JP2557414B2 - Fuel assembly for boiling water reactor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Field of Industrial Application) The present invention relates to a fuel assembly for a boiling water nuclear reactor with improved fuel economy.

(Prior Art) In a boiling water reactor core, voids are generated in the coolant along with the flow of the coolant from the lower part of the core to the upper part, so that the moderator density is large in the lower part of the core and small in the upper part. Become. Because of this, power peaking is likely to occur in the lower part of the core, and reducing it has been the purpose of previous development. For this reason, control rod patterns that flatten the output distribution during operation, and fuels that are divided into upper and lower two regions and in which the reactivity of the upper part is higher than that of the lower part have been proposed. Specifically, the purpose was to realize an output distribution close to the Haling distribution, which has been proven to be the most flat throughout the operation period. An example of such fuel is shown in FIG. 2 which shows the vertical enrichment / gadolinia distribution.

(Problems to be solved by the invention) However, in recent years, as the mechanical strength of the fuel element has improved, it is not always necessary to make the output distribution flat, but rather power generation within the allowable range of output peaking. There has been a demand for improving fuel economy in order to reduce costs.

From this point of view, the distribution of the moderator density in the vertical direction of the core can be used as a means for improving fuel economy. That is, in one operation period, the power distribution is greatly operated in the lower core from the start of operation to the middle period,
In the final stage, the power distribution is moved to the upper part of the core. As a result, plutonium can be accumulated in the upper part of the core during operation, and this plutonium can be actively burned in the final stage.

However, the fuel developed for the purpose of flattening the above-mentioned power distribution is not designed for such operation.

An object of the present invention is to provide a fuel assembly for a boiling water reactor, which has improved the above points and improved fuel economy.

[Configuration of the Invention] (Means for Solving Problems) In the present invention, a fuel assembly used in a boiling water reactor is divided into two regions, a first region and a second region, at upper and lower sides at an arbitrary boundary. It was designed so that the difference between the upper and lower reactivity obtained by subtracting the lower reactivity from the upper reactivity is positive and increases with combustion. Here, the upper reactivity and the lower reactivity mean an average of the reactivity from the center position of the fuel assembly to the upper end and an average of the reactivity from the lower end regardless of the boundary position of the region. Furthermore, in the present invention, the reactivity difference is 1.0 to 1.5% Δk at the beginning of operation and 1.5 to 3.0% Δk at the end of operation.
And the arbitrary boundary is 4 / of the total length from the lower end of the fuel assembly.
Designed to be in the 12-10 / 12 position.

That is, in the present invention, the first boundary is set in the vertical axis direction at an arbitrary boundary.
Area and a second area, and the uranium enrichment and gadolinia concentration of each of the first area and the second area are the upper reactivity of the fuel assembly (from the center position of the fuel assembly). The upper and lower reactivity differences obtained by subtracting the lower reactivity (average reactivity from the center position of the fuel assembly to the lower end) from the average reactivity up to the upper end are positive and distributed so as to increase with combustion. The difference is at the beginning of operation
1.0 to 1.5% Δk, 1.5 to 3.0% Δk at the end of operation, and the arbitrary boundary is 4/12 to 10/1 of the total length from the lower end of the fuel assembly.
The present invention relates to a fuel assembly for a boiling water reactor characterized by being located at the position 2.

(Operation) In the core loaded with the fuel assembly of the present invention, since the difference in vertical reactivity is not so large at the initial stage of operation, the power distribution is allowed to be peaking due to the moderator density distribution peculiar to the boiling water reactor. Although it occurs in the lower part of the core within the range, the difference in vertical reactivity increases with combustion, so it becomes possible to overcome the moderator density distribution and move the power distribution to the upper part at the end of operation. As a result, plutonium can be accumulated in the upper part of the core during operation, and this plutonium can be actively burned in the final stage.

(Example) The Example of this invention is described with reference to drawings.

FIG. 1 is a vertical enrichment and gadolinia distribution of a boiling water reactor fuel assembly according to an embodiment of the present invention. The fuel assembly of this example is divided into an upper portion (first region) and a lower portion (second region) at the center, the enrichment is 0.10 wt% higher in the first region, and the number of gadolinia rods is higher and lower. The number is equal to three, and the concentration is 1.0 wt% lower in the first region.

Fig. 2 is a conventional example designed for the purpose of flattening the output distribution, but it is divided into upper and lower parts at the same position as in the example of Fig. 1, and the enrichment is 0.18 wt% higher in the first region, The number of gadolinia rods is the same at the top and bottom, and the concentration is 0.5 wt% lower in the first region.

The difference between the upper and lower reactivity of these two fuels is shown in FIG. In FIG. 3, 31 is an embodiment of the present invention and 32 is a conventional example. In the conventional example, there is a constant upper and lower reactivity difference of about 2.5% Δk over the entire combustion period, whereas in the embodiment of the present invention, it is 1.3% Δk at the early stage of combustion and 3.0% Δk near the end stage.

Next, the axial power distributions when these fuels are loaded in all cores are shown in FIG. 4 (initial stage of operation) and FIG. 5 (end stage of operation). In these figures, 41 and 51 show the embodiment of the present invention, and 42 and 52 show the conventional example. In the conventional example, the output is extremely flat over the entire operation period, whereas in the example of the present invention, a peak appears in the lower part of the core in the initial stage of operation and a peak in the upper part in the final stage. As a result of controlling the power distribution in this way, in the embodiment of the present invention, it is possible to operate the reactor for the same period at an average enrichment lower by 0.04 wt% than the conventional example. This corresponds to an improvement of about 2% in utilization efficiency of natural uranium resources.

The embodiment of the present invention shown in FIG. 1 was a case where the fuel assembly was divided into upper and lower parts at the exact center, but next, a case where the boundary position is changed will be described.

Fig. 6 shows a natural uranium resource effective compared to the conventional example (Fig. 2) of a fuel assembly in which the difference in vertical reactivity falls within the range of the diagonal line in Fig. 7 by devising the distribution of enrichment and gadolinia. It is the improvement rate of the utilization rate. The improvement was possible only when the upper and lower boundary positions were within the range of 4/12 to 10/12 of the total length from the lower end. In FIG. 7, wherever the upper and lower boundary positions are, the average value of the reactivity from the center to the upper end of the fuel assembly is shown as the upper reactivity, and the average value of the reactivity from the center to the lower end of the fuel assembly is shown. It is called the lower reactivity and the difference is defined as the upper and lower reactivity difference. As can be seen from the figure, in order to use natural uranium resources more efficiently,
The difference in vertical reactivity is within the range of 1.0 to 1.5% Δk at the beginning of operation, increases with combustion, and is 1.5 to 3.0% near the end of operation.
It must be in the range of Δk. If the difference in upper and lower reactivity in the initial stage is smaller than this range, the output becomes too large in the lower core, and if it is large, the output cannot be sufficiently generated in the lower core and plutonium cannot be accumulated. . Also, if the difference in vertical reactivity at the end of operation is smaller than this range, it is not possible to generate sufficient output in the upper core, so it is not possible to burn the accumulated plutonium, while if it is large, the output peak in the upper core is increased. Grows too large.

[Effects of the Invention] As described above, the fuel assembly used in the boiling water reactor according to the present invention is divided into two regions, an upper region and a lower region, at an arbitrary boundary, and the difference in the vertical reactivity is positive, and the combustion is performed. If the enrichment and gadolinia distributions in the above two regions are distributed so as to increase with the above, it is possible to make the power distribution of the core loaded with this into a lower peak from the early stage to the middle stage of operation and an upper peak in the final stage. . As a result, the utilization efficiency of natural uranium resources can be improved.

In the fuel assembly in which the low enrichment blanket is arranged at the upper end or the lower end, the first and second regions are
This is a division of the part excluding the blanket part, and the difference in vertical reactivity is also defined for the part excluding the blanket part.

[Brief description of drawings]

FIG. 1 is a diagram showing the enrichment and gadolinia axial distribution of a fuel assembly, which is an embodiment of the present invention, and FIG. 2 is a diagram showing the enrichment and gadolinia axial distribution of a conventionally designed fuel assembly, FIG. 3 is a diagram showing changes in the vertical reactivity difference during the operation period of each fuel assembly shown in FIGS. 1 and 2, and FIG.
Figures 5 and 5 show the axial power distribution at the beginning of operation and the axial power distribution at the end of operation when the above fuels are loaded into the core, respectively. FIG. 7 is a diagram showing the relationship between the upper and lower boundary positions of the fuel assembly so as to fall within the range and the uranium resource utilization efficiency, and FIG. 7 is a diagram showing the difference in vertical reactivity during the operating period in the fuel assembly of the present invention.

Claims (2)

(57) [Claims] 1. An arbitrary boundary is divided into two regions, a first region and a second region, in the vertical axis direction, and the uranium enrichment and gadolinia concentration of each of the first region and the second region are divided. , The upper and lower reactivity difference (average reactivity from the center position of the fuel assembly to the upper end) of the fuel assembly minus the lower reactivity (average reactivity from the center position of the fuel assembly to the lower end) is positive. Moreover, it is distributed so as to increase with combustion, and the reactivity difference is 1.0 to 1.5% at the initial stage of operation.
Δk, 1.5 to 3.0% Δk at the end of operation, and the arbitrary boundary is located at a position of 4/12 to 10/12 of the entire length from the lower end of the fuel assembly, the fuel assembly for a boiling water reactor body. 2. The concentration range of the first region is higher than that of the second region, and the gadolinia concentration of the first region is lower than the gadolinia concentration of the second region. Boiling water reactor fuel assembly.