JPS6276490A - Control rod 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 [Technical Field of the Invention] The present invention relates to a control rod for a nuclear reactor, such as a boiling water reactor, which increases the reactor shutdown margin.

[Technical past of invention and its problems]

Generally, in a boiling water reactor, one control rod is installed in every four fuel assemblies, excluding the area around the core, to control the power distribution. FIG. 6 shows a cross section of four fuel assemblies and one control rod assembled in this way 1°, that is, 1 is a fuel rod consisting of a large number of fuel rod bundles 1a and a channel box 1b surrounding the fuel rod bundles. The assembly 2 is a control rod for a nuclear reactor in which neutron absorption rods 3 are bundled.

Further, FIG. 7 shows the overall perspective configuration of a control rod for a nuclear reactor.

The control rod 2 for a nuclear reactor is constructed by installing a large number of neutron absorption rods 3 within a plurality of wings formed by attaching an elongated U-shaped sheath 5 to a central structural member 4. The neutron absorption rod 3 has a material with a large neutron absorption cross section such as B4C (hereinafter referred to as neutron absorption material) in the cladding tube made of SUS.
The cladding tube is filled with partitions arranged at regular intervals to prevent powder movement. Further, the same number of neutron absorption rods 3 (of the same length) are arranged in each wing.

84C in the neutron absorption rod 3 absorbs neutrons and gradually loses its neutron absorption ability, and during that time 10s reacts with the neutrons to generate 1-10 gas and increase the pressure inside the cladding tube. Hereinafter, the lifetime determined by the neutron absorption capacity will be referred to as the nuclear lifetime, and the lifetime determined by the pipe gas pressure will be referred to as the mechanical lifetime.

One of the unique features of boiling water nuclear reactors is a zero reactivity that reduces the neutron multiplication factor, which allows the reactor to operate safely and stably. This negative reactivity occurs due to changes in the conditions inside the reactor, including changes in the temperature of the moderator water (moderator hot water reactivity, void reaction), and fuel rod temperature changes. There is a change (Doppler reactivity). Due to changes in the composition of the moderator, the density of the moderator decreases, and when the saturation of the moderator reaches 4 degrees, bubbles are generated in the moderator, and the proportion of bubbles increases as the temperature rises. To increase. This reduction in the density of the moderator and the increase in the number of bubbles in the moderator work to reduce the rate of neutron fission within the reactor (this is called the neutron multiplication factor). Temperature changes in the fuel rods are caused by increases in the temperature of the fuel rods, which intensifies the thermal motion of the atomic nuclei, increases the relative velocity between the neutron-absorbing atomic nuclei, and increases the number of neutrons absorbed. This will reduce the rate of neutron fission in the reactor.

When the temperature inside the reactor increases in this way, the neutron multiplication factor decreases and moves in the direction of lowering the temperature rise, which is called negative reactivity. When the temperature inside the reactor rises, the neutron multiplication factor decreases due to negative reactivity, but when the temperature inside the reactor falls, the negative reactivity becomes negative, so the neutron multiplication factor increases. Therefore, the neutron multiplication factor is highest at a water temperature of 20°C, which is the lowest temperature in a boiling water reactor. The neutron multiplication factor (effective multiplication factor), which takes into account the leakage of neutrons inside a conventional reactor, must be kept below 1°0.In an actual reactor core, the maximum Even when one control rod with reactivity value is completely handled, the effective multiplication factor of the core is always kept below 0.99.In addition, the effective multiplication factor at this time is set to 1.0. The amount subtracted from this is called the reactor shutdown margin.

In a boiling water reactor, a subcooled single-phase flow flows from the bottom of the reactor core, and voids occur due to heat generation within the reactor, and 70% of the voids occur in the upper part of the reactor core. Therefore, when comparing the upper and lower parts of the reactor core, the thermalization of neutrons is more advanced in the lower part, and in boiling water reactors, the position of the neutron peak becomes larger in the lower part, and the power distribution behaves in the same way. In this way, the upper part of the core has a high void ratio during operation (the power density is somewhat low, so there is a relatively large amount of fissile material U-235 remaining, and the void ratio is high, so Pu-2
39 production rate increases.

Therefore, after the reactor is in operation, the concentration of fissile material in the upper part of the reactor core increases, reducing the margin for reactor shutdown in that area.

With conventional fuel, the enrichment of uranium was low, so it was possible to increase the margin for reactor shutdown even with conventional control rods.
As times change, the enrichment of uranium tends to increase due to reasons such as the lengthening of reactor operating periods and fuel economy, and as a result, the margin for reactor shutdown is decreasing.

[Purpose of the invention]

The present invention was made in view of the above circumstances, and can increase reactor shutdown margin compared to conventional control rods, thereby extending the reactor operation period and improving the operating rate. The purpose is to provide a control rod for a nuclear reactor that can achieve this.

(Summary of the Invention) In order to achieve the above object, the present invention provides a system in which neutron absorbing rods made by sealing a neutron absorbing substance in a metal cladding tube are installed in a nuclear reactor control rod d3 arranged in a wing, and the neutron absorbing rod is The feature is that the number of absorption rods is increased in areas where the axial distribution of the reactor shutdown margin is small, and is decreased in areas where the axial distribution of the reactor shutdown margin is large.

[Embodiments of the Invention] Hereinafter, one embodiment of the present invention will be described in detail with reference to FIGS. 1 to 5. Although the general structure of J3 and the control 211 rod itself is almost the same as that of the conventional one,
Also refer to FIG.

Figure 1 shows the arrangement of neutron absorption rods 3 used as atomic control rods. The arrangement configuration of neutron absorption rods 3 in one of the four wings of a nuclear reactor control rod is shown. Note that the arrangement configuration of the neutron absorption rods 3 in the other three wings is also the same.

That is, the neutron absorption rod 3 is configured to have two different lengths, and the length of each neutron absorption rod 3 is determined as follows.

FIG. 5 shows a known experimental example of the relationship between the number of neutron absorption rods and the reduction value of the effective multiplication factor of the core. From this figure, it can be seen that there is a proportional relationship between the number of neutron absorption rods and the reduction value of the effective multiplication factor.

The method of arranging the neutron absorption rods 3 will be explained as follows.

FIG. 2 (Δ) shows the axial distribution of the reactor shutdown margin (subcriticality) when the reactor is operated using conventional control rods with a uniform reactivity distribution in one direction. From this figure,
It can be seen that the degree of subcriticality is large in the upper part of the core, and is 1μ smaller at a position slightly lower than the upstream tJ. As mentioned above, when the core axis length is set, the upper part of the core, which is 1/2 L-L from the bottom, has a high void ratio during operation and has a slightly low power density, so the remaining time of U-235, which is a fissile material, is low. Since the amount of Pu-239 is relatively large and the void rate is high, the production rate of Pu-239 is high. As a result, the concentration of fissile material in the upper part of the reactor core increases after the reactor is in operation, reducing the margin for reactor shutdown in that area.

In this embodiment, the number and length of the neutron absorption rods 3 are determined in consideration of the subcriticality distribution in the axial direction of the core as described above.

That is, as shown in FIG. 2(C), the proportion of the neutron absorption rods 3 is determined in the form of a broken line that roughly approximates the subcriticality axial distribution curve of FIG. 2(A). In other words, subcriticality (reactor shutdown margin)
The number of neutron absorption rods in the small part is increased.

In FIG. 1, the number of neutron absorption rods 3 at the lower end 1/2L to L is determined to match the broken line in FIG. 2(C).

Here, a cross-sectional view of the neutron absorption rod 3 of FIG. 1, which has increased as shown by the broken line in FIG. 2(C), is shown at 30 in FIG.

As shown by 3a in Fig. 4, the part corresponding to 1/2 L-1- of the increased neutron absorption rod 3 in Fig. 1 contains neutron absorption H3ar, and the other part contains water, which is a moderator. Hollow tube 3 with a small hole on the side that can flow freely
It is 8″.

As mentioned above, since the axial reactivity distribution is adjusted depending on the number of neutron absorption rods, according to the atomic reactor control rod 2 according to this embodiment, ? As shown in the actual tfA in the t52 diagram (B), the axial distribution of the subcriticality (furnace shutdown margin) can be made almost uniform.

The number of neutron absorption rods 3 that can be added to the reactor control rods of existing boiling water reactors can be increased to about 4, based on the arrangement of the fuel assembly 1 and the reactor control rods 2. Therefore, Noboru 11! ! When applying Mokkomei to a water reactor, the number of neutron absorption rods can be adjusted within this range.

Figure 3 shows another embodiment, and Figure 3 (A) shows a nuclear reactor using conventional control rods with uniform reactivity distribution in the same axial direction as in Figure 2 (A). It shows the axial distribution of subcriticality (reactor shutdown margin) of the reactor during operation. In FIG. 3(C), the ratio of the neutron absorption rods 3 is defined in a broken line shape that approximates the subcriticality axial distribution curve in FIG. 3(A) to be thinner by b than in the m2 diagram (C).

However, in Figure 2 (C), the type of neutron absorption rod 3 is 2.
The number of types has been increased to three in Fig. 3 (-C).

In other words, as shown at 38' and 38'' in Figure 4, 1/2 corresponds to the increased number of neutron absorption rods on the broken line in Figure 3 (C).
L-L, 1/3L-1 contains neutron absorbers 3a', 3a'', and the other parts are hollow tubes 3a', 3a''2
The number of types of neutron absorption rods has been increased to three types.

As mentioned above, the axial distribution of the number of neutron absorption rods 3 is
Since it more accurately approximates the axial distribution of subcriticality in Figure 3(Δ), the subcriticality e as shown by the solid line in Figure 3(B)
, (furnace shutdown margin) can be made more uniform in the axial direction.

Note that the present invention is not limited to the above embodiments.

For example, the number of neutron absorption rods 3 in the core axis direction is divided into three or more types, and the number of neutron absorption rods 3 in the core axis direction is divided into three or more types.
It is also possible to determine the array configuration of

In addition, the position where the neutron absorption rods 3 are increased is not limited to the upper end in the axial direction of the reactor core, but can be adjusted to a subcritical distribution in the axial direction of the reactor core, and the neutron absorption rods 3 can be installed at the center or lower end in the axial direction of the reactor core. Even if you increase it by 3, it will not work. Furthermore, the number of neutron absorption rods may be reduced in a region along the core axis where the degree of subcriticality is large.

〔Effect of the invention〕

As explained above, according to the nuclear reactor control rod of the present invention, the number of neutron absorbing rods is increased in the region where the axial distribution of the reactor shutdown margin is small, and the axial distribution of the reactor shutdown margin is large. Since there are fewer neutron absorption rods in the control rods, the reactor shutdown margin is increased compared to conventional control rods, and it is sufficient to cope with higher uranium enrichment and longer reactor operating periods. Operation rate can be improved.

[Brief explanation of drawings]

Fig. 1 is a layout diagram of a neutron absorption rod according to an embodiment of the present invention, Fig. 2 (△), (B), and (C) are J-characteristic diagrams showing Ll< action in Fig. Figure 3 (△). (B) and (C) are dimensional characteristic diagrams showing the functions based on other embodiments of the present invention, FIG. A graph showing the relationship between the number of neutron absorption rods and the reduction value of the effective multiplication factor of the core. Figure 6 is a side view of the fuel r (assembly and control rod blades) of a boiling water reactor. Figure 7 is a side view of the R, IJ It is a perspective view of the entire control rod. 2... Nuclear reactor control rods, 3.3a', 3a''...
Neutron absorption rod. Applicant's agent Kuge Hatano 4th Chuse Sensu Station Katsumoto, Shikimo 5 Illustration 6 Illustration

Claims (1)

[Claims] 1. In a nuclear reactor control rod in which neutron absorption rods made by sealing a neutron absorption material in a metal cladding tube are arranged in a wing, the number of the neutron absorption rods is adjusted to the position where the axial distribution of the reactor shutdown margin is small. A control rod for a nuclear reactor, characterized in that the control rod is increased in areas where the axial distribution of the reactor shutdown margin becomes large, and decreased in areas where the axial distribution of the reactor shutdown margin becomes large.