JPS60224092A - 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 with excellent heat removal characteristics used in a boiling water nuclear reactor.

[Technical background of the invention]

Generally, as shown in FIG. 6, a boiling water nuclear reactor has a reactor core 3 in which a large number of fuel assemblies 2 are loaded in the center of a reactor pressure vessel 1. A steam separator 5 and a steam dryer 6 are provided in this order above the core 3, and the steam separator 5 is normally filled with cooling water 4 up to its middle. The cooling water 4 is forcibly circulated by a jet pump, an internal pump (both not shown), and flows upward within the core 3. Further, a main steam outlet nozzle 7 is provided in the surrounding wall soil portion of the reactor pressure vessel 1, and a water supply inlet nozzle 8 is provided below the main steam outlet nozzle 7.

Further, as shown in FIGS. 7 and 8, the fuel assembly 2 includes a large number of fuel rods 1 in a long square channel 9.
The fuel rods 10 are arranged in a square grid (usually 8x8-64 rods), and the upper and lower ends of each fuel rod 10 are supported by an upper tie plate 11F3 and a lower tie plate 12, and furthermore, at multiple locations between the upper and lower tie plates 11 and 12. By supporting the fuel rods 10 with spacers 13, the mutual spacing between the fuel rods 10 is kept constant.

As shown in FIG. 9, this fuel rod 10 is formed by stacking a large number of cylindrical pellets 15 inside a cladding tube 14 and enclosing the pellets while elastically pressing them from above with a spring 16. Further, each pellet 15 is formed into a certain shape by sintering uranium oxide powder.

The boiling water reactor thus constructed is operated as follows. That is, the nuclear fission of uranium in each pellet 15 in the reactor core 3 causes the cooling water 4 in the reactor pressure vessel 1 to boil, and the steam generated thereby is passed through the steam separator 5 and the steam dryer 6, and then The steam is taken out from the steam outlet nozzle 7 as steam for driving a turbine in a power plant.

On the other hand, the steam that rotates the turbine and is used for power generation is condensed in a condenser and is returned as cooling water 4 through a water supply inlet nozzle 8 into the reactor pressure vessel 1 .

[Problems with background technology]

In the conventional fuel assembly 2, all 64 fuel rods 10
were formed to have the same shape and dimensions.

Therefore, when examining the thermal neutron flux distribution of each fuel assembly 2 as shown in FIG. It's getting lower. Therefore, the output P at various points in the horizontal section of the fuel assembly 2 during reactor operation is the 10th
As shown in the figure, the outer peripheral portion of the fuel assembly 2 becomes higher. Therefore, heat removal from the fuel rods 10 due to boiling of the cooling water 4 is more difficult at the outer periphery of the fuel assembly 2 than at the center, and the thermal margin differs in various parts of the fuel assembly 2. I end up.

[Purpose of the invention]

The present invention has been made in view of these points, and it is an object of the present invention to provide a fuel assembly that has uniform thermal margins at various locations in a horizontal cross section and has excellent heat removal characteristics.

[Part of the invention]

The fuel assembly of the present invention is characterized in that a suitable number of fuel rods located in the second row from the outside are formed shorter than other fuel rods.

[Embodiments of the invention]

Generally, in a fuel assembly, if some of the fuel rods are made short, the following advantages arise.

That is, in a fuel rod located in a thermally severe position, if the upper part of the fuel rod, which is most thermally severe, is removed and shortened, the thermal margin will be significantly increased. This is because in a normal fuel assembly, the output becomes abnormally high and insufficient heat removal occurs at the top of the fuel rod or near the second spacer, so it is necessary to remove the fuel rods in these areas. If so, the thermal problems of the short fuel rods themselves would be eliminated.

At the same time, the thermal margin of the fuel rods adjacent to the shorter fuel rods also increases. This is because removing the upper part of the fuel rod increases the amount of cooling water per surrounding fuel rod. In other words, as shown in Figure 5, the limit output of a fuel rod tends to increase as the amount of cooling water increases, and the limit output of a normal long fuel rod adjacent to a short fuel rod is the same. This will improve the score from A to 8 in the figure.

-hl If some of the fuel rods are made short, the amount of fissile material in the fuel assembly will be reduced, resulting in a disadvantage that the output load per remaining long fuel rod will increase.

Therefore, in the present invention, the number of shortened fuel rods is reduced and the short fuel rods are arranged at effective positions to equalize the thermal margin. Form fuel rods in rows short. This is because the thermal margin in the outer periphery of the fuel assembly is severe, and the fuel rods in the second row from the outside can adjust the thermal margin in almost the entire outer periphery. In addition, an appropriate number of the fuel rods in the second row from the outside are formed short, and in particular, the four rods located at the four corners of the second row are placed in the corners of the fuel assembly where the output is maximum. Therefore, the effect of equalizing the thermal margin is maximized.

Next, embodiments of the present invention will be specifically described with reference to FIGS. 1 to 5.

FIGS. 1 and 2 show an embodiment of the fuel assembly 18 of the present invention, in which four fuel rods located at the four corners of the second row from the outside are arranged in an 8×8 square grid. This fuel rod 17 is made shorter.

The 60 fuel rods are made up of the same long fuel rods 10 as before. The exothermic length of the short fuel rod 17, that is, the fuel pellet loading length p (Fig. 1), may be determined by taking into account the advantages and disadvantages of adopting the short fuel rod, and one guideline is: It may be approximately 3/4 of the exothermic length L (FIG. 1) of a normal fuel rod 10. As a result, compared to the normal fuel rod 10, the fuel rod 17 can remove the rods at the top of the fuel rod and the second spacer part, which are thermally severe, so the thermal margin can be dramatically improved. can be increased to In addition, short fuel rod 1
7 is attached with its lower end screwed into the lower tie plate 12 and supported by a spacer 13 in the middle.

Since it is formed in this way, as shown in FIG. 2, the thermal margin of the 32 fuel rods 10 (shaded area in the figure) adjacent to the short fuel rod 17 is improved, and these fuel rods are Since the rods 10 are located at the four corners of the outer periphery of the fuel assembly 18, the thermal margin of the entire fuel assembly 18 is uniform throughout, resulting in excellent heat removal characteristics.

In addition, as shown in FIG. 3, short fuel rods 17 are provided at each side in addition to the four corners of the second row from the outside, so that all the regular fuel rods 10 on the outer periphery are short. The thermal margin may be improved by the thick fuel rods 17. In this case, the amount of uranium loaded in the fuel assembly 18 is prevented from decreasing abnormally.

Furthermore, the present invention is applicable not only to fuel rods arranged in an 8x8 square lattice, but also to those arranged in a 9x9 square lattice or other square lattice as shown in FIG. The same applies. The one in Figure 4 is similar to Figure 3,
The thermal margin of all the ordinary fuel rods 10 in the outer periphery is improved, and the heat removal characteristics are excellent.

The fuel assembly may be of any type, such as one in which one or two water rods are arranged in the center. This water rod may be used conventionally.

〔Effect of the invention〕

In this way, the fuel assembly of the present invention has uniform thermal margins at various points in the horizontal cross section, has a large thermal margin as a whole, and has excellent heat removal characteristics. This has the effect of increasing the safety of the nuclear reactor.

[Brief explanation of drawings]

1 to 4 show examples of the fuel assembly of the present invention, in which FIG. 1 is a longitudinal side view, FIG. 2 is a sectional view taken along the line ■-■ in FIG. 1, and FIGS. Fig. 4 is a sectional view similar to Fig. 2 showing other embodiments, Fig. 5 is a characteristic diagram showing the relationship between cooling water flow rate and limit output, and Fig. 6 is a schematic sectional view of a boiling water reactor. , FIG. 7 is a sectional view similar to FIG. 2 showing a conventional fuel assembly, FIG. 8 is a partially cutaway perspective view of the fuel assembly, FIG. 9 is a partially cutaway side view of a fuel rod, and FIG. 10 is a power distribution diagram of a conventional fuel assembly. 9... Channel, 10... Ordinary long fuel rod, 1
7...Short fuel rod, 18...Fuel assembly. Applicant's agent Inomata Kiyoshi 1st Garden 1152 Zuman 3rd Eye 4th Garden! R Figure 5 Cooling P water flow rate 6th figure O decoy Figure 9 Leopard Figure 10 Collection size position

Claims (1)

[Claims] 1. In a fuel assembly formed by arranging long fuel rods in a square lattice in a channel, an appropriate number of the fuel rods located in the second row from the outside of the fuel assembly. A fuel assembly characterized by having shorter fuel rods than other fuel rods. 2. The fuel assembly according to claim 1, wherein the four fuel rods located at the four corners of the second row from the outside of the fuel assembly are formed shorter than the other fuel rods. body.