JPS63231293A - Core for nuclear reactor - 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] [Industrial Application Field] The present invention relates to the core of a yX child reactor, and in particular, to securing margin for reactor shutdown and stability of a reactor loaded with highly enriched fuel, and furthermore,
The present invention relates to a boiling water reactor core suitable for flattening the power distribution in the axial direction of the core.

[Conventional technology]

Figure 2 shows the core configuration of a conventional boiling water reactor. The reactor core consists of a large number of fuel assemblies 1 arranged at equal intervals in a grid pattern.
, a control rod 2 inserted between the fuel assemblies 1 and an in-core instrumentation system 3.

Also shown in FIG. 3 is a cross-sectional view of a conventional 8x8 fuel assembly. The fuel assembly is a square tube channel box 1.
1 and a fuel bundle housed inside this channel box 11. The fuel bundle is composed of a plurality of fuel rods 6 regularly arranged in a square lattice shape and water rods 7 that are neutron moderation rods. On the other hand, the area around the channel box 11 is filled with gap water 8, which is a region of saturated water, so that the control rod 2 or the neutron detector instrumentation tube 3 can be inserted therein.

Coolant flowing from the lower part of the core to the upper part flows between the fuel bodies through the holes in the lower tie plate of the fuel assembly, is heated, becomes a two-phase flow, and flows out through the holes in the upper tie plate. As a result, a void fraction distribution occurs in the core axis direction,
The neutron moderation effect differs between the upper and lower parts of the core, which distorts the axial power distribution.

Furthermore, in current light water-cooled nuclear reactors, increasing the average enrichment of the fuel assembly and increasing the extraction burnup are being considered as a way to effectively utilize uranium resources. However, when an assembly is composed of highly enriched fuel, the amount of coolant relative to the fissile material decreases, which increases the average neutron energy and increases the absolute value of the void coefficient.
Control rod value decreases. These impair the stability of the nuclear reactor and are also unfavorable from the standpoint of effective utilization of fissile material. In addition, as the void coefficient increases, the reactivity that occurs during cold shutdown increases, and the reactor shutdown margin, which is a design standard established as an indicator of whether a reactor has the ability to safely shut down, decreases. .

To deal with the axial power distribution, there is a method of making the concentration in the upper part higher than in the lower part, as described in Japanese Patent Publication No. 58-29878. In addition, as a countermeasure for the reactor shutdown margin, there is a method described in Subcommittee F 4 (p. 320) of the Autumn 1986 of the Atomic Energy Society, which uses a large fuel body that can lengthen the blade length of the neutron absorption section, as shown in Ebisu.

Further, as a measure to reduce the void coefficient and the reactivity generated during cold shutdown, there is a method of reducing the amount of fuel loaded, as described in Japanese Patent Application Laid-Open No. 135989/1989.

[Problem that the invention seeks to solve]

The above-mentioned conventional technology is effective in either flattening the axial power distribution or ensuring margin for reactor shutdown, but cannot solve both problems at the same time. Furthermore, the method using long-blade control rods did not take into account the increase in reactivity at cold temperatures due to the increase in average neutron energy, and the void coefficient (absolute value) also increased.

On the other hand, the method of reducing the amount of fuel loaded does not take into account the spatial distribution of light water, which is a moderator, and requires a significant reduction in the amount of fuel loaded to reduce the increase in reactivity at cold temperatures. This will impair fuel economy.

An object of the present invention is to provide a nuclear reactor core that can solve this problem by increasing the cross-sectional area of the gap water region relative to the cross-sectional area of the fuel pellets in the fuel assembly, thereby reducing the average neutron energy. .

[Means for solving problems]

The above purpose is to remove fuel assemblies loaded on the outer periphery of the core in a boiling water reactor, as well as the central 80% of the fuel assemblies, excluding the upper and lower ends of the fuel assemblies. In the section perpendicular to the axial direction of
This is achieved by setting the ratio of the cross-sectional area of the gap region outside the channel box to 1.0 or more.

In the present invention, the gap water region is the diagonally shaded region shown in FIG.
Indicates the area outside the channel box within the area surrounded by . Further, the fuel pellet cross-sectional area refers to the total cross-sectional area of the fuel pellets existing in the area surrounded by the straight line A.

[Effect]

The premise of not changing the core structure of the present invention is to not change the interval P at which fuel assemblies are loaded (hereinafter abbreviated as fuel assembly pitch). Current, BW
Most R fuel assembly pitches are 6 inches (15,2cm).
m), and the following explanation will be made using 6 inches as an example, but the same effect can be obtained in the case of 6 inches as well.

Figures 4 and 5 show the increase in cold reactivity and the change in cold control rod value when the average enrichment is increased in the current assembly shown in Figure 3. Currently, the typical replacement fuel for BWR is enrichment 3w10. The average extraction burnup is 28 GWd/l. Average extraction burnup is 1.5
In order to more than double the amount, an enrichment level of 4w10 or more is required, but the increase in reactivity at cold temperatures will increase by about 1% compared to the current level, and the value of control rods will decrease by about 0.7%. As a result, the reactor shutdown margin, which is a design standard established as an indicator of whether the reactor has the ability to safely shut down, is approximately 2% Δkeit.
expected to decrease.

It is known to substantially improve this nuclear property by increasing the moderator-to-fuel ratio and decreasing the average neutron energy. The present invention focuses on the effect of the moderator existing outside the channel box.

Considering backfitting to the current reactor core, it is desirable to secure the same coolant flow area in the channel box as the current one from the standpoint of heat removal and stability due to pressure loss. To increase the moderator-to-fuel ratio while keeping the coolant flow area the same: (1) Decrease the fuel loading and increase the saturated water area (water rod area) inside the channel box.

(2) Reduce the fuel loading and increase the saturated water area (gap area) outside the channel box.

There are two possibilities.

FIG. 6 shows a comparison between the two in terms of their effectiveness in reducing the increase in reactivity at cold temperatures. (2) is about 1.3 times more effective. This is because moderator concentration is higher in the regap water region than in the water rond region, and neutrons are effectively moderated before they are absorbed by the fuel. As a result, in method (2), the thermal neutron flux in the gap region increases, and the value of the control rod also increases. On the other hand, in method (1), as the number of water rods is increased, the number of fuel rods must be reduced, and the thermal margin is reduced compared to method (2).

FIGS. 7 and 8 show the effects of the present invention.

It can be seen that the effects of the present invention can be obtained by increasing the amount of gap water relative to the fuel compared to the current fuel assembly described above.

Furthermore, according to this embodiment, since the amount of change in the moderator-to-fuel ratio due to the change in void ratio in the channel box is reduced, there is also the effect of flattening the power distribution in the core axial direction.

It can be seen that when the cross-sectional area ratio as shown in FIGS. 7 and 8 is set to 1.0 or more, sufficient reactor shutdown margin can be secured even for a fuel assembly with an enrichment of 4w10 or more.

〔Example〕

<Example 1> FIG. 1 shows the configuration of the core of a nuclear reactor of this example. This reactor core is a boiling water reactor of 1100 MWe class and is composed of 764 fuel assemblies. The aggregate pitch is 15.24 cm, and the present invention is applied to an aggregate having an average enrichment of about 6W10 as shown in FIG. 9 as a replacement fuel. Fuel pellets 21 to 26 shown in Table 1 are used.

27 is a cross-shaped water rod, and the cross-sectional area of the water rod area is approximately 9 a.
m2, which is larger than the current 3 am2. This serves to eliminate the distortion in the neutron flux distribution due to the increase in the gap water region and improve the thermal margin and fuel economy.

Table 1 In the present invention, the outer diameter of the fuel pellet is increased to 8.6 mm, and the outer diameter of the channel box is increased from the conventional 13.8 cm to 13 mm.
．． 6 cm, resulting in a ratio of the gap water region cross-sectional area to the sum of the cross-sectional areas of the pellets in all fuel rods present in the channel box of 1.11. As a result, by setting the enrichment level to 6w10, the cold reactivity increased by 2.5%Δ/k, and the control rod value decreased by 1.
This was solved by reducing the cold reactivity increase to 3.5% Δ/k and increasing the control rod value to 0.5% Δ/k. In addition, in this embodiment, the void reactivity difference can be made to the same level as the current one, so the core axial power distribution can also be made flat to the same level as in the case of the current enrichment.

Therefore, it is possible to realize a high burnup core that can extend the current take-out burnup by more than twice without making any changes to the core internal structure of the current core. The uranium saving effect of this core is approximately 20% compared to the current core, and the reprocessing amount is approximately 40% per unit output.
Can be reduced.

<Example 2> In this example, the present invention is applied to a core with an aggregate pitch of 15.5 cm. The enrichment distribution of the replacement fuel was the same as in Example 1.

In the present invention, by setting the outer diameter of the fuel pellet to 8.76 mm and the outer diameter of the channel box to 13.9 cm, the ratio of the cross-sectional area of the gap water area to the cross-sectional area of the pellet area is 1.
04. In this example, since the blade length of the control rod can be increased due to the increase in the aggregate pitch, Example 1
The same effect can be obtained.

〔Effect of the invention〕

According to the present invention, the gap water region cross-sectional area is made larger than the fuel pellet cross-sectional area, the neutron average energy is reduced,
Since the difference in neutron moderation effects existing above and below the core can be reduced, the reactor shutdown margin can be increased and the core axial power distribution can be flattened.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view of the core of an embodiment of the present invention, and FIG.
FIG. 3 is a cross-sectional view of a conventional reactor core, and FIG. 3 is a cross-sectional view of a fuel assembly.
Figure 4 is a diagram showing the relationship between concentration and increase in reaction at cold temperatures;
FIG. 5 is a diagram showing the relationship between enrichment and control rod value, FIGS. 6 to 8 are diagrams showing the effects of the present invention, and FIG. 9 is a cross-section of another actual flow side of the present invention. It is a diagram.

Claims (1)

[Scope of Claims] 1. In a nuclear reactor core loaded with a large number of fuel assemblies, both upper and lower ends of the effective length of the core, excluding the outermost periphery of the core, to which fuel is being delivered. 80 in the center excluding
%, the cross-sectional area of the coolant existing outside the channel box of the fuel assembly and the fuel in all the fuel rods existing inside the channel box in a cross section taken along a plane perpendicular to the axial direction of the core. A nuclear reactor core characterized in that the ratio of the cross-sectional area of the pellets to the total cross-sectional area of the pellets is 1.0 or more. 2. The nuclear reactor core according to claim 1, wherein the weight ratio of fissile material to nuclear fuel material in the unburned state of the fuel assembly is 4% or more.