JPH0437391B2 - - 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.

[Technical background of the invention and its problems]

Generally, a boiling water reactor has a configuration as shown in FIG. Reference numeral 1 in the figure indicates a reactor pressure vessel. Inside the reactor pressure vessel 1, a reactor core 3 consisting of a plurality of fuel assemblies 2, control rods (not shown), etc. is installed, and cooling water 4 is accommodated. A steam separator 5 is installed above the reactor core 3, and a steam dryer 6 is further installed above the steam separator 5. A main steam outlet nozzle 7 is connected to the upper part of the peripheral wall of the reactor pressure vessel 1, and a water supply inlet nozzle 8 is connected below it. The main steam outlet nozzle 7 is connected to a main steam pipe (not shown). That is, the cooling water 4 rises from the bottom to the top of the core 3, and as it does so, its temperature rises, resulting in a two-phase flow state of water and steam. The cooling water 4 in a two-phase flow state is passed through a steam-water separator 5
The steam is separated into water and steam, and the steam flows into a steam dryer 6, where it becomes dry steam and is introduced into a power generation drive turbine via a main steam outlet nozzle 7 and a main steam pipe, and is used as a turbine drive source. The steam that has passed through the turbine is cooled and liquefied by a condenser (not shown) and flows into the reactor pressure vessel 1 again through the water supply inlet nozzle 8. On the other hand, the water descends through the annulus and flows into the lower part of the reactor core 3 again, and this cycle is repeated thereafter.

Next, the structure of the fuel assembly 2 will be explained with reference to FIGS. 2 to 4. 9 in FIG. 2 indicates a rectangular cylindrical channel. A plurality of fuel rods 10 are arranged in a matrix (for example, 8 rows x 8 columns) within this channel 9, and their upper and lower ends are supported by an upper tie plate 11 and a lower tie plate 12, respectively. . Spacers 13 are provided at a plurality of locations between the upper tie plate 11 and the lower tie plate 12, and the spacers 13 maintain a constant spacing between the fuel rods 10.

The fuel rod 10 has a configuration as shown in FIG. That is, inside the cylindrical cladding tube 14, a plurality of cylindrical pellets 15 made by sintering uranium oxide powder into pellets are stacked in the axial direction, and are pressed from above by an upper end plug 17 via a spring 16. It is structured as follows. Further, a lower end plug 18 is attached to the lower end.

In the above configuration, the thermal neutron flux distribution of each fuel assembly 2 is as shown in FIG. 5 during reactor operation. In FIG. 5, the horizontal axis represents the position in the fuel assembly 2, and the vertical axis represents the thermal neutron flux Φ (indicated by the broken line in the figure), the fission cross section Σ _f (indicated by the two-dot chain line in the figure), and the power P ( FIG. Thermal neutron flux Φ is high at the outer periphery of the fuel assembly 2 and decreases as it approaches the center. This is because the outer peripheral portion comes into contact with a large amount of cooling water 4. Further, it is desirable that the output P at various points in the horizontal section of the fuel assembly 2 during nuclear reactor operation be approximately constant as shown in FIG. On the other hand, this output P is obtained as the product (Σ _f・Φ) of the thermal neutron flux Φ and the fission cross section Σ _f , and therefore, the fission cross section Σ _f is calculated as By making the distribution low and high in the center, the output P can be made almost constant. In this way, the power distribution during nuclear reactor operation can be adjusted by the fission cross section Σ _f . This nuclear fission cross section Σ _f changes depending on the uranium enrichment, so by adjusting the distribution of the uranium enrichment, the output of each part of the fuel assembly 2 can be kept almost constant. . Further, during reactor operation, approximately 60% of the cooling water 4 in the channel 9 in the upper part of the fuel assembly 2 is steam. For this reason, as shown in FIG. 6, there is an insufficient deceleration state (as shown in the figure), and therefore the axial output distribution tends to be distorted downward. In Figure 6, the horizontal axis shows water/
It is a diagram showing the relationship between the water/uranium ratio and the infinite multiplication factor K∞, with the uranium ratio taken and the infinite multiplication factor K∞ plotted on the vertical axis.

Next, when the nuclear reactor is shut down, as shown in FIG. 6, the structure is such that over-deceleration occurs as the infinite multiplication factor K∞ decreases. That is, when the nuclear reactor is shut down, the temperature drops, the amount of steam decreases, and the amount of cooling water increases accordingly. Therefore, the water/uranium ratio increases and the infinite multiplication factor K∞ decreases, resulting in over-deceleration.

Incidentally, the fuel assembly 2 has a structure in which a plurality of fuel rods 10 are arranged in a matrix as described above, and one or two water rods 19 are usually mixed therein. Therefore, we prepared four water rods 19 and connected them to Fig. 7A,
When the water rods 19 are placed at various locations in the fuel assembly 2 as shown in B, C, and D, and the relationship between the position of the water rod 19 and the infinite multiplication factor K∞ is investigated, the results shown in FIG. 8 can be obtained. did it. FIG. 8 is a diagram showing the change in the infinite multiplication factor K∞ depending on the position of the waterrod 19, with the horizontal axis representing the position of the water rod 19 and the vertical axis representing the infinite multiplication factor K∞. As is clear from this, when the four waters 19 are located in the center (Fig. 7A), the infinite multiplication factor K∞
is the highest, Figure 7B, Figure 7C, and Figure 7D
As the water rod 19 approaches the periphery of the fuel assembly 2, the infinite multiplication factor K∞ decreases, and it decreases the most when it is located at the four corners of the fuel assembly 2 (FIG. 7D). It can be seen that the rate of decrease increases as the distance approaches the periphery. Therefore, in order to keep the infinite multiplication factor K∞ as low as possible using the same number of waterrods 19, it is optimal to position the waterrods 19 around the fuel assembly 2, especially at the four corners.

As mentioned above, in a boiling water reactor, it is necessary to suppress the infinite multiplication factor K∞ as small as possible when the reactor is shut down to suppress the reactivity at the time of the shut down, and to improve safety. Sometimes it is necessary to flatten the axial power distribution, which tends to be distorted downward, by promoting deceleration in the upper part of the core 3 and increasing the infinite multiplication factor K∞.

[Purpose of the invention]

The purpose of the present invention is to improve safety by suppressing the reactivity when nuclear reactor operation is stopped, and to flatten the axial output during operation. Our objective is to provide fuel assemblies for nuclear reactors.

[Summary of the invention]

The boiling water reactor fuel assembly according to the present invention comprises:
A fuel assembly for a boiling water reactor consisting of a plurality of fuel rods arranged in a lattice in a rectangular cylindrical channel, in which fuel rods with the upper fuel missing are arranged at the four corners of the channel. It is.

Therefore, in the present invention, the water-to-fuel ratio in the upper part of the fuel assembly increases, and when the reactor is shut down, the infinite multiplication factor K
Since ∞ is the lowest, the degree of reactivity can be effectively suppressed. Additionally, during reactor operation, the void ratio in the upper part of the fuel assembly decreases due to an increase in the water-to-fuel ratio in the upper part of the fuel assembly, which improves the neutron moderation effect, so it is possible to flatten the axial power distribution. can be achieved.

[Embodiments of the invention]

Referring to FIGS. 9 to 12, the first aspect of the present invention
An example will be explained. FIG. 9 is a diagram showing a schematic configuration of a boiling water reactor. Reference numeral 101 in the figure indicates a reactor pressure vessel. Inside this reactor pressure vessel 101, a reactor core 103 consisting of a plurality of fuel assemblies 102, control rods (not shown), etc. is installed, and cooling water 104 is accommodated. A steam separator 105 is installed above the core 103, and a steam dryer 106 is further installed above the steam separator 105. At the upper part of the peripheral wall of the reactor pressure vessel 101, there is a main steam outlet nozzle 107.
is connected thereto, and a water supply inlet nozzle 108 is connected below it. The main steam outlet nozzle 107 is connected to a main steam pipe (not shown). That is, the cooling water 104 is
3 rises from the bottom to the top, and at the same time, the temperature rises, resulting in a two-phase flow state of water and steam. The cooling water 104, which has become a two-phase flow state, is separated into water and steam in the steam-water separator 105, and the steam flows into the steam dryer 106 and becomes dry steam, which generates electricity through the main steam outlet nozzle 107 and the main steam pipe. It is introduced into a power turbine and used as a turbine drive source. The steam that has passed through the turbine is cooled and liquefied by a condenser (not shown), and then flows into the reactor pressure vessel 101 again through the water supply inlet nozzle 108. On the other hand, water descends through the annulus and flows into the lower part of the reactor core 103 again, and this cycle is repeated thereafter.

The fuel assembly 102 has a configuration as shown in FIG. In other words, fuel rods 110 are arranged in a matrix (8
The four fuel rods 110 located at the four corners are arranged in rows and eight columns, with their upper portions cut out. The channel 109 at the position of the missing fuel rod 110 is diagonally inwardly recessed to form a recessed portion 109A. Then, the cross section of the portion where this recessed portion 109A is formed is
1 and its lower cross section is shown in FIG. 12, respectively. In other words, the amount of uranium is reduced by lacking fuel (uranium) at the top of the four corners,
In addition, by forming the recessed portion 109A, the cooling water flow rate is increased and the water/uranium ratio is increased.

Therefore, when the reactor is shut down, the water/uranium ratio increases, which lowers the infinite multiplication factor K∞ and suppresses the reactivity. Since a large amount of plutonium (Pu) has been accumulated in the reactor core 103 and the reactivity in the upper part of the reactor core 103 tends to be higher than the lower part when the operation is stopped, it is effective to reduce the amount of uranium located in the upper part of the four corners and increase the flow rate of cooling water. It is possible to reduce the infinite multiplication factor K∞. In addition, during normal operation, by increasing the water/uranium ratio, the deceleration in the upper part can be improved and the infinite multiplication factor K∞ can be increased, which conventionally tends to cause downward distortion. It is possible to level out the axial power distribution.

Next, a second embodiment will be described with reference to FIGS. 13 to 15. In other words, 4 located at the four corners
It has a structure in which the upper part of a real fuel rod 110 is removed and a substantially L-shaped member 120 is attached. FIG. 14 shows a cross section of the portion where the substantially L-shaped member 120 is attached, and FIG. 15 shows a cross section below it. This makes it possible to reduce the amount of uranium at the top of the four corners and increase the flow rate of cooling water.
The same effects as in the first embodiment can be achieved. Further, a cooling water inlet 109B may be formed in the channel 109 located on the outer peripheral side of the member 120.

Next, a third embodiment will be described with reference to FIG.
That is, a hollow tube 122 is connected above a shorter fuel rod 121 than the conventional one in a state where the upper part lacks fuel, a cooling water inlet 123 is formed in the lower part of this hollow tube 122, and a cooling water outlet 124 is formed in the upper part. It is. Like conventional fuel rods, the fuel rod 121 has a structure in which a plurality of cylindrical pellets 126 are stacked inside a cladding tube 125 and pressed by an upper end plug 128 via a spring 127, and a lower end plug 129 is attached to the lower end. has been done. Further, an upper end plug 130 is attached to the upper end of the hollow tube 122, and is supported between an upper tie plate and a lower tie plate (not shown) via the upper end plug 130 and the lower end plug 129. be.

According to the above configuration, the amount of uranium in the upper portions of the four corners can be reduced and the flow rate of cooling water can be increased, so that the same effects as in the first and second embodiments can be achieved.

In the first to third embodiments described above, the water rods normally used in conventional fuel assemblies may be arranged as they are. In addition, when a neutron detector is placed in one of the four corners, in order to reduce the measurement error of the neutron detector, that corner is configured as conventionally, and the remaining one to three corners are placed with the present invention. Just apply.

〔Effect of the invention〕

The boiling water reactor fuel assembly according to the present invention comprises:
A fuel assembly for a boiling water reactor consisting of a plurality of fuel rods arranged in a lattice in a rectangular cylindrical channel, in which fuel rods with the upper fuel missing are arranged at the four corners of the channel. It is.

Therefore, in the present invention, the water-to-fuel ratio in the upper part of the fuel assembly increases, and when the reactor is shut down, the infinite multiplication factor K
Since ∞ is the lowest, the degree of reactivity can be effectively suppressed. Additionally, during reactor operation, the void ratio in the upper part of the fuel assembly decreases due to an increase in the water-to-fuel ratio in the upper part of the fuel assembly, which improves the neutron moderation effect, so it is possible to flatten the axial power distribution. The effects are great, such as the ability to improve

[Brief explanation of drawings]

Figures 1 to 4 are diagrams showing conventional examples. Figure 1 is a vertical cross-sectional view of a boiling water reactor, Figure 2 is a perspective view of a fuel assembly, and Figure 3 is a cross-sectional view of a fuel assembly. , Figure 4 is a partial vertical cross-sectional view of a fuel rod, Figure 5 is a power distribution diagram of a fuel assembly, Figure 6 is a diagram showing the relationship between water/uranium ratio and infinite multiplication factor, and Figure 7 is a diagram of a water rod. Figure 8 is a diagram showing the relationship between the position of the water rod and the infinite multiplication factor, Figures 9 to 12 are diagrams showing the first embodiment of the present invention, and Figure 9 is a diagram showing the relationship between the position of the water rod and the infinite multiplication factor. Fig. 10 is a perspective view of the upper part of the fuel assembly, and Fig. 11 is a cross-sectional view of the reactor.
The sectional view, FIG. 12, is a sectional view taken along the line XII-XII in FIG. 13 to 15 are views showing the second embodiment, in which FIG. 13 is a perspective view of the upper part of the fuel assembly, FIG. 14 is a cross-sectional view of FIG. 13, and FIG. - It is a cross-sectional view. FIG. 16 is a longitudinal sectional view showing the third embodiment. 102...Fuel assembly, 109...Channel, 109A...Recess, 110...Fuel rod.

Claims (1)

[Claims] 1. In a fuel assembly for a boiling water reactor in which a plurality of fuel rods are arranged in a grid in a rectangular cylindrical channel, fuel rods with the upper fuel missing are arranged at the four corners of the channel. A fuel assembly for a boiling water reactor characterized by: