JPS6190084A - Nuclear fuel aggregate - 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 nuclear fuel assembly for a boiling water nuclear reactor, and more particularly to a nuclear fuel assembly with improved operating performance due to increased reactor shutdown margin.

[Technical background of the invention and its problems]

One of the unique features of boiling water reactors is negative reactivity, which reduces the neutron multiplication factor, and this allows the reactor to operate safely and stably.

This negative reactivity is caused by changes in the conditions inside the reactor, including changes in the temperature of the moderator water (moderator temperature reactivity, void reaction rgI), and changes in the temperature of the fuel rods (moderator temperature reactivity, void reaction rgI). Doppler reactivity). Temperature changes in the moderator cause the density of the moderator to decrease as the temperature of the moderator increases, and when the saturation temperature of the moderator is reached, bubbles are generated in the moderator, and as the temperature rises, the bubbles increase. 〃
j ratio increases. This decrease in the density of the moderator and the increase in the number of bubbles in the moderator work to decrease the rate of neutron fission (this is called the neutron multiplication factor) in the reactor. Due to temperature changes in the fuel rods, when the temperature of the fuel rods increases,
The thermal motion of the atomic nucleus increases, and the relative velocity between the neutron and the atomic nucleus that easily absorbs neutrons increases, increasing the number of neutrons absorbed and decreasing the rate of neutron fission in the reactor.

As the temperature inside the reactor increases in this way, the neutron multiplication factor decreases, working in the direction of lowering the temperature rise, which is called a 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 disappears, so the neutron multiplication factor increases. do. Therefore, the neutron multiplication factor is highest at 20°C, which is the lowest temperature of water in a boiling water reactor.
At this moment, the reactor core is ν with the 1III control rod fully inserted.
In order to prevent neutrons from escaping, the neutron multiplication factor (effective multiplication factor), which takes into account the leakage of neutrons in conventional reactors, must be less than 1.0. In an actual reactor core, the effective multiplication factor of the reactor core is always less than 0.99 even when all the control rods are inserted to allow a safety margin, and even when one control rod with the maximum reactivity value is completely withdrawn. I'm trying to make it happen. Also, the value obtained by subtracting the effective multiplication factor at this time from 1.0 is called the reactor shutdown margin.

With conventional fuel, the enrichment level of uranium was low, which made it possible to increase the margin for reactor shutdown. As a result, the reactor shutdown margin is becoming smaller.

In a boiling water reactor, a single-phase flow in a subcooled state flows from the bottom of the reactor core, and voids are generated due to heat generation within the reactor, and as many as 70% of the voids are generated in the upper part of the reactor core. Therefore, when comparing the upper and lower parts of the core, the thermalization of neutrons is more advanced in the lower part, and in pressurized water reactors there is an axial neutron distribution close to a cosine distribution, whereas in boiling water reactors the neutron peak The position of becomes larger at the bottom, and the output distribution behaves similarly. However, it is desirable that the power distribution be uniform within the core.

As a means of suppressing excessive reactivity in the reactor core, a small amount of gadolinia (Gd203) is injected into the fuel. Gadolinia has an unprecedented ability to absorb neutrons among naturally occurring elements, and also has excellent properties as a burnable poison. .

In conventional fuel rods, in order to flatten the distribution of axially protruding horns, the fuel rod is divided into two regions, an upper half and a lower half. uranium was mixed in, and the uranium was distributed so that the concentration was higher in the upper half than in the lower half. In such a fuel rod, the lower half of the fuel rod is made of gadolinia, which suppresses the output (neutron flux), and the upper half of the fuel rod has a high uranium enrichment, so output is produced in the upper half, and the output is 20 When the temperature is as low as ℃, there are no voids, so the output with a peak at the top becomes I[. For this reason,
Due to the neutron peak in the upper region of the fuel rods, the effective multiplication factor of the core increases, and when one control rod with the maximum reactivity (IIIi value) is completely withdrawn, the effective multiplication factor of the core becomes 0.99. As a result, the margin for reactor shutdown is small, and there is a limit to the margin for safely operating the core.

These factors lead to the use of fuel that has a high uranium enrichment, has a large reactor shutdown margin, and can flatten the axial power distribution.

[Purpose of the invention]

An object of the present invention is to provide a fuel assembly that can increase reactor shutdown margin while reducing uranium enrichment, prolonging nuclear reactor operation, and improving fuel economy.

(Summary of the Invention) The present invention divides the concentration distribution of burnable poisons such as gadolinia throughout a plurality of fuel rods into an upper region, a central region, and a lower region from the upper end of the effective fuel length along the axis of the fuel rod. In this case, by making both the upper region J5 and the lower region higher than the central region, the effective multiplication factor of neutrons is substantially lowered, and the reactor shutdown margin is increased.

[Embodiments of the invention] An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a perspective view showing an embodiment of the present invention.

As shown in FIG. 1, the nuclear fuel assembly 11 includes a plurality of fuel rods 21 arranged in a square shape, and an upper coupling plate 22 and a lower coupling plate that support the upper and lower ends of the fuel rods 21, respectively. 23, several spacers 24 located in the middle of the fuel rods 21 to regulate the gaps between the fuel rods, and a channel box 25 that covers these spacers. The fuel rods 21 of this nuclear fuel assembly 11 contain uranium dioxide (UO2), which is a fissile material, and gadolinia (GdO2), which is a burnable poison.
3) is loaded. The amount of uranium dioxide mixed in is almost the same over the entire length of the fuel rods 21, but gadolinia is mixed in some of the fuel rods 21, and the concentration distribution varies in the axial direction. It is mixed with

There are two ways to control the reactivity using gadolinia. One is to change the number of fuel rods containing gadolinia, and the other is to change the amount of gadolinia contained. In order to specifically understand this, the combustion behavior due to the difference in gadolinia concentration and the fuel behavior due to the difference in the number of fuel rods 21 containing gadolinia will be explained in detail with reference to FIG. 2.

In Figure 2, the vertical axis shows the negative reactivity due to gadolinia (△K G
) is shown as a percentage, and the horizontal axis shows the burnup (GW+) /
S T ). In the figure, the line △ indicates a fuel assembly consisting of five fuel rods mixed with 2W10 gadolinia;
Line B shows the behavior of gadolinia for a fuel assembly with five fuel rods mixed with 4W10 gadolinia, and line C shows the behavior of gadolinia for a fuel assembly with six fuel rods mixed with 4W10 gadolinia. There is. We describe this behavior and the technical costs of understanding it.

Gadolinia contains several types of gadolinium isotopes, among which Gd15 and Gd mainly act as reactive pollutants that easily absorb neutrons. These two types of iso-1 absorb neutrons and change into other nuclides that hardly function as neutrons m. This process is called ``Gadolinia burns.'' Since the neutron absorption cross section of Gadolinia is very large, most of the neutrons are captured only on the surface of Gadolinia distributed within the fuel pellets of the fuel rods 21, and the fuel pellets 1 to 1. Therefore, during the combustion period 1υ1, gadolinia exists up to the outermost surface of the fuel pellet, but as it burns, the effective surface of gadolinia as a neutral foal gradually becomes smaller in the shape of a cylinder, and finally disappears. I end up.

The rate at which the effective surface area decreases becomes slower as the concentration of gadolinia increases, so by adjusting the concentration it is possible to control the time when the reactivity toxicity of gadolinia disappears.

It goes without saying that the larger the number of fuel rods containing gadolinia, the larger the effective surface area of the entire gadolinia as a nuclear fuel assembly, and the greater the reactivity toxicity.

Therefore, as can be seen from FIG. 2, if the number of gadolinia-containing fuel rods 21 is the same but the zero temperature is different, the speed of the loss of gadolinia due to combustion will be different. That is, if the number of gadolinia-containing fuel rods 21 is the same and the concentration is small, it will disappear quickly (case A), and if left unattended, it will remain until later (case B). Also, if the concentration of gadolinia is the same but the number of gadolinia-containing fuel rods 21 is different, are the speeds of loss due to combustion the same?
0 reactivity ΔKG is different. That is, the gadolinia volume lα
If GJ is the same and the number of gadolinia-containing fuel rods 21 is large, the negative reactivity ΔKG becomes large (in the case of C), and if it is small, the negative reactivity ΔKG becomes small (in the case of B).

The effective multiplication factor is uranium (23
5U) causes nuclear fission, so the concentration of uranium decreases and the effective multiplication factor also decreases. Figure 3 shows an example of how the effective multiplication factor in the core decreases due to combustion when one control rod with the highest reactivity value is completely withdrawn from a state where all control rods are inserted at a cold temperature of 20°C. It is something that As shown in Figure 3, the effective multiplication factor of the core increases at the early stage of combustion, and if the reactor shutdown margin at this time is satisfied, all reactor shutdown margins are satisfied for the fuel that has progressed through combustion. It turns out.

The average power distribution in the axial direction of the core at time zero at J3 in FIG. 3, which is the initial stage of combustion, is as shown in FIG. 4.

In FIG. 4, the vertical axis shows the core height in percentage, and the horizontal axis shows the output distribution normalized to 1.0.

That is, the average power density is set to 1.0.

The circle indicates the behavior of the core according to the present invention, and the solid line indicates the behavior of the core where the gadolinia distribution is divided into upper and lower regions at 40% of the core height from the lowest part of the effective fuel length, with the lower part being 5% and the upper part being 4%. Show each. As can be seen from FIG. 4, in the nuclear reactor J3 of the present invention, the power density in the range of 80% to 100% of the core height is suppressed compared to the conventional core. This means that gadolinia absorbs neutrons well in the range of 80% to 100% of the core height, suppressing the peak of output power, and effectively reducing the effective multiplication factor of neutrons. There is. Therefore, in the present invention, the reactor shutdown margin is increased compared to the conventional method.

This effective multiplication factor is shown in the table below in concrete numerical values.

Here, the conventional example, Example 1, and Example 2 are reactor cores made of nuclear fuel assemblies including fuel rods 1 having a gadolinia distribution as shown in FIG. 5, respectively.

That is, the conventional example corresponds to FIG. 5a, and has a height of 0 to 40
Example 1 is a reactor core consisting of a nuclear fuel assembly made of 10 fuel rods with 5W10 gadolinia in the range of 10% and 8 fuel rods with 4W10 gadolinia in the range of 40% to 100% of the core height. Corresponds to Figure 5, core height 0 to 40
% and 10 fuel rods with 5W10 gadolinia in the range of 85-100% and 4 with core height in the range of 40-85%.
A core consisting of a nuclear fuel assembly comprising 8 fuel rods with gadolinia of W10, Example 2 corresponds to FIG.
10 fuel rods with gadolinia and core height 40~7
The core consists of a nuclear fuel assembly comprising 8 fuel rods with 4W10 gadolinia in the 5% range.

As is clear from the table above, Example 1 and Example 2
It can be seen that in both cases, the effective multiplication factor is lower than in the conventional example, and the reactor shutdown margin is increased.

In the present invention, when the fuel rod is divided from the upper end of its effective fuel length into an upper W4 region, a central region, and a lower region along the axis of the fuel rod, both the -F region and the lower region are the central region J:
It is sufficient that the burnable poison, such as gadolinia, is distributed in a high concentration. Therefore, the concentration of gadolinia may be distributed lower in the upper region than in the lower region, and each fuel rod is not necessarily divided into an upper region, a central region, and a lower region, and the ci degree of gadolinia in the central region is high. The distribution of gadolinia across the plurality of fuel rods along the axis of the fuel rods may be such that the concentration of gadolinia is low in the central region as described above.

Regarding the gadolinia distribution within the fuel rod, the reactor shutdown margin increases as the range of the upper region, which has a higher gadolinia concentration than the central region, increases; however, the rate of increase in the reactor shutdown margin increases as the range of the upper region increases. Therefore, it will decrease. FIG. 6 shows this situation. In FIG. 6, the II axis shows the reactor shutdown margin, the horizontal axis shows the range of the upper region, and the range of the upper region on the horizontal axis is shown by the number of nodes, where the total core height is 24.

The boundary between the upper region and the center region is within the range of 16/24 to 22/24 from the bottom of the effective fuel length, and the concentration of gadolinia is most effective in a fuel assembly in which the upper region is distributed higher than the center region. It is possible to increase the margin for reactor shutdown.

In the present invention, in order to suppress the output peak at the lower part of the fuel rod, the 11 degrees of gadolinia is distributed higher in the lower region than in the center region, but the boundary between the center 'W4 region and the lower region is from the lowest end of the effective fuel length. The output peak is most effectively suppressed in the range of 8/24 to 14/24, and in other ranges, the output peak at the lower part of the fuel rod becomes large, and the output distribution becomes distorted toward the lower part.

〔Effect of the invention〕

As explained above, in the reactor core composed of the nuclear fuel assembly of the present invention, the effective multiplication factor of neutrons is substantially reduced and the reactor shutdown margin is increased, and the margin for safely shutting down the reactor core is increased compared to the conventional method. The permeability of driving is expanded. Therefore, it is possible to cope with a longer operating period, and therefore the operating rate can be improved.

[Brief explanation of the drawing]

FIG. 1 shows a diagonal 61 showing an embodiment of the nuclear fuel assembly of the present invention.
Figure 2 is a graph showing the combustion characteristics due to differences in gadolinia a degree, Figure 3 is a graph showing the combustion characteristics with reactor shutdown margin, and Figure 4 is a graph showing the effect of the present invention compared to the conventional output. Distribution characteristic diagrams, Figures 5 (a), (b), and (c) are schematic vertical cross-sectional views of fuel rods showing the state of gadolinia distribution, respectively.
The figure shows changes in the reactor shutdown margin depending on the range of the upper region, which has a gadolinia distribution with a higher concentration than the central region. 11... Nuclear fuel assembly, 21... Fuel rod, 22...
- Upper coupling plate, 23... Lower coupling plate, 24... Spacer, 25... Channel box. Applicant's agent Hisashi Hatano Figure 1 Flute 4F! A Figure 3 Time (Monday) Thursday → 2 Output 2 degrees (1,01-, *Tadakashi Ishi) Figure 5 Figure 6'' 2nd part 1st place 1st relative (Note Kei) Procedural amendment form) % Formula % 1. Indication of the case Patent Application No. 211253 @ 1983 2. Name of the invention Nuclear fuel assembly 3. Relationship with each case to be amended Patent applicant (307) Toshiba Corporation (and 1 other person) 4. Agent Address: 14-2-5 Shinbashi 5-chome, Minato-ku, Tokyo 105 Date of amendment order: IJ: 19, 1985 (Shipping date: January 29, 1985) 6. Drawings of the specification subject to amendment Column 7 of ``Brief explanation of the contents of the amendment'', line 7 to line 9 of page 15 of the statement of contents of the amendment, ``Figure 5 (a), (b), and (c) are... Longitudinal sectional view,
is corrected to ``Figure 5 is a schematic vertical cross-sectional view of a fuel rod showing the distribution state of gadolinia.''

Claims (1)

[Claims] In a nuclear fuel assembly formed by bundling a plurality of fuel rods containing fuel mixed with gadolinia and surrounding them with a channel box, the fuel rods are divided into an upper region, a central region, and a lower region from the upper end of the effective fuel length along the axis of the fuel rods. When divided into regions, the concentration distribution of gadolinia throughout the plurality of fuel rods is such that both the upper region and the lower region are higher than the central region, and the boundary between the upper region and the central region is the same as the fuel effective length. 16/24 to 22/24 from the lowest end of the fuel effective length, and the boundary between the central region and the lower region is 8/24 to 14/24 from the lowest end of the fuel effective length.