JPH03214095A - Fuel assembly for boiling water reactor - Google Patents

Abstract

PURPOSE:To improve economics of a fuel of high burnup by a method wherein the content of a fissile material is made smaller in the upper part of a fuel assembly than in the lower part thereof and the number of fuel rods containing burnable poison is made larger 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 3.09wt.% and the number of its fuel rods containing gadolinia to be seven, while the concentration of the gadolinia is set to be about 2.5wt.%. The average enrichment of a lower fuel assembly is set to be about 3.35wt.% and the number of its fuel rods containing the gadolinia to be five, while the concentration of the gadolinia is set to be about 2.5wt.%. In the case when a lower peak is extreme in the middle period of a cycle, the lower peak can be eased by making the concentration of the gadolinia in the lower part higher than in the upper part, according to this constitution. When there is a sufficient allowance for output peaking, to the contrary, the lower peak can be made still higher by making the concentration of the gadolinia in the upper part lower than in the lower part.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Application Field) The present invention relates to a fuel assembly for a boiling water nuclear reactor that is particularly suitable for high burnup and has improved economic efficiency and safety.

(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 upwards, so the density of the moderator decreases at the bottom of the core. becomes smaller in parts. For this reason, power beaking tends to occur in the upper part of the reactor core, and reducing this has been an important issue 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 5-output molding. Ta. From this point of view, the above-mentioned moderator density distribution in the vertical direction of the core can be utilized to improve fuel economy. Namely.

In one cycle of operation, the output distribution is at the lower peak from the beginning to the middle, and the output distribution is at the upper peak at the end. This suppresses the combustion of uranium-235 in the upper part of the fuel and accumulates plutonium during operation.
Plutonium multiplied by 35 and Tj can be efficiently burned.

As an example of a fuel assembly that has been proposed to fully demonstrate this effect, Japanese Patent Application Laid-Open No. 58-196483 proposes a fuel assembly in which the uranium enrichment in the upper part is higher than that in the lower part, and gadolinia, which is a burnable poison, is added. The number of fuel rods contained in the upper part is greater than that in the lower part.

(Problem to be Solved by the Invention) Currently, as a means of improving fuel economy, increasing the enrichment of fuel and reducing the extraction burnup to the current level of approximately 30,000 MW
The plan is to gradually increase the burnup starting from d. 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 has a lower horn peak 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 power distribution has its lowest peak at the beginning of the first cycle, and gradually flattens out as combustion progresses, from the end of the first cycle to the 2.3rd cycle, and so on. This tendency is even stronger due to the final length and the extension of the fuel residence period in the reactor, and the power distribution tends to peak upwards.Also, with high burnup fuel, the number of fuel rods per fuel assembly is It is being considered to increase the number of engines from the current 60 to 62 to more than 70, and in that case, the restriction on output peaking will be relaxed compared to the current one, and this will be actively used to improve fuel economy. When operating at a lower peak, the burnup distribution tends to have an upper peak.

Due to the effects of such burnup distribution, in the fuel assembly according to the invention mentioned in section 2, the power distribution cannot be made into a sufficiently one-sided peak from the beginning to the middle of the operating cycle.

Here, if the enrichment in the upper part of the fuel is further increased in order to make the power distribution at the end of the cycle reach a sufficiently upper peak, the reactor shutdown margin, which is an index of subcriticality at the time of reactor shutdown, decreases. That is, since the neutron flux distribution at cold temperatures has an upper peak, increasing the enrichment in the upper part of the fuel causes an even higher peak, and as a result, the reactivity at cold temperatures increases and the reactor shutdown margin decreases. In order to avoid this decrease in the reactor shutdown margin, it is possible to increase the number of fuel rods containing burnable poison in the upper part or increase the burnable poison concentration, but such methods cannot sufficiently improve the reactor shutdown margin. Normally, from the perspective of improving economic efficiency, gadolinia is designed so that it burns out at the end of the cycle, so it is almost impossible to improve the reactor shutdown margin at the end of the cycle, when the reactor shutdown margin is considered to be the most severe. .

The first object of the present invention is to solve the problems of 5 or more, and to solve the above problems without reducing reactor shutdown margin, especially in high burnup fuel.
The objective is to improve fuel economy by enabling operation at a sufficiently low peak from the early to middle stages of the M conversion cycle and at a sufficiently high peak at the end of the cycle.

[Structure of the invention]

(Means for solving s problem) In order to solve the above problems, the present invention has the following features.

In a reactor fuel assembly, a boiling water type material consisting of a large number of fuel rods is bundled, and the number of fuel rods containing fissile material is smaller in the upper part than in the lower part of the fuel assembly, and the number of fuel rods containing burnable poison is reduced. More at the top of the fuel assembly than at the bottom.

(Function) Since the upper part of the fuel has a lower enrichment degree than the lower part, the reactivity is lower, so that the power distribution can be made to have a sufficiently downward peak. However, when combustion occurs at the lower peak, the burnup in the lower part advances faster than in the upper part, so the reactivity in the F part decreases faster, so no difference in reactivity occurs anywhere in the enrichment distribution. Therefore, by increasing the number of fuel rods containing burnable poison in the upper part, the power distribution can be made to have a double peak, especially from the early stage to the middle stage of combustion. Furthermore, since the upper part of the fuel is less enriched than the lower part, the reactor shutdown margin can be avoided even if the power distribution reaches an upper peak, especially at the end of the cycle.

(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 3.09v.
t%, the number of fuel rods containing gadolinia is 7, the gadolinia concentration is 2.5 wt%, and the average enrichment in the lower part is 3.
The number of fuel rods containing gadolinia is 5, and the gadolinia concentration is 2.5 wt%. FIG. 3 shows the combustion change with an infinite multiplication factor when the void ratio is 40%, where curve 4 is the fuel part and curve 5 is the lower part of the fuel. The infinite multiplication factor in the lower part is 1 due to the difference in enrichment and the difference in the number of fuel rods containing gadolinia.
′, greater than part.

As a conventional example for comparison, JP-A-58-196483
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 number of fuel rods containing gadolinia and the same degree of gadolinia, but the enrichment levels are reversed.

The infinite multiplication factor of the conventional fuel is shown by dotted lines in FIG. 3, where one curve 6 is the upper part of the fuel and the curve 7 is the lower part of the fuel. In this fuel, the infinite multiplication factor reverses to - just before the burnup when 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 difference between the upper and lower core average infinite multiplication factors (the value obtained by subtracting the lower infinite multiplication factor from the lower infinite multiplication factor). 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 in the vertical multiplication factor shifts to the negative side. do. 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 from the beginning of the cycle to just before the end of the cycle than in the conventional example, so that the output distribution has a downward peak. Figure 6 shows the core average void fraction. The core average void fraction increases as the power distribution peaks downward and its peaking value increases, and conversely, as the power distribution peaks upward and its peaking value increases, it decreases. In FIG. 6, the solid line 10 represents the present embodiment, the dotted line 11 represents the conventional example, and it can be seen that according to the present invention, the void ratio is high (that is, a downward peak) from the beginning of the cycle to just before the end.

In FIGS. 5 and 6, the characteristics at the end of the cycle do not differ much between the embodiment of the present invention and the conventional example.

However, during operation, the present example operates at a lower peak than the conventional example, so during this time, depletion of uranium-235 is more suppressed and more plutonium is accumulated in the upper part of the fuel. As a result, although the Hi average concentration is the same, in this example, the effective multiplication factor at the end of the cycle is increased to about 0.3%Δ compared to the conventional example.

In this example, the gadolinia concentration was equalized in two parts.
This may be varied by 1-1:. In other words, if there is an extremely downward peak in the middle of the cycle, the downward peak can be alleviated by making the gadolinia concentration in the lower part higher than in the upper part, and conversely, if there is sufficient margin for output peaking, [By increasing the concentration of a part of gadolinia higher than 5 parts, it is possible to make the peak even more extreme, especially in the middle of the cycle.

A second embodiment of the invention is shown in FIG. This fuel is an ultra-high burnup fuel with an extraction burnup of approximately 45,000 MVd/l, and as shown in the cross-sectional view of Fig. 8, 74 fuel rods 1
and two large-diameter water rods 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 reactor core, the total length of the upper end is 2724 mm, and the total length of the lower end is 1 mm. The part 24 is made of natural uranium that does not contain gadolinia, and the enrichment of the part 3724 of the total length immediately below the upper end is changed to reduce reactivity loss due to residual gadolinia at the end of the cycle and increase margin for reactor shutdown. Low gadolinia loading rat is reduced. The present invention is applied to the central portion, which occupies 18/24 of the total length excluding these.
Compared to the lower part, the average enrichment in the second part is 0.4 νL% lower, the number of fuel rods containing gadolinia is 4 more, and the gadolinia concentration in the fuel rods is 1.0 wt% lower.

As in the second embodiment, the present invention can fully exhibit its functions if applied to most of the center excluding the ends.

〔Effect of the invention〕

According to the invention, especially in high burnup fuels.

It is possible to operate at a downward peak from the beginning to the middle of the operating cycle, during which time the depletion of uranium-235 in the upper part of the fuel is suppressed and plutonium is accumulated, and the power distribution can be brought to a single peak at the end of the cycle. The upper uranium-235 and plutonium can be burned efficiently.

Furthermore, since the enrichment in the upper part of the fuel is lower than in the r part, the reactor shutdown margin can be improved. These effects can improve fuel economy without compromising safety.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the vertical enrichment 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. The first figure shows the combustion change of the upper and lower part of the fuel of Example A, Figure 4 shows the two-part enrichment and gadolinia distribution of the fuel of the conventional example, and Figure 5 shows the FIG. 6 is a diagram showing the vertical difference in the infinite multiplication factor in the core loaded with the fuel of the first embodiment of the invention, and FIG. 6 is a diagram showing the average void fraction in the core loaded with the fuel of the first embodiment of the invention. FIG. 7 shows the second embodiment of the present invention.
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 water rod

Claims (1)

[Claims] In a fuel assembly for a boiling water reactor consisting of a large number of fuel rods bundled together, the content of fissile material is lower in the upper region than in the lower region of the fuel assembly, and the number of fuel rods that contain burnable poison. 1. A fuel assembly for a boiling water reactor, characterized in that the amount of water is larger in an upper region than in a lower region of the fuel assembly.