JPH0376434B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a fuel assembly for a boiling water nuclear reactor that is capable of suppressing reactivity at a low level during shutdown of a nuclear reactor.

[Technical background of the invention and its problems]

In a boiling water nuclear reactor, as shown in FIG. 1, a large number of fuel assemblies 2 are loaded in the center of a reactor pressure vessel 1 to form a reactor core 3. Moreover, cooling water 4 is accommodated inside the pressure vessel 1 up to the upper part of the reactor core 3 . In addition, a steam separator 5 is installed in the upper part of the pressure vessel 1.
and a steam dryer 6 are housed therein. Further, a main steam outlet nozzle 7 is provided at the upper part of the peripheral wall of the pressure vessel 1, and a water supply inlet nozzle 8 is provided below the main steam outlet nozzle 7.

The fuel assembly 2 includes a large number of fuel rods 10 in a rectangular cylindrical channel 9 as shown in FIGS. 2 and 3.
are arranged in a matrix (usually 8 x 8 pieces), their upper and lower ends are supported by an upper tie plate 11 and a lower tie plate 12, and multiple locations between the upper and lower tie plates 11 and 12 are supported by spacers 13. As a result, the fuel rods 10 are constructed so as to keep the mutual spacing constant.

As shown in FIG. 4, each fuel rod 10 has a structure in which a large number of cylindrical pellets 15 are stacked inside a cladding tube 14 and sealed by a spring 16 from above, and each pellet 15 is filled with uranium oxide. is formed into a certain shape by sintering the powder.

Therefore, the cooling water 4 in the pressure vessel 1 is boiled by nuclear fission of uranium in the reactor core 3, and the steam generated thereby is transferred to a steam separator 5 and a steam dryer 6.
The main steam is extracted through the main steam outlet nozzle 7 for use in driving the turbine of the power plant. Further, the steam that has passed through the turbine is cooled and liquefied in a condenser, and is again supplied into the pressure vessel 1 as cooling water 4 through a water supply inlet nozzle 8.

Figure 5 shows each fuel assembly 2 during reactor operation.
The thermal neutron flux Φ is high at the outer periphery of the assembly because it is in contact with a large amount of cooling water 4, and conversely, it is low at the center of the assembly.

On the other hand, it is desirable that the output P at various points in the horizontal cross section of the fuel assembly 2 during nuclear reactor operation has a substantially uniform distribution as shown in FIG. Therefore, since the output P is obtained as the product (Σ・Φ) of the thermal neutron flux Φ and the fission cross section Σ,
As shown in FIG. 5, the nuclear fission cross section Σ of the fuel assembly 2 is set to be low at the outer periphery of the assembly and high at the center, so that the output P at each part of the fuel assembly is approximately constant. In this way, the power distribution during operation can be adjusted by the fission cross section Σ, but the fission cross section Σ _f also changes depending on the uranium enrichment level. Therefore, by appropriately setting the distribution of uranium enrichment, the output during operation of each part of the fuel polymer can be made approximately constant.

On the other hand, when the operation is stopped, the infinite multiplication factor K∽
It is designed so that excessive deceleration occurs as the speed decreases. FIG. 6 shows the change in the infinite multiplication factor K∽ at cold temperatures using the water/uranium ratio as a parameter. Because of this design, when the operation is stopped, steam decreases as the temperature decreases, and the amount of cooling water increases accordingly, increasing the water/uranium ratio, decreasing the infinite multiplication factor K∽, and preventing over-deceleration. It becomes.

Incidentally, in the fuel assembly 2, one or two water rods 17 are mixed among a large number of fuel rods 10. For example, four such water rods 17 were prepared, and they were placed at various locations in the fuel assembly 2, and the relationship between the position of the water rods 17 and the infinite multiplication factor K∽ was investigated. We obtained the results shown in the figure. In other words, Water Rod 17...
When placed at the center of the aggregate (Fig. 7A), the infinite multiplication factor K∽ is the highest, and decreases as it approaches the outer periphery in order of B and C.
It decreases the most. Furthermore, it has been found that the degree of decrease becomes more remarkable as the distance is moved closer to the outer periphery.
From this, the same number of water rods 17...
In order to keep the infinite multiplication factor K∽ as low as possible by using ..., it is found that it is most effective to arrange them at the outer periphery of the aggregate, especially at the four corners.

[Purpose of the invention]

The present invention was made based on the above study results, and an object of the present invention is to provide a fuel assembly for a boiling water reactor that can suppress the reactivity during shutdown to a level lower than that of the conventional fuel assembly.

[Summary of the invention]

The fuel assembly according to the present invention is characterized in that the amount of uranium accommodated in the fuel rod accommodating areas at the upper portions of the four corners of the assembly is smaller than the average uranium accommodating value of all the fuel rod accommodating areas.

Note that the fuel rod accommodation area here is S×L/N However, S is the cross-sectional area inside the channel. L is the length of the pellet storage part in the fuel rod N is the number of fuel rods that can be accommodated in the channel shall mean.

With the above configuration, the infinite multiplication factor K∽ can be most effectively lowered by reducing the amount of uranium at the outer periphery of the aggregate, as is clear from the results shown in FIG.

[Embodiments of the invention]

First, a first embodiment shown in FIG. 8 will be described.

Figure 8A shows a cross section of the fuel assembly.
A large number of fuel rods 10 in a rectangular cylindrical channel 101
2... are arranged in a matrix (8 x 8 rods), and the upper portions of the four fuel rods 105 located at the outermost periphery, especially at the four corners, are made smaller in diameter than the other fuel rods 12... There is.

In this way, the amount of uranium sealed in the fuel rods 105 is smaller than in the other fuel rods 102, and the water-to-uranium ratio in the upper part of the assembly is increased. Therefore, the infinite multiplication factor K∽ when the operation is stopped decreases, and in particular, by reducing the amount of uranium at the four corners, the degree of decrease in the infinite multiplication factor K∽ becomes remarkable.

Therefore, the reactivity during shutdown of the nuclear reactor can be kept low.

In boiling water reactors, a large amount of plutonium (Pu) is usually accumulated in the upper part of the reactor core, and when the reactor is shut down, the reactivity in the upper part of the core tends to be higher than in the lower part. Therefore, if the fuel rods 105 at the four corners of the assembly are made smaller in diameter, especially in the upper part, to reduce the amount of uranium in that part, the infinite multiplication factor K∽ at the time of shutdown can be effectively reduced by a small amount of uranium. .

Based on the same idea, as in the second embodiment shown in FIG. The desired effect can also be obtained by filling the upper part of the fuel rod with a moderator instead of uranium, or by filling uranium fuel in the form of hollow pellets or low-density pellets to reduce the amount of uranium. can.

Note that the present invention is not necessarily limited to the above embodiments. For example, conventional fuel polymers typically have one or two water rods located in the center of the assembly, and such water rods may be used in a conventional manner.

〔Effect of the invention〕

As described above based on various embodiments, the fuel assembly of the present invention has a fuel rod accommodation area with a uranium capacity smaller than the average value of the uranium capacity of all fuel rod accommodation areas at the upper part of the four corners of the assembly. That is.

Therefore, in the present invention, the infinite multiplication factor K∽ at the time of shutdown is effectively reduced, the reactivity at the time of shutdown can be kept low, and the safety of the nuclear reactor can be improved.

Furthermore, since the void coefficient can be reduced and transient phenomena such as sudden pressure increases in the reactor can be alleviated, safety can be improved in this respect as well.

[Brief explanation of drawings]

Figure 1 is a schematic diagram of a boiling water reactor, Figure 2 is a cross-sectional view of a fuel assembly, Figure 3 is a perspective view of a fuel assembly, and Figure 4 is a partially cutaway side view of a fuel rod. figure,
Figure 5 is a diagram showing the power distribution of each fuel assembly, Figure 6 is a diagram showing the relationship between the water/uranium ratio and the infinite multiplication factor, and Figure 7 is a diagram showing the relationship between the water rod position and the infinite multiplication factor. , FIG. 8 shows a first embodiment of the present invention, where A is a cross-sectional view of a fuel assembly, and B is a cross-sectional view of A and B.
-B sectional view and FIG. 9 are cross-sectional views of a fuel assembly showing the second embodiment. 101... Channel, 102, 105, 10
6...Fuel rod.

Claims (1)

[Claims] 1. A fuel assembly for a boiling water reactor, characterized in that the amount of uranium accommodated in the fuel rod accommodating areas at the upper portions of the four corners of the assembly is smaller than the average uranium accommodating value of all fuel rod accommodating areas. body. 2. The fuel assembly for a boiling water nuclear reactor according to claim 1, wherein the upper portions of the fuel rods located at the four corners of the assembly are made smaller in diameter than the other fuel rods. 3. The fuel assembly for a boiling water nuclear reactor according to claim 1, characterized in that the upper portions of the fuel rods located at the four corners of the assembly are missing. 4. The fuel assembly for a boiling water nuclear reactor according to claim 1, wherein the uranium sealed in the upper part of the fuel rods located at the four corners of the assembly is a hollow pellet or a low-density pellet.