JPS6295492A - Fuel aggregate - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] (Technical Field of the Invention) The present invention relates to a fuel assembly applied to a boiling water type Kyoryo reactor, etc., and in particular to a fuel assembly for preventing and protecting a fuel cladding tube from FII loss. Regarding fuel assemblies.

(Technical Precedence of the Invention) Boiling water
:1 assembly is loaded to form the reactor core H6. As shown in Fig. 4, the fuel assembly is housed in a rectangular cylindrical channel box 1 whose cross section is approximately square.
are held and accommodated at predetermined intervals by the spacer 3. The fuel rods 2 are arranged, for example, in seven or eight rows in the vertical and horizontal directions, and both upper and lower ends of each fuel rod 2 are restrained and fixed by an upper tie plate 4 and a lower tie plate 5.

Upper tiebrae i~4 is installed on channel box 1 C
However, the lower tie plate 5 is not fixed and has a free lower end structure. Each fuel rod 2 of the fuel assembly has a Zircaloy fuel cladding tube loaded with a large number of fuel pellets such as UO2, and its upper and lower ends are each plugged with one upper end plug 6 and one lower end plug 7. A spring (not shown) is interposed in the upper blemish of the fuel cladding tube to prevent the fuel pellet from moving or shifting its position.

Also, an upper tie plate 4 that restrains the upper part of each fuel rod 2 is provided.
, there is a hole 8 into which the upper end plug 6 is inserted and a cooling hole (not shown).
A large number of water outflow holes are provided, and an operating handle 9 for handling the fuel assembly is fixed. On the other hand, the lower tie plate 5 is formed with a hole 10 for supporting the lower end plug 7 and a cooling water inflow hole (not shown). This lower tie plate 5 has a tie plate main body 11 shown in a partially enlarged cross section in FIG.
A nose piece 13 is integrally formed at the frontage 5 and guides the cold 741 water flowing in from the cold 7JI water inlet 12 into the channel box 1.

As shown in FIGS. 6(A) and 6(B), which are a plan view and a side sectional view of the lower tie plate 5, the support portion 14 of the lower tie plate 5 has a plurality of screw holes 15, for example, eight screw holes.
A fuel rod (or a Talorad) 2 is screwed into each screw hole 15, and the lower end of each fuel rod 2 is restrained by a lower tie plate 5. Note that the support portion 14 of the lower tie plate 5 is connected to the second portion via a bridge 16.

A large number of such fuel assemblies are loaded into a reactor pressure vessel in the form of four fuel assemblies, forming the core of a nuclear reactor.

[Problems with backcountry technology] By the way, when assembling fuel assemblies, replacing them, or transporting them, the fuel assemblies are lifted up by cranes, etc., and moved.
be transported. If, for some reason, the fuel assembly falls from the crane and collides with a rigid body such as a work floor during these operations, the impact force may deform or damage the fuel rods.

In the unlikely event that the spent fuel cladding is damaged due to a fall,
Fission products could have been released and the work opening could have been exposed to radiation.

In reality, there is almost no damage to the fuel cladding due to a fall, but we are concerned about the deformation of the fuel assembly if it were to fall from a crane and collide with a rigid part such as a work floor. It is assumed that the fall at this time is a vertical fall with the lower tie plate 5 facing downward. In this case, the falling energy of the fuel assembly is absorbed by the fuel assembly itself as it collides with a rigid body such as a floor.

For example, when a fuel assembly falls and the tip nose piece 13 of the lower tie plate 5 collides with a floor or the like, an impact load is generated on each part of the fuel assembly. In this case, since the channel box 1 and the lower tie plate 5 are free and not fixed, the load is transferred to the support part 1 of the lower tie plate 5.
4 to each fuel rod 2.

When an impact horn acts on the support part 14 of the lower tie plate 5, when considering the strength of the lower tie plates 1 to 5, the tip nose piece 13 is the weakest, followed by the support part 14, and then the tie plate body 11. There is.

Therefore, the falling energy caused by the collision of the fuel assembly causes deformation of the nosepiece 13 first, and then deformation of the tie brake 1 to the main body 11 occurs, and is absorbed in sequence. Therefore, when the falling energy is small, the impact force can be absorbed only by the deformation of the nose piece, but when the falling energy becomes large, the supporting part 14 receives a part of the falling energy, and the supporting part 14 absorbs the impact force. It's deformed. For this reason, the support part 14 and the bridge 16 undergo large deformations (), and the lower end plug 7 of the fuel rod 2 supported by the support part 14 changes from Fig. 7(A) to (B).
) acts as a bending moment.

As shown in FIG. 8, the conventional lower end plug 7 has a constricted portion 7 directly below the welding portion 7a where the lower end plug 7 is welded to the fuel cladding tube.
b, and this constricted portion 7b is used as a weakened region.

Due to the presence of this weakened region, when a bending moment is applied, the largest stress is generated in the constricted portion 7b, causing deformation. The bending moment was absorbed by this deformation of the constricted portion 7b, and there was no possibility that the fuel cladding tube would be damaged.

However, in fuel rods that will be developed in the future, there is a tendency for end plug welds to be inspected by ultrasonic probes, and in this case, a constriction will be formed in the lower end plug to prevent disturbance of the ultrasonic waves. becomes difficult.

If a constriction is not formed in the lower end plug, the bending moment will be transmitted to the fuel cladding tube through the lower end plug, causing a risk of the cladding tube breaking or being damaged, which has been a problem.

[Purpose of the invention]

The present invention has been made in consideration of the above-mentioned circumstances, and even if the fuel assembly falls due to an unexpected accident, damage to the fuel cladding tube can be prevented reliably, and safety measures can be taken sufficiently. The purpose is to provide a fuel assembly with

[Summary of the invention]

In order to achieve the above object, the present invention accommodates a large number of fuel rods in a rectangular cylindrical channel box via spacers, and connects the upper and lower ends of the fuel rods with an upper tie plate and a lower tie plate, respectively. In the fixed fuel assembly, a lower end plug is fixed to the lower end of the fuel rod, and a cut groove is cut in the shank portion of the lower end plug in the circumferential direction.

[Embodiments of the invention]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a fuel assembly according to the present invention will be described below with reference to the accompanying drawings.

In explaining this embodiment, the overall structure of the fuel assembly is substantially the same as the conventional fuel assembly shown in FIGS. Omitted.

The fuel assembly according to the present invention is an improved lower end plug 20 for each fuel rod housed within a channel box.

The lower end plug 20 is constructed as shown in FIG.
A welded portion 22 is formed to weld the fuel cladding tube 0 to a fuel cladding tube (not shown), while a shank portion 23 is integrally adjacent to the lower part of the barrel portion 21, and a cut groove 24 is cut in the circumferential direction in this shank portion 23. , a weakened part is formed.

Although it is desirable that the cut grooves 24 be formed over the entire circumference, the cut grooves 24 are not necessarily limited to this. Also, the depth of cut is 24mm.
may be a single groove or a plurality of grooves.

This lower end plug 20 is for butt welding as shown in FIG. 2, and since there is no constriction between the welding part 22 and the barrel part 21, the barrel part 21 is longer than the conventional lower end plug, While the strength of the lower end plug 20 is improved, there is no disturbance of ultrasonic waves when inspecting the welded portion, and the welded portion can be inspected accurately and reliably. The lower end plug shown in FIG. 1 is constructed by making a notch 24 in the shank portion 23 of the lower end plug 2o shown in FIG.

On the other hand, it is conceivable to carve the groove in the middle or lower part of the barrel part 21, but in general, a predetermined barrel length is required in order to carve a stamp on the barrel part 21 to identify the fuel rod. . The length of this barrel portion is, for example, the sum of the constriction of a conventional lower end plug and the barrel portion.

Therefore, when the constriction is provided below or in the middle of the barrel, it is necessary to increase the overall length of the lower end plug by the length of the constriction. However, even if the lower end plug is lengthened, the overall length of the fuel rod cannot be changed, so the length of the upper plenum portion of the fuel cladding tube will be reduced. Reducing the Blenheim length makes the mechanical installation 31 of the fuel rod extremely difficult, which is undesirable. For this reason, it is desirable not to change the length of the lower end plug, the position or shape of the contact surface with the lower tie plate.

From this point of view, as shown in FIG.
In response to the action of ~, a large stress is generated in the cut groove portion, which has the same effect as the constriction portion of the conventional lower end plug. The cut groove 24 has an appropriate cut depth to avoid excessive stress concentration.

FIG. 3 shows the stress curve A that acts on the cut groove of the lower end plug 20 due to the action of bending moment during 3G seismic analysis (G is gravitational acceleration), and this stress curve Δ is
The horizontal axis shows the depth of the cut groove, and the vertical axis shows the generated stress.

Further, reference numeral 13 indicates stress generated at the welded portion between the lower end plug 20 and the fuel cladding tube.

From FIG. 3, the depth of cut 24 is required to prevent damage to the fuel cladding during an earthquake, and for this purpose, the stress generated in the cut groove of the lower end plug 20 must be calculated by dividing the material yield stress by the stress. The condition is that the concentration ratio is less than or equal to the value C and greater than the stress B generated in the welded portion 22. Therefore, the material of the lower end plug 20 is Zircaloy-2, and the seal 11 of the material is Zircaloy-2.
If the diameter of 3 is about 7.5, the material yield stress is 24
．． 6Kg/rnrA, and if the stress concentration rate for stress concentration to occur in the cut groove is 5.0, then the cut groove 2
The upper limit of the cutting depth in No. 4 is that the generated stress A is 4.92 Ky/
It is necessary to set the value to be less than mtA (material yield stress ÷ 5.0), which is about 1.5 mm as shown in Fig. 3.

Furthermore, when the fuel assembly falls, a large bending moment acts depending on the falling distance, and there is a risk that the fuel cladding tube will be damaged. It can be damaged at the cut groove. In other words, the stress A generated in the cut groove portion 24 may be made larger than the maximum stress B generated in the welded portion with the fuel cladding tube, and the lower limit of the cut depth is set at the third
It is approximately 0.3 mm from the figure. Therefore, it is desirable that the optimum depth of cut is in the order of 0.3 to 1.5.

(Effects of the Invention) As described above, the fuel assembly according to the present invention has a lower end plug fixed to the lower end of the fuel rod, and a cut groove is cut in the shank portion of the lower end plug in the circumferential direction. , a weakened region can be formed in the shank portion, and stress can be actively concentrated in this weakened region. Therefore, if the fuel assembly falls due to some reason, the fuel rod support will be deformed following the deformation of the nosepiece when it hits the floor, but the bending moment will act due to the deformation of the support. However, before the fuel cladding tube breaks, a large stress is generated in the cut groove of the lower end plug, causing it to break or break, thereby absorbing the bending moment.

Therefore, it is possible to effectively and reliably prevent the fuel cladding from breaking or breaking 10, prevent the adverse effects of the fuel cladding from breaking, and improve the safety of the fuel assembly.

[Brief explanation of drawings]

FIG. 1 is a diagram showing a lower end plug attached to each fuel rod of a fuel assembly according to the present invention, FIG. 2 is a diagram showing a state before cut grooves are carved in the lower end plug, and FIG. Figure 3 shows the stress generated at the welded part of the fuel cladding tube and the lower end plug, based on the resistance analysis.
A graph showing the relationship between the stress generated in the cut groove portion and the cut depth. Fig. 4 is a side view showing a conventional fuel assembly. Fig. 5
Figure 6 shows the lower tie plate of the fuel assembly.
Figure (A) is a plan view of the lower tie plate, and Figure 6 (B) is a plan view of the lower tie plate.
) is a side sectional view taken along line A-A in Fig. 6 (△), and Fig. 7 (
Δ) and ([3) are diagrams showing the states before and after a bending moment is applied to the fuel rods of the fuel assembly, and Figure 0'58 is a diagram showing the lower end plug of the conventional fuel assembly. 20... Lower end plug, 21... Barrel part, 22...
Welded part, 23...Shank part, 24... Cut groove. Representative Patent Attorney Noriyuki Ken Yudo Mitsumata Hirofumi t” Katana Ai,
Figure 3 Figure 4 (A) Figure 6 Figure 8 (B) Figure 7

Claims (1)

[Claims] 1. In a fuel assembly in which a large number of fuel rods are housed in a rectangular cylindrical channel box via spacers, and both upper and lower ends of the fuel rods are fixed by an upper tie plate and a lower tie plate, respectively. A fuel assembly, characterized in that a lower end plug is fixed to the lower end of the fuel rod, and a cut groove is cut in the shank portion of the lower end plug in the circumferential direction. 2. The fuel assembly according to claim 1, wherein the notch is carved around the entire circumference of the shank portion of the lower end plug, and a weakened region is formed in the shank portion. 3. The scope of claims in which the depth of the cut groove carved in the shank portion is set so that the stress generated therein is always greater than the stress generated at the weld between the lower end plug and the fuel cladding tube. The fuel assembly according to paragraph 1.