group of fuel rods as a boundary, the lower fuel rod bundle is traversed. The area average fissile material concentration is smaller than the cross-sectional average fissile material concentration of the upper fuel rod bundle. According to a second aspect of the present invention, a plurality of fuel pellets are bundled in a cladding tube and sealed at both ends, and a plurality of fuel rods are bundled in a lattice shape. A water rod is provided at the center of the fuel rod bundle. In a fuel assembly formed by surrounding a fuel rod bundle with a channel box, the plurality of fuel rods are a first group of fuel rods,
The second group of fuel rods has an effective portion shorter than that of the first group of fuel rods.
Fissile material of fuel pellets filled in each fuel rod
The quality concentration is upper and lower at least inside the upper and lower ends.
The fuel rods of the first group except the upper and lower ends.
Fissile material concentration of fuel pellets filled inside
Consists of at least two types of fuel rods with different
The fuel pellets filled in the effective portion of the second group of fuel rods.
The fissile material concentration of the
Fuel pellets filled inside except the upper and lower ends of the rod
Less than the lowest concentration of fissile material in
However, it is higher than the uranium concentration and the fuel rods of the second group are present.
The fuel rod bundle was divided into upper and lower parts with the upper end of the effective portion as a boundary.
If the average cross-section fissile material concentration of the lower fuel rod bundle is
Lower than the average cross-section fissile material concentration of the upper fuel rod bundle
The feature is that The invention of claim 3 is the first
Some of the fuel rods in the group had combustible poisons and
Cross section of the fuel rod bundle
Characterized by greater than average cross-sectional average burnable poison concentration
It The invention of claim 4 is characterized in that the total length of the plenums of the second group of fuel rods is shorter than the total length of the plenums of the first group of fuel rods.

[0020]

In the fuel assembly according to the present invention, since the fissionable material concentration of the fuel pellets filled in each fuel rod can be made uniform inside except the upper and lower ends, the manufacturability of the fuel rod is improved. Is greatly improved.

Further, since the amount of fission product gas produced depends on the number of fission in the fuel rod, that is, the output, by lowering the enrichment of the short fuel rod, the output is lowered and the rise in the fuel rod internal pressure is suppressed. be able to. Therefore, the plenum of the short fuel rod can be made shorter than the plenum of the long fuel rod as in the invention of claim 4 , and the fuel economy and the reactor shutdown margin can be improved.

Further, a part of the liquid film formed on the surface of the short fuel rod is transmitted to the adjacent long fuel rod to cool it, but since the output of the short fuel rod becomes small, the liquid film is not formed on the surface. Since the liquid film to be formed becomes thicker, the cooling efficiency of the long fuel rod is further increased and the limit output is improved.

[0023]

【Example】

(First Embodiment) A first embodiment of the fuel assembly according to the present invention will be described with reference to FIG. The first embodiment applies the high burnup fuel assembly shown in FIG. The fuel rod arrangement is a 9 × 9 square lattice arrangement, and the fuel rod bundle is composed of 66 first group long fuel rods 2, eight second group short fuel rods 3 and two large diameter water rods. It is composed of a rod 6. The total length of the long fuel rod 2 is about 410 cm, of which about 370 cm is the effective portion filled with fuel pellets. Short fuel rod 3
The total length is about 260 cm, of which about 220 cm is the effective part.

At the effective portion of the long fuel rod 2, the upper end is approximately 30 cm.
And the natural uranium pellets are packed in the lower part of about 15 cm, but the concentration of the inner part of about 325 cm is uniform in the axial direction. The enrichment of each fuel rod 2, 3 is A>B>C> D
> E in the order of> E, and the enrichment E of the short fuel rods 3 is lower than the enrichments A to D of the long fuel rods 2 than any of the upper and lower ends except natural uranium. As a result of constituting each of the fuel rods in this manner, fuel assembly average enrichment, the length fuel rods 3 effective
The upper end parts bordering to the direction of the upper about than the lower 0.34%
It's getting higher.

In the present embodiment, the gadolinia is contained inside the fuel rods indicated by the numbers 6 and 7 excluding the natural uranium portion at the upper and lower ends of the 14 long fuel rods, and the concentration thereof is HG>MG> L.
Highest in G order. Of these, the eight gadgets 6 have a higher gadolinia concentration in the lower part than in the upper part, with the upper end position of the effective portion of the short fuel rod 3 indicated by the number 5 as a boundary. Such a gadolinia concentration distribution can suppress the lower peak particularly in the middle of the operation cycle.

FIG. 2 shows the axial power distribution in the equilibrium cycle of the core loaded with the fuel assembly of this embodiment in the core.
Shown in (A) and (B). Due to the low enrichment of the short fuel rods, the cross-sectional average enrichment was made lower in the lower part than in the upper part and the gadolinia concentration distribution maintained the axial output peaking value at around 1.3 or less throughout the operating cycle. The thermal margin can be sufficiently satisfied without complicating the enrichment distribution in the axial direction in the fuel rod.

In the boiling water reactor, since the coolant does not boil outside the channel box 7 shown in FIG. 5, the fuel rods closer to the outer side in the cross section of the fuel rod bundle tend to have higher output. is there. Therefore, in the first embodiment shown in FIG. 1, the closer the fuel rods are to the outside, the lower the enrichment is to reduce the local output peaking in the cross section of the fuel rod bundle.

Therefore, the short fuel rods 3 arranged inside should originally have higher enrichment than the long fuel rods 2 and 3. However, in this embodiment,
By placing some short fuel rods diagonally adjacent to the fuel rod No. 4 at the four corners to reduce its enrichment,
The power of the fuel rods numbered 3 and 4 is lowered.

In this embodiment, the local output peaking can be suppressed to 1.4 or less in the lower section and 1.3 or less in the upper section. Therefore, as in the present embodiment, the second group of fuel rods, that is, the short fuel rods, can be arranged at least diagonally adjacent to the fuel rods at the four corners of the cross section of the channel box to reduce the enrichment. is important.

In this embodiment, since the enrichment of the short fuel rods 3 is made lower than that of any of the long fuel rods 2, the output of the short fuel rods 3 is less than half the output of the long fuel rods 2. .
Therefore, a thicker liquid film is formed on the surface of the upper end portion of the short fuel rod 3, and the liquid film becomes water droplets and is transmitted to the adjacent long fuel rod 2 to be cooled.

As a result, the cooling efficiency of the long fuel rods 2 can be increased without lowering the enrichment of the long fuel rods 2 adjacent to the short fuel rods 3, thus improving the limit output.

Further, the concentration of the short fuel rods 3 is reduced based on this embodiment, and further, the first method disclosed in Japanese Patent Laid-Open No. 144290/1988.
By providing a coolant flow stirring blade as shown in the figure at the upper end of the short fuel rod, the coolant flow with many water droplets at the upper end of the short fuel rod is further enhanced by cooling the adjacent long fuel rods. It can be used efficiently.

(Second Embodiment) In the first embodiment, the long fuel rod 2 has an upper portion of about 40 cm, and the short fuel rod 3 has an upper portion of about 40 cm.
There is a plenum 14 of 25 cm and about 15 cm at the bottom. The lower plenum of the short fuel rod 3 is a region just corresponding to the lower natural uranium portion of the long fuel rod 2. That is, the lengths of the long fuel rods 2 and the short fuel rods 3 are substantially equal to each other in the plenum 14.

However, in the first embodiment, the enrichment of the short fuel rods 3 is set to be lower than that of any of the long fuel rods 2. Therefore, the output of the short fuel rods 3 is equal to that of the long fuel rods 2. It is less than half. Therefore, even if the length of the plenum 14 of the second group fuel rods, that is, the short fuel rods 3 is shorter than that of the first group fuel rods, that is, the long fuel rods 2, the internal pressure increase due to the fission product gas is sufficiently suppressed. You can

Therefore, in the second embodiment shown in FIG. 3, the effective length of the short fuel rod 3 is extended upward by 15 cm to about 235 cm, and the upper plenum 14 is shortened to about 10 cm. In this embodiment, the effective length of the short fuel rod can be extended, and the fuel inventory can be increased by about 0.5% without affecting the core characteristics, so that the fuel economy is improved accordingly. The length of the upper plenum of the short fuel rod must be at least 5 cm or more in order for the spring to work effectively.

In the present embodiment, the upper and lower boundary positions of the fuel rod with gadolinia number 6 are aligned with the upper end of the effective portion of the short fuel rod.

(Third Embodiment) FIG. 4 shows a third embodiment of the present invention. In the present embodiment, there are two types of short fuel rods 3 of Nos. 5 and 8, and one short fuel rod of No. 8 has a gadolinia concentration LLG lower than the gadolinia concentrations LG, MG, HG of the other fuel rods. Is mixed in. The enrichment of each fuel rod is F>G>H>J> I in the order of increasing, and the enrichment J of the short fuel rod is lower than the average enrichment of the cross section, but is higher than the enrichment I of the long fuel rod of No. 4. Is high. Concentration F of long fuel rods
~ I is more than the enrichments A to D of the long fuel rods of the first embodiment.
0.1% to 0.2% lower. As a result, the difference in average cross-sectional enrichment between the upper part and the lower part is 0.17%, which is about half that in the first embodiment.

The axial power distribution of this embodiment is shown in FIG.
In this example, since the number of gadolinia fuel rods increased by one in the lower cross section, the reactivity of the lower part of the fuel assembly declines, so the axial power distribution is sufficiently flat with a smaller vertical enrichment difference than in the first example. Has been converted.

(Fourth to Seventh Embodiments) In the fourth to sixth embodiments of the present invention, the cross section and enrichment distribution of the fuel assembly are the same as those in FIGS. 1 (A) and 1 (B). However, the boundary position of the gadolinia concentration of the fuel rod with gadolinia number 6 is different from that of the first embodiment. That is, in the first embodiment shown in FIG. 1 (B), the boundary position of the gadolinia concentration of the fuel rod No. 6 was made to coincide with the upper end of the effective portion of the short fuel rod No. 5, but the fourth and the fourth In the fifth embodiment, the boundary positions are moved up and down by about 60 cm. Further, in the sixth embodiment, the gadolinia density is uniform in the axial direction.

Since the difference in gadolinia concentration has little effect on the output distribution in the early stage of the operation cycle, the output distribution in the early stage of the operation cycle of these examples is the same as that of the first example shown in FIG. 2 (A). Indistinguishable from the distribution. Although a slight difference occurs at the end of the operation cycle, it is well within the limit in any of the examples. However, in the sixth example in which the gadolinia concentration is uniform in the upper and lower directions, the output distribution is distorted downward from the middle to the end of the cycle, and peaking increases, but it is still within about 1.3. As described above, in the sixth embodiment, the output peaking is slightly increased, but since the enrichment and the gadolinia concentration are uniform inside all the fuel rods except the upper and lower ends, the production is most simplified.

The seventh embodiment differs from the first embodiment shown in FIGS. 1A and 1B only in the enrichment distribution of the short fuel rod of No. 5. In this embodiment, natural uranium pellets are filled in place of the enriched uranium of enrichment E in the upper portion 5 to 15 cm of the effective portion of the short fuel rod. Since the short fuel rod has the coolant above the upper end portion and is surrounded by more moderator than the long fuel rod, an output peak occurs. In this embodiment,
This peak is reduced by sufficiently lowering the concentration of the portion where the output peak is likely to occur.

The fourth to seventh embodiments described above are the first
It is a modification of the embodiment described above, but it is obvious that the same modification can be obtained for the second and third embodiments. Further, in the above embodiments, the enrichment of the short fuel rods is different from that of any of the long fuel rods, but an embodiment having the same enrichment of any of the long fuel rods can be applied.

[0043]

According to the present invention, the average enrichment of the fuel assembly can be made higher in the upper part than in the lower part. As a result, the enrichment of the fuel pellets filled in each fuel rod can be made uniform in the inside excluding the upper and lower ends, so that the manufacturing process of the fuel rods can be greatly simplified. As a result, fuel production costs can be reduced.

Further, by sufficiently reducing the enrichment of the short fuel rods, it is possible to reduce the amount of fission product gas produced and suppress an increase in the internal pressure of the fuel rods. Therefore,
The plenum of short fuel rods can be made shorter than the plenum of long fuel rods, and the fuel economy and the reactor shutdown margin can be improved by increasing the fuel inventory.

Further, since the liquid film layer on the surface of the short fuel rod can be made thicker, the cooling water having many water droplets can be efficiently sent to the long fuel rod adjacent to the short fuel rod.
The limit output is improved.

[Brief description of drawings]

FIG. 1A is a plan view showing an arrangement state of fuel rods in a first embodiment of a fuel assembly according to the present invention, and FIG. 1B is a fuel rod enrichment and gadolinia distribution in FIG. 1A. Distribution map.

FIG. 2 (A) is an axial power distribution diagram of the core at the beginning of the cycle in which the fuel assemblies of the first and third embodiments of the present invention are loaded, and FIG. 2 (B) is the axial power output of the core at the end of the cycle. Distribution map.

FIG. 3 is a distribution diagram showing enrichment and gadolinia distribution in a second embodiment of the fuel assembly according to the present invention.

FIG. 4A is a plan view showing an arrangement state of fuel rods in a third embodiment of the fuel assembly according to the present invention, and FIG. 4B is a fuel rod enrichment and gadolinia distribution in FIG. 4A. Distribution map.

5A is an elevation view showing a conventional fuel assembly in a partial cross section, FIG. 5B is a cross sectional view taken along the line BB in FIG. 5A, and FIG. 5C is CC in FIG. FIG.

6 (A) is an elevation view showing a part of a long fuel rod of the fuel assembly in FIG. 5, and FIG. 6 (B) is an elevation view showing a part of a short fuel rod of the fuel assembly.

[Explanation of symbols]

1 ... Fuel assembly, 2 ... Long fuel rod, 3 ... Short fuel rod, 4
... Upper tie plate, 5 ... Lower tie plate, 6 ... Water rod, 7 ... Channel box, 8 ... Spacer,
9 ... External spring, 10 ... Fuel pellet, 11 ... Cladding tube,
12 ... upper end plug, 13 ... lower end plug, 14 ... plenum, 15 ... spring, 16 ... first embodiment, 17 ... third embodiment.

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Atsuji Ebikawa 8 Shinsugita-cho, Isogo-ku, Yokohama, Kanagawa Prefecture Toshiba Corporation Yokohama office (72) Inventor Takeshi Nakajima 8 Shinsugita-cho, Isogo-ku, Yokohama, Kanagawa Prefecture Toshiba Corporation (56) References JP-A-2-103491 (JP, A) JP-A-63-133086 (JP, A) JP-A-4-120497 (JP, A) JP-A-54-59594 (JP, A) JP-A-56-166488 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) G21C 3/30-3/62

Claims (4)

(57) [Claims] 1. A plurality of fuel rods formed by filling a plurality of fuel pellets in a cladding tube and sealing both ends thereof in a lattice shape,
In a fuel assembly in which a water rod is provided in the center of the fuel rod bundle and the fuel rod bundle is surrounded by a channel box, the plurality of fuel rods are a first group of fuel rods and a first group of fuel rods. A second group of fuel rods having an effective portion shorter than that of the first group of fuel rods, and the fuel pellets respectively filled in the first group of fuel rods and the second group of fuel rods have at least a fissile material concentration. The fuel rods of the first group are uniform in the vertical direction in the interior excluding the upper and lower end portions, and the first group of fuel rods are composed of at least two types of fuel rods having different fissile material concentrations in the fuel pellets filled in the interior. The fissionable material concentration of the fuel pellets configured and filled in the effective portion of the fuel rods of the second group is equal to that of all the fuel rods of the first group arranged at positions other than the four corners of the fuel assembly. Inside except upper and lower end Lower than the minimum concentration of the fissile material concentration of the filled fuel pellets and higher than the natural uranium concentration, and the fuel rod bundle is vertically moved up and down with the upper end of the effective portion of the fuel rods of the second group as a boundary. If divided
A fuel assembly characterized in that the average cross-section fissile material concentration of the lower fuel rod bundle is smaller than the average cross-section fissile material concentration of the upper fuel rod bundle. 2. A plurality of fuel rods, each of which is filled with a plurality of fuel pellets in a cladding tube and sealed at both ends thereof, is bundled in a lattice shape,
In a fuel assembly in which a water rod is provided in the center of the fuel rod bundle and the fuel rod bundle is surrounded by a channel box, the plurality of fuel rods are a first group of fuel rods and a first group of fuel rods. A second group of fuel rods having an effective portion shorter than that of the first group of fuel rods, and the first group of fuel rods and the first group of fuel rods.
The core of the fuel pellets filled in the two groups of fuel rods
The degradable substance concentration is at least in the interior except the upper and lower ends.
It is uniform in the vertical direction, and the first group of fuel rods has upper and lower end portions.
Fissionable material of fuel pellets filled inside
Consists of at least two types of fuel rods with different quality concentrations
The fuel filled in the effective portion of the second group of fuel rods.
The fissile material concentration of the material pellets is
Of the fuel rods filled inside except the upper and lower ends of
Less than the lowest concentration of fissile material in the let
And a concentration of natural uranium greater than that of the second group of fuel
The fuel rod bundle is moved up and down with the upper end of the effective portion of the rod as a boundary.
Cross section of the lower fuel rod bundle, if divided
The cross-sectional average fissile material concentration of the upper fuel rod bundle
A fuel assembly characterized by being much smaller . 3. A portion of the fuel rods of the first group contains burnable poisons.
The cross section of the lower fuel rod bundle has an average burnable poison concentration.
Than the average burnable poison concentration in the cross section of the upper fuel rod bundle
The fuel assembly according to claim 2, wherein the fuel assembly is large. 4. The fuel according to claim 1, wherein the length of the plenum of the second group of fuel rods is shorter than the length of the plenum of the first group of fuel rods. Aggregation.