JPS6161080B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for storing spent fuel assemblies, and particularly to a method for storing spent fuel assemblies taken out from boiling water nuclear reactors.

Spent fuel assemblies taken out from the core of a nuclear reactor are stored in a fuel storage pool. FIGS. 1 and 2 show a longitudinal sectional view and a plan view of the fuel storage pool. In the figure, reference numeral 1 indicates the entire fuel storage pool, 2 is the pool wall, and 3 is a fuel storage rack installed within the fuel storage pool 1. Third
Figure 4 is a plan view of a fuel storage rack loaded with spent fuel assemblies taken out from a boiling water reactor using a conventional method, and Figure 4 is a front view of Figure 3. In the storage rack 3, the spent fuel storage assemblies are uniformly loaded in the same direction with the handles facing up. In the figure, 5 indicates the anchor base of the fuel storage rack 3.

FIG. 5 is a diagram showing the combustion dependence of the fuel assembly due to the infinite multiplication factor, where the solid line indicates the void ratio of 70%,
The dotted lines indicate the case where the void ratio is 0%. Further, FIG. 6 is a diagram showing the void distribution in the axial direction of the core of a boiling water reactor, where the symbol V represents the void ratio and the symbol W represents the heat flux, respectively.
From the same figure, it can be seen that the void ratio in the upper part of the core is approximately 70%, and in the lower part of the core, the void ratio is approximately 0%.

As is clear from Figures 5 and 6, 70% of the fuel in the reactor is in the upper part.
Since combustion proceeds with different void ratios, such as 0% void ratio in the lower part, the formation ratio of plutonium isotope composition is different in the upper and lower parts of the core, and generally speaking, if the void ratio is high, the conversion of fuel isotopes is The rate increases, the rate of plutonium production increases, and the reactivity of the fuel (neutron multiplication rate, hereinafter the same) increases.

Therefore, if spent fuel assemblies taken out from boiling water reactors are stored uniformly in the same direction within a fuel storage pool, as is commonly done in the past, If high-reactivity areas and low-reactivity areas occur and the upper and lower reactivity balance is disrupted, and if high-reactivity areas concentrate, there is a risk that the subcriticality of the fuel storage pool as a whole will not be maintained.

For this reason, in order to meet the criticality standards for the fuel storage pool as a whole, the fuel enrichment level has been kept below a certain level, and the spacing between spent fuel assemblies within the fuel pool has been reduced. I try to make it bigger.

However, according to the above-mentioned conventional method of storing spent fuel assemblies taken out from boiling water reactors, it becomes impossible to use so-called highly flammable fuel with a high fuel concentration, or the capacity of the fuel storage pool is reduced. Considering the future increase in spent fuel assemblies, immediate improvements are desired.

Furthermore, JP-A-52-92090 titled "Reactor Fuel Assembly Storage Device" states that it "increases the storage rate of spent fuel assemblies per unit volume of fuel storage equipment." , the same publication stipulates that ``fuel assemblies that have a reactive part that contains fuel and a non-reactive part that does not contain fuel are to be stored along the axial direction.'' According to this, it is possible to increase the storage rate per horizontal unit volume of fuel storage equipment. Even if possible, the height dimensions of the fuel storage facility are not recognized. In other words, when adjoining spent fuel assemblies are arranged offset in the axial direction as in JP-A-52-92090, the height of the storage rack becomes high, and the spent fuel assemblies above it are Considering movement, it is necessary to deepen the water depth of the fuel storage pool.

An object of the present invention is to improve the storage rate of spent fuel fuel assemblies per unit volume in a fuel storage pool without increasing the horizontal and vertical dimensions of the fuel storage pool. An object of the present invention is to provide a method for storing spent fuel assemblies taken out from boiling water nuclear reactors.

A feature of the present invention is that a part of the spent fuel assembly taken out from a boiling water reactor is loaded into a fuel storage rack in a fuel storage pool with the handle turned upside down. The other spent fuel assembly is loaded into the fuel storage rack with the handle down, and the spent fuel assembly with the handle down and the other spent fuel assembly with the handle down. and the spent fuel assemblies are disposed adjacent to each other in the fuel storage rack.

Hereinafter, the present invention will be explained based on FIGS. 7 to 18.

7 to 14 show a fuel assembly direction changing device (hereinafter abbreviated as a changing device) suitable for use in carrying out the method of the present invention. In FIGS. 7 and 8, 7 is a support stand for the conversion device 6 installed at the bottom of the fuel storage pool 1, and the support stand 7 includes:
A rotation guide plate 8 is fixedly attached. Reference numeral 9 denotes a rotating body attached to the rotation guide plate 8 via a shaft 10, and on the back side of both upper and lower ends of the rotating body 9,
A roller 11 is installed, and this roller 11
is configured to rotate while being guided by the peripheral edge of the rotation guide plate 8. Furthermore, fuel assembly fitting caps 12 and 13 are removably attached to the upper and lower ends of the rotating body 9, respectively. FIG. 9 shows the structure of the fuel assembly mounting cap 12 that supports the upper end of the spent fuel assembly 4 taken out from the boiling water reactor. 9a is a partially cutaway plan view of section A in FIG. 7, and FIG. 9b is a partially cutaway front view of section A in FIG. 7. Therefore, inside the cap 12, there is a cavity 1 into which the upper end of the spent fuel assembly 4 is fitted.
4 is formed. Reference numeral 15 denotes a cap attachment key provided on the back surface of the cap 12, and this cap attachment key 15 is removably fitted into a key groove 16 formed in the rotating body 9. Next, a fuel assembly fitting cap 1 supporting the lower end of the spent fuel assembly 4 is installed.
The structure of No. 3 is shown in FIG. Figure 10 is Figure 7B
It is a partially cutaway side view of the section. Thus, a cavity 17 is formed inside the cap 13 into which the lower end of the spent fuel assembly 4 is fitted, and the cap 13, like the cap 12, can be attached to and detached from the rotating body 9. It is fitted. In FIGS. 7 and 8, reference numeral 18 indicates a fuel assembly support member attached to the center of the rotating body 9, and 19 indicates a stopper for preventing unstable movement of the rotating body 9 when it is stopped.

The configuration of the conversion device 6 suitable for carrying out the method of the present invention is as described above, and next, the vertical direction of the spent fuel assembly 4 taken out from the boiling water reactor using this conversion device 6 is as follows. We will explain the case of reversing.

7 and 8 show the spent fuel assembly 4 fitted into the conversion device 6. As shown in FIG. In this way, in order to fit the spent fuel assembly 4 into the conversion device 6, the fuel assembly fitting cap 12 is first removed from the rotating body 9 via the hoist mechanism. next,
The spent fuel assembly taken out from the core via the fuel gripper is inserted into the fuel assembly fitting cap 13 from above the rotating body 9 (the state shown in FIG. 10). The lower part of the spent fuel assembly 4 is the cap 13
Once the spent fuel assembly 4 has been fitted into the space 17 of the spent fuel assembly, the cap 12 is again attached to the upper end of the rotating body 9, and the upper part of the spent fuel assembly 4 is fitted into the space 14 of the cap 12 (the state shown in Fig. 9). . In this way, once the spent fuel assembly 4 is set at a predetermined position on the rotating body 9, the stopper 19 is moved back to release the previous restraint on the rotating body 9, and it is rotated via the rotating body drive mechanism. Rotate body 9 180 degrees. FIGS. 11 and 12 show a state in which the rotating body 9 is in the middle of rotation. The rotating body 9
Once it has rotated 180 degrees, the rotating body drive mechanism is stopped and the cap 12 is restrained by the stopper 19. This state is shown in FIGS. 13 and 14. next,
The cap 13 is removed from the rotating body 9, and then the spent fuel assembly 4, which has been reversed in the vertical direction, is taken out from the cap 12 and mounted on a fuel storage rack. Figure 15 shows the spent fuel assembly 4a with the handle turned down by reversing the vertical direction as described above, and the spent fuel assembly 4a taken out from the boiling water reactor with the handle turned up without performing the reversal operation. FIG. 16 is a front view of FIG. 15, FIG. 17 is a bottom view of section C in FIG.
7 is a sectional view taken along line DD in FIG. 7. FIG.

FIG. 19 is a diagram showing the neutron multiplication factor in the fuel storage pool obtained by implementing the method of the present invention in comparison with that of the conventional one, where the symbol X indicates the case where the storage method of the present invention is used. This shows the case where Y is stored using a conventional storage method. As is clear from the figure, the neutron multiplication factor in the fuel storage pool obtained by implementing the method of the present invention is suppressed to about half that of the conventional method. This shows that if the neutron multiplication factor in the fuel storage pool is kept at the same level as before, the pitch spacing of the fuel storage racks can be reduced to about half of the conventional one.

According to the present invention, in the storage of spent fuel assemblies taken out from boiling water reactors, spent fuel assemblies whose vertical directions are reversed and those whose vertical directions are not reversed are mixed and loaded into a fuel storage rack. Therefore, the reactivity distribution within the fuel storage pool can be made more uniform than before, and there is no need to increase the height of the fuel storage rack. As a result, when storing spent fuel assemblies taken out from boiling water reactors in the fuel storage pool, it is possible to store the spent fuel assemblies in the fuel storage pool without increasing the horizontal and height dimensions of the fuel storage pool. When the neutron multiplication rate of is maintained at the same level as before, the storage rate of fuel assemblies per unit volume in the pool can be improved.

[Brief explanation of the drawing]

Figure 1 is a vertical cross-sectional view showing the internal structure of the fuel storage pool, Figure 2 is a plan view of Figure 1, and Figure 3 shows the spent fuel assembly mounted in a conventional manner. A plan view of the fuel storage rack, Fig. 4 is a front view of Fig. 3, Fig. 5 is a diagram showing combustion dependence due to infinite multiplication factor of the fuel assembly, and Fig. 6 is the core axis of a boiling water reactor. Diagram showing directional void distribution,
7, 11 and 13 are front views of a fuel assembly direction changing device suitable for carrying out the method of the present invention, and FIGS. 8, 12 and 14 are respectively shown in FIGS. Figures 11 and 13 are side views, Figure 9a is a partially cutaway plan view of section A in Figure 7, Figure 9b is
7 is a partially cutaway front view of section A in FIG. 7, FIG. 10 is a partially cutaway side view of section B in FIG. 7, and FIG. 15 is a fuel storage rack in which spent fuel assemblies are mounted by the method of the present invention. 16 is a front view of FIG. 15, FIG. 17 is a bottom view of section C in FIG. 16, FIG. 18 is a sectional view taken along line DD in FIG. 17, and FIG. It is a diagram showing the neutron multiplication factor in the fuel storage pool obtained in comparison with that of the conventional method. 1...Fuel storage pool, 3...Fuel storage rack, 4, 4a and 4b...Spent fuel assembly,
6...Fuel assembly direction changing device.

Claims (1)

[Claims] 1. A part of the spent fuel assembly taken out from the boiling water reactor is loaded into a fuel storage rack in the fuel storage pool with the handle upside down so that it is at the bottom. The spent fuel assembly is loaded into the fuel storage rack with the handle facing up, and the spent fuel assembly with the handle facing down and the spent fuel assembly with the handle facing up. A method for storing spent fuel assemblies, characterized in that the spent fuel assemblies are placed adjacent to each other in the fuel storage rack.