JPH03214094A - Fuel assembly for boiling water reactor - Google Patents

Abstract

PURPOSE:To improve fuel economics of a fuel of high burnup by a method wherein the number of fuel rods containing burnable poison is made larger in the upper part of a fuel assembly than in the lower part thereof and the concentration of the burnable poison is made higher in the upper part of the fuel assembly than in the lower part thereof. CONSTITUTION:A fuel assembly is constructed sectionally of sixty-two fuel rods 1 and two water rods 2 arranged in the shape of a square lattice and surrounded by a channel box 3. The average enrichment of an upper fuel assembly is set to be about 3.39wt.% and the number of its fuel rods containing gadolinia to be eight, while the concentration of the gadolinia is set to be about 3.0wt.%. The average enrichment of a lower fuel assembly is set to be about 3.06wt.% and the number of its fuel rods containing the gadolinia to be five, while the concentration of the gadolinia is set to be about 2.0wt.%. According to this constitution, a core average void rate becomes large as a peaking value of an output distribution becomes large at its lower peak, while it becomes small as the peaking value of the output distribution becomes large at its upper peak, to the contrary. As the result, an effective multiplication constant in the end of a cycle becomes larger than usual, though the average enrichment is equal.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a boiling water nuclear reactor which is particularly suitable for high burnup and is economically oriented. related to fuel assemblies for use.

(Prior art) In the core of a boiling water reactor, voids occur in the coolant along the flow of coolant from the bottom of the core in the -L direction. becomes smaller. For this reason, power beaking tends to occur in the lower part of the reactor core, and reducing this has been one of the three important issues to date. However, in recent years, with improvements in the thermal and mechanical strength of fuel elements, it has become necessary to improve fuel economy in order to reduce power generation costs within the allowable range of output peaking. Ta. From this point of view, the above-described moderator density distribution in the downward direction of the core can be utilized to improve fuel economy. That is, in one cycle of operation, the output distribution is operated at the lower peak from the beginning to the middle period, and the output distribution is set to one of the four peaks at the end. As a result, during operation, the combustion of uranium-235 in the upper part of the fuel is suppressed and plutonium is accumulated, and at the end of the operation, the uranium-235 that remains in a part of the fuel and the accumulated plutonium can be efficiently burned. .

As an example of a fuel assembly proposed to fully exhibit such effects, Japanese Patent Application Laid-Open No. 58-19fi483
Now, let's make the uranium enrichment in the upper part larger than that in the lower part, and
The number of fuel rods containing gadolinia, a burnable poison, is greater in the upper part than in the lower part.

Furthermore, in Japanese Patent Application Laid-Open No. 82-276494, in addition to the above-mentioned example, the gadolinia concentration in the upper part is lower than that in the lower part. In either example, the output distribution can have a downward peak at the beginning of the operation cycle and an upward peak at the end.

(Problems to be Solved by the Invention) Currently, as a means to improve fuel economy, the concentration of fuel is increased, and the extraction burnup is reduced from the current level of about 3 (1000M).
It is planned to gradually increase the burnup starting from Wd/l. However, it has become clear that the above-described invention cannot sufficiently exhibit its effects on such high burnup fuels.

That is, since the power distribution peaks downward due to the moderator density distribution described above, combustion of the fuel progresses in the lower part than in the upper part, and as a result, the reactivity of the fuel decreases faster in the lower part. Therefore, the output distribution has its lowest peak in period 1) of the first cycle, and furthermore at the end of the first cycle and in the second period.
．． As the combustion progresses through 3 cycles, it gradually flattens out. With high burnup fuel, this tendency is even stronger due to the longer cycle length and the length of time the fuel stays in the reactor, and the power distribution tends to peak upward.

In addition, for high burnup fuel, it is considered that the number of fuel rods per fuel assembly will be increased from the current 60-132 to 70 or more, and in that case, the restriction on output peaking will be relaxed from the current level. By actively utilizing this and driving at a lower peak to improve fuel economy,
The tendency towards an upward peak becomes stronger due to the burnup distribution. Due to the effects of such burnup distribution, in the fuel assembly according to the invention described above, the power distribution cannot sufficiently reach a downward peak from the beginning to the middle of the operating cycle.

By the way, in fuels containing burnable poisons, the reactivity increases with combustion for a period of time until the burnable poisons are burned out, so if the lower part of the fuel burns faster than the upper part, the reactivity of the lower part will increase in the middle of the cycle. The output distribution then peaks downward. In Japanese Patent Application Laid-Open No. 02-278494, in order to avoid the downward peak of polarization in the middle of the cycle due to this reason, the concentration of four combustible substances in the lower part of the fuel is made higher than in the upper part.

However, as mentioned in [2], high burnup fuel tends to have an upward peak due to the burnup distribution, so this kind of burnable poison concentration distribution will conversely hinder the downward peak during the cycle, and as a result, the It becomes impossible to peak the output distribution sufficiently upward.

An object of the present invention is to solve the above-mentioned problems, and to enable operation with a sufficiently lower peak from the beginning to the middle of the operating cycle and an upper peak at the end of the cycle, especially when using high burnup fuel 1. The aim is to improve economic efficiency.

[Structure of the Invention] (Means for Solving the Problems) In order to solve the above problems, the present invention provides a fuel assembly for a boiling water reactor that is constructed by bundling a large number of fuel rods. The number of fuel rods containing poisonous substances is greater in the upper part of the fuel assembly than in the lower part (and the concentration of the flexible poisonous substance is higher in the upper part of the fuel assembly than in the lower part.Furthermore, in order to increase the effect, The content of fissile material fm may be greater in the lower part of the fuel assembly than in the lower part.

(Effect) In the upper part of the fuel, there are more fuel rods containing burnable poison than in the lower part, so the reactivity decreases, especially in the early stages of combustion. There is a downward peak at . Furthermore, since the concentration of burnable poisons in the upper part is high, the reactivity in the upper part can be maintained low until the burnup at which the burnable poisons burn out, so the power distribution of the core loaded with this fuel can be maintained at a downward peak until the middle of the cycle. I can do it. In addition, by increasing the amount of fissile material -1:i' in the upper part of the fuel than in the lower part, the power distribution at the end of the cycle can be made to have an even higher peak.

(Example) An example of the present invention is shown in FIG. FIG. 2 is a cross-sectional view of the fuel assembly of this embodiment, in which 62 fuel rods 1 and two water rods 2 are arranged in a square lattice shape, which is surrounded by a channel box 3. . As shown in Figure 1, in this example, the average concentration in the upper part is 3.39v.
t%, the number of fuel rods containing gadolinia is 8, and the gadolinia concentration is 3. Ovt%, and the average concentration at the bottom is 3.0
[iwt%, the number of fuel rods containing gadolinia is 5, the gadolinia concentration is 2, Ovt%. FIG. 3 shows the combustion change with infinite multiplication factor when the void ratio is 40%, where curve 4 is the upper part of the fuel and curve 5 is the lower part of the fuel. Due to the difference in the number of fuel rods containing gadolinia and the gadolinia concentration, the infinite multiplication factor in the lower part is larger than the upper part from the initial combustion stage until the burnup at which gadolinia burns out, and after gadolinia burns out, it reverses due to the enrichment difference.

As a conventional example for comparison, JP-A-62-278494
An example in which this is applied to the fuel assembly shown in FIG. 2 is shown in FIG. Compared to the embodiment shown in FIG. 1, this conventional fuel has the same enrichment and the same number of fuel rods containing gadolinia, but the gadolinia concentration is reversed below. The infinite multiplication factor of this conventional fuel is shown by dotted lines in FIG. 3, where curve 6 is the second part of the fuel and curve 7 is the lower part of the fuel.

In this conventional fuel, the infinite multiplication factor is reversed approximately halfway between the initial stage of combustion and the burnup at which gadolinia burns out.

The characteristics of the core loaded with the fuel of this example and the conventional fuel are shown below. Figure 5 shows the vertical difference in the core average infinite multiplication factor (the value obtained by subtracting the bottom infinite multiplication factor from the upper infinite image magnification). The solid line 8 represents this embodiment, and the dotted line 9 represents the conventional example. The infinite multiplication factor in Figure 3 is a comparison of the upper and lower parts with the same void ratio for explanation, but when loaded into the core, the void ratio is high in the upper part and lower in the lower part, so the difference between the upper and lower infinite multiplication factors shifts to the negative side. do. In addition, in this reactor core, approximately 173 fuels in the core are replaced with new fuel during fuel exchange, so approximately the same number of 1st cycle, 2nd cycle, and 3rd cycle fuels are mixed in the core. , these are each in the range shown in FIG. As can be seen from Fig. 5, in the fuel according to the present invention, the difference in reactivity between the upper and lower sides is smaller than in the conventional example, especially in the middle of the cycle, so the power distribution has a downward peak, and as a result, the difference in reactivity between the upper and lower sides at the end of the cycle is It becomes even larger and the output distribution peaks upward.

Figure 6 shows the core average void fraction instead of the power distribution. The core average void fraction increases as the power distribution peaks at a lower peak and its peaking value increases, and conversely, the core average void fraction decreases as the power distribution peaks at an upper peak and its peaking value increases. In FIG. 6, the solid line IO is the present example, and the dotted line II is the conventional example. According to the present invention, the void ratio is high from the beginning to the middle of the cycle (i.e., the lower peak), and the void ratio is low at the end of the cycle (i.e., the upper peak). )
You can see that it is done. As a result, although the average concentration is the same, in this example, the effective multiplication factor at the end of the cycle is increased to about 0.4Δ compared to the conventional example.

A second embodiment of the invention is shown in FIG. This fuel is a fuel for ultra-high burnup of approximately 45,000 MWd for extraction combustion.
As shown in Figure 8, there are 74 fuel rods 1 and 2.
It consists of a large diameter water rod 12. In this second embodiment, several measures have been taken to improve the fuel economy, and in order to reduce the leakage of neutrons out of the core, the total length of the two ends is 2124, and the total length of the lower end is 2124. 1/24th part is made of natural uranium that does not contain gadolinia, and 3/24th of the total length immediately below the upper end is used to reduce reactivity loss due to residual gadolinia at the end of the cycle and increase reactor shutdown margin. The concentration of gadolinia is lowered and the amount of gadolinia loaded is reduced. The present invention has a total length of 1
It is applied to the central part that occupies 8/24, and the number of gadolinia-containing fuel rods in the upper part is 4 more than in the lower part, and the gadolinia concentration in the fuel rods is 1 Ovt% higher.
Furthermore, the average concentration is 0.4vt% higher. As in the second embodiment, the present invention can fully exhibit its functions if applied to most of the center excluding the ends.

[Effects of the Invention] According to the present invention, especially in high burnup fuel, it is possible to operate at a lower peak from the beginning to the middle of the operation cycle, and during this period, depletion of uranium-235 in the upper part of the fuel is suppressed, plutonium is accumulated, and Since the power distribution can have an upward peak, the uranium-23 at the top of the fuel
5 and plutonium can be burned efficiently. This effect can improve fuel economy.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the concentrated skin and gadolinia distribution of the fuel according to the first embodiment of the present invention, FIG. 2 is a cross-sectional view of the fuel according to the first embodiment of the present invention, and FIG. FIG. 4 is a diagram showing the upper and lower enrichment levels (Δ rate combustion changes) of the fuel of the first embodiment of the invention, FIG. is a diagram showing the 4-day difference in infinite multiplication factor in the core loaded with the fuel of the first embodiment of the present invention, and FIG. 6 is the average void fraction in the core loaded with the fuel of the first embodiment of the present invention. FIG. 7 is a diagram showing the LT ja reduction and gadolinia content 41 of the fuel according to the second embodiment of the present invention, and FIG. 8 is a cross-sectional view of the fuel according to the second embodiment of the present invention. 1...Fuel rod 2...Water rod 3...Channel box 12...Large diameter tube Talorad

Claims (1)

[Claims] In a fuel assembly for a boiling water reactor consisting of a large number of fuel rods bundled together, the number of fuel rods containing burnable poison is greater in the upper region than in the lower region of the fuel assembly, and the concentration of the burnable poison is low. 1. A fuel assembly for a boiling water reactor, wherein the fuel assembly is higher in an upper region than in a lower region of the fuel assembly.