JPH0339692A - Nuclear fuel assembly - Google Patents

Abstract

PURPOSE:To improve SCC resistance by improving the structure of a hold-down spring in the upper nozzle of the fuel assembly for a pressurized water reactor, thereby dispersing the strains concentrated in the neck part thereof and preventing the cracking in this part from occurring. CONSTITUTION:A top plate 2 is disposed in the upper part of the upper nozzle 1 and an adapter plate 3 in the lower part. A leaf spring 4 as the hold-down spring is placed between these plates. This leaf spring 4 is formed flat and one end thereof is fixed to the under surface of the top plate 2 by means of a bolt 6 and a clamp 7 so that the spring can deflect downward when loaded. On the other hand, a bar 5 is detained to the leaf spring 4 and remain upright at all times to maintain the perpendicular contact with an upper core plate even if the leaf spring deflects. The bar 5 is pressed when the upper nozzles 1 rises on increasing of the elongation rate of fuel. The force thereof is then transmitted to the leaf spring 4 and, therefore, the leaf spring deflects downward and reaction force is generated by the elastic force thereof, by which the floating of the fuel assembly is prevented and the force to withstand the stress corrosion cracking (SCC) is improved.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a nuclear fuel assembly, particularly a nuclear fuel assembly for a pressurized wooden nuclear reactor. This invention relates to the above-mentioned nuclear fuel assembly in which the hold-down spring has been improved.

(Prior Art) A conventional fuel assembly for a pressurized water reactor has a large number of fuel rods αD arranged in parallel as shown in FIG. It consists of a fuel support part supported by During furnace operation, hold-down springs αQ are attached to the four sides to prevent the assembly from floating due to the fluid force of the upwardly flowing coolant.

And this hold-down spring 0 [9 is usually -
The boat fishing configuration is made by bending thick plate material as shown in Figures 7 and 8, and the upper nozzle aω is connected to the pole) Q7.
), and a clamp α is attached to the attachment part for the purpose of covering the joint part and protecting the attachment bolt 07), and as shown in Figure 8, the surface stress due to the load due to the displacement of the spring is It is processed into a tapered shape to make it as even as possible.

(Problems to be Solved by the Invention) However, in the case of the conventional fuel assembly as described above, the hold-down spring is bent near the attachment part as described above, so the base part is bent during use. (
The maximum strain occurred in a), and tarawaku was likely to occur, which caused problems in terms of SCC (stress corrosion cracking) resistance.

In particular, in recent years, research has been carried out to improve fuels with the aim of improving the economic efficiency of nuclear power generation, and high burnup cores that extend the length of the operating cycle are attracting attention as an effective measure. When fuel is used to a high burnup, the amount of elongation of the fuel assembly increases, so the above-mentioned problem becomes more serious.

By the way, in order to deal with the occurrence of the above-mentioned distortion and to reduce it, it is also a good idea to change the hold-down spring to one with a multi-piece structure by making the plate thickness even thinner and increasing the number of pieces. However, this method has dimensional limitations for assembly and is difficult to adapt to all usage conditions, so it has not been a fundamental solution.

The present invention deals with this situation by improving the structure of the hold-down spring in the upper nozzle of the nuclear fuel assembly, thereby dispersing the strain concentrated at the base of the hold-down spring and preventing the occurrence of cranking in that part. The purpose of this is to improve SCC resistance.

(Means for Solving the Problem) That is, the feature of the present invention that meets the above problem is that the hold-down spring in the nuclear fuel assembly is formed of a flat leaf spring that is not actually bent. The basic idea is to place a leaf spring between the top plate of the upper nozzle and the adapter plate and fasten it at the end, while a bar whose upper end abuts the upper core plate is loosely inserted through the top plate. A first aspect of the present invention is a structure in which one end is attached to the lower surface of the top plate, the other end is a free end that is movable up and down, and the bar is rotatably locked to the free end.

Further, in the invention as claimed in claim 2, both ends of the leaf spring are attached to the lower surface of the top plate, and a bar is secured to the approximately central portion thereof.

In addition to having a constant cross-sectional area over the entire length of the temporary spring, it is also possible to taper the temporary spring so that the cross-sectional area becomes substantially larger toward the attachment part.
It is also possible to use stacked springs with the same thickness facing in the -1 direction in the mounting portion, and the invention described in claim J[13] has such a configuration.

(Function) In the nuclear fuel assembly according to the present invention, when the fuel assembly extends upward due to the above configuration, a downward load is applied to the bar that contacts the upper core plate perpendicularly (ν), and this load causes Deflect the leaf spring downward.

At this time, the elastic force of the leaf spring acts as a reaction force that presses the upper nozzle downward, thereby preventing the fuel assembly from floating.

Since the above-mentioned bar is pivotally supported with respect to the leaf spring so that it can swing and rotate, it can always press against the upper core plate perpendicularly even if the temporary spring is bent downward.

- On the other hand, since the temporary spring does not have any bent parts, large distortions do not concentrate even in the vicinity of the attachment part, and cracks are prevented from occurring.

Further, by constructing the hold-down spring with the flat temporary spring and the bar as described above, the hold-down force is always appropriately and reliably applied to the fuel assembly.

Furthermore, if the temporary spring is tapered or stacked, the stress will be approximately averaged over the entire length of the leaf spring, making it even more effective in preventing cranking.

(Example) Examples of the present invention will be further described below based on the accompanying drawings.

Fig. 1 shows the upper nozzle (11) of the fuel assembly according to the present invention, with top plates 1 and (2) arranged at the upper part and an adapter plate (3) at the lower part, with an l-led down between the two. A leaf spring (4) forming a spring is arranged.

This leaf spring (4) is formed substantially flat without bending, and its one end is fixed to the lower surface of the top plate (2) with a bolt (6) and a clamp (7), and Below the spring (4) is an adapter plate (3).
) so that the leaf spring (4) can bend downward without any hindrance when a load is applied, as will be described later.

On the other hand, (5) is a bar that is locked to the leaf spring (4), and the bar (5) penetrates the top plate (2) so as to be able to move up and down approximately vertically, and its lower end is attached to the leaf spring (4). ) is pivoted on the free end side. And the bar (5) is a leaf spring (4)
Even if the leaf spring (4) is bent, the bar (5) can always stand vertically and contact the upper core plate (not shown) perpendicularly. It becomes. Further, unlike the leaf spring (4), the material of the bar (5) does not require particular elasticity.

When the amount of elongation of the fuel assembly increases and the upper nozzle (1) rises as described above, the upper end portion of the bar (5) is first pressed against the upper core plate, and this force is applied to the shaft support as a downward load. It is transmitted to the leaf spring (4) through the minute. At this time, the leaf spring (4) is bent downward, and its elastic force generates a reaction force, which can prevent the fuel assembly from floating up, and has the effect of a hold-down spring.

Moreover, because the leaf spring is essentially flat, unlike conventional hold-down springs that are bent, there is no ζ strain concentrated in one place, and the occurrence of cranks can be reliably prevented. .

In the configuration shown in Figure 1 above, the leaf spring (4) has a flat shape with a uniform thickness over its entire length, but it does not need to be separately installed or cauterized, and it can be used as long as it is substantially flat. , as shown in Figures 2 and 3, the fixed mounting end is tapered so that its cross-sectional area increases toward the end, or leaf springs of different lengths and the same or different thicknesses ( 4a),
(4b) and (4C) may be superimposed, and in this way, the stress acting on the leaf spring (4) is averaged and dispersed, making it even more effective in improving SCC resistance.

In addition, in the present invention, since a flat temporary spring without bending is used as described above, stress concentration does not occur in the attachment part, and therefore, it is possible to eliminate the clamping α seen in FIG. The height of the nozzle can be lowered.

FIG. 2 shows an example of such an upper nozzle (1) that does not require a clamp, and the same reference numerals as in FIG. 1 indicate the same parts.

Such an upper nozzle is particularly advantageous for a high burnup fuel assembly, and it is easy to increase the distance between the upper nozzle and the upper core plate to accommodate large elongation of the fuel assembly.

In addition, the leaf spring (4) has one end hardened and the other end with a bar (5).
) is attached, but this is not necessarily limited, and if the leaf spring is flat and the bar (5) is swingable, the purpose can be achieved in the same way.

FIG. 5 shows another embodiment in which a leaf spring (4) has both ends fixed to the lower surface of the top plate (2), and a bar (5).
This figure shows a configuration in which the plate spring (4) is locked to a substantially central portion of the leaf spring (4).

That is, in this case as well, the upper end portion of the bar (5) comes into contact with the upper core plate, and when the upper nozzle (1) force (E) rises, the leaf spring (4)
) can prevent the fuel assembly from floating. Particularly in this case, the load can be distributed to the left and right from approximately the center of the temporary spring (4), and stress can be distributed from a different perspective than the above.

In addition, the locking portion of the bar (5) is preferably located at the center of the leaf spring (4) in consideration of the deflection of the leaf spring (4), but it may be located in the vicinity as long as it is within a practical range.

Of course, in this case as well, leaf springs of various thicknesses (
4d), (4e), and C4f) can be overlapped or tapered, and by applying such means toward both ends of the leaf spring (4) and the fixed part,
It goes without saying that stress can be averaged and SCC resistance can be further improved.

(Effects of the Invention) As described above, the present invention provides a hold-down spring for an upper nozzle in a nuclear fuel assembly using a substantially flat leaf spring installed horizontally on the lower surface of the top plate of the upper nozzle; It consists of a bar that is attached so that it can swing and rotate, and the upper end of the bar is brought into contact with the upper core plate perpendicularly.Since it is a leaf spring and does not have a bent part, the bending part has a maximum It does not have the disadvantages of conventional hold-down springs where strain is concentrated, and especially when the temporary spring is tapered as described in claim 3 to equalize the stress, it prevents the occurrence of cracks and improves durability. S
It has a remarkable effect on improving CC.

In addition, since the leaf spring does not have a bending part, there is no need to use a clamp on the top plate, which reduces the height of the upper nozzle and eliminates the need for a large distance between the truss plate and the upper core plate. It can effectively handle high burnup type fuel assemblies.

Furthermore, since only the bar protrudes from the top plate, there is more space for the inscription on the top plate, and the engraving can be made larger to make it easier to confirm the number.

However, such a hold spring requires t in its formation.
There is also the advantage that there is no need for +h-edge processing, and the amount of processing can be reduced compared to the conventional method.

[Brief explanation of drawings]

FIG. 1 shows one part of the upper nozzle related to the main part of the fuel assembly of the present invention.
FIG. 2 is a front view showing another example of the upper nozzle, FIGS. 3 and 4 are front views showing each side of the leaf spring shape of the upper nozzle, and FIG. 5 is the same. A front view showing still another example of the upper nozzle, FIG. 6 is a front view showing another shape of the leaf spring in FIG. 5, FIG. 7 is a schematic diagram of the structure IIi and i11 of the nuclear fuel assembly, and FIG. FIG. 2 is a front view showing an example of the configuration of a conventional upper nozzle. (11...Top nozzle, (2)...Top plate, (3)...Adapter plate, (4)...Temporary spring, (5)...Bar 1 figure

Claims (1)

[Claims] 1. A nuclear fuel assembly in which a large number of fuel rods are supported by a plurality of support grids, the upper and lower sides are fixed by an upper nozzle and a lower nozzle, respectively, and a hold-down spring is attached to the upper nozzle. In the body, the hold-down spring is formed of a flat leaf spring that is not substantially bent, and the leaf spring is disposed between the top plate of the upper nozzle and the adapter plate, and one end of the leaf spring is attached and installed horizontally. A nuclear fuel assembly characterized in that a bar that is loosely inserted into a plate and whose upper end portion abuts an upper core plate is rotatably locked to the other end of the leaf spring. 2. In a nuclear fuel assembly in which a large number of fuel rods are supported by a plurality of support grids, the upper and lower sides are fixed by an upper nozzle and a lower nozzle, and a hold-down spring is attached to the upper nozzle, the hold-down The spring is formed of a flat leaf spring that is not substantially bent, and the leaf spring is arranged between the top plate of the upper nozzle and the adapter plate, and both ends of the leaf spring are fixed and installed laterally, while a loose leaf spring is formed in the center of the top plate. 1. A nuclear fuel assembly characterized in that a bar is inserted into the plate and whose upper end portion abuts an upper core plate is locked to the central portion of the leaf spring. 3. The nuclear fuel assembly according to claim 1 or 2, wherein the cross-sectional area of the leaf spring is formed to become substantially larger toward the attachment portion.