JP2626867B2 - Fuel assembly for pressurized water reactor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement of a presser spring provided in an upper nozzle of a pressurized water reactor, for example, and particularly to secure the integrity of a fuel assembly accompanying a high burnup of fuel. And a fuel assembly for a pressurized water reactor.

[0002]

2. Description of the Related Art From the standpoint of improving economy and reducing the amount of radioactive waste generated, the burnup of nuclear fuel assemblies has been promoted.

[0003] When the fuel is used up to a high burnup, one of the items to be considered is an increase in the elongation of the fuel assembly. The fuel rod cladding tube and the control rod guide tube are extended in the axial direction by neutron irradiation during combustion of the fuel, so that the overall length of the fuel assembly is increased. The amount of increase in the total length, that is, the amount of elongation depends on the burnup of the fuel. As the burnup increases, the amount of elongation also increases.

[0004] As a result, the distance between the upper part of the assembly and the upper core plate is reduced by the amount of extension of the entire length of the assembly. On the other hand, a presser spring is attached to the upper part of the assembly in order to prevent the fuel assembly from being lifted by the fluid during operation of the reactor.

FIG . 4 is an explanatory view showing the structure of a fuel assembly for a pressurized water reactor. 5 is an explanatory view showing the configuration of the upper nozzle of FIG. 4 , wherein FIG. 5A is a front view of the upper nozzle, and FIG. 5B is an enlarged view of a main part. 6A and 6B are explanatory diagrams showing the configuration of another upper nozzle. FIG. 6A is a front view of the upper nozzle, and FIG. 6B is an enlarged view of a main part. 7A and 7B are explanatory views showing a leaf spring and its strain distribution, FIG. 7A is a front view showing the configuration of the leaf spring, and FIG. 7B is a diagram showing a strain amount corresponding to the position in FIG.

As shown in FIG . 4 , a fuel assembly (0) for a pressurized water reactor has several support grids (2) in which a plurality of fuel rods (1) are bundled in a bundle, and upper and lower fixing grids for fixing them. Nozzle
It consists of (3) and (4). The upper nozzle (3) contacting the upper core plate (5) and the lower nozzle (4) contacting the lower core plate (6) are connected by a plurality of control rod guide tubes (7).

In order to prevent the fuel assembly (0) from floating, the upper nozzle (3) is provided with presser springs (8) attached to four sides of the fuel assembly as shown in FIG. I have. The holding spring (8) is composed of a plurality of leaf springs (9).

This leaf spring (9) is an upper nozzle
It is made by bending a thick plate material as shown in the figure for the purpose of facilitating attachment to (3), and is attached to the upper nozzle (3) with bolts (10).

A clamp (11) is attached to the leaf spring (9) for the purpose of covering the joined portion and protecting the mounting bolt (10), so that the leaf spring (9) is attached. In some cases, the bolt (10) does not attach the clamp (11) as shown in FIG .

The individual leaf springs (9) are gradually thinned to the tip so that the surface stress caused by the load based on the displacement of the spring is averaged as much as possible. S _{1 to} S ₄ and thickness t ₁ at the tip
~t ₄ are related as follows. t ₁ = t ₂ = t ₃ = t ₄ <S ₁ = S ₂ = S ₃ = S ₄

As described above, the shape of the leaf spring (9) should be a shape suitable for holding the components of the fuel assembly in a small space and further holding the elements for ensuring safety. I understand.

This is because, as shown in FIG. 7A, the uppermost leaf spring (9a) is provided with a bent portion (12),
The hanging part (13) extending downward from (12) has a narrow width, and is passed through a through-hole (14) provided at the tip of the lower leaf springs (9b) to (9d). Leaf spring (9
b) to (9d) are devised so as not to fall off even if broken.

Now, when the amount of extension of the fuel assembly (0) is increased and the uppermost leaf spring (9a) is displaced downward, the normal width portion of the bent portion becomes a through hole of the lower leaf spring (9b). (14), the lower leaf spring (9b) is also displaced downward, and when the amount of extension of the fuel assembly further increases, the lower leaf spring (9c) and the lower leaf spring (9d) are moved further. , In order.

By the way, as the amount of elongation of the assembly increases, the amount of downward displacement of the pressing spring (8) constituted by the leaf springs (9) also increases. Furthermore, when the reactor is shut down, the core size becomes smaller due to the reactor temperature drop, and the distance between the upper part of the fuel assembly (0) and the upper core plate (5) is further reduced.As a result, the displacement of the presser spring (8) The amount is even larger.

As described above, as the burnup of fuel increases,
The allowable displacement required for the holding spring at the top of the fuel assembly is further increased. In addition, since the spring force required as the holding force is very large, a holding spring having a large spring constant is required, while the displacement amount is very large, and especially when the reactor is stopped at the end of combustion, the holding spring is generated. The distortion amount becomes very large. If this amount of distortion exceeds a certain value, it will be damaged, making it impossible to use it in the next cycle. Therefore, in order to guarantee the mechanical integrity of the fuel assembly at the time of high burnup, it is important to reduce the strain level of the presser spring when the reactor is stopped.

[0016]

However, in the bent portion near the mounting portion of the leaf spring, the surface stress due to the bent shape increases, and the stress distortion level is higher than other portions. The uppermost, middle and lowermost leaf springs (9a) to (9d) constituting the upper holding spring (8)
When the maximum stress (strain) for the same displacement amount is compared, a difference as shown in FIG. 7 is recognized due to a difference in bending angle and the like.

[0017] That is, in the structure of the conventional pressing spring, the difference as shown in FIG. 7 to each of the leaf spring is permitted, the soundness of the fuel assemblies associated with high burnup of the fuel
However, there was a problem that the safety margin was not sufficient .

According to the present invention, it is possible to reduce the distortion level of the holding spring when the reactor is stopped without reducing the spring force of the holding spring during operation. It is another object of the present invention to provide a fuel assembly for a pressurized water reactor which can relatively easily ensure the soundness of the fuel assembly accompanying the high burn-up of the fuel.

[0019]

In the fuel assembly for a pressurized water reactor according to the first aspect of the present invention, one end is fixed to the upper nozzle and the other end extending obliquely upward contacts the upper core plate. In a fuel assembly for a pressurized water reactor, comprising a plurality of leaf springs, which constitute a presser spring and are gradually thinned in the direction of the other end, each leaf is formed according to a strain amount distribution of each leaf spring. The thickness distribution of the spring is different.

Further, in the fuel assembly for a pressurized water reactor according to the second aspect of the present invention, in the fuel assembly for a pressurized water reactor according to the first aspect, the uppermost leaf spring of the individual leaf springs may be used. the thickness t ₁ of the base portion thickness S ₁ and the tip portion, and the thickness t _n of the thickness S _n and the distal end portion of the root portion of the lowermost leaf spring, the root portion of the intermediate position of the leaf spring thickness is is obtained by S ₂ ... and the tip portion thickness t ₂ ... and the following relationship. (T ₁ <S ₁ , t ₂ <S ₂ ,..., T _n <S _n , and t
₁ <t _n <... <t ₂ , S ₁ ≤S _n ≤ ... ≤S ₂ )

[0021]

According to the present invention, one end is fixed to the upper nozzle, and the other end extending obliquely upward constitutes a presser spring contacting the upper core plate, and is gradually thinned toward the other end. In a fuel assembly for a pressurized water reactor equipped with a plurality of leaf springs, the thickness distribution of each leaf spring is made different according to the strain distribution of each leaf spring. By changing the thickness distribution of each leaf spring without changing the configuration of the spring, the maximum generated stress of the leaf spring can be averaged.

Thus, the strain level of the presser spring when the reactor is stopped can be reduced without reducing the spring force of the presser spring during operation, and based on the current fuel design and manufacturing technology. The soundness of the fuel assembly accompanying the high burnup of the fuel can be relatively easily secured.

[0023] More specifically, the thickness t ₁ of the thickness S ₁ and the distal end portion of the root portion of the uppermost leaf spring of each leaf spring, and the thickness S _n of the root portion of the lowermost leaf spring the thickness t _n of the tip of the root portion of the intermediate position of the leaf spring thickness S ₂ ... and the tip portion thickness t ₂ ... and, t ₁
<S ₁ , t ₂ <S ₂ ,..., T _n <S _n , and t ₁ <t
As _{n <... <t 2, S} 1 ≦ S n ≦ ... ≦ S 2, is obtained by averaging the maximum stress generated individual leaf spring.

For example, when there are three leaf springs,
t ₁ <t ₃ <t ₂ , S ₁ ≦ S ₃ ≦ S _2, and when there are four leaf springs, t ₁ <t ₄ <t ₃ <t ₂ , S ₁ ≦ S
_By setting ₄ ≦ S ₃ ≦ S ₂ , the maximum generated stress of each leaf spring is averaged, and the maximum amount of distortion of the entire upper holding spring can be reduced. This takes advantage of the fact that a thinner plate having a large strain or stress reduces the surface maximum stress strain.

The reason why the maximum stress distortion of the uppermost leaf spring of the conventional holding spring is larger than that of the other leaf springs is mainly because the rising angle of the uppermost leaf spring becomes maximum. . The reason why the maximum stress strain increases successively from the lowest leaf spring to the higher order is mainly that the shorter the distance from the fulcrum of deformation to the tip of the spring, the greater the stress strain (see FIG. 7) . ).

For this purpose, when the thickness distribution of each leaf spring is made different and the stress-strain distribution is averaged,
t ₁ <S ₁ , t ₂ <S ₂ ,..., t _n <S _n , and t ₁
When <t _n <... <T ₂ , S ₁ ≦ S _n ≦... ≦ S ₂ , the maximum generated stress of each leaf spring is averaged, and the maximum amount of distortion of the upper holding spring can be reduced. be able to.

[0027]

FIG. 1 is an explanatory view showing the structure of an upper holding spring of one embodiment of a fuel assembly for a pressurized water reactor according to the present invention.
2 is an explanatory diagram showing the leaf spring of FIG. 1 and its strain distribution, FIG. 2A is a front view showing the configuration of the leaf spring, and FIG. 2B is a diagram showing the amount of strain corresponding to the position of FIG. FIG. 3 shows another leaf spring of FIG. 1 and its strain distribution.
FIG. A is a diagram illustrating a configuration of a leaf spring.
FIG. B shows the distortion amount corresponding to the position in FIG. A.
FIG.

As shown in FIG. 1, of the leaf springs constituting the presser spring (20), the uppermost leaf spring (21)
The thickness of the base plate is S ₁ , the thickness of the tip portion is t _1, and the base plate thicknesses of the lower two intermediate leaf springs (22) and (23) are S ₂ and S ₃ , respectively. T ₂ and t
₃ , the base plate thickness of the lowermost leaf spring (24) is S ₄ , and the plate thickness of the tip portion is t ₄ .

As shown in FIG. 2B, the thickness t ₁ -t ₄ at the tip portion is made different from the thickness S ₁ -S ₄ at the base of each leaf spring (21)-(24). When the stress strain of the leaf spring is averaged, it has been found that the upper holding spring (20) of the present invention has the following relationship. That is, t ₁ <S ₁ , t ₂ <S ₂ , t ₃ <S ₃ , t ₄ <S
₄ and t ₁ <t ₄ <t ₃ <t ₂ , S ₁ ≦ S ₄ ≦ S ₃
A ≦ S _2.

This is based on the fact that, when a plate having a large strain stress is generally made thinner, the maximum surface stress strain becomes smaller.

More specifically, for the conventional upper holding spring of t ₁ = t ₂ = t ₃ = t ₄ <S ₁ = S ₂ = S ₃ = S ₄ , the assumed maximum displacement is In many cases, the surface strain distribution at the time of application is as shown in FIG . In this case, there is a large variation in the maximum strain of each spring, and the maximum strain of the upper spring is significantly larger than the others.

Here, for example, t ₁ <t ₄ <t ₃ <t ₂ By changing the thickness of each leaf spring in the relationship of S ₁ = S ₂ = S ₃ = S ₄ , the maximum strain of each spring can be averaged, and the maximum strain of the upper spring can be greatly reduced. Can be reduced. in this case,
The surface strain distribution is as shown in FIG. Further, S ₁ <S
_{When the} relationship ₄ <S ₃ <S ₂ is introduced, t _{1 to} t
_A similar effect can be obtained by setting ₄ to an appropriate value.

Further, if FIG. 2, _{t 1 <t 4 <t 3} <t 2
One _{S 1 = S 2 = S 3} = individual in S ₄ consisting relation Rifusupu
Although the thickness of the ring was changed, as shown in FIG.
In this case, the distortion amount of the top leaf spring (21)
It is the smallest compared to the pulling (22)-(24). Therefore, distortion
Considering the amount distribution from averaging, the highest leaf split
Ring (21) the thickness t ₁ of the tip section lowest Leafs pudding
Grayed also be thicker than the thickness t ₄ of the tip portion (24)
It is.

FIG . 3 shows t ₁ <S ₁ , t ₂ <S ₂ , t ₃ <S
₃ , t ₄ <S ₄ , and t ₄ <t ₁ = t ₃ <t ₂ , S ₁
= S ₄ = S ₃ = S ₂ and the leaf spring and its distortion
It is an explanatory view showing a distribution, and FIG.
FIG. B shows the distortion amount corresponding to the position in FIG.
FIG. As shown in FIG. 3, the top leafsp
Thickness t ₁ of the tip portion of the ring (21) is the lowest Rifusupu
If thicker than the thickness t ₄ of the tip portion of the ring (24),
The strain distribution is more averaged than under the condition shown in FIG.
Was confirmed.

As described above, in comparison with the conventional upper holding spring composed of a plurality of leaf springs having the same base plate thickness and tapered portion plate thickness, the upper plate thickness of the present invention is optimized for each leaf spring. The pressing spring can average the maximum amount of distortion generated in each spring, and as a result, the maximum amount of distortion of the entire upper pressing spring can be significantly reduced.

Although FIG. 1 shows a four-sheet pressing spring, the present invention is also applicable to a three-sheet pressing spring composed of three leaf springs, a five-sheet pressing spring, and the like.

[0037]

According to the present invention, as described above, one end is fixed to the upper nozzle and the other end extending obliquely upward constitutes a presser spring which comes into contact with the upper core plate, and gradually moves toward the other end. In a fuel assembly for a pressurized water reactor equipped with a plurality of leaf springs formed to be thin, the thickness distribution of each leaf spring is made different according to the strain distribution of each leaf spring. By changing the thickness distribution of the individual leaf springs without changing the basic configuration of the holding spring, the maximum generated stress of the leaf springs can be averaged.

As a result, it is possible to reduce the distortion level of the presser spring when the reactor is stopped without lowering the spring force of the presser spring during operation, and based on the current fuel design and manufacturing technology. The soundness of the fuel assembly accompanying the high burnup of the fuel can be relatively easily secured.

[0039] Specifically, the thickness t ₁ of the thickness S ₁ and the distal end portion of the root portion of the leaf spring of the uppermost individual leaf springs, the thickness S _n of the root portion of the lowermost leaf spring the thickness t _n of the tip of the root portion of the intermediate position of the leaf spring thickness S ₂ ... and the tip portion thickness t ₂ ... and, t ₁
<S ₁ , t ₂ <S ₂ ,..., T _n <S _n , and t ₁ <t
_{When n} <... <t ₂ , S ₁ ≤S _n ≤ ... ≤S ₂ , the maximum generated stress of each leaf spring is averaged, and the maximum amount of distortion of the upper holding spring can be reduced. There is an effect that can be.

[Brief description of the drawings]

FIG. 1 is an explanatory view showing a configuration of an upper presser spring of one embodiment of a fuel assembly for a pressurized water reactor of the present invention.

FIGS. 2A and 2B are explanatory diagrams showing the leaf spring of FIG. 1 and its strain distribution. FIG. 2A is a front view showing the configuration of the leaf spring, and FIG. 2B is a diagram showing a strain amount corresponding to the position of FIG. .

FIG. 3 shows another leaf spring of FIG . 1 and its strain distribution.
FIG. A is a diagram showing a configuration of a leaf spring.
FIG. 1B is a front view, and FIG. 2B is a line showing a distortion amount corresponding to the position in FIG.
FIG.

FIG. 4 is a view showing a configuration of a fuel assembly for a pressurized water reactor .
FIG.

FIG. 5 is an explanatory view showing a configuration of an upper nozzle of FIG . 4,
Figure a is a front view of the upper nozzle, and Figure b is an enlarged view of the main part.
You.

FIG. 6 is an explanatory diagram showing a configuration of another upper nozzle, and FIG.
a is a front view of an upper nozzle, and FIG. b is an enlarged view of a main part.

FIG. 7 is an explanatory view showing a leaf spring and its strain distribution.
FIG. A is a front view showing the configuration of the leaf spring;
FIG. B is a diagram showing the distortion amount corresponding to the position in FIG.

[Explanation of symbols]

(20)… Top holding spring, (21)… Top leaf spring,
(22) (23) ... middle leaf spring, (24) ... lowest leaf spring,

Claims (2)

(57) [Claims] 1. A plurality of presser springs, one end of which is fixed to an upper nozzle and the other end of which extends obliquely upward constitutes a presser spring which abuts on an upper core plate, and is gradually thinned toward the other end. A fuel assembly for a pressurized water reactor having a leaf spring according to claim 1, characterized in that the thickness distribution of each leaf spring is different according to the strain distribution of each leaf spring. body. 2. The fuel assembly for a pressurized water reactor according to claim 1, wherein a thickness S ₁ of a root portion of the uppermost leaf spring of the individual leaf springs and a thickness t ₁ of a tip portion thereof, the thickness S _n and the distal end portion of the thickness t of the root portion of the lowermost leaf spring
_n and the thickness S ₂ at the root of the intermediate leaf spring.
Pressurized water reactor fuel assembly in which the tip portion thickness t ₂ ... and is characterized in that the following relationship with. (T ₁ <S ₁ , t ₂ <S ₂ ,..., T _n <S _n , and t
₁ <t _n <... <t ₂ , S ₁ ≤S _n ≤ ... ≤S ₂ )