JPS58223780A - Fuel assembly - 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 that is loaded vertically into the core of a nuclear reactor.

[Technical background of the invention]

FIG. 1 shows a schematic configuration of a boiling water reactor gland, where 1 is a reactor pressure vessel and 2 is a reactor core provided within the pressure vessel 1. A plurality of fuel assemblies 3 are vertically loaded in the reactor core 2, and a coolant 4 is accommodated in the pressure vessel 1.

In addition, in the pressure vessel 1, a steam separator 5 is installed above the core 2.
A steam dryer 6 is housed above it, and a plurality of jet pumps f7 are arranged around the core 2.

And each jet pump 7''7...
A recirculation ring 8 for circulating the coolant 4 is provided, and a main steam pipe 11 connects the turbine 10 for driving the generator 9 and the pressure vessel 1. A main steam isolation valve 121.12B is inserted in the main steam pipe ll. In addition, in the figure, 13 is a condenser that cools and liquefies the main steam that has passed through the turbine I0, and 14 is a feed water sparger installed above the core 2 in the pressure vessel 1. The space is connected by a water supply pipe 15.

A water supply bong f16 is inserted into the water supply pipe 15.

Therefore, the coolant 4 in the reactor pressure vessel 1 is sent to the lower part of the reactor core 2 via the pipes 7, and flows upward in the reactor core 2. At this time, it is heated by the reaction heat of the nuclear fuel loaded in the core 2, and a part of it is converted into steam and is supplied to the main steam pipe 11 and the turbine 10 through the steam separator 5 and the steam dryer 6. After the turbine 10 is driven, the water is cooled and liquefied in the condenser 13 and sent to the pressure vessel 1 through the selective water supply ponzo 16 through the water supply piping 15, and then supplied to the water supply sparger 14.

By the way, the fuel assembly 3 is constructed as shown in FIGS. 2 to 4. In other words, numerals 17 in the figure indicate a plurality of fuel rods arranged in a square lattice, and the inside of each fuel rod 17 is filled with nuclear fuel such as UO2 pellets. These plurality of fuel rods 17 .
19 are held at constant intervals by a plurality of spacers 20... And these spacers 20...
A plurality of fuel rods 17 . . . , which are integrated with both tie plates 1B and 19, are housed in a rectangular cylindrical channel 2. Further, a lifting metal fitting 22 is fixed to the upper tie plate 18.

Note that there are a small number of fuel rods (for example, two) in the fuel rods 17.
The water rods 23, 23 are mixed together as shown in FIG. 4, and the water rods 23, 23 are configured to hold the plurality of spacers 20 at a constant interval. , as shown in FIGS. 5 and 6, a plurality of dividers 25 .
. . . are assembled vertically and horizontally to form a plurality of squares (usually 8×8 squares), and the fuel rod 17 . . . or water rod 23 is inserted through each square.

In addition, at the intersection and difference between the divider 25 and the par 26,
Rectangular cylindrical holders 27 are incorporated every other in any of the vertical, horizontal, and diagonal directions, and each holder 270
The center portions of the four sides are bent to protrude outward to form V-shaped spring pieces 28. Furthermore, the dividers 25 and /4-25 have S-shaped spring pieces 29 corresponding to each square.
... is formed by cutting and bending, and the outer peripheral surface of each fuel rod 17 or water rod 23 is covered with two S-shaped spring pieces 29.2.
9 and one V-shaped spring piece 28 to elastically hold it. Further, oval shaped bulges 30 are formed on the outer surface of the outer frame 24.
. . are configured to be brought into close contact with the inner surface of the channel 21.

[Problems with background technology]

Coolant 4 flows through the core 2 from downward to upward.

At this time, the cooling rods 4 come into contact with the fuel rods 17 in each fuel assembly 3 and flow through them, so horizontal fluid vibrations occur in the fuel rods 17. do. When considering such vibrations of the fuel rods 17, each fuel rod 17 is
It can be considered that the amplitude becomes zero at the support point by each spacer 2θ, and the amplitude becomes maximum at the center position between each spacer 20. Since the amplitude of such fluid vibrations should be suppressed within a certain tolerance range, a fluid vibration test is performed on each fuel assembly 3 in advance. In this test, the fuel assembly 3 is loaded into a simulated nuclear reactor to form a simulated reactor core, a high-temperature, high-pressure liquid single-phase flow similar to the actual reactor is generated in the simulated core, and fluid vibrations of the fuel rods 17 are measured. FIG. 7 shows the measurement results. As is clear from FIG. 7, in the conventional fuel assembly 3, the fluid vibration of the fuel rods 17 increases from the lower end to the upper end. This appears to be because when the fluid (coolant) flows from the lower end side to the upper end side of the fuel assembly 3, turbulence increases each time it passes through one spacer 20.

On the other hand, the amplitude y of the fluid vibration of the fuel rod 17 is 1. It is known that it increases in proportion to the square of the fluid flow velocity. Therefore, when designing a nuclear reactor, it is necessary to set the maximum flow velocity of the coolant based on the maximum amplitude ym□ at the upper end of the fuel rod, and the reactor output is determined by this. Therefore, in order to increase the reactor output, it is necessary to make it possible to increase the flow velocity of the coolant by reducing the maximum amplitude "maK" of the fuel rods 17.

Furthermore, if the amplitude of the fluid vibration is large, the fuel rod 17 may suffer from fatigue failure, and corrosion may occur at the portion in contact with the spacer 20, which may impair the integrity of the fuel rod.

2 [Object of the invention] The present invention was made based on the above circumstances,
The purpose is to reduce the amplitude of fluid vibrations in the fuel rods to maintain the integrity of the fuel rods, and to provide a fuel assembly that can increase the coolant flow rate and increase the reactor output. be.

[Summary of the invention]

The fuel assembly according to the present invention includes a plurality of fuel rods arranged in a square grid and loaded vertically into the core of a nuclear reactor, and an upper tie plate that supports the upper and lower ends of the plurality of fuel rods, respectively. and a lower tie plate, a plurality of spacers that hold the plurality of fuel rods at regular intervals between the two tiles, and whose spacing becomes narrower from the lower end side to the upper end side, and these spacers and the two tie plates. A channel for accommodating the plurality of fuel rods integrated with the grate.

[Embodiment J of the Invention Fig. 8 shows a fuel assembly 100 for a boiling water reactor, and in the figure 101... are a plurality of fuel rods arranged in a square lattice. Inside each fuel rod 101 is a UO. A plurality of fuel rods 101 are filled with nuclear fuel such as pellets.
... is the upper end and the lower end, respectively, with the upper tie rate 1.
02 and lower tie plate 103, and a plurality of spacers 1 are provided between both tie plates 102 and 103.
04..., are maintained at constant intervals. In addition,
The distance in the axial direction between these plurality of spaces (-sans 104...) is as follows:
It is set to become narrower from the bottom end to the top end. A plurality of fuel rods 101 . . . , which are integrated with these spools 104 . is the lifting metal fittings 10
6 is fixed.

The fuel assembly configured as described above is loaded into the core of a boiling water nuclear reactor, and the coolant flows along the fuel rods 101 from the lower end to the upper end. At this time, fluid vibration occurs in each fuel rod 101, but the amplitude of this vibration is zero at the support point by each spacer 104 and maximum at the center position between each spacer 104. Then, the maximum amplitude yrn□ at the center position between each spacer 104 is determined by the following formula.

Here, W is each spacer 104 for the fuel rod 101.
- A load that acts in the vertical direction at the central position between the two, and corresponds to the acting force caused by the flow of coolant. Therefore, as the coolant flow is disturbed, the load W
becomes large.

Further, t is the interval between the spacers 104, and the spacer interval t becomes smaller as it goes from the lower end side to the upper end side of the through-through fuel rod 101. In addition, E is the Young's modulus of the fuel rod 101,
Similarly, (2) is the moment of inertia of the fuel rod 101.

As is clear from this equation, the maximum amplitude ymax of the fluid vibration between each spacer 104 of the fuel rods 10 is proportional to the load W and at the same time proportional to the cube of the spacer interval t.

Therefore, when the fuel assembly of this embodiment is loaded into the core of a nuclear reactor and coolant is caused to flow along the fuel rods 101..., the load W on the upper end side of the fuel rods 101 is large (< 11). However, at the same time, the maximum amplitude y mhx is suppressed by decreasing the spacer interval t. Therefore, by changing the spacer interval t in accordance with the change in the load W, each spacer 4 can be adjusted over the entire length of the fuel rod 101.
- The maximum amplitude 7m1LX between the terminals 104 and 104 can also be made constant. To give a specific example, as shown in Figure 9, 7
spacers 104... are used. Then change the distance between each spacer from the bottom side to the top side.tl t ts p
'a t t4 e ts t L-, then t
1-1. The ratio of to ts is 0.7, respectively.
0.8, 0.82, 0.89, and 0.94, the fluid vibration at the upper end of the fuel rod ioi is reduced, and each spacer 104 and
...The maximum amplitude 'max between t1! , could be kept constant.

As described above, the fluid vibration at the upper end of the fuel rod 101 is reduced.1. By keeping the maximum amplitude y7m□ #1 constant between each spacer 104 over the entire length of the fuel rod 101, the flow velocity of the coolant can be increased, and thereby the reactor output can be increased. . In addition, when the fluid vibration becomes smaller, it is possible to suppress the occurrence of fretting corrosion at the part of the fuel rod 101 that contacts the parts 104 and 2-0104, and fatigue damage due to vibration is less likely to occur, so the fuel rod 101 can maintain its health.

[Effects of the Invention] As detailed above, in the fuel assembly of the present invention, the intervals between the spacers that hold the plurality of fuel rods at constant intervals are narrowed from the lower end to the upper end. Fluid vibration at the upper end of the fuel rod can be reduced, thereby maintaining the integrity of the fuel rod.

[Brief explanation of the drawing]

Figures 1 to 7 show the background art; Figure 1 is a schematic diagram of a boiling water reactor plant, Figure 2 is a partially cutaway perspective view of a fuel assembly, and Figure 3 is a schematic diagram of a boiling water reactor plant. 4 is a longitudinal sectional view of the fuel assembly, FIG. 4 is a sectional view taken along the line ■-■ in FIG. 3, FIG. 5 is a plan view of the spacer, and FIG. Fig. 7 is a diagram showing the amplitude of fluid vibration occurring at each position of the fuel rod, Fig. 8 and Fig. 9 show an embodiment of the present invention, and Fig. 8 shows a fuel assembly partially cut away. FIG. 9 is a diagram showing changes in the spacing between the supports that hold the fuel rods. 101... Fuel rods, 102... Upper tie plate. 103... Lower tie plate. Great, 104...Spacer, 105...Channel, t1 to t6...Spacer interval. Applicant's agent Patent attorney Name: Takehiko E 5th factor, 6th factor, 7th factor 2 Fig. 9

Claims (2)

[Claims] (1) A plurality of fuel rods arranged in a square grid and loaded vertically into the core of a nuclear reactor, and an upper tie plate and a lower tie plate that respectively support the upper and lower ends of the plurality of fuel rods; Between these two tie rates □
A plurality of spacers that hold the plurality of fuel rods at constant intervals and whose intervals become narrower from the lower end side to the upper end side, and the plurality of fuel rods that are integrated by these spacers and both tiles. A fuel assembly characterized by comprising a channel for accommodating the fuel. (2) The fuel rods are held by seven spacers, and the spacing between the spacers at the bottom position is 0.7, 0.8°, 0.82°, and 0.82° from the top position, respectively. 8
9. The fuel assembly according to claim 1, wherein the ratio is set to 0.94.