JPH0519671B2 - - 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 Problems thereof] A boiling water nuclear reactor is generally constructed as shown in FIG. 1, and reference numeral 1 in the figure indicates a reactor pressure vessel.
A reactor core 3 loaded with a large number of fuel assemblies 2 is installed in the center of the reactor pressure vessel 1. Further, cooling water 4 is accommodated inside the reactor pressure vessel 1 up to the upper part of the reactor core 3 . A steam separator 5 and a steam dryer 6 are housed above the reactor pressure vessel 1. Furthermore, reactor pressure vessel 1
A main steam outlet nozzle 7 is connected to the upper part of the peripheral wall, and a water supply nozzle 8 is connected to the lower part thereof.

The fuel assembly 2 is constructed as shown in FIGS. 2 and 3. FIG. 2 is a perspective view of the fuel assembly, and FIG. 3 is a cross-sectional view of the fuel assembly. That is, the fuel assembly 2 has a large number of fuel rods 10 arranged in a matrix (e.g. 8×8) in a rectangular cylindrical channel 9, and these fuel rods 10...
The upper end portion and the lower end portion are supported by an upper tie plate 11 and a lower tie plate 12, respectively,
Further, by supporting the duplicated portions between the upper tie plate 11 and the lower tie plate 12 with spacers 13, the mutual spacing between the fuel rods 10 is maintained constant.

The fuel rod 10 is constructed as shown in FIG. FIG. 4 is a partially cutaway side view of a fuel rod, for example, a cladding tube 1 made of Zircaloy.
A large number of uranium fuels 15 formed as cylindrical pellets are stacked in the spring 1 from above.
Each uranium fuel 15 is formed into pellets of a certain shape by sintering uranium oxide (UO ₂ ) powder. In addition,
The upper and lower end openings of the cladding tube 14 are connected to upper end plugs 17.
A and each are sealed by a lower end plug 17B. In addition, in FIG. 3, the fuel rod 10A (indicated by G in the figure) is a fuel rod in which pellets of sintered mixed powder of UO ₂ and Gd ₂ O ₂ (Gatlinia) as a burnable poison are enclosed. Hereinafter, it will be referred to as a Gd-containing fuel rod.
In this case, the concentration of Cadolinia is about 5 wt%, and the number of Gd-containing fuel rods is about 7.

According to the above configuration, the cooling water 4 rises from below to above the reactor core 3 and is boiled by the thermal energy generated as a result of nuclear fission of the uranium fuel in the reactor core 3. The generated steam is taken out from a main steam outlet nozzle 7 via a steam separator 5 and a steam dryer 6 for use in driving a turbine of a power plant. In addition, the steam that has passed through the turbine (not shown) is transferred to a condenser (not shown).
The water is cooled and liquefied, and is again supplied to the pressure vessel 1 from the water supply inlet nozzle 8 as cooling water 4.

By the way, in the conventional fuel assembly, the enrichment of the uranium fuel 15 sealed in the fuel rods 10 is all the same, and ^{the enrichment} of the uranium fuel 15 in the vertical direction of the reactor core 3 is the same.
The concentration distribution was uniform as shown in Figure 5. Figure 5 is a diagram showing the ²³⁵ U enrichment distribution in the axial direction of the fuel rods, with the horizontal axis representing the enrichment (wt%) and the vertical axis representing the fuel rod position.
3.0 (wt%).

Furthermore, the amount of Gd mixed in was also uniform in the axial direction of the fuel rods, and for example, as shown in FIG. 6, Gd was mixed over the entire length of about seven fuel rods. Note that FIG. 6 is a diagram showing the distribution of the amount of Gd mixed in the axial direction, with the number of Gd-containing fuel rods plotted on the horizontal axis and the fuel rod position plotted on the vertical axis.

By the way, in general, the effective multiplication factor K _eff of the core 3 is proportional to the effective average infinite multiplication factor _∞ . Further, when the core power at position Z downward from the top of the core 3 is P and the infinite multiplication factor is K _∞ , the effective average infinite multiplication factor _∞ is approximately expressed by the following equation (). .

_∞ = ∫K _∞・P・dZ／∫P・dZ ... In particular, from the viewpoint of using fuel effectively, it is necessary to increase the effective average infinite multiplication factor _∞ at the end of the driving cycle.

The above infinite multiplication factor K _∞ is approximately proportional to the ²³⁵ U enrichment. Furthermore, when the burnup exceeds a certain value (10 ³ MWd/t), it becomes inversely proportional to the burnup as shown in Figure 7. Figure 7 shows the burnup on the horizontal axis and the infinite multiplication factor K _∞ on the vertical axis.
This is a diagram showing the infinite multiplication factor K _∞ change as the burnup changes. Diagram A is for an enrichment of 3.0 (wt%), and diagram B is for an enrichment of 3.5 (wt%). If the diagram C
indicates the case where the concentration is 4.0 (wt%).

However, conventionally, the vertical direction inside the fuel rod 10
²³⁵ U enrichment distribution and Gd contamination amount are almost uniform,
On the other hand, the distribution is such that combustion does not progress much at the upper and lower ends and burns well at the center, so the infinite multiplication factor K _∞ at the end of the cycle is 8th in the distribution.
The result will be as shown in the figure. Figure 8 is a diagram showing the axial distribution of the infinite multiplication factor (K _∞ ) and core power at the end of the cycle, with the horizontal axis representing the infinite multiplication factor (K _∞ ) and the core power. The infinite multiplication factor (K _∞ ) is shown by a solid line. In other words, the core power distribution is opposite to the infinite multiplication factor (K _∞ ) distribution, and the infinite multiplication factor (K _∞ ) is low where the core power is high. Therefore, as is clear from the above equation (), the effective average infinite multiplication factor
K _∞ could not be increased, which was not desirable in terms of increasing the reactivity of the core 3.

[Objective of the Invention] The present invention has been made based on the above points, and its purpose is to ensure that the vertical distribution of the infinite multiplication factor K _∞ exhibits the same tendency as the vertical power distribution of the core at the end of the cycle. By setting the ²³⁵ U enrichment distribution and the distribution of the number of Gd-filled fuel rods to make effective use of neutrons and increasing the average infinite multiplication factor _∞ , boiling can increase core reactivity without increasing uranium fuel. An object of the present invention is to provide a fuel assembly for a water reactor.

[Summary of the Invention] That is, the fuel assembly for a boiling water nuclear reactor according to the present invention is a boiling water nuclear reactor fuel assembly in which a plurality of fuel rods are housed in a channel and the upper and lower ends are supported by an upper tie plate and a lower tie plate. In a fuel assembly for a reactor, the fuel enrichment at the upper and lower ends in the axial direction of the fuel assembly is lower than in the center, and the content of burnable poisons at the upper and lower ends in the axial direction of the fuel assembly is lower than in the center. This is the configuration.

By providing such enrichment and Gd distribution, the areas with higher core power receive more uranium fuel and neutron irradiation at the end of the cycle, which makes it possible to utilize neutrons effectively and to reduce core reactivity. It becomes possible to increase the

[Embodiment of the Invention] An embodiment of the present invention will be described below with reference to FIGS. 9 to 16.
This will be explained with reference to the figures.

FIG. 9 is a longitudinal cross-sectional view showing one of the many fuel rods constituting a fuel assembly for a boiling water reactor. The fuel rod 101 is constructed by stacking a large number of uranium fuel rods 103A, 103B, ..., 103N formed as cylindrical pellets inside an elongated cladding tube 102, and pressing them from the upper end with a spring 104. The lower ends are sealed with an upper plug 105 and a lower plug 106, respectively. The above uranium fuel 103A, 103B,
..., 103N are cylindrical pellets formed by sintering uranium oxide powder;
^{The 235} U concentration is not constant as in the past;
As shown in FIG. 10, the concentration at the upper and lower ends is lower than that at the center. Figure 10 is a diagram showing the axial distribution of enrichment, with the horizontal axis representing the enrichment (wt%) and the vertical axis representing the fuel rod position.
2.50 (wt%) at the top and bottom, and at the middle
3.25 (wt%).

Note that it is not necessary to provide such an enrichment distribution to all the fuel rods 101, and it is sufficient that the enrichment distribution is provided in the axial direction as an average of the fuel assemblies.

Next, the Gd distribution will be explained. That is, as shown in FIG. 11, there are six Gd-containing fuel rods 111 in the cross section at the upper and lower ends of the fuel assembly, whereas in the center, there are eight Gd-containing fuel rods 111. configured to exist. Looking at this in each cross section, at the upper and lower ends, the 12th
As shown in the figure, the central portion is constructed as shown in FIG. 13. For example, as shown in FIG.
The Gd-filled fuel rod 111 has Gd at its upper end and lower end.
The fuel pellets in between are normal fuel pellets. In addition, Gd-filled fuel rod 1 shown in Fig. 13
11 has a Gd-containing fuel pellet in the center and an upper end,
The lower end is a regular fuel pellet.

Based on the above configuration, the operation will be explained with reference to FIGS. 14 to 16. In Figure 14, burnup (MWd/t) is plotted on the horizontal axis and infinite multiplication factor (K _∞ ) is plotted on the vertical axis.
This is a diagram showing the infinite multiplication factor (K _∞ ) change at the center (indicated by a broken line in the figure) and the infinite multiplication factor (K _∞ ) change at the upper and lower ends (indicated by a solid line in the figure). Fig. 15 is a diagram showing the axial distribution of the core power at the beginning of the cycle (indicated by the broken line in the figure) and the infinite multiplication factor (K _∞ ), and Fig. 16 is a diagram showing the core power at the end of the cycle (indicated by the broken line in the figure). ) and the axial direction of the infinite multiplication factor (K _∞ ). In other words, at the beginning of the cycle, the infinite multiplication factor (K _∞ ) at the center is large due to the _large amount of Gd despite the high fuel enrichment. It is lower than the multiplication factor (K _∞ ) (solid line _E1 point in the figure). For this reason, the core power distribution at the beginning of the cycle has an infinite multiplication factor (K _∞ ) as shown in Figure 15.
According to the distribution of , it is high at the top and bottom and low at the center.

Since combustion proceeds in such a state, the burnup distribution at the end of the cycle is similar to that shown in FIG. 15, that is, combustion proceeds at the upper and lower ends and does not proceed at the center. As a result, the infinite multiplication factor K _∞ at the upper and lower ends reaches point E ₂ on the solid line in Figure 14, whereas the infinite multiplication factor (K _∞ ) at the center reaches point D ₂ on the broken line. .

And the infinite multiplication factor (K _∞ ) distribution at the end of the cycle,
The core power distribution is as shown in FIG.
In other words, the core power distribution follows the distribution of the infinite multiplication factor (K _∞ ), with high power at the center and low power at the upper and lower ends. Therefore, the above formula ()
In , the effective average infinite multiplication factor _∞ increases because the core power is increasing in areas where the infinite multiplication factor K _∞ is high. At the end of the cycle, neutrons are effectively used for fuel in the central area with high reactivity, and by lowering the output at the upper and lower ends, neutrons leak to the upper and lower reflectors (not shown). This makes it possible to reduce the amount of fuel used, thereby making it possible to use fuel more effectively.

Note that the present invention is not limited to the configuration of the above embodiment; for example, as shown in FIG. 17, ²³⁵ U
The concentration distribution may be in three stages. Furthermore, regarding the Gd distribution, as shown in Figure 18, the cuts at the tip and center may be configured to be slightly shifted from the ²³⁵ U enrichment distribution, and various other configurations are possible, such as a three-stage configuration. .

[Effects of the Invention] As detailed above, the fuel assembly for a boiling water reactor according to the present invention accommodates a plurality of fuel rods in a channel and supports the upper and lower ends with an upper tie plate and a lower tie plate. In a fuel assembly for a boiling water reactor, the fuel enrichment at the axial upper and lower ends of the fuel assembly is lower than that at the center, and the burnable poison content at the axial upper and lower ends of the fuel assembly is lowered. This is a configuration in which the number of parts is smaller than that of the central part.

Therefore, it is possible to utilize fuel effectively, and the core reactivity can be increased without increasing the amount of fuel.

[Brief explanation of the drawing]

Figures 1 to 8 are diagrams showing conventional examples.
The figure is a schematic diagram of a boiling water reactor, Figure 2 is a perspective view of a fuel assembly, Figure 3 is a cross-sectional view of the fuel assembly, and Figure 4 is a side view showing a partially cut away fuel rod. Figure 5 is a diagram showing ^{the 235} U enrichment distribution in the core of a boiling water reactor, Figure 6 is a diagram showing the distribution of Gd mixed in, and Figure 7 is based on the burnup of infinite multiplication factor K _∞ with enrichment as a parameter. FIG. 8 is a diagram showing the core power distribution and infinite multiplication factor K _∞ distribution at the end of the cycle, and FIGS. 9 to 16 are diagrams showing an embodiment of the present invention.
Figure 9 is a vertical cross-sectional view of a fuel rod, Figure 10 is ^{a 235} U enrichment distribution map of uranium fuel in the fuel rod, and Figure 11 is a Gd
A diagram showing the mixed amount distribution, Figure 12 is a cross-sectional view of the upper and lower ends of the fuel assembly, Figure 13 is a cross-sectional view of the central part of the fuel assembly, and Figure 14 is the burnup at infinite multiplication factor K _∞ . Figure 15 shows the core power and infinite multiplication factor _K∞ distribution at the beginning of the cycle, Figure 16 shows the changes.
The figure shows the core power and infinite multiplication factor at the end of the cycle.
Figures 17 and 18 are diagrams showing the distribution of K _∞ , and Figures 17 and 18 are diagrams showing other examples. Figure 17 is ^{a 235} U enrichment distribution diagram, and Figure 18 is a Gd contamination distribution diagram. 2... Fuel assembly, 3... Core, 9... Channel, 11... Upper tie plate, 12... Lower tie plate, 101... Fuel rod, 103A~1
03N...Uranium fuel, 111...Gd-filled fuel rod.

Claims (1)

[Claims] 1. In a fuel assembly for a boiling water reactor, in which a plurality of fuel rods are accommodated in a channel and the upper and lower ends are supported by an upper tie plate and a lower tie plate, the fuel at the upper and lower ends in the axial direction of the fuel assembly is 1. A fuel assembly for a boiling water nuclear reactor, characterized in that the enrichment level is lower than that of the central part, and the burnable poison content of the upper and lower ends in the axial direction of the fuel assembly is lower than that of the central part.