JPH0758340B2 - Fuel assembly - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a nuclear fuel assembly, and more particularly to a fuel assembly suitable for suppressing leakage of cooling water from a gap between a channel box and a lower tie plate. .

[Conventional technology]

The fuel assembly includes a plurality of fuel rods, spacers for supporting these fuel rods in an aligned manner and enclosing lateral vibrations, an upper tie plate and a lower tie plate for supporting upper and lower end plugs of each fuel rod, respectively. .

In this fuel assembly, a finger spring installed in the lower tie plate is inserted into a gap formed between the channel box and the lower tie plate.

[Problems to be Solved by the Invention]

The above-mentioned finger springs prevent the cooling water in the channel box from leaking out of the channel box from the flat surface of the four sides excluding the corners of the gap formed between the channel box and the lower tie plate. It is provided in. However, the outward force of the finger spring promotes the swelling of the lower end of the channel box due to the creep, and the outward deformation of the channel box increases. This increase in deformation leads to an increase in the amount of leaked cooling water near the lower tie plate corner where there is no finger spring.

An object of the present invention is to provide a fuel assembly capable of suppressing the leakage of the coolant from between the lower tie plate and the channel box without using a finger spring.

Another object of the present invention is to provide a fuel assembly that can reduce the creep deformation of the channel box.

[Means for Solving the Problems] The first feature of achieving the object of the present invention is that the lower tie plate is distributed in a first region on the upper surface of the lower tie plate at a predetermined distance from the periphery of the upper surface. The plurality of first coolant supply holes and the plurality of second coolant supply holes distributed in the second region surrounded by the first region, and all the first coolant supply holes in the first region are provided. The total cross-sectional area of one coolant supply hole is larger than the total cross-sectional area of all the second coolant supply holes in the second region.

A second feature of achieving the other object of the present invention is to act on a part of the channel box when the coolant leaks from the leakage flow passage formed between the lower tie plate and the channel box. There is a means for generating a force toward the side.

[Action]

Since the total cross-sectional area of all the first coolant supply holes in the first area is larger than the total cross-sectional area of all the second coolant supply holes in the second area, a large amount of water is provided above the outer periphery of the lower tie plate. Coolant can be supplied. Therefore, when the lower tie plate is surrounded by the channel box, it is possible to reduce the pressure of the part connected to the gap formed between the lower tie plate and the channel box, and to use the finger spring without using the finger spring. It is possible to suppress the coolant leakage flow rate from the gap.

Since the force generating means can pull the channel box toward the lower tie plate side, the creep deformation spreading to the outside of the channel box can be reduced.

〔Example〕

Next, a fuel assembly which is an embodiment of the present invention will be described with reference to the drawings. This fuel assembly is used in a boiling water reactor.

2 and 3, the fuel assembly 1 of this embodiment is shown.
Comprises an upper tie plate 2, a lower tie plate 4, a fuel rod 5, a water rod 6 and a fuel spacer 7. Both ends of the fuel rod 5 and the large diameter water rod 6 are held by the upper tie plate 2 and the lower tie plate 4. The upper and lower tie plates 2, 4 are joined by a joining fuel rod (not shown). Each fuel rod 5 is horizontally supported by a fuel spacer 7. Therefore, a cooling water passage 11 having a predetermined width is formed between the fuel rods 5.

As shown in FIG. 1 of JP-A-62-217186, the fuel assembly 1 has fuel rods 5 arranged in 9 rows and 9 columns, and two large diameter water rods are arranged in the central portion. is there. However, in this embodiment, the two large diameter water rods are disclosed in JP-A-62-2.
As shown in FIG. 7 of Japanese Patent No. 17186, they are arranged so as to form a gap therebetween.

The channel box 8 is a tubular body having a square cross section, and is attached to the corner post 3 of the upper tie plate 2 by the cap screw 10 provided in the channel asuna 9. The lower end of the channel box 8 is a free end. Such a channel box 8 surrounds the bundle of fuel rods 5 bundled by the fuel spacer 7.

The lower tie plate 4 has a square cross section having a grid portion 40 that provides a fuel support portion, four outer side surfaces 41A as side surfaces of the lower tie plate 4, and an inner side surface 41B that defines the internal space 42. Cylindrical side wall connected to part 40
41 and a nozzle portion 43 that is connected to the cylindrical side wall portion 41 and guides the coolant to the internal space 42.

As shown in FIGS. 1 and 4, the grid portion 40 has the fuel rod mounting hole 45 into which the lower end portion of the fuel rod 5 is inserted, the connecting fuel rod mounting hole 46, and the lower end of the water rod 6. It has a water rod mounting hole 47. Further, the grid portion 40 guides the cooling water guided to the internal space 42 between the mounting holes 45, 46, 47 above the grid portion 40, that is, to the cooling water passage 11, and has different flow passage cross-sectional areas. Cooling water supply holes 18, 21, 22, 23, 24 are provided. Of these cooling water supply holes, the outermost cooling water supply hole 18
Reduces the driving force for leaking cooling water from the gap (cooling water passage 17 described later) formed between the channel box and the lower tie plate when the fuel assembly 1 is loaded in the core, A flow path means for generating a coolant flow that suppresses leakage of cooling water from the gap is configured. Hereinafter, the coolant flow flowing out from the cooling water supply hole 18 will be referred to as a jet flow.

The cooling water supply hole 18 is located outside the inner side surface 41B and an inner portion 18a that penetrates a portion of the grid portion 40 that is located inside the inner side surface 41B of the tubular side wall portion 41, and the cylindrical side wall portion 41. Outer part 1 that bites in and opens to the inner side surface 41B
With 8b. Therefore, as shown in FIG. 1, the inner side surface 41B can be seen through the cooling water supply hole 18. The outlet of the cooling water supply hole 18 is located closer to the outer side surface 41A of the lower tie plate 4 than the outlet of the cooling water supply hole located on the outermost periphery in the conventional example.

It can also be said that the cooling water supply hole 18 is configured as follows. That is, in FIG. 4, a point P (the inner peripheral surface portion closest to the outer side surface 41A) located on the outer side surface 41A side of the lower tie plate 4 on the inner peripheral surface of the outlet of the cooling water supply hole 18 and the outer side surface. The dimension t ₁ between the lower tie plate 4 and 41A is smaller than the wall thickness (wall thickness of the cylindrical side wall portion 41) t ₂ of the lower tie plate 4. This is because the point P is the inner side surface of the cylindrical side wall portion 41.
It is located on the outer side surface 41A side of 41B. From another point of view, the dimension L ₁ between the point P at the outlet of the cooling water supply hole 18 and the central axis of the lower tie plate 4 is the same as that of the inner side surface 41 B of the lower tie plate 4 and the lower tie plate 4. It can be said that it is larger than the dimension L ₂ with the central axis. The cooling water supply hole 18 has a fuel rod insertion hole (a fuel rod attachment hole 45 and a coupling fuel rod attachment hole (46) into which the lower end portion of the fuel rod 5 positioned at the outermost part of the fuel rod 5 is inserted. It can be said that it is located on the outer side (side of the outer side surface 41A) than the).
The entire cooling water supply hole 18 may be located outside the fuel rod insertion hole.

In order to increase the cooling water leakage suppression function or the sealing effect due to the jet flow, the total flow passage cross-sectional area of the cooling water supply hole 21 in the outer peripheral portion should be maximized and the inner supply holes 22, 23, 24 should be
It is important to reduce the cross-sectional area of the channel.

Therefore, the distribution of the flow passage cross-sectional areas of the cooling water supply holes 18, 21 to 24 will be described. That is, one flow channel cross-sectional area of the cooling water supply hole 18 is a, one flow channel cross-sectional area of the cooling water supply hole 21 is b, and one flow channel cross-sectional area of the cooling water supply hole 22 is one. Let c be the flow passage cross-sectional area, d be the flow passage cross-sectional area of the cooling water supply hole 23 inside thereof, and e be the flow passage cross-sectional area of the central cooling water supply hole 24. The total flow passage cross-sectional area in the cooling water supply hole area is expressed by the following equation.

The total flow passage cross-sectional area A is a value in the cooling water supply hole area A outside the alternate long and short dash line 50A as shown in FIG. The total flow passage cross-sectional area B is a value in the cooling water supply hole region B formed between the alternate long and short dash line 50A and the alternate long and short dash line 50B. Total channel cross section C
Is a value in the cooling water supply hole region C formed between the alternate long and short dash line 50B and the alternate long and short dash line 50C. The total flow passage cross-sectional area D is a value in the cooling water supply hole region D formed between the alternate long and short dash line 50C and the alternate long and short dash line 50D. The total flow path cross-sectional area E is 50D
It is a value in the cooling water supply hole region E formed inside. n _a , n
_b, n _c and n _d is the number of the corresponding cooling water supply hole provided in the cooling water supply hole area. In this embodiment, n _a = 32, n _b
= 28, n _c = 20, and n _b = 18.

It is desirable that the total flow passage cross-sectional area ratio in each of the cooling water supply hole regions be within the range shown in Table 1.

That is, the total flow passage cross-sectional area A of the cooling water supply hole 18 is set as large as possible and is set to 25 to 35%, and the total flow passage cross-sectional area of the cooling water supply hole 21 is set to 40 to 50% at the maximum, and the cooling water in the central portion is set. As the supply holes are narrowed, the total flow passage cross-sectional area of the cooling water supply holes 22 is 15 to 25%, the total flow passage cross-sectional area of the cooling water supply holes 23 is 2 to 10%, and the cooling water supply holes 24 are
The total cross-sectional area of the channel is 0 to 5%. In addition, cooling water supply hole
The reference numeral 24 is used as a mounting hole for a machine tool when processing the lower tie plate 4.

Here, as a specific example, Table 2 shows an example of a fuel assembly in which the fuel rods 5 are arranged in 9 rows and 9 columns. As shown in Table 2, the ratio of the total flow passage cross-sectional area of each region in this specific example satisfies the range of Table 1, but the numerical values such as the diameter and number of these cooling water supply holes are one example. However, the present invention is not limited to these numerical values. The ratio of the above-mentioned total flow passage cross-sectional area is the total flow passage cross-sectional area for each cooling water supply hole region relative to the total flow passage cross-sectional area (= A + B + C + D + E) of all the cooling water supply holes provided in the lower tie plate 4. The ratio is the ratio of the cross-sectional area of the coolant supply holes. Lower tie plate 4
The cross-sectional area ratio with respect to the cooling water supply hole 21 arranged in the cooling water supply hole region B located away from the outer side surface 41A of the cooling water supply hole region B is the cooling water supply hole region C existing inside the cooling water supply hole region B. It is larger than the above-mentioned cross-sectional area ratio to the cooling water supply hole 22 inside. The above-mentioned cross-sectional area ratio with respect to the cooling water supply holes in the cooling water supply hole area (the combined area of the cooling water supply hole areas D and E) existing further inside the cooling water supply hole area C is the smallest. The cross-sectional area ratio in the cooling water supply hole area B is larger than the cross-sectional area ratio for the cooling water supply hole 18 in the cooling water supply hole area A outside the cooling water supply area B and adjacent to the outer side surface 41A. large.

Further, in the present embodiment, the opening area ratio of the first area (for example, the cooling water supply hole area B) on the upper surface of the lower tie plate 4 which is spaced a predetermined distance from the periphery of this upper surface is obtained by the first area. A second area surrounded (for example, cooling water supply hole areas C and D)
It can also be said that it is larger than that of E). The opening area ratio of each region corresponds to the surface area of the corresponding region (the area of the corresponding region on the upper surface of the lower tie plate 4, excluding the portion where the fuel rod and the water rod are inserted). It is the ratio occupied by the total cross-sectional area (value on the upper surface of the lower tie plate 4) of all cooling water supply holes in the region.

The tubular side wall portion 41 of the lower tie plate 4 is shown in FIGS.
As shown in the figure, it has a recess 12 provided on each of its four side surfaces 41A. The recess 12 has a rectangular shape as a whole and is open toward the outside. The upper portion of the recess 12 is an inclined surface inclined at an angle θ as shown in FIG. 8 (A).
It's 13. The upper end of the inclined surface 13 is the lower tie plate 4
At a distance a from the upper end of the. The side surface 41A at this distance a is a plane perpendicular to the upper surface of the grid portion 40. The bottom surface 12A of the inclined surface 13 of the concave portion 12 is continuous with the inclined surface 13 and is a plane located at a depth b from the side surface 41A (side surface 4).
It is parallel to 1A). Due to the arrangement of the recesses 12 as described above, the inner surface of the channel box 8 and the lower tie plate 4 are
Between the outer surface of the channel box and the outer surface of the channel, bench bench means for suppressing outward deformation of the lower end of the channel box 8 due to the depressurizing action is formed. The bench lily means includes a cooling water passage 17 formed between the lower tie plate 4 and the channel box 8.

In this embodiment, as shown in FIG. 2, a finger spring is not provided between the lower tie plate 4 and the channel box 8 as in the conventional example.

In the present embodiment, the flow path means for generating a jet flow provided in the grid portion 40 described above and the bench Yury means formed between the channel box 8 and the lower tie plate 4 are used in combination. Due to the synergistic effect of the function of suppressing the leakage of the cooling water of the jet flow and the function of suppressing the deformation of the channel box due to the depressurizing action of the bench Yury, the leakage of the cooling water is effectively suppressed while suppressing the deformation of the channel box 8. can do. Hereinafter, these points will be described in detail.

During operation of the boiling water reactor, cooling water is supplied into the fuel assembly 1 mounted on the core. This cooling water is supplied to the cooling water supply holes 18 and 21 formed in the grid portion 40 of the lower tie plate 4.
It will be led into the Channel Box 8 through ~ 24. Most of the cooling water introduced into the channel box 8 rises in the cooling water passage 11 and flows upward from the upper tie plate 2. A part of the cooling water passes through the cooling water passage 17 (including the gaps 14 to 16 shown in FIG. 8A) formed between the channel box 8 and the lower tie plate 4 and the fuel assembly 1
Leaks out. However, the leakage amount of this cooling water is
First, it is suppressed by the jet flow ejected from the outer peripheral portion of the lower tie plate 4 toward the inner surface of the channel box 8 by the flow passage means including the cooling water supply hole 18 at the outermost periphery.

The principle of the jet flow effect is to reduce the static pressure at the inlet of the cooling water passage 17 (the upper end of the gap 14) by the jet flow ejected from the outer peripheral portion of the lower tie plate 4 to reduce the driving force of the leakage flow. is there. In the present embodiment, the flow rate of the cooling water increases near the channel box 8 as compared with the conventional example. In this embodiment having such a cooling water flow rate distribution, in the vicinity of the inner surface of the channel box 8, that is, in the vicinity of the inlet of the cooling water passage 17, below the jet flow flowing out from the cooling water supply hole 18 at the outermost periphery. The low pressure region is formed by the action of the jet flow. Therefore, the cooling water in the channel box 8 can be suppressed from leaking to the outside through the cooling water passage 17.

The effect of the jet flow is that the outlet of the cooling water supply hole 18 at the outermost periphery of the grid portion 40 is brought closer to the outer side surface 41A of the lower tie plate 4 (point P is close to the outer side surface 41A), and as shown in Table 1, the center It is further effective to distribute the flow passage cross-sectional areas of the cooling water supply holes 21 to 24 so that the total flow passage cross-sectional area of the outer peripheral portion is larger than the total flow passage cross-sectional area of the portion. As described above, the ratio of the total flow passage cross-sectional area (opening area ratio) in the outer peripheral portion of the grid portion 40 is larger than that in the central portion thereof, so that the cooling water flow rate near the inner surface of the channel box 8 is large. Therefore, the flow rate of the cooling water leaking from the cooling water passage 17 is reduced. In this way, by increasing the cross-sectional area of the supply hole flow passage in the outer peripheral portion of the lower tie plate and reducing the cross-sectional area of the supply hole flow passage in the central portion, the cooling water leakage suppression function or sealing effect due to the jet flow can be increased. However, by further defining the supply hole flow passage cross-sectional area in this way, the flow velocity distribution in the fuel assembly 1 after exiting the lower tie plate 4 is shown in FIG. The effect that it can be made flatter than that is also obtained.

Particularly, in this embodiment, the cooling water supply hole located at the outermost peripheral portion
Reference numeral 18 denotes a fuel rod whose width in the direction parallel to the outer side surface of a portion (outer portion 18b) of the multiple fuel rod insertion holes located outside the outermost fuel rod insertion hole is located at the outermost periphery. The part located in the area between the insertion holes (inner part 18
It is wider than the width of a) in that direction. For this reason,
It is possible to supply the jet flow substantially uniformly over the entire circumference of the inner surface of the channel box 8 except the carna portion. This is effective in reducing the leakage cooling water flow rate.

Next, the bench yury means formed between the channel box 8 and the lower tie plate 4 will be described.

The outer side surface 41A including the concave portion 12 of the lower tie plate 4 and the inner surface of the channel box 8 form the cooling water passage 17 including the gaps 14 to 16 shown in FIG. In the cooling water passage 17, the flow passage cross-sectional area is small in the gap 14, the flow passage area gradually increases toward the gap 15 in the gap 16, and the flow passage area increases in the gap 15. ing. The cooling water passage 11 above the gap 14 has a large flow passage area. For this reason, it can be said that the outer side surface 41A, the inclined surface 13 and the bottom surface 12A and the inner surface of the channel box 8 facing them constitute the bench-free means including the cooling water passage 17 therein.

That is, in the cooling water passage 17, a gap having a small flow passage cross-sectional area is formed.
In the portion 14, the cooling water has a high speed and the static pressure becomes lower than that in the cooling water passage 11. In particular, the static pressure becomes lowest at the lower end of the gap 14 (starting point of the inclined surface 13). Therefore, the pressure P ₁ inside the channel box 8 becomes smaller than the pressure P ₂ outside it in the gap 14, and the pressure difference ΔP (= P ₁ −
P ₂ ) becomes negative pressure. In other words, Channel Box 8
The pressure to push inward is the pressure to push channel box 8 outward (force to expand channel box 8).
Will be larger than. The pressure gradually recovers in the gap 16, and the pressure difference ΔP between the inside and outside of the channel box 8 becomes zero in the gap 15.

Since the pressure difference ΔP becomes a negative pressure in this way, the channel box 8 moves toward the outer side surface 41A of the lower tie plate 4.
Is pressed toward. Therefore, the channel box 8
The outward deformation at the lower end of the is significantly suppressed, and the amount of deformation is significantly reduced. This keeps the widths G ₁ and G _{2 of the} gaps 14 and 15 substantially constant from the beginning to the end of the operating cycle.

The pressure difference ΔP of the negative pressure described above is obtained by utilizing the bench Yury effect shown by the following equation (1).

P _S = P ₀ −ρv ² / 2g (1) where P ₀ is the total pressure of the cooling water flowing in the cooling water passage 17, P _S
Is the static pressure of the cooling water, and ρv ² / 2g is the dynamic pressure of the cooling water.

The effect of the above bench yury means will be further described with reference to FIG.

FIG. 10 shows the distribution of the total pressure P ₀ and the static pressure P _S in the cooling water passage 17 at the position below the upper end of the lower tie plate 4,
The figure shows the amount of deformation of the channel box at a position below the upper end of the lower tie plate 4. In these figures, the broken line shows the characteristic of the present embodiment shown in FIG. 1, and the solid line shows that the outer side surface 41A overlapping the channel box 8 without forming the recess 12 on the outer side surface 41A of the lower tie plate has a straight shape. The characteristic of the conventional fuel assembly is shown. The vertical axis in FIGS. 10 and 11 represents the distance from the upper end of the lower tie plate 4.

In the conventional example, as shown by the solid line in FIG. 10, a force (positive static pressure P _S ) that deforms the channel box 8 outward acts on a portion below the upper end of the lower tie plate 4 of the channel box 8. ing. According to this embodiment, the force of pushing the channel box 8 toward the lower tie plate 4 is greater than the force of pushing the channel box 8 outward, as shown by the broken line in FIG. Also grows. Therefore, in this embodiment, as shown in FIG. 11, the amount of deformation of the channel box 8 toward the outside is smaller than that in the conventional example.

Next, the effects of this embodiment described above will be described with reference to FIGS.

FIG. 12 shows the dependence of the leakage flow rate of the cooling water on the differential pressure ΔP inside and outside the channel box, that is, the differential pressure which is the driving force of the leakage flow rate.

The jet flow from the cooling water supply hole 18 provided on the outer peripheral portion of the lower tie plate grid portion 40 reduces the leakage flow rate. The effect is shown by the dotted line in FIG. When the jet flow is used, the leakage flow rate increases as the differential pressure inside and outside the channel box increases, but there is an effect of suppressing the leakage flow rate by about 20% compared to the conventional example. That is, when viewed when the differential pressure inside and outside the channel box is ΔP ₀ , the leakage flow rate in this example is reduced by ΔQ from the conventional example, which is because the jet flow causes the differential pressure inside and outside the channel box to drop by ΔP _i. , Corresponding to reducing the leakage flow rate by ΔQ.

Further, in the fuel assembly of the present embodiment in which the bench flow means is added to the jet flow effect, the effect of suppressing the deformation amount of the channel box by the bench flow means has the effect of further reducing the leakage flow rate as compared with the case where only the jet effect is used. . This effect has been confirmed and verified by tests.

The greatest effect of this example is the burnup dependency of the leakage flow rate of the cooling water during the operation cycle. That is, the channel box 8 is constantly subjected to a pressure from the inside to the outside during the operation of the reactor due to the static pressure of the cooling water in the cooling water passage 11 or the like.
For this reason, creep deformation occurs toward the outside. In the conventional example,
Due to the creep deformation of the channel box, as shown by the solid line in FIG. 13, the flow rate of the cooling water leaking from between the lower tie plate 4 and the channel box 8 increases as the burnup increases. The leakage flow rate of the cooling water is reduced by the effect of the jet flow by the flow path means 18, as shown by the broken line. Further, in the present embodiment in which the bench unit is also used, as described above, the force for pressing the channel box 8 toward the lower tie plate 4 becomes large, and the increase in creep deformation amount of the channel box 8 can be suppressed. Therefore, the effect of the jet flow is added to the effect of suppressing the creep deformation amount of the channel box, and the increase in the leakage flow rate of the cooling water due to the increase in the burnup is further effectively suppressed,
It can be almost flattened as shown by the alternate long and short dash line in FIG.

The effect of reducing the leakage flow rate of the cooling water by using the jet flow and the bench Yury means together will be further described with reference to FIG.

The right half of FIG. 14 shows the change in the channel box deformation amount with respect to the increase in burnup during the operation cycle in the case of using the bench Yury means in the conventional example and the present example. In the conventional example, the channel box deformation amount of a ₁ occurs, but when the bench Yury means is used, the channel box deformation amount of a ₂ decreases.

The left half of Fig. 14 shows the effect of suppressing the leakage flow rate of the cooling water with respect to the deformation amount of the channel box. In the prior art, if it results in a leakage flow rate c ₁ of the cooling water at the point b ₁ on the solid line corresponds to channel dead issue hex deformation amount a _1, using the effect of Jietsuto flow in the same channel pop box deformation amount a ₁ is ,
The cooling water leakage flow rate is c ₁ corresponding to point b ₁ on the broken line,
(C ₁ -c ₁ ') have the effect of cooling water leakage flow reduction of. Further, in the present embodiment, when only the bench Yury means is used, the points on the solid line correspond to the channel box deformation amount a _2.
cause b ₂ 'coolant leak rate c ₂ in', with respect to prior art there is a leak rate reducing effect of (c ₁ -c ₂ '), but in combination the effect of Jietsuto flow in the same channel pop box deformation amount a ₂ In this case, the cooling water leakage flow rate is c ₂ corresponding to point b ₂ on the broken line.
Therefore, the conventional example has a remarkable leakage flow reduction effect of (c ₁ −c ₂ ), and even when only the effect of the jet flow is used and when only the bench Yury effect is used, (c _{1 '-c 2), (c} 2' it is leak rate reduction effect -c _2).

As described above, according to the present embodiment, the effect of the jet flow and the effect of reducing the leakage flow rate of the cooling water are remarkable as compared with the conventional example which does not use the bench yury means. Further, the effect of the jet flow and the bench yury means are provided. The leakage flow rate of the cooling water is also reduced compared to the case where each of them is used alone.

By the way, in the present embodiment, since the effect of the jet flow and the bench Yury means are used together, the pressure at the inlet of the cooling water passage 17 of the bench Yury means is reduced by the effect of the jet flow, compared to the case where only the bench Yury means is used. The flow of the cooling water in the cooling water passage 17 has different static pressure distributions. First
FIG. 15 shows this static pressure distribution, and similarly to FIG. 11, the case where the effect of the jet flow is also used is shown by a broken line. Further, the case where only the bench Yury means is used is shown by a solid line.

When the jet flow is also used, the pressure at the tie plate upper end position, that is, the inlet of the cooling water passage 17 decreases by ΔP _j . Therefore, the flow velocity of the leaked cooling water is reduced, and the static pressure at the starting point S of the bench Yury means inclined surface 13 is increased. However, the pressure that produces the amount of deformation of the channel box 8 is the total of the static pressure distribution from the upper end of the tie plate of the static pressure distribution to the lower end of the channel box, and the area difference of the static pressure distribution with reference to pressure 0 in FIG. Is. As can be seen from the figure, the area difference of the static pressure distribution has the effect A _{1 of} decreasing the positive static pressure area due to the decrease of the pressure of ΔP _{j and} the effect A _{1 of} decreasing the negative static pressure area due to the decrease of the flow velocity. ₂ becomes almost equal, and the effect is almost the same as when the bench Yury means is used alone. In other words, the static pressure effect ΔP _j at the upper end position of the tie plate 4 caused by the effect of the jet flow and the increase ΔP _{v in the} flow velocity due to this ΔP _j become almost equal, and as a result, The effect of the bench Yury means is not affected by the combined use of the effect of the jet flow.

As described above, in the present embodiment, the leakage flow rate of the cooling water can be kept substantially constant throughout the operation cycle, so that the heat output of the fuel assembly 1 can be further stabilized.

Further, the effects described above in the present embodiment, the cooling water supply hole 18 is provided in the outer peripheral portion of the grid portion 40 of the lower tie plate,
The effect of the jet style is obtained, and the channel box 8
Water passage 17 formed between the lower tie plate 4 and the lower tie plate 4
It can be obtained by a simple structure in which the flow path area is changed and the bench Yury effect is exerted.

In the present embodiment, it is desirable to consider the following in order to more effectively bring out the function of the bench yury means.

In FIG. 1 and FIG. 6, one outer side surface 41A of the lower tie plate 4 is smoothly radiused to the adjacent outer side surface 41A by a radius R.
To improve the insertability of the channel box 8 into the lower tie plate 4 during assembly. Here, the width of each outer side surface 41A of the lower tie plate 4 is L, when the width of the flat portion of the side surface 41A and the L _f, is L _f = L-2R. And, letting the width of the recess 12 for providing the bench lily means be 1, this width 1 is preferably in the range of 0.5 to 1.0 with respect to the width L _f of the flat portion of the lower tie plate 4. That is, it is preferable that l / L _f = 0.5 to 1.0 (2).

As can be seen from the above formula (1), the effect of the bench Yury means utilizes the leakage flow velocity of the cooling water, that is, the dynamic pressure. At this time, the pressure difference ΔP of the negative pressure suppresses the outward deformation amount of the channel box 8, and the effect that the pressure difference ΔP acts on the four corners of the channel box 8 rather than acting near the corners of the four sides of the channel box 8. It has a greater effect on.
In other words, the effect of suppressing the outward deformation of the channel box 8 is small even if bench bench means are provided near the corners on the four sides of the channel box 8.

In addition, the pressure loss coefficient of the cooling water leakage flow path in the portion provided with the bench oily means is about half that in the case where the bench oily means is not provided, and the cooling water easily leaks. Therefore, when the bench unit is provided near the corners on the four sides of the channel box 8, the leakage flow rate of the cooling water is only increased unnecessarily.

FIG. 16 shows the maximum deflection amount (equivalent value) of a beam having a length L when a uniformly distributed load is applied. The amount of deflection increases as the length l to which the uniformly distributed load is applied increases, but when the ratio of the length l of the evenly distributed load to the beam length L becomes 0.5 or more, the increase in the amount of deflection decreases.

From the above viewpoint, the width 1 of the concave portion 12 which constitutes the bench Yuri means is preferably 0.5 or more and 1.0 or less with respect to the width L _f of the flat portion of the outer side surface 41A as in the above-described formula (2). .

This will be further described with reference to FIG. FIG. 17 shows the relationship between the width (l / L _f ) of the recess 12 provided on the side surface 41A of the tie plate and the leakage flow rate (relative value) between the lower tie plate 4 and the channel box 8 and the rigidity of the channel box 8. Is shown as a parameter. The rigidity of the channel box 8 was changed within a range of about 1.5 mm to 4 mm including the zirconium alloy plate (thickness 2 mm to 3 mm) which is the current material.

As can be seen from this figure, when the wall thickness of the channel box 8 is large (high rigidity), the deformation amount when a constant load is applied to the channel box is smaller than when the wall thickness is thin (low rigidity). Even if the width of the recess 12 is widened, the amount of relative suppression of the leakage flow becomes small. Then, in the figure, the diagonal line is the range in which the leakage of the cooling water is minimum, but the width of the recess 12 in this range approaches 1.0 as the rigidity of the channel box becomes smaller. Therefore, also from this fact, it is understood that the effect of the bench Yuly means can be effectively exhibited if the width 1 of the recess 12 is in the range of 5.0 to 1.0 with respect to L _f .

Next, the distance a from the upper end of the lower tie plate 4 to the start point of the inclined surface 13 and the distance L ₀ to the lower end of the channel box 8 shown in FIG. 8 are the new fuel assembly 1 before core loading.
It is desirable that a / L ₀ satisfy the following conditions.

0.3 ≦ a / L ₀ <α (3) where α is the slope of the inclined surface 13 even if the lower end of the channel box 8 moves upward as shown in FIG. 8 (B) due to an increase in burnup. This limit value is set so that the lower end does not go below the lower end of the channel box 8. This limit value depends on the setting condition of the burnup of the fuel assembly 1.
When the lower end of the inclined surface 13 is located below the channel box 8, the bench-yurie effect eliminates the bench-yurie effect. In this case, the leakage flow rate of the cooling water rapidly increases.

On the other hand, if the distance a is increased, the wear pressure loss between the channel box 8 and the lower tie plate 4 increases, and the increase in the leakage flow rate of the cooling water is suppressed. However, since the starting point (upper end) of the inclined surface 13 approaches the lower end of the channel box 8, the negative pressure and the zero pressure region of the pressure difference ΔP that can suppress the outward deformation of the channel box 8 are at the lower end of the channel box 8. As the distance becomes closer, the creep deformation of the channel box 8 becomes larger than that when the distance a is small.

FIG. 18 shows the dependence of the leakage flow rate of the cooling water on the distance a when the creep deformation of the channel box 8 is considered. From the characteristics shown in Fig. 18, the leakage flow rate of cooling water is significantly reduced in the region of a / L ₀ ≥ 0.3. Therefore, a / L ₀
It is desirable to satisfy ≧ 0.3. In particular, since the leakage flow rate of the cooling water is minimized at a / L ₀ ≧ 0.6, better to satisfy the a / L ₀ ≧ 0.6.

〔The invention's effect〕

According to the first feature of the present invention, the leakage of the coolant from between the lower tie plate and the channel box can be suppressed without using the finger spring.

According to the second feature of the present invention, the creep deformation spreading to the outside of the channel box can be reduced.

[Brief description of drawings]

1 is a plan view of a lower tie plate of a fuel assembly which is a preferred embodiment of the present invention, and FIG. 2 is a fuel assembly which is a preferred embodiment of the present invention having the lower tie plate of FIG. Fig. 3 is a longitudinal sectional view of the body, Fig. 3 is an enlarged perspective view near the lower tie plate of Fig. 2, Fig. 4 is a sectional view taken along line IV-IV of Fig. 1, and Fig. 5 is each of the lower tie plate of Fig. 1. Explanatory drawing showing the distribution of cooling water supply holes in the cooling water supply hole area, FIG. 6 is a side view of the lower tie plate, FIG. 7 is a sectional view taken along line VII-VII of FIG. 6,
FIG. 8 (A) is an enlarged vertical sectional view of the vicinity of the lower end of the channel box of the fuel assembly before core loading, and FIG. 8 (B).
Is an enlarged vertical sectional view near the lower end of the channel box of the fuel assembly after core loading, FIG. 9 is an explanatory view showing the flow velocity distribution of the cooling water in the channel box at the position of the upper surface of the lower tie plate, and FIG. 10 is the lower part. Fig. 11 is an explanatory diagram showing the pressure distribution at each level from the upper end of the tie plate, Fig. 11 is an explanatory diagram showing the distribution of the deformation amount of the channel box at each level from the upper end of the lower tie plate, and Fig. 12 is the internal and external differential pressure of the channel box. Fig. 13 is a characteristic diagram showing the relationship between the cooling water leakage flow rate, Fig. 13 is a characteristic diagram showing the change of the cooling water leakage flow rate with respect to burnup during the operation cycle of the reactor, and Fig. 14 is shown in Fig. 1 and Fig. 2. FIG. 15 is a characteristic diagram showing the effect of the embodiment of the present invention, and FIG. 15 shows the static pressure distribution in the cooling water passage of the bench yuly means of the embodiment of the present invention in which the function of the jet flow and the function of the bench yuly are used together. Characteristic diagram, Fig. 16 is a characteristic diagram showing the relationship between the range to which a load is applied and the maximum deflection of the channel box, and Fig. 17 is a characteristic diagram showing the change of the cooling water leakage flow rate with respect to the width of the concave portion that constitutes the bench Yuly means. , First
FIG. 18 is a characteristic diagram showing the relationship between the cooling water leakage flow rate and a / L ₀ , which is a value related to the relative positional relationship between the lower end of the channel box and the upper end of the lower tie plate. 1 ... Fuel assembly, 4 ... Lower tie plate, 5 ... Fuel rod, 7 ... Fuel spacer, 8 ... Channel box, 12 ... Recess, 13 ... Inclined surface, 17 ... Cooling water passage, 1
8,21,22,23,24 …… cooling water supply hole, 40 …… grid part, 4
1 …… Cylinder side wall part, 41A …… Outside side surface, 45 …… Fuel rod mounting hole, 46 …… Coupling fuel rod mounting hole.

Front page continued (72) Inventor Akira Maru 3-1-1, Saiwaicho, Hitachi City, Ibaraki Hitachi Ltd. Hitachi factory (72) Inventor Yoshinari Kawada 3-1-1, Saiwaicho, Hitachi City, Ibaraki Prefecture Hitachi Ltd., Hitachi Works (72) Inventor Yoshihiko Yanagi 3-1-1, Saiwaicho, Hitachi City, Ibaraki Hitachi Ltd. (72) Inventor, Hiromasa Hirakawa 3 Saiwaicho, Hitachi City, Ibaraki Prefecture 1-1-1, Hitachi Ltd., Hitachi Factory (72) Inventor Junjiro Nakajima 3-1-1, Saiwaicho, Hitachi City, Ibaraki Hitachi Ltd., Hitachi, Ltd. (72) Inventor Hajime Umehara Hitachi, Ibaraki Prefecture 3-1, 1-1 Kochi-cho, Ichi, Hitachi, Ltd. Hitachi factory (56) References JP-A-53-122089 (JP, A) JP-A 1-212391 (JP, A) JP-A-2-268290 (JP, A)

Claims (18)

[Claims] 1. A plurality of fuel rods, and a lower tie plate having a plurality of coolant supply holes for supporting a lower end portion of the fuel rods and supplying a coolant upward, wherein the lower tie plate is the lower portion. On the upper surface of the tie plate, a plurality of first coolant supply holes distributed in a first area at a predetermined distance from the periphery of the upper surface, and in a second area surrounded by the first area. A plurality of second coolant supply holes, and the total cross-sectional area of all the first coolant supply holes in the first region is all the second coolant supply holes in the second region. Is larger than the total cross-sectional area of the fuel assembly. 2. A plurality of fuel rods, and a lower tie plate having a plurality of coolant supply holes for supporting a lower end portion of the fuel rods and supplying a coolant upward, wherein the lower tie plate is the lower portion. On the upper surface of the tie plate, a plurality of first coolant supply holes distributed in a first area at a predetermined distance from the periphery of the upper surface, and in a second area surrounded by the first area. A plurality of second coolant supply holes, and the opening area ratio of the first region determined by all the first coolant supply holes in the first region is all in the second region. The fuel assembly is larger than the opening area ratio of the second region determined by the second coolant supply hole. 3. A plurality of fuel rods, and a lower tie plate having a plurality of coolant supply holes for supporting a lower end portion of the fuel rods and supplying a coolant upward, wherein the lower tie plate is the lower portion. On the upper surface of the tie plate, a plurality of first coolant supply holes distributed in a first area at a predetermined distance from the periphery of the upper surface, and in a second area surrounded by the first area. And a plurality of second coolant supply holes each having a smaller cross-sectional area than the first coolant supply holes. 4. The fuel assembly according to claim 1, 2 or 3, wherein the first and second coolant supply holes have a circular cross section. 5. The lower tie plate is at least outside a fuel rod insertion hole provided in the lower tie plate and having a lower end portion of the fuel rod located at the outermost periphery inserted therein. 4. The fuel assembly according to claim 1, 2 or 3, wherein the fuel assembly has a plurality of coolant supply holes, a part of which is arranged, and these coolant supply holes are located at an outermost peripheral portion of the lower tie plate. 6. The lower tie plate has a plurality of coolant supply holes located at an outermost peripheral portion of the lower tie plate, and the coolant supply holes are provided in the lower tie plate. The width of the portion of the fuel rod insertion hole located outside the fuel rod insertion hole into which the lower end portion of the fuel rod located at the outermost periphery is inserted, in the direction parallel to the outer side surface of the lower tie plate, 4. The fuel assembly according to claim 1, 2 or 3, wherein a width of a portion located in a region between the fuel rod insertion holes is wider than the width in the direction. 7. A plurality of fuel rods and a lower tie plate having a plurality of coolant supply holes for supporting a lower end portion of the fuel rods and supplying a coolant upward, wherein the lower tie plate is the lower portion. On the upper surface of the tie plate, a plurality of first coolant supply holes distributed in a first area at a predetermined distance from the periphery of the upper surface, and in a second area surrounded by the first area. A plurality of second coolant supply holes, and the total cross-sectional area of all the first coolant supply holes in the first region is all the second coolant supply holes in the second region. Is larger than the total cross-sectional area of the lower tie plate, and the lower tie plate is loaded with the fuel assembly in the core with the channel box attached, and the coolant leaks from between the channel box and the lower tie plate. And the channel nervo Fuel assembly, characterized in that it comprises means for generating a force to draw the box on the lower tie plate side. 8. A plurality of fuel rods and a lower tie plate having a plurality of coolant supply holes for supporting a lower end portion of the fuel rods and supplying a coolant upward, wherein the lower tie plate is the lower portion. On the upper surface of the tie plate, a plurality of first coolant supply holes distributed in a first area at a predetermined distance from the periphery of the upper surface, and in a second area surrounded by the first area. And a plurality of second coolant supply holes each having a cross-sectional area smaller than that of the first coolant supply holes, and the lower tie plate is configured such that the fuel assembly is loaded into the core with the channel box attached. And a means for generating a force for drawing the channel box toward the lower tie plate when a coolant leaks between the lower tie plate and the lower tie plate. Coalescence. 9. A plurality of fuel rods, and a lower tie plate having a plurality of coolant supply holes for supporting a lower end portion of the fuel rods and supplying a coolant upward, wherein the lower tie plate is the lower portion. On the upper surface of the tie plate, a plurality of first coolant supply holes distributed in a first area at a predetermined distance from the periphery of the upper surface, and in a second area surrounded by the first area. A plurality of second coolant supply holes, and the total cross-sectional area of all the first coolant supply holes in the first region is all the second coolant supply holes in the second region. And a flat portion perpendicular to the upper surface that occupies the upper end portion of the tubular side wall portion on the outer surface of the tubular side wall portion of the lower tie plate. Connecting to the flat side, the part that is inclined to the vertical Beauty fuel assembly, characterized in that a and recesses having parallel vertical portions to lead the flat portion to the inclined portion. 10. A plurality of fuel rods, and a lower tie plate having a plurality of coolant supply holes for supporting a lower end portion of the fuel rods and supplying a coolant upward, wherein the lower tie plate is the lower portion. On the upper surface of the tie plate, a plurality of first coolant supply holes distributed in a first area at a predetermined distance from the periphery of the upper surface, and in a second area surrounded by the first area. And a plurality of second coolant supply holes each having a cross-sectional area smaller than that of the first coolant supply holes, and the lower tie plate has an upper end of the cylindrical side wall portion on an outer surface of the cylindrical side wall portion. A flat portion that occupies a portion and is perpendicular to the upper surface, a portion of the lower portion of the flat portion that is connected to the flat portion, is inclined with respect to the vertical, and is connected to the inclined portion and is parallel to the flat portion. And a recess having a vertical portion is provided. Fuel assemblies that. 11. A plurality of fuel rods, a lower tie plate having a plurality of coolant supply holes for supporting a lower end portion of the fuel rods and supplying a coolant upward, and a lower end portion surrounding a bundle of the fuel rods. When a coolant leaks from a channel box that surrounds the lower tie plate and a leakage flow passage formed between the lower tie plate and the channel box, it acts on a part of the channel box and the lower tie plate side. A plurality of first coolant supplies distributed in a first region of the upper surface of the lower tie plate at a predetermined distance from the periphery of the upper surface of the lower tie plate. All the first coolants in the first region, the holes having a plurality of second coolant supply holes distributed in the second region surrounded by the first region. The fuel assembly of the total cross-sectional area of the Kyuana to being larger than the total cross-sectional areas of all of the second coolant supply hole of the second region. 12. A plurality of fuel rods, a lower tie plate having a plurality of coolant supply holes for supporting a lower end portion of the fuel rods and supplying a coolant upward, and a lower end portion surrounding a bundle of the fuel rods. When a coolant leaks from a channel box that surrounds the lower tie plate and a leakage flow passage formed between the lower tie plate and the channel box, it acts on a part of the channel box and the lower tie plate side. A plurality of first coolant supplies distributed in a first region of the upper surface of the lower tie plate at a predetermined distance from the periphery of the upper surface of the lower tie plate. All the first coolants in the first region, the holes having a plurality of second coolant supply holes distributed in the second region surrounded by the first region. The opening area ratio of the first region defined by the supply holes is larger than the opening area ratio of the second regions defined by all the second coolant supply holes in the second region. Fuel assembly. 13. A plurality of fuel rods, a lower tie plate having a plurality of coolant supply holes for supporting a lower end portion of the fuel rods and supplying a coolant upward, and a lower end portion surrounding a bundle of the fuel rods. When a coolant leaks from a channel box that surrounds the lower tie plate and a leakage flow passage formed between the lower tie plate and the channel box, it acts on a part of the channel box and the lower tie plate side. A plurality of first coolant supplies distributed in a first region of the upper surface of the lower tie plate at a predetermined distance from the periphery of the upper surface of the lower tie plate. A hole and a plurality of second coolant supply holes distributed in a second region surrounded by the first region and having a smaller cross-sectional area than the first coolant supply holes. Fuel assembly, characterized in that. 14. The fuel assembly according to claim 11, 12 or 13, wherein the force generating means is a bench Yury portion of the leakage flow passage. 15. A plurality of fuel rods, a lower tie plate having a plurality of coolant supply holes for supporting a lower end portion of the fuel rods and supplying a coolant upward, and a lower end portion surrounding a bundle of the fuel rods. A channel box surrounding the lower tie plate, and a leakage flow passage formed between the lower tie plate and the channel box, the inner surface of the channel and an outer surface of the lower tie plate facing the inner surface of the channel. And a plurality of first flow passages, the leakage flow passage including a bench-yurie portion defined by the lower tie plate, the lower tie plate being distributed in a first region at a predetermined distance from a periphery of the upper surface of the lower tie plate. 1 coolant supply holes and a plurality of second coolant supply holes distributed in a second region surrounded by the first region, all in the first region Fuel assembly, characterized in that a total cross-sectional area of the first coolant supply hole is larger than the total sectional area of all of the second coolant supply hole of the second region. 16. A plurality of fuel rods, a lower tie plate having a plurality of coolant supply holes for supporting a lower end portion of the fuel rods and supplying a coolant upward, and a lower end portion surrounding a bundle of the fuel rods. A channel box surrounding the lower tie plate, and a leakage flow passage formed between the lower tie plate and the channel box, the inner surface of the channel and an outer surface of the lower tie plate facing the inner surface of the channel. And a plurality of first flow passages, the leakage flow passage including a bench-yurie portion defined by the lower tie plate, the lower tie plate being distributed in a first region at a predetermined distance from a periphery of the upper surface of the lower tie plate. No. 1 cooling material supply hole and a plurality of second cooling areas distributed in a second area surrounded by the first area and having a smaller cross-sectional area than the first cooling material supply hole. Fuel assembly, characterized in that it comprises a supply hole. 17. The inner surface of the channel box is vertical, and the outer surface of the lower tie plate has a flat portion perpendicular to the upper surface and a recess below the flat portion and inclined to the vertical. The fuel assembly according to Item 15 or 16. 18. The width of the recess in the lateral direction of the outer surface of the lower tie plate is between the flat portion of one outer surface of the lower tie plate and the flat portion of the other outer surface opposite thereto. 0.5 to 1.0 times the fuel assembly of claim 9, 10 or 17.